Cumulative

Question 1

What is the structure of the cell-surface membrane?

Answer 1

The cell surface membrane is a phospholipid bilayer. Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails, so the heads point outwards and the tails point inwards. It also contains intrinsic and extrinsic proteins, glycolipids and glycoproteins on the exterior surface, and cholesterol molecules.

Question 2

Explain the role of cholesterol in cell membranes.

Answer 2

Restricts movement of other molecules making up the membrane, increasing rigidity

Question 3

Describe how movement across membranes occurs by simple diffusion

Answer 3

Lipid-soluble, non-polar or very small substances move from an area of higher concentration to an area of lower concentration down the concentration gradient across the phospholipid bilayer. This is a passive process, meaning it requires no energy from ATP.

Question 4

Describe how movement across membranes occurs by facilitated diffusion

Answer 4

Water-soluble, polar, large molecules move down the concentration gradient through specific channel and carrier proteins. This is a passive process, meaning it requires no energy from ATP.

Question 5

Explain the role of carrier and channel proteins in facilitated diffusion

Answer 5

Channel proteins have a hydrophilic pore filled with water which allows water-soluble molecules to diffuse across the membrane. Carrier proteins facilitate the diffusion of larger molecules. When complimentary substances bind to the binding site, the carrier protein will change shape via conformational change to transport the substance across the membrane.

Question 6

Describe how movement across membranes occurs by osmosis

Answer 6

Water moves from an area of high water potential to an area of low water potential down a water potential gradient through a partially permeable membrane until equilibrium. This is a passive process, meaning it requires no energy from ATP.

Question 7

What is the maximum water potential and what substance would have this amount?

Answer 7

0ψ or 0kPA, distilled water

Question 8

Describe how movement across membranes occurs by active transport

Answer 8

Substances move from an area of lower concentration to an an area of high concentration, against the concentration gradient. This requires energy from the hydrolysis of ATP and also requires specific carrier proteins

Question 9

Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport

Answer 9

The complementary substance binds to specific carrier protein, and ATP also binds to the carrier protein, resulting in it being hydrolysed into ADP + Pi, releasing energy. This energy is used to change the shape of the carrier protein, transporting the substance to the other side of the membrane. Then, the inorganic Phosphate is released, causing the protein to return to the original shape

Question 10

Describe how movement across membranes occurs by co-transport

Answer 10

Two different substances bind to a
co-transporter protein (type of carrier protein) and move through it simultaneously. Often this involves movement of one substance against its concentration gradient and the movement of the other down its concentration gradient

Question 11

Name 4 factors affecting the rate of movement across cell membranes and explain how they affect it.

Answer 11

• Surface area of membrane: Increasing surface area increases the rate of movement
• Number of channel/carrier proteins: Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport
• Concentration gradient: Increasing concentration gradient increases rate of facilitated diffusion (until number of channel / carrier proteins becomes a limiting factor as all are saturated), simple diffusion and osmosis.
• Water potential gradient: Increasing water potential gradient increases rate of osmosis

Question 12

Explain the adaptations of some specialised cells in relation to the rate of transport across their membranes

Answer 12

• The membrane is folded, for example in microvilli in ileum. This increases surface area, increasing the rate of transport.
• More protein channels / carriers for facilitated diffusion or active transport (Carrier Proteins only)
• Large number of mitochondria to release more ATP and therefore releasing more energy by aerobic respiration for active transport

Question 13

What is genetic diversity?

Answer 13

Number of different alleles of genes in a population

Question 14

What are alleles and how do they arise?

Answer 14

Variations of a particular gene, they arise by mutation

Question 15

What is a population?

Answer 15

A group of interbreeding individuals of the same species.

Question 16

Explain the importance of genetic diversity

Answer 16

Enables natural selection to occur, as in certain environments, a new allele of a gene might be beneficial. Having advantageous genes increases chances of survival and reproductive success.

Question 17

What is evolution and how does it occur?

Answer 17

Change in allele frequency over many generations in a population. It occurs through the process of natural selection

Question 18

Explain the principles of natural selection in the evolution of populations

Answer 18

MARIA
* Mutation: Random gene mutations can result in new alleles of a gene
* Advantage: In certain environments, the new allele might benefit its possessor, so the organism has a selective advantage
* Reproductive success: Possessors are more likely to survive and have increased reproductive success
* Inheritance: Advantageous allele is inherited by members of the next generation (offspring)
* Allele frequency Over many generations, the advantageous allele increases in frequency in the population

Question 19

Describe 3 types of adaptations

Answer 19

• Anatomical - structural / physical features that increase chance of survival
• Physiological - processes / chemical reactions that increase chance of survival
• Behavioural - ways in which an organism acts that increase chance of survival

Question 20

What is directional selection?

Answer 20

Organisms with an extreme variation of a trait has a selective advantage over the other extreme. This results in an increased frequency of organisms with alleles for the extreme trait. An example is antibiotic resistance in bacteria.

Question 21

What is stabilising selection?

Answer 21

Organisms with an average variation of a trait has a selective advantage. This results in an increased frequency of organisms with alleles for the average trait. An example is human birth weight.

Question 22

What is disruptive selection?

Answer 22

Organisms with either extreme
variation of a trait has a selective advantage. This results in an increased frequency of organisms with alleles for both extremes of the trait. An example is bird beak size

Question 23

Name 2 groups of Lipids

Answer 23

Triglycerides and Phospholipids

Question 24

Describe the structure of a fatty acid.

Answer 24

Has a variable R Group which is a hydrocarbon chain which is either saturated or unsaturated. It also has a Carboxyl group.

Question 25

What is the difference between saturated and unsaturated fatty acids?

Answer 25

Unsaturated fatty acids contain one or more Carbon-Carbon double bonds, creating kinks in the hydrocarbon chain. Saturated fatty acids contain no Carbon-Carbon double bonds, and therefore all Carbons are fully saturated with Hydrogen.

Question 26

How do triglycerides form?

Answer 26

Triglycerides consist of 1 glycerol molecule and 3 fatty acids, which join via condensation reactions. Therefore, 3 molecules of water are released and 3 ester bonds form.

Question 27

Explain how the properties of triglycerides are related to their structure.

Answer 27

The hydrolysis of the fatty acid chains releases a lot of energy, meaning that triglyceride molecules are ideal energy stores. Also, the fatty acid chains are hydrophobic, meaning that triglyceride molecules are insoluble in water. Therefore, the molecules do not affect water potential, so no osmotic action occurs, damaging cells.

Question 28

Describe the difference between the structure of triglycerides and phospholipids.

Answer 28

One of the fatty acids of a triglyceride is substituted for a phosphate group in a phospholipid.

Question 29

Describe how the properties of phospholipids relate to their structure.

Answer 29

The phosphate group is hydrophilic and the fatty acid chains are hydrophobic. Therefore they can form membranes as they are selectively permeable, allowing certain substances into the cell and not others.

Question 30

Describe the test for lipids.

Answer 30

Add ethanol and shake, and then add water. If the test is positive, a milky emulsion will form on top.

Question 31

What is independent measures design? Give a strength and a weakness

Answer 31

Each participant only takes part in one of the conditions.
+ Order effects will not affect the results
- Differences between conditions may be due to participant variables

Question 32

What is repeated measures design? Give a strength and a weakness

Answer 32

Each participant takes place in all of the conditions
+ Participant variables will not affect the results
- Order effects may affect the results

Question 33

How can order effects be reduced?

Answer 33

Counterbalancing

Question 34

What is the matched pairs design? Give a strength and a weakness

Answer 34

Each participant takes part in one condition, however each participant is
matched according to certain characteristics with another participant in the
other condition.
+ The effect of order effects and participant variables are reduced
- Can be time consuming or difficult to match participants

Question 35

What is a laboratory experiment? Give a strength and a weakness

Answer 35

A laboratory experiment is an experiment carried out in a highly controlled environment, such as a laboratory.
+ Researchers can control for extraneous variables and ensure the procedure is
fully standardised and replicable, increasing reliability
- Results may lack ecological validity, as the artificial environment may cause unnatural behaviour from the participants

Question 36

What is a field experiment? Give a strength and a weakness

Answer 36

A field experiment is an experiment carried out the participants’ natural setting
+ Results are likely to be high in ecological validity as participants’ behaviour will be more natural
- Researchers have little control over extraneous variables, therefore decreasing reliability

Question 37

What is a quasi experiment? Give a strength and a weakness

Answer 37

A quasi experiment is an experiment in which the IV is not directly manipulated by the researcher
+ Useful for conducting research into naturally occurring variables or variables that would be unethical or impractical to manipulate
- Low reliability as the researcher has no control over the IV

Question 38

What is a monomer?

Answer 38

A smaller, repeating molecule from which larger polymers are made

Question 39

What is a polymer?

Answer 39

A larger molecule which is made up of identical repeating monomers.

Question 40

What is a condensation reaction?

Answer 40

A reaction in which two molecules join together, forming a covalent bond and releasing a molecule of water.

Question 41

What is a hydrolysis reaction?

Answer 41

A reaction in which the covalent bond between two molecules is broken, which uses up a molecule of water.

Question 42

Which polymer consists of repeating nucleotides?

Answer 42

Polynucleotides (DNA or RNA)

Question 43

Which polymer consists of repeating monosaccharides?

Answer 43

Polysaccharides

Question 44

Which polymer consists of repeating amino acids?

Answer 44

Polypeptides (Proteins)

Question 45

What is the difference between alpha and beta glucose?

Answer 45

In α-glucose, the H is above the OH whereas in β-glucose, the OH is above the H

Question 46

Draw a molecule of alpha glucose.

Answer 46

https://static.aqa.org.uk/assets/image/0018/235440/00055366-DA00046397-DB.png

Question 47

Draw a molecule of beta glucose

Answer 47

https://static.aqa.org.uk/assets/image/0008/235439/00055366-DA00046396-DB.png

Question 48

What are isomers?

Answer 48

Molecules with the same molecular formula but differently arranged atoms.

Question 49

What are disaccharides and how are they formed?

Answer 49

A disaccharide is two monosaccharides joined together with a glycosidic bond. They are formed by a condensation reaction, which releases a molecule of water.

Question 50

Which disaccharide does Glucose and Glucose make?

Answer 50

Maltose

Question 51

Which monosaccharides make Maltose?

Answer 51

Glucose and Glucose

Question 52

Which disaccharide does Glucose and Fructose make?

Answer 52

Sucrose

Question 53

Which monosaccharides make Sucrose?

Answer 53

Glucose and Fructose

Question 54

Which disaccharide does Glucose and Galactose make?

Answer 54

Lactose

Question 55

Which monosaccharides make Lactose?

Answer 55

Glucose and Galactose

Question 56

What are polysaccharides and how are they formed?

Answer 56

Polysaccharides are made up of many (more than 2) monosaccharides, joined together with glycosidic bonds. They are formed by many condensation reactions, releasing water molecules.

Question 57

Describe the basic function and structure of starch.

Answer 57

Starch acts as an energy source in plant cells. It is a polysaccharide of alpha glucose. It contains both Amylose and Amylopectin. Amylose has 1-4 glycosidic bonds and is therefore unbranched. Amylopectin has both 1-4 and 1-6 glycosidic bonds, meaning that it is branched.

Question 58

Describe the basic function and structure of glycogen.

Answer 58

Glycogen acts as an energy store in animal cells. It is a polysaccharide of alpha glucose. It has both 1-4 and 1-6 glycosidic bonds. Therefore, it is branched.

Question 59

Describe the basic function and structure of cellulose.

Answer 59

Cellulose acts as structural support in plant cells. It is found in the cell walls of plant cells. It is a polysaccharide of beta glucose, held together by 1-4 glycosidic bonds forming straight unbranched chains. Chains are joined together with hydrogen bonds forming microfibrils.

Question 60

Explain how the structure of starch is related to its function.

Answer 60

Starch is helical, so it is compact for storage in cell. Also, it is a large molecule, meaning it can not leave the cell. It is also insoluble in water, meaning that it does not affect the water potential of the cell, so no osmotic action which could damage the cell occurs. Also, it is branched to increase the surface area for faster hydrolysis.

Question 61

Explain how the structure of glycogen is related to its function.

Answer 61

Glycogen is branched are is therefore compact and more molecules can fit in small area. Also, the branching results in a larger surface area for the enzymes to quickly hydrolyse the glycosidic bonds to release glucose. Also, it is a large molecule meaning it can not leave the cell. It is also insoluble in water, meaning that it does not affect the water potential of the cell, so no osmotic action which could damage the cell occurs

Question 62

Explain how the structure of cellulose is related to its function.

Answer 62

Every second β-glucose molecule is inverted in a long, straight, unbranched chain. Many hydrogen bonds link parallel strands (crosslinks) to form microfibrils. Hydrogen bonds are strong in large quantities, so provides strength to plant cell walls

Question 63

Describe the test for reducing sugars

Answer 63

Firstly, add Benedict’s solution, which is blue to the sample. Next, heat the sample in a water bath. A positive test will result in a green, yellow, orange or red precipitate depending upon on the concentration of reducing sugars.

Question 64

Describe the test for non-reducing sugars.

Answer 64

If the result of the Benedict’s test is negative, there still could be non-reducing sugar present. Heat in a boiling water bath and add acid in order to hydrolyse into reducing sugars. Next, neutralise with alkali (e.g. sodium bicarbonate) Now, carry out the Benedicta test as normal by heating sample in a boiling water bath with Benedict’s solution. Positive result = green / yellow / orange / red precipitate.

Question 65

What is the one example of a non reducing sugar (that’s on the specification)?

Answer 65

Sucrose

Question 66

Suggest a method to measure the quantity of sugar in a solution without a colorimeter.

Answer 66

Carry out a Benedict’s test, then filter and dry the precipitate. Then, measure the mass of the precipitate.

Question 67

Suggest a method to measure the quantity of sugar in a solution using a colorimeter.

Answer 67

First, make a dilution series of sugar solutions of known concentrations. Next, heat a set volume of each sample with a set volume of Benedict’s solution. Use a colorimeter to quantitively measure the absorbance of each known concentration, and plot a calibration curve with . concentration on the x axis and absorbance on the y axis, and draw a line of best fit. Repeat the Benedict’s test with the unknown sample and find the concentration on the graph which is associated with the absorbance of the unknown sample.

Question 68

Describe the biochemical test for starch.

Answer 68

Add iodine dissolved in potassium iodide, which is orange/brown. A positive result will cause a colour change to blue/black

Question 69

Draw the general structure of an amino acid.

Answer 69

https://cdn.sanity.io/images/p28bar15/green/e7a792fa03f686507301a1d00a82af36777d3f91-960x670.png

Question 70

What is the molecular formula of the carboxyl group?

Answer 70

COOH

Question 71

What is the molecular formula of the amine group?

Answer 71

NH₂

Question 72

How many amino acids are common in all organisms and how do they vary?

Answer 72

There are 20 amino acids that are common in all organisms and they differ only in their R group side chain.

Question 73

How do amino acids join together?

Answer 73

Amino Acids join together via a condensation reaction, removing a water molecule. A Peptide bond is formed between the carboxyl group of one amino acids and the amine group of the other.

Question 74

What is a dipeptide?

Answer 74

2 amino acids joined together with peptide bonds.

Question 75

What is a polypeptide?

Answer 75

More than 2 amino acids joined together with peptide bonds.

Question 76

What is the primary structure of a protein?

Answer 76

Sequence of amino acids in a polypeptide chain, joined by peptide bonds.

Question 77

What is the secondary structure of a protein?

Answer 77

The folding of the polypeptide chain into alpha helices or beta pleated sheets due to hydrogen bonding between amino acids (between NH group of one amino acid and C=O group of the other)

Question 78

What is the tertiary structure of a protein?

Answer 78

The 3D folding of the polypeptide chain due to interactions between the R groups of the amino acids. Therefore, hydrogen bonds, ionic bonds and disulphide bridges are formed.

Question 79

What is the quaternary structure of a protein?

Answer 79

Made up of more than one polypeptide chain, formed by interactions between polypeptides.

Question 80

Describe the test for proteins.

Answer 80

Add Biuret reagent (sodium hydroxide + copper II sulphate), which is blue. A positive result will cause a colour change to lilac. This indicates the presence of peptide bonds and therefore the presence of proteins.

Question 81

How do enzymes act as a biological catalyst?

Answer 81

Enzymes catalyse reaction by lowering the activation energy. This speeds up the rate of reaction as less energy is needed to start it.

Question 82

Describe the lock and key model of enzyme action.

Answer 82

Substrate binds to the active site, which is completely complimentary to it. They bind to form an enzyme-substrate complex.

Question 83

Describe the induced fit model of enzyme action.

Answer 83

The substrate binds to the active site, which is not completely complimentary. This causes the active site to change shape so that it becomes complimentary to the substrate and an enzyme-substrate complex forms. This causes bonds in the substrate to change, lowering activation energy.

Question 84

How have models of enzyme action changed over time?

Answer 84

Initially, the accepted model of enzyme action was the lock and key model, which stated that the active site is fixed and it is exactly complimentary to the substrate. New molecular evidence has suggested the induced fit model, which states that the active site changes shape slightly in order for the substrate to be able to fit.

Question 85

Explain the specificity of enzymes.

Answer 85

The specific tertiary structure determines the shape of the active site. This is dependent on the sequence of amino acids (primary structure). The active site is complimentary to a specific substrate, and only this substrate can bind to the active site. The active site slightly changes shape in order for the substrate to fit, forming an enzyme-substrate complex.

Question 86

Describe and explain the effect of enzyme concentration on the rate of enzyme-controlled reactions.

Answer 86

As enzyme concentration increases, the rate of reaction also increases. The rate increases because the concentration of enzymes is increasing, and there are more available active sites for substrate molecules to bind to, and therefore more enzyme-substrate complexes are formed. At this point, enzyme concentration is the limiting factor as there is excess substrate. At a certain point, the substrate concentration becomes the limiting factor, so the rate will stop increasing because all of the substrates will be in use.

Question 87

Describe and explain the effect of substrate concentration on the rate of enzyme-controlled reactions.

Answer 87

As substrate concentration increases, the rate of reaction also increases. The rate increases because the concentration of substrates is increasing, and there are more substrate molecules that can bind to active sites, and therefore more enzyme-substrate complexes are formed. At this point, substrate concentration is the limiting factor as there is an excess of enzyme active sites, but at a certain point, the enzyme concentration becomes the limiting factor, so the rate will stop increasing because all of the enzymes will be saturated.

Question 88

Describe and explain the effect of temperature on the rate of enzyme-controlled reactions.

Answer 88

As temperature increases up to the optimum, the rate of reaction increases because the molecules have more kinetic energy, meaning that more collisions between enzymes and substrate molecules will occur, leading to more enzyme-substrate complexes being formed. After the optimum, the rate of reaction decreases as the enzymes denature. Due to the high temperatures, the hydrogen bonds and ionic bonds in the tertiary structure of the enzyme break, changing the shape of the active site. Therefore, the active site is no longer complimentary, meaning that fewer enzyme-substrate complexes can form.

Question 89

Describe and explain the effect of pH on the rate of enzyme-controlled reactions.

Answer 89

If the pH decreases or increases too much beyond the optimum, the rate of reaction decreases. This is because the H+ and OH- ions interfere with the hydrogen bonds and ionic bonds in the tertiary structure of the enzyme. Therefore, the shape of the active site changes and is no longer complimentary to the substrate, meaning that fewer enzyme-substrate complexes can form.

Question 90

Describe and explain the effect of competitive inhibitors on the rate of enzyme-controlled reactions.

Answer 90

As the concentration of competitive inhibitors increases, the rate of reaction decreases. Competitive inhibitors have a similar shape to the substrate, and they therefore compete for the active site. This means that the active site is occupied, so the substrate can’t bind to the active site and fewer enzyme-substrate complexes can form. Increasing the concentration of substrate will reduce the effect of the competitive inhibitor as the substrate will begin to outcompete the inhibitor.

Question 91

Describe and explain the effect of non-competitive inhibitors on the rate of enzyme-controlled reactions.

Answer 91

As the concentration of non-competitive inhibitor increases, the rate of reaction increases. Non-competitive inhibitors bind to the allosteric site, away from the active site. This leads to the tertiary structure of the enzyme changing, and the shape of the active site changing. Therefore, the active site is no longer complimentary to the substrate, so the substrate can not bind and fewer enzyme-substrate complexes form. Increasing substrate concentration will have no effect on the rate of reaction as the change to the tertiary structure is permanent.

Question 92

Draw and label the general structure of a nucleotide.

Answer 92

https://images.squarespace-cdn.com/content/v1/5c5aed8434c4e20e953d6011/1600528627495-YZ3SZ4AQ5LOSINZEEX1I/nucleotide.jpg

Question 93

Name 5 variables that could affect the rate of enzyme-controlled reaction.

Answer 93

Enzyme concentration, Substrate concentration, Temperature, pH, Inhibitor concentration.

Question 94

What is the basic function of RNA in living cells?

Answer 94

Transfers genetic information from DNA to ribosomes.

Question 95

What does a ribosome consist of?

Answer 95

RNA and Proteins

Question 96

What is the basic function of DNA in living cells?

Answer 96

Carries genetic information, which codes for proteins.

Question 97

Describe 3 differences between DNA and RNA.

Answer 97

In a DNA nucleotide, the pentose sugar in deoxyribose, whereas in RNA it is ribose. DNA contains the nitrogenous bases A, T, G and C, whereas RNA contains A, U, G and C. DNA molecules are double stranded and much larger than RNA molecules, which are much smaller and usually single stranded.

Question 98

How do nucleotides join together to form polynucleotides?

Answer 98

Nucleotides join via condensation reactions, which remove water molecules. This forms phosphodiester bonds between the phosphate group of one nucleotide and the deoxyribose/ribose of the other.

Question 99

Why did many scientists originally doubt that DNA carried the genetic code?

Answer 99

DNA is a chemically simplistic molecule which consists of very few components. Many scientists believed that the molecule that carries the genetic code would be much more complex.

Question 100

Describe the structure of DNA.

Answer 100

DNA is a polynucleotide. Each nucleotide is formed from a deoxyribose pentose sugar, a phosphate group, and a nitrogenous base. Adjacent nucleotides are joined by phosphodiester bonds, and the 2 polynucleotide chains are joined by hydrogen bonds between complimentary base pairs (Adenine and Thymine, Guanine and Cytosine) This forms a double helix structure.

Question 101

Describe the structure of RNA.

Answer 101

RNA is Polynucleotide. Each nucleotide is formed from a ribose pentose sugar, a phosphate group and a nitrogenous base (Adenine, Uracil, Guanine or Cytosine). Phosphodiester bonds join adjacent nucleotides, forming a single helix.

Question 102

How does the structure of DNA relate to it’s function?

Answer 102

• Double stranded - Both can act as template strands for semi-conservative replication.
• Hydrogen bonds between complimentary base pairs are weak - strands can be easily separated for replication.
• Complimentary base pairing - Accurate replication
• Many hydrogen bonds between complimentary base pairs - strong, stable molecule
• Double helix with sugar phosphate back bone - protects the bases and hydrogen bonds
• Long molecule - can store lots of genetic information, which codes for proteins.
• Coiled double helix - compact for storage

Question 103

How can you use incomplete information about the frequency of bases on DNA strands to find the frequency of other bases?

Answer 103

The % of adenine in strand 1 is equal to the % of thymine in strand 2 and vice versa. The % of guanine in strand 1 is equal to the % of cytosine in strand 2 and vice versa. This is because of complimentary base pairing between the 2 strands.

Question 104

How many hydrogen bonds are there between the 2 complimentary base pairings in DNA?

Answer 104

There are 2 Hydrogen bonds between Adenine and Thymine, and 3 Hydrogen bonds between Guanine and Cytosine

Question 105

Why is semi-conservative replication important?

Answer 105

It ensures genetic continuity between generations of cells.

Question 106

Why is DNA replication known as “semi-conservative”?

Answer 106

Each new DNA molecule consists of one strand from the parent DNA molecule, and one new strand.

Question 107

Describe the process of semi-conservative DNA replication.

Answer 107

1. DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix
2. Both strands act as template strands.
3. Free DNA nucleotides in the nucleoplasm are attracted to exposed bases and join by complementary base pairing
4. Hydrogen bonds form between complimentary base pairs (adenine-thymine and guanine-cytosine)
5. DNA Polymerase moves along the polynucleotide chain, joining adjacent nucleotides on new strand by condensation reactions, forming phosphodiester bonds.

Question 108

Why does DNA Polymerase move in opposite directions along the polynucleotide chains?

Answer 108

DNA has antiparallel strands, so the arrangements of nucleotides on each end are different. DNA Polymerase is an enzyme, and therefore has a specific active site, so it can only bind to substrate with a complimentary shape, which is the Phosphate end of each strand.

Question 109

Which two scientists proposed models of the chemical structure of DNA and DNA replication?

Answer 109

Watson and Crick

Question 110

Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication.

Answer 110

1. Bacteria was grown in medium containing heavy nitrogen (15N), so nitrogen is incorporated into the DNA bases. The DNA was extracted & centrifuged, so it settled near the bottom, as all DNA molecules contain 2 heavy strands
2. Bacteria that was grown in medium containing heavy nitrogen (15N) and was transferred to medium containing light nitrogen (14N) and was left to divide once - The DNA was extracted & centrifuged, so it settled in the middle, as all DNA molecules contain 1 original heavy strand and 1 new light strand
3. Bacteria in light nitrogen (14N) was allowed to divide again.
   The DNA was extracted & centrifuged, and half settled in middle, as
   contains 1 original heavy and 1 new light strand; half
   settles near top, as contains 2 light strands.

Question 111

What components make up ATP?

Answer 111

A ribose sugar, Adenine and 3 Phosphate groups.

Question 112

How is ATP broken down?

Answer 112

ATP is broken down via a hydrolysis reaction, using a molecule of water. ATP → ADP + Pi. This reaction is catalysed by the enzyme ATP Hydrolase

Question 113

Give two ways in which the hydrolysis of ATP is used in cells

Answer 113

• The hydrolysis of ATP releases energy, so this energy can be used to provide energy for energy requiring reactions within cells.
• The hydrolysis of ATP releases an inorganic phosphate, which can be used to phosphorylate other molecules in the cell, making them more reactive.

Question 114

How is ATP is resynthesised in cells?

Answer 114

ADP + Pi → ATP. This is a condensation reaction, and therefore releases a molecule of water. This is catalysed by the enzyme ATP Synthase and it occurs during respiration and photosynthesis.

Question 115

Suggest how the properties of ATP make it a suitable immediate source of energy for cells

Answer 115

• Releases energy in small, manageable amounts
• Hydrolysis of ATP is a single reaction as only one bond is hydrolysed to release energy. This means that the energy is released immediately.
• ATP molecules can not exit the cell

Question 116

Explain how hydrogen bonds occur between water molecules

Answer 116

Water is a polar molecule. The slightly negatively charged oxygen atoms attract the slightly positively charged hydrogen atoms of other water molecules.

Question 117

Explain the Specific Heat Capacity of Water

Answer 117

Due to the Hydrogen bonding, water molecules take more energy to separate them, so the boiling point is higher than expected. As water therefore has a higher specific heat capacity, it is a buffer to sudden temperature changes. This is important when organisms are mostly water

Question 118

Explain the Latent Heat of Vaporisation of Water

Answer 118

The Hydrogen Bonding also means that a lot of energy is required to evaporate 1kg of water. This means that sweat in mammals is a very efficient way of cooling as body heat is used to evaporate water.

Question 119

Explain Cohesion and Surface Tension in Water

Answer 119

Cohesion allows water to be pulled through tubes e.g. xylem vessels. Where water molecules meet the air, they are pulled back to the body of water, resulting in surface tension. This allows water to support small organisms.

Question 120

Give 3 ways that water is important to living organisms

Answer 120

• Metabolite - Water is used in hydrolysis and released in condensation, as well as it being a raw material in photosynthesis
• Solvent - Water dissolves gasses, wastes, ions and small hydrophilic molecules.
• Other - Evaporation cools organisms, isn’t easily compressed so allows turgor in plants, transparent so aquatic plants can photosynthesise.

Question 121

Describe the role of Hydrogen ions

Answer 121

They maintain pH levels in the body, therefore affecting enzyme action.

Question 122

Describe the role of Iron ions

Answer 122

They are a component of haemoglobin, therefore allowing oxygen to bind for transport as oxyhaemoglobin.

Question 123

Describe the role of Sodium ions

Answer 123

They are involved in the co-transport of glucose and amino acids into cells. They are also involved in action potentials in neurons and they affect the water potential of cells, causing osmosis into and out of the cell.

Question 124

What is Observational Research?

Answer 124

A research method in which data is collected by watching participants’ behaviour

Question 125

Give 2 strengths of observational research.

Answer 125

• Observational research often produces valid results as we can investigate how people actually behave, rather than how they said they would, like in self report methods.
• Observations can be used when it would be unethical or impractical to manipulate the Independent Variable.

Question 126

Give 2 weaknesses of observational research.

Answer 126

• Difficult to replicate as the way people act in a certain scenario may not be exactly the same every time.
• Results may be affected by observer bias, reducing the validity of the results.

Question 127

What is a structured observation?

Answer 127

An observation in which observers collect data by recording the frequency of predetermined behaviours using a coding scheme.

Question 128

Give 2 strengths of structured observations.

Answer 128

• Produces quantitative data which is easy to analyse and compare between conditions
• More objective and reduces chance of observer bias

Question 129

Give 2 weaknesses of structured observations.

Answer 129

• May produce invalid results as the behaviours recorded may not be representative of all behaviours that could occur
• Produces quantitative data which lacks detail, therefore doesn’t provide detail and reasons for behaviour

Question 130

What is an unstructured observation?

Answer 130

An observation in which observers collect data by writing down all behaviour that occurs in the observation period

Question 131

Give 2 strengths of unstructured observations.

Answer 131

• Collects qualitative data which is rich in detail and can explain reasons for behaviour
• Unlike in structured observations, all behaviours will be recorded, so the results will be more valid

Question 132

Give a 2 weaknesses of unstructured observations.

Answer 132

• As observers are recording everything that occurs, they may miss certain behaviours, therefore decreasing the reliability of results.
• Collects qualitative data, which is difficult to analyse and can be subjective, decreasing the validity of conclusions made.

Question 133

What is a controlled observation?

Answer 133

An observation carried out in a controlled environment in which extraneous variables can be controlled, for example a laboratory.

Question 134

Give 2 strengths of controlled observations

Answer 134

• High levels of control over extraneous variables, so causality can be established
• Can be replicated easily, so they have high external reliability.

Question 135

Give 2 weaknesses of controlled observations

Answer 135

• Low ecological validity as the environment is highly artificial
• Participants are more likely to show demand characteristics, decreasing the validity of the results

Question 136

What is a naturalistic observations?

Answer 136

An observation which takes place in the participants’ natural environment

Question 137

Give 2 strengths of naturalistic observations

Answer 137

• More ecologically valid as the environment is natural.
• Participants are less likely to show demand characteristics, increasing the validity of the results

Question 138

Give 2 weaknesses of naturalistic observations

Answer 138

• Low levels of control over extraneous variables, so it is difficult to establish causality
• Can not be replicated easily, so they have low external reliability.

Question 139

What is a participant observation?

Answer 139

An observation where the observer takes part in the situation they are observing and becomes part of the observed group

Question 140

Give 2 strengths of participant observations

Answer 140

• Observer can gain a more in-depth understanding of the participants’ behaviour as they are interacting with them
• The observer will understand the situation that they are observation better because they take part in it themself.

Question 141

Give 2 weaknesses of participant observations

Answer 141

• Observer will likely have to record behaviours afterwards, and therefore may forget some behaviours, decreasing the reliability of results
• May lead to observer bias as the researcher is interacting with the participants
• Usually involves deceiving participants, and therefore is unethical

Question 142

What is a non-participant observation?

Answer 142

An observation in which the observer doesn’t participate in the situation that they are observing and does not become part of the observed group

Question 143

Give 2 strengths of non-participant observations

Answer 143

• Less likely to result in observer bias as the researcher does not interact with the participants
• Observer can record behaviours as they happen, therefore not relying on memory

Question 144

Give 2 weaknesses of non-participant observations

Answer 144

• It may be difficult to understand behaviour of participants if the observer doesn’t interact with them
• The observer may not understand the situation they are observing if they do not participate in it

Question 145

What is a covert observation?

Answer 145

An observation in which the participants are not aware that they are being observed

Question 146

Give a strength of covert observations

Answer 146

Reduces the chance of demand characteristics, increasing the validity of results.

Question 147

Give a weakness of covert observations

Answer 147

They are unethical as participants do not give informed consent

Question 148

What is a overt observation?

Answer 148

An observation in which the participants are aware that they are being observed

Question 149

Give a strength of overt observations

Answer 149

They are usually ethical as participants are aware they are being observed and have therefore consented.

Question 150

Give a weakness of overt observations

Answer 150

Results may be affected by demand characteristics as participants know they are being observed

Question 151

What is time sampling?

Answer 151

When observers pick a time interval and only record behaviours that occur at those times.

Question 152

Give a strength of time sampling

Answer 152

Observers are more likely to be able to record behaviours accurately and in detail as they have time to write down what they have observed, meaning what they write down will be valid

Question 153

Give a weakness of time sampling

Answer 153

Any behaviours that occur outside of the time intervals will not be recorded so the results may not be representative of all behaviours, and so are less likely to be valid

Question 154

What is event sampling?

Answer 154

When observers observe all behaviour through the entire length of the observation, recording all events that occur using a coding scheme

Question 155

Give a strength of event sampling

Answer 155

No behaviours will be missed because the observer records behaviour that occurs throughout the whole length of the observation

Question 156

Give a weakness of event sampling

Answer 156

It may not be possible for researchers to write down every behaviour that they see without missing some behaviours, therefore results may be invalid.

Question 157

What is inter-rater reliability in observations?

Answer 157

A measure of agreement between multiple observers in what they have seen

Question 158

What is observer bias?

Answer 158

When the observer interprets what they see in a way which is influenced by what they expect to see

Question 159

What is correlational research?

Answer 159

Research method which investigates the relationship between two co-variables without the researcher manipulating any of them

Question 160

What is a positive correlation?

Answer 160

As one co-variable increases, the other co-variable also increases

Question 161

What is a negative correlation?

Answer 161

As one co-variable increases, the other co-variable decreases

Question 162

What is no correlation?

Answer 162

No significant relationship is found between the co-variables

Question 163

Write a null hypothesis for correlational research

Answer 163

There will be no significant correlation between (Variable A) and (Variable B). Any correlation will be due to chance.

Question 164

Write a one-tailed alternate hypothesis for correlational research

Answer 164

There will be a significant positive/negative correlation between (Variable A) and (Variable B).

Question 165

Write a two-tailed alternate hypothesis for correlational research

Answer 165

There will be a significant correlation between (Variable A) and (Variable B).

Question 166

What correlation co-efficients indicate a strong positive correlation?

Answer 166

0.7 , 0.8 , 0.9 , 1

Question 167

What correlation co-efficients indicate a strong negative correlation?

Answer 167

-0.7 , -0.8 , -0.9 , -1

Question 168

What correlation co-efficients indicate a moderate positive correlation?

Answer 168

0.4 , 0.5 , 0.6

Question 169

What correlation co-efficients indicate a moderate negative correlation?

Answer 169

-0.4 , -0.5 , -0.6

Question 170

What correlation co-efficients indicate a weak positive correlation?

Answer 170

0.1 , 0.2 , 0.3

Question 171

What correlation co-efficients indicate a weak negative correlation?

Answer 171

-0.1 , -0.2 , -0.3

Question 172

What correlation co-efficients indicate no correlation?

Answer 172

0

Question 173

Give 2 strengths of correlational research

Answer 173

• Good as preliminary research. We can investigate if there is a link between variables before investigating them further experimentally
• Useful for investigating variables that would be unethical or impractical to manipulate.

Question 174

Give 2 weaknesses of correlational research.

Answer 174

• Impossible to establish cause and effect as a relationship is established, but it is unclear which variable causes the effect in the other
• There may be a third variable which is affecting the variables, therefore making the results invalid

Question 175

What is the difference between interviews and questionnaires?

Answer 175

In interviews, the questions are asked verbally by an interviewer, whereas in questionnaires, questions are written down

Question 176

Give a strength and a weakness of the self report method

Answer 176

Strength:
Is a cheaper, easier method of obtaining data about people’s thoughts and behaviours

Weakness:
Results are often affected by social desirability bias, meaning results may be invalid.

Question 177

Give a strength and a weakness of Questionnaires

Answer 177

Strength:
* Easier and cheaper way to collect lots of data as easy to distribute
* Results are less likely to be affected by social desirability bias because there is nobody else present when the participant completes the questionnaire

Weakness:
There is no way for the participants to clarify questions that they don’t fully understand. Therefore, results may be less valid as participants may not have answered how the researcher intended

Question 178

Give a strength and a weakness of Interviews

Answer 178

Strength:
Participants can ask interviewer to clarify any questions they don’t understand. This means the results are likely to be more valid as participants are more likely to answer how the research intended
Weakness:
* Much more time consuming as each participants needs to be individually interviewed
* More likely to be affected by social desirability bias because the interviewer is present, so the participant may give untruthful answers in an attempt to make themself appear more socially desirable

Question 179

What are closed questions? Give a strength and a weakness

Answer 179

Questions which are answered by the participant choosing from a pre-determined set of answers.
+ Produces quantitative data, which is easy to analyse and compare across conditions
- Lacks detail and doesn’t allow researcher to gain an insight into the reasons for behaviour.

Question 180

What are open questions? Give a strength and a weakness

Answer 180

Questions that the participant can answer in any way they choose
+ Provides detail and insight into the reasons for behaviour
- Produces qualitative data, which is difficult to analyse and compare across conditions

Question 181

What is a Likert Scale?

Answer 181

Questions in which participants indicate on a scale how much they agree with a statement given

Question 182

What is a Semantic Differential Scale?

Answer 182

Questions in which the participants is given a statement, as well as a scale with two contrasting adjectives on the ends. They have to say where on the scale their opinion on the statement lies between the two adjectives.

Question 183

Name the 3 types of Interview and define each

Answer 183

• Structured: Interviewers ask a predetermined list of questions in the same order and in the same way
• Semi-structured: Interviewers ask a predetermined list of questions but they can deviate from the list and ask follow up questions
• Unstructured: The topic of the interview is the only thing that is predetermined. The interviewer asks anything they believe will give them useful answers.

Question 184

Give a strength and a weakness of structured interviews

Answer 184

+ As the questions are standardised, the research will be more reliable as it can be easily replicated
- The interviewer can’t ask follow up questions, meaning they may not be able to gain a full understanding of the participant’s thoughts and feelings. Therefore, results may lack validity

Question 185

Give a strength and a weakness of unstructured interviews

Answer 185

+ As the interviewer is able to ask follow up questions, they can gain more valid data
- Low reliability as the interview can not be replicated as the questions are not standardised

Question 186

What are the 4 principles of the BPS code of ethics?

Answer 186

Respect, competence, responsibility, integrity

Question 187

Which ethical guidelines form part of the ‘Respect’ principle of the BPS code?

Answer 187

Informed consent, right to withdraw, confidentiality

Question 188

Which ethical guidelines form part of the ‘Responsibility’ principle of the BPS code

Answer 188

Protection from Physical and Psychological harm, Debrief

Question 189

Which ethical guideline forms part of the ‘Integrity’ principle of the BPS code

Answer 189

Deception

Question 190

Name the 5 non-parametric tests on the specification, what are the conditions for the use of each one, and how should you compare the observed and critical value for each?

Answer 190

• Chi-Squared: Independent Measures Design, Nominal Data. Observed value must be greater than or equal to the critical value for findings to be significant.
• Mann-Whitney U Test: Independent Measures Design, Interval/Ordinal Data. Observed value must be less than or equal to the critical value for findings to be significant.
• Binominal Sign Test: Repeated Measures Design, Nominal Data Observed value must be less than or equal to the critical value for findings to be significant.
• Wilcoxon Test: Repeated Measures Design, Interval/Ordinal Data. Observed value must be less than or equal to the critical value for findings to be significant.
• Spearman’s Rho: Correlations. Observed value must be greater than or equal to the critical value for findings to be significant.

Question 191

Name 3 measures of Central Tendency

Answer 191

Mode, Median, Mean

Question 192

How do you calculate the Mode?

Answer 192

Identify what is the most frequently occurring number in the data set

Question 193

How do you calculate the Median?

Answer 193

Arrange the numbers from smallest to largest and find the midpoint.

Question 194

How do you calculate the Mean?

Answer 194

Add together all numbers in the data set, and divide by how many numbers there are in the data set

Question 195

Give a strength and weakness of using the mode

Answer 195

• Strength: Not skewed by extreme values
• Weakness: Doesn’t take all the scores in the data set into consideration

Question 196

Give a strength and weakness of using the median

Answer 196

• Strength: Not skewed by extreme values
• Weakness: Doesn’t take all the scores in the data set into consideration

Question 197

Give a strength and weakness of using the mean

Answer 197

• Strength: Takes all of the scores in the data set into consideration
• Weakness: Can be skewed by extreme values

Question 198

Name 3 measures of Dispersion

Answer 198

Range, Variance, Standard Deviation

Question 199

How do you calculate the Range?

Answer 199

Calculate the largest value minus the smallest value

Question 200

How do you calculate the Variance and standard deviation?

Answer 200

Subtract the mean from each number in your sample, and square the result of these calculations. Add these numbers together and divide this by how many numbers you have in the sample minus 1. This will give the variance. Square root the variance to find the standard deviation

Question 201

What does the standard deviation show?

Answer 201

It is a measure of how dispersed the data is in relation to the mean. A small standard deviation indicates data is clustered tightly around the mean, and a large standard deviation indicates data is more spread out.

Question 202

What are the levels of data and what is the definition of each?

Answer 202

• Nominal - Data that is sorted into categories
• Ordinal - Data which is on a rating scale
• Interval - Data with an exact value

Question 203

What are the types of data and what is the definition of each?

Answer 203

• Qualitative - Data which involves in depth descriptions
• Quantitative - Data which involves numerical values
• Primary - Data that has been collected directly by the researcher, solely for their research
• Secondary - Data that the researcher has not collected themself.

Question 204

What are the distinguishing features of eukaryotic cells?

Answer 204

Eukaryotic cells have membrane bound organelles, and their genetic material is enclosed in a nucleus.

Question 205

What is the structure of the cell-surface membrane?

Answer 205

The cell surface membrane is a phospholipid bilayer. Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails, so the heads point outwards and the tails point inwards. It also contains intrinsic and extrinsic proteins, glycolipids and glycoproteins on the exterior surface, and cholesterol molecules.

Question 206

What is the function of the cell-surface membrane

Answer 206

It is selectively permeable, selectively allowing some substances in and out of the cell. It also has receptors on its surface, allowing recognition of foreign and abnormal cells.

Question 207

Describe the structure of the nucleus

Answer 207

The nucleus is surrounded by a double membrane called the nuclear envelope, which has nuclear pores in it. The interior of the nucleus consists of the nucleoplasm, within which is the nucleolus. Also in the nucleoplasm is chromatin.

Question 208

Describe the function of the nucleus

Answer 208

Holds and stores genetic information, which codes for proteins. It is also the site of DNA replication as well as the site of transcription, producing mRNA. Also, Ribosomes are produced in the nucleolus.

Question 209

Describe the structure of a ribosome

Answer 209

Made up of ribosomal RNA and 2 subunits of protein. It is not membrane bound.

Question 210

Describe the function of a ribosome

Answer 210

Site of protein synthesis

Question 211

Describe the function of the RER

Answer 211

Ribosomes on the surface synthesise proteins, which are then processed, in the RER and are then packaged into vesicles for transport around the cell.

Question 212

Describe the function of the SER

Answer 212

Synthesises and processes lipids

Question 213

Describe the structure of the Endoplasmic Reticulum

Answer 213

Consists of a system of fluid filled channel-like membranes. The RER has attached Ribosomes.

Question 214

Describe the function of the Golgi Apparatus and Vesicles

Answer 214

The Golgi modifies and packages proteins and lipids, and also produces lysosomes. The Golgi vesicles transport proteins and lipids around the cell.

Question 215

Describe the function of lysosomes.

Answer 215

They release hydrolytic enzymes called lysozymes, which hydrolyse pathogens.

Question 216

Describe the structure of lysosomes.

Answer 216

They are surrounded by a membrane and contain lysozymes, which are digestive enzymes.

Question 217

Describe the structure of mitochondria

Answer 217

Mitochondria consist of an outer membrane, Cristae, which are the inner membrane folds, and the matrix containing small 70S ribosomes and circular DNA

Question 218

Describe the function of mitochondria

Answer 218

Mitochondria are the site of aerobic respiration, producing ATP for energy release.

Question 219

Describe the structure of chloroplasts

Answer 219

Chloroplasts have a double membrane, and stroma, which contain thylakoid membrane, 70S ribosomes, circular DNA, starch granules and lipid droplets. Chloroplasts also contain lamellae, and grana (stacks of thylakoid)

Question 220

Describe the function of chloroplasts

Answer 220

Absorb light energy for photosynthesis

Question 221

Describe the structure of the cell wall

Answer 221

Composed of Cellulose in plants and algae. Composed of Chitin in fungi, and of Murein in bacteria.

Question 222

Describe the function of the cell wall

Answer 222

Provides structural support to the cell, preventing the cell bursting due to osmotic action.

Question 223

Describe the structure of the vacuole

Answer 223

Contains cell sap and is surrounded by a tonoplast membrane.

Question 224

Describe the function of the vacuole

Answer 224

Maintains turgor pressure in the cell, stopping the plant wilting. Also contains cell sap which stores sugars, amino acids, pigments and any waste chemicals.

Question 225

What is a tissue?

Answer 225

Group of specialised cells with a similar structure working together to perform a specific function.

Question 226

What are the 2 main distinguishing features of prokaryotic cells?

Answer 226

They lack membrane-bound organelles, and therefore their genetic material is not enclosed in a nucleus

Question 227

What is an organ?

Answer 227

Group of tissues working together to perform a specific function

Question 228

What is an organ system?

Answer 228

Group of organs working together to perform specific functions.

Question 229

Describe the general structure of prokaryotic cells

Answer 229

They consist of cytoplasm, and within the cytoplasm are 70S Ribosomes and circular DNA, which is free in the cytoplasm and not associated with proteins. Sometimes, this DNA is in the form of plasmids. The cells are surrounded by a cell-surface membrane and a murein cell wall. Some prokaryotic cells are also surrounded by a capsule, and some also have flagella.

Question 230

Name 5 differences between Eukaryotic and Prokaryotic Cells

Answer 230

• Eukaryotic Cells have membrane bound organelles, whereas Prokaryotic Cells have no membrane-bound organelles.
• Eukaryotic Cells have a nucleus which contains DNA, whereas Prokaryotic Cells have no nucleus, so their DNA is free in the cytoplasm.
• In Eukaryotic Cells, the DNA is long, linear, and associated with histone proteins, whereas in Prokaryotic Cells, the DNA is short, circular, and not associated with any proteins.
• Eukaryotic Cells contain larger (80S) Ribosomes, whereas Prokaryotic Cells contain smaller (70S) Ribosomes.
• Prokaryotic Cells have a Cell Wall made of Murein, whereas in Eukaryotic Cells, where present, the Cell Wall is made of Cellulose or Chitin in fungi.

Question 231

Why are Viruses described as acellular and non-living?

Answer 231

They are acellular because they are not made of cells, and they have no organelles. They are non-living because they can not independently replicate.

Question 232

Describe the general structure of a virus particle.

Answer 232

Contains Nucleic acids surrounded by a capsid (protein coat) They have attachment proteins on surface to allow attachment to specific host cells. Some also surrounded by a lipid envelope e.g. HIV

Question 233

What is magnification?

Answer 233

The number of times larger the image is than size of the real object

Question 234

What is the equation for magnification?

Answer 234

Magnification = Size of Image / Size of real object

Question 235

What is Resolution?

Answer 235

Minimum distance apart 2 objects can be to be distinguished as separate objects

Question 236

Optical Microscopes: How do they work, What kind of image do they produce, and what is the resolution and magnification like?

Answer 236

• Light focused using
  glass lenses
• Light passes through specimen, different structures absorb different amounts & wavelengths
• Generates a 2D image of a
  cross-section
• Low resolution due to long wavelength of light
• Can’t see internal structure of
  organelles
• Specimen has to be thin
• Low magnification (x 1500)
• Can view living organisms
• Simple preparation
• Can show colour

Question 237

Transmission Electron Microscopes: How do they work, What kind of image do they produce, and what is the resolution and magnification like?

Answer 237

• Electrons focused using
  electromagnets
• Electrons pass through specimen,
  denser parts absorb more and
  appear darker
• Generates a 2D image of a
  cross-section
• Very high resolution due to short
  wavelength of electrons
• Can see internal structures of organelles
• Specimen has to be very thin
• High magnification (x 1,000,000)
• Can only view dead / dehydrated
  specimens as uses a vacuum
  Can only view dead / dehydrated
  specimens as uses a vacuum
• Complex preparation so
  artefacts often present
• Does not show colour

Question 238

Scanning Electron Microscopes: How do they work, What kind of image do they produce, and what is the resolution and magnification like?

Answer 238

• Electrons focused using
  electromagnets
• Electrons deflected / bounce
  off specimen surface
• Generates a 3D image
  of surface
• High resolution due to short
  wavelength of electrons
• Can’t see internal structures
• Specimen does not need to be thin
• High magnification (x 1,000,000)
• Can only view dead / dehydrated
  specimens as uses a vacuum
• Complex preparation so
  artefacts often present
• Does not show colour

Question 239

What may occur if there are issues in the preparation of microscope slides?

Answer 239

May result in air bubbles or particles of dust etc. in the slide. This could result in artefacts which could be mistaken for cell organelles and other structures that are not actually present.

Question 240

What is the order of units of measurement, getting 1000 times smaller each time?

Answer 240

Metre (m), Millimetre (mm), Micrometre (µm), Nanometre (nm)

Question 241

Describe how the size of an object viewed with an optical microscope can be measured

Answer 241

1. Line up scale of eyepiece graticule with scale of stage micrometre
2. Calibrate eyepiece graticule - use stage micrometre to calculate size of divisions on eyepiece graticule
3. Take micrometre away and use graticule to measure how many divisions make up the object
4. Calculate size of object by multiplying number of divisions by size of division
5. Recalibrate eyepiece graticule at different magnifications

Question 242

Describe and explain how cell fractionation and
ultracentrifugation can be used to separate cell components

Answer 242

1. Homogenise tissue to disrupt cell membrane, breaking open cells and releasing the organelles
2. Place in a cold, isotonic, buffered solution (Cold to reduce enzyme activity → so organelles not broken down / damaged, Isotonic so water doesn’t move in or out of organelles by osmosis → so organelles don’t burst, Buffered to keep pH constant → so enzymes don’t denature
3. Filter homogenate to remove large, unwanted debris
4. Ultracentrifugation - separates organelles in order of density. Centrifuge homogenate in a tube at a relatively low speed. Remove pellet of heaviest organelle and respin supernatant at a higher speed. Repeat at increasing speeds until separated out.

Question 243

What order do the organelles separate during Ultracentrifugation?

Answer 243

1. Nuclei
2. Chloroplasts and Mitochondria
3. Lysosomes
4. Endoplasmic Reticulum
5. Ribosomes

Question 244

Describe the stages of the cell cycle in eukaryotic cells.

Answer 244

1. Interphase - DNA undergoes semi-conservative replication, leading to 2 sister chromatids joined at the centromere. Also, the number of organelles and the volume of cytoplasm increases.
2. Mitosis - The nucleus divides to produce 2 nuclei with identical copies of DNA
3. Cytokinesis - The cytoplasm and cell membrane divide to form 2 new genetically identical daughter cells.

Question 245

What happens in Prophase?

Answer 245

The Chromosomes condense, becoming visible. They will appear as 2 sister chromatids joined at the centromere. The nuclear envelope breaks down and the centrioles move to opposite poles, forming the spindle network.

Question 246

What happens in Metaphase?

Answer 246

The spindle fibres attach to the chromosomes at the centromere, and chromosomes align along the equator.

Question 247

What happens in Anaphase?

Answer 247

The spindle fibres contract and the centromere divides, pulling sister chromatids to opposite poles of the cell.

Question 248

What happens in Telophase?

Answer 248

Chromosomes uncoil, and the nuclear envelope reforms to form 2 new nuclei. Spindle fibres and centrioles are broken down.

Question 249

Explain the importance of mitosis in the life of an organism

Answer 249

Cells divide to produce 2 genetically identical daughter cells for:
* Growth
* Replacing cells in damaged tissue
* Asexual reproduction

Question 250

How do tumours and cancers form?

Answer 250

Mitosis is a controlled process. Mutations in the gene controlling mitosis (oncogenes) can lead to uncontrolled cell division. This may result in a mass of abnormal cells, forming a tumour. If this tumour metastasises, it becomes malignant and cancerous.

Question 251

Suggest how cancer treatments control rate of cell division.

Answer 251

Some cancer treatments disrupt spindle fibre formation and activity, so that the spindle fibres can not attach to the chromosomes at the centromere, and the chromatids can’t be separated to opposite poles during anaphase, therefore preventing mitosis. Others prevent DNA replication by inhibiting DNA Polymerase, so that cells can’t make 2 copies of each chromosome, so prevents mitosis.

Question 252

What is the main issue with most cancer treatments?

Answer 252

Most cancer treatments kill cells or prevent mitosis, therefore healthy cells will also be destroyed or their cell cycle will be disrupted.

Question 253

Describe how prokaryotic cells replicate.

Answer 253

They replicate via Binary Fission:
* Replication of Circular DNA and any Plasmids that are present.
* Division of the cytoplasm, producing 2 new daughter cells
* The 2 new daughter cells each contain a single copy of the Circular DNA and a variable number of copies of plasmids.

Question 254

How do viruses replicate?

Answer 254

1. Attachment proteins attach to the complimentary receptors on host cell
2. They inject viral nucleic acid into host cell
3. Infected host cell replicate the virus particles nucleic acid, producing viral proteins.
4. The virus is assembled then released.

Question 255

What is an antigen?

Answer 255

A foreign protein which stimulates an immune response, leading to the production of a complimentary antibody.

Question 256

How are cells identified by the immune system?

Answer 256

Each cell has specific antigen on it’s cell surface. These antigens are proteins and have a specific tertiary structure, which help the immune system to recognise them as self or foreign.

Question 257

What types of cells and molecules can the immune system identify?

Answer 257

• Pathogens (microorganisms that cause disease)
• Cells from other organisms (e.g. organ transplants)
• Abnormal body cells (e.g. tumour cells or virus infected cells)
• Toxins (Harmful chemicals release by some bacteria)

Question 258

What is the non-specific immune response?

Answer 258

Phagocytosis

Question 259

What are the specific immune responses?

Answer 259

Cellular and humoral response.

Question 260

Describe phagocytosis of pathogens

Answer 260

1. Phagocyte is attracted to chemicals made by pathogen or recognises it’s foreign antigens on cell surface membrane.
2. Phagocyte engulfs pathogen by endocytosis, surrounding it with its cell membrane.
3. Pathogen contained in phagosome within the cytoplasm of the phagocyte.
4. Lysosome fuses with phagosome to produce a phagolysosome and releases lysozymes.
5. Lysozymes hydrolyse the pathogen and the products are absorbed into the phagosome if they are soluble, but are expelled from the cell if they are insoluble.

Question 261

How does phagocytosis lead to the cellular and humoral responses?

Answer 261

Phagocytosis leads to presentation of antigens, where antigens are displayed on the phagocyte cell-surface membrane, stimulating the specific immune response (cellular and humoral response).

Question 262

Describe the response of T lymphocytes to a foreign antigen (the cellular response).

Answer 262

T-Lymphocytes recognise antigens on the surface of antigen presenting cells. Specific T-Helper cells with complimentary receptors on their cell surface bind to the antigen on the antigen-presenting cell, causing the T-Helper cells to divide rapidly by mitosis, which then stimulate the production of Cytotoxic T Cells (which produce chemicals such as perforin to kill infected cells and pathogens), Specific B Cells (which stimulate the humoral response, including the production of antibodies) and Phagocytes (which engulf pathogens by phagocytosis).

Question 263

Describe the response of B lymphocytes to a foreign antigen (the humoral response)

Answer 263

B Lymphocytes can recognise free antigens. When they come across a foreign antigen, specific B-Lymphocytes with complimentary receptors bind to the antigens, and is stimulated by T-Helper cells to divide rapidly by mitosis. Some of these clones differentiate into B-Plasma cells, which secrete large amounts of antibodies, which are specific to the foreign antigen. Others differentiate into B-Memory cells, which remain in the blood until the secondary immune response.

Question 264

What are antibodies?

Answer 264

Antibodies are quaternary structure proteins and they consist of 4 polypeptide chains. They are secreted by B-Plasma cells in response to specific antigen. They are complimentary to their specific antigen, and when bind to it, they form an antigen-antibody complex.

Question 265

Draw and label the structure of an antibody.

Answer 265

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.thesciencehive.co.uk%2Fimmune-response-a-level&psig=AOvVaw18Tp-0NiivX2iWLKfl9f_u&ust=1712164476099000&source=images&cd=vfe&opi=89978449&ved=0CBIQjRxqFwoTCMizj-2DpIUDFQAAAAAdAAAAABAE

Question 266

How do antibodies lead to the destruction of pathogens?

Answer 266

Antibodies bind to complimentary antigens on the surface of pathogens . Each antibody can bind to 2 pathogens at a time, causing agglutination. This allows the antibodies to attract phagocytes, which destroy the pathogens via phagocytosis.

Question 267

What are the differences between the primary and secondary immune response?

Answer 267

The primary response refers to the immune response which occurs upon the first exposure to an antigen. Antibodies are produced slowly and at a lower concentration as it takes time for specific B-Plasma cells to be stimulated to produce the specific antibodies. Memory cells are produced in the primary response. The secondary response refers to the immune response that occurs upon the second exposure to an antigen. In the secondary response, antibodies are produced faster and at a higher concentration. This is because B memory cells are present in the blood, and they rapidly undergo mitosis in order to rapidly produce specific antibodies.

Question 268

What is a vaccine?

Answer 268

The injection of antigens from dead, weakened or inactive pathogens, stimulating a weak immune response, producing memory cells.

Question 269

How do vaccines provide protection to individuals against disease?

Answer 269

1. Specific B lymphocyte with complementary receptor binds to antigen
2. Specific T helper cell binds to antigen-presenting cell and stimulates B cell
3. B lymphocyte divides by mitosis to form clones
4. Some differentiate into B plasma cells which release antibodies
5. Some differentiate into B memory cells
6. On secondary exposure to antigen, B memory cells are already present, so they can rapidly divide by mitosis to produce B plasma cells
7. These release antibodies faster and at a higher concentration

Question 270

How do vaccines provide protection for entire populations against disease?

Answer 270

Herd Immunity - If a large proportion of the population is vaccinated, then the majority of people will not become ill from infection. This results in fewer infected people to pass the pathogen, and therefore unvaccinated people are less likely to come into contact with someone with the disease. This helps to protect vulnerable people such as young children and elderly people, as well as other people who do not get vaccinated due to personal or medical reasons.

Question 271

What are the differences between active and passive immunity?

Answer 271

• In active immunity, the person is exposed to the antigen, either natural (via primary infection) or artificially (via vaccination), whereas in passive immunity, the person has not been exposed to the antigen.
• In active immunity, the antibodies were produced and secreted by the person’s own B-plasma cells, but in passive immunity the antibodies that are introduced into the body were produced by another organism (e.g. through breast milk)
• Active immunity is slower as it requires a full immune response. Passive immunity is much quicker as the antibodies can act immediately.

Question 272

Explain the effect of antigen variability on disease and disease prevention.

Answer 272

The tertiary structure of the antigens on the surface of pathogens changes due to a mutation, causing a change in shape. Therefore, the B-memory cells can no longer bind to the antigen when exposed to the antigen, and the specific antibodies are no longer complimentary so can not bind to the antigen. Therefore, there is no more immunity.

Question 273

Draw and label the structure of a HIV particle.

Answer 273

https://images.squarespace-cdn.com/content/v1/5c5aed8434c4e20e953d6011/1591278433693-1D7XKEM9RC99Y8B861EY/hiv+structure.jpg

Question 274

Describe the replication of HIV in helper T-Cells.

Answer 274

1. HIV attachment proteins attach to receptors on helper T cell.
2. Lipid envelope fuses with T-Helper cell-surface membrane, releasing capsid into cell.
3. Capsid breaks down, releasing RNA and reverse transcriptase into the cell.
4. Reverse transcriptase converts viral RNA to DNA.
5. Viral DNA is inserted into helper T cell DNA, so it is replicated when cells replicate.
6. DNA is used to make HIV RNA and proteins at the host ribosomes.
7. Virus particles are assembled, which bud off from the cell membrane and continue infecting other cells.

Question 275

How does HIV cause AIDS?

Answer 275

HIV infects and kills T-Helper cells. Therefore, T-Helper cells can’t stimulate Cytotoxic T Cells , B-Cells and Phagocytes. This means that B Plasma cells can’t release as many antibodies, leading to the deterioration of the immune system. This makes the patient more susceptible to opportunistic infections, leading to many of the patient’s healthy body cells being destroyed.

Question 276

Name the 3 main ways that HIV can be transmitted

Answer 276

• Having unprotected sex with an infected person.
• Close contact with the blood of an infected person (via the sharing of needles for drug use or the use of infected blood in blood transfusions)
• Mother to child (Shared through the placenta or through breast milk).

Question 277

Why are antibiotics ineffective against viruses?

Answer 277

Viruses do not have structures and processes that antibiotics inhibit. Viruses do not have metabolic processes and they do not have bacterial enzymes or a murein cell wall.

Question 278

How can monoclonal antibodies be used in medical treatments?

Answer 278

Monoclonal antibodies have a specific tertiary structure and therefore their antigen binding site is complementary to the receptor of an antigen found only on a specific cell type (e.g. cancer cell). Therapeutic drugs can be attached to the monoclonal antibodies, so the antibody will bind to the specific cell, forming antigen-antibody complex, delivering the drug to the specific cell.

Question 279

How can monoclonal antibodies be used in medical diagnosis?

Answer 279

Monoclonal antibodies have a specific tertiary structure and therefore their antigen binding site is complementary to the receptor of the antigen associated with diagnosis. Fluorescent markers can be attached to the monoclonal antibody, and if the antibody binds to the receptor of the specific antigen, it will form an antigen-antibody complex which will be visible so that scientists can diagnose diseases by identifying which antigen is present.

Question 280

Explain the use of antibodies in the ELISA test to detect antigens.

Answer 280

1. Attach sample with potential antigens to well
2. Add complementary monoclonal antibodies with enzymes attached. They will bind to antigens if present.
3. Wash well to remove unbound antibodies (to prevent false positive)
4. Add substrate so that the enzymes create products that cause a colour change (positive result)

Question 281

Explain the use of antibodies in the ELISA test to detect antibodies.

Answer 281

1. Attach antigens to well that are complimentary to the antibody you are testing for.
2. Add sample with potential antibodies, wash to remove unbound antibodies.
3. Add complementary monoclonal antibodies
   with enzymes attached, which will bind to antibodies if
   present,
4. Wash well to remove unbound antibodies.
5. Add substrate so that enzymes create products that
   cause a colour change (positive result).

Question 282

What is the purpose of a control well in the ELISA test?

Answer 282

So that you can compare it to the test well to ensure that only the enzyme caused the colour change and that all unbound antibodies had been washed away.

Question 283

How do pregnancy tests use monoclonal antibodies?

Answer 283

1. Mobile monoclonal antibodies that are complimentary to the hCG protein are attached to a coloured bead.
2. If there is any hCG in the urine (indicating pregnancy) then it will bind to the antibodies, forming a HCG-antibody complex.
3. The urine moves up the strip, until in reaches a line of immobilised monoclonal antibodies which only binds to the HCG-antibody complex. If the test is positive, this first this line will change colour.
4. The urine continues up the test strip to a line of immobilised monoclonal antibodies which only bind to the mobile monoclonal antibodies, regardless of whether there in hCG attached or not. This line indicates that the test worked.

Question 284

Suggest some points to consider when evaluating methodology relating to the use of vaccines and monoclonal antibodies.

Answer 284

• Was the sample size large enough to be representative?
• Were participants diverse in terms of age, sex, ethnicity and health status?
• Were placebo / control groups used for comparison?
• Was the duration of the study long enough to show long-term effects?
• Was the trial double-blind to reduce bias?

Question 285

Suggest some points to consider when evaluating data relating to the use of vaccines and monoclonal antibodies.

Answer 285

• What side effects were observed, and how frequently did they occur?
• Was a statistical test used to see if there was a significant difference between start & final results?
• Was the standard deviation of final results large, showing some people did not benefit?
• Did standard deviations of start & final results overlap, showing there may not be a significant difference?
• What dosage was optimum? Does increasing dose increase effectiveness enough to justify extra cost?
• Was the cost of production & distribution low enough?
• What are the ethical issues surrounding the use of vaccines and monoclonal antibodies?

Question 286

What is the relationship between the size of an organism and its surface area to volume ratio?

Answer 286

As size increases, surface area to volume ratio decreases.

Question 287

How is Surface Area to Volume Ratio calculated?

Answer 287

Surface area (for regular shapes: side length x side width x number of sides) divided by volume (length x width x depth)

Question 288

What is metabolic rate and how is it usually measured?

Answer 288

Metabolic rate is the amount of energy used up by an organism within a given period of time. It is often measured by oxygen uptake because oxygen is used in aerobic respiration to make ATP for energy release.

Question 289

What features do Unicellular Organisms have in relation to Exchanging Substances and what are the advantages and disadvantages of these features?

Answer 289

They have a large surface area to volume ratio, so they can absorb any substances required. They have a short diffusion distance between the outside of the organism to the centre of it, so they can quickly absorb substances from the environment. An advantage of this is that they can exchange materials with their environment. A disadvantage is that they lose heat energy and water quickly, so they can not survive extreme temperatures

Question 290

What features do Multicellular Organisms have in relation to Exchanging Substances and what are the advantages and disadvantages of these features?

Answer 290

They have a small surface area to volume ratio so can not absorb enough substances through small outer surface to support large volume. They have a large diffusion distance between the outside and the centre of the organism so diffusion through outer surface is too slow to supply cells efficiently. An advantage of this is that they lose less heat energy, so can survive in cold environments. A disadvantage is that they often need internal mass transport systems in order to supply the body with vital substances.

Question 291

State and explain the relationship between surface area to volume ratio and metabolic rate.

Answer 291

Organisms with a larger surface area to volume ratio (smaller organisms) have a higher metabolic rate as they lose heat more easily. Therefore, more energy and a higher metabolic rate is required to maintain a constant internal temperature. Per unit of body mass, metabolic rate is higher in small organisms.

Question 292

What are the behavioural and physical adaptations that organisms in cold environments have to prevent heat loss?

Answer 292

• Behavioural: Small mammals with a large surface area to volume ratio will lose heat easily so they need to eat high energy foods such as nuts and seeds to help maintain body temperature. They may also hibernate during winter.
• Physical: Adapted animals will have a compact body shape, giving a smaller surface area to volume ratio. Small mammals with larger surface area to volume ratio may have thick layers of fur to insulate and reduce heat loss.

Question 293

What are the behavioural and physical adaptations that organisms in hot environments have to prevent overheating?

Answer 293

• Behavioural: Large organisms such as hippos spend much of the day in water to help lose heat. Some other organisms may be nocturnal so that they are only active in cold temperatures (at night).
• Physical: Large organisms with low surface area to volume ratio often have large ears which increase their surface area allowing them to lose more heat.

Question 294

What are the behavioural and physical adaptations that organisms in dry environments have to prevent water loss?

Answer 294

• Behavioural: Organisms may be nocturnal so that they are most active in cooler temperatures, reducing the need for cooling by evaporative water loss (sweating), therefore conserving water.
• Physical: Small mammals with a high surface area to volume ratio have structural kidney adaptations so that they produce less urine to conserve water.

Question 295

What is the function of the Waxy Cuticle?

Answer 295

It is waterproof to prevent water loss by evaporation, and transparent to allow light to pass through.

Question 296

What is the function of the Upper Epidermis?

Answer 296

It protects the leaf and is 1 cell thick to allow the light to pass through

Question 297

What is the function of the Palisade Mesophyll?

Answer 297

It is a layer of cells containing large amounts of chloroplasts for photosynthesis.

Question 298

What is the function of the Spongy Mesophyll?

Answer 298

It has air spaces which increases the surface area for gas exchange. The cells within the spongy mesophyll also contain lots of chloroplasts.

Question 299

What is the function of the Xylem?

Answer 299

Transports water from the roots up the plant to the leaves.

Question 300

What is the function of the Phloem?

Answer 300

Transports nutrients, sugars and respiratory products up and down the plant

Question 301

What is the function of the Lower epidermis?

Answer 301

Gases enter and exit via the stoma, which opens and closes

Question 302

Describe the role of the Stomata and how they carry out this role.

Answer 302

Stomata control how much water leaves the plant by transpiration. If there is a higher water potential outside than there is inside the cell, water will move in via osmosis, and if there is a lower water potential outside than inside , water will move via osmosis. When plants have enough water, guard cells are turgid which keeps the pores open, and when plants don’t have enough water, guard cells become flaccid causing the pores to close.

Question 303

Draw and label a plant leaf.

Answer 303

https://biology-igcse.weebly.com/uploads/1/5/0/7/15070316/7148326.gif?565

Question 304

Describe the pathway of air through the tracheal system of an insect

Answer 304

Air enters the trachea through pores on the surface of the exoskeleton called spiracles, which can open and close. Carbon Dioxide and Oxygen will diffuse in and out of the spiracles down the concentration gradient. The tracheae divide into smaller tubes called tracheoles which continue to divide until they branch off into individual body cells. The tracheoles are permeable to allow gas exchange

Question 305

Explain how an insect’s tracheal system is adapted for gas exchange

Answer 305

• Tracheoles have thin walls, so short diffusion distance to cells
• High numbers of highly branched tracheoles, so larger surface area for gas exchange
• Rhythmic contraction of abdominal muscles changes pressure in body, causing air to move in / out, maintaining concentration gradient for diffusion
• Fluid in end of tracheoles drawn into tissues by
  osmosis during exercise increases rate of diffusion
• Spiracles can open and close to maintain the concentration gradient

Question 306

Describe ventilation in insects

Answer 306

By contracting muscles between each body segment, the insect can compress the trachea and therefore pump gases in and out of its body.

Question 307

Describe the structure of fish gills

Answer 307

Each gill is made of lots of thin gill filaments which are attached to a bony gill arch. The gill filaments a covered in small, thin folds called lamellae, which have lots of blood capillaries and a thin layer of cells.

Question 308

What is Counter Current flow?

Answer 308

• Blood and water flow over the lamellae in opposite directions
• So blood is always flowing next to water that has a higher oxygen concentration
• So maintains a concentration gradient of oxygen between water and blood
• For diffusion happens along whole length of lamellae

Question 309

How are gills adapted for gas exchange?

Answer 309

• Gills made of many filaments covered with many lamellae, increasing surface area for diffusion
• Thin lamellae wall / epithelium, so short diffusion distance between water and blood
• Lamellae have a large number of capillaries which remove O2 and bring CO2 quickly so maintains concentration gradient
• Counter current flow system to maintain concentration gradient across the full length of the gill lamellae.

Question 310

Explain how the leaves of dicotyledonous plants are adapted for gas
exchange

Answer 310

They have many stomata , resulting in a large surface area for gas exchange. The spongy mesophyll contains air spaces, allowing a large surface area for gases to diffuse through. The leaves are thin, resulting in a short diffusion distance

Question 311

Explain structural and functional compromises in xerophytic plants that
allow efficient gas exchange while limiting water loss

Answer 311

• Leaves are spikes and therefore have a small surface area, reducing evaporation rate
• Sunken stomata trap water to maintain maintain humid air around the stomata to reduce the water potential gradient
• Stomatal hairs trap water to maintain maintain humid air around the stomata to reduce the water potential gradient
• Extensive root systems maximise water uptake. Some xerophytes have wide, shallow roots to collect rainwater, and other have deep roots to collect groundwater
• Reduced amount of stomata, reducing the amount of places water can evaporate from
• Thicker waxy cuticle to waterproof leaves and stem to reduce evaporation

Question 312

Describe the gross structure of the human gas exchange system

Answer 312

The Trachea spits into Bronchi, when then split into smaller Bronchioles. At the end of each Bronchiole is an Alveolus, surrounded by a capillary network

Question 313

Explain the essential features of the alveolar epithelium that make it
adapted as a surface for gas exchange

Answer 313

• 1 cell thick → short diffusion distance
• Folded → large surface area
• Permeable → allows diffusion of gases
• Moist → gases can dissolve for diffusion
• Good blood supply from large network of capillaries → maintains large concentration gradient

Question 314

Describe how gas exchange occurs in the lungs

Answer 314

Oxygen diffuses from the alveoli, across the alveolar epithelium and the capillary endothelium into blood down its concentration gradient. Carbon dioxide diffuses from the blood, across the capillary endothelium and the alveolar epithelium into the alveoli down its concentration gradient.

Question 315

Explain the importance of ventilation

Answer 315

Brings in air containing higher concentration of oxygen and removes air with lower concentration of oxygen, maintaining concentration gradients

Question 316

Explain how humans breathe in

Answer 316

1. Diaphragm muscles contract and the diaphragm therefore flattens
2. External intercostal muscles contract, and the internal intercostal muscles relax, so the ribcage is pulled up and out
3. This increases volume and decreases pressure in thoracic cavity
4. Air moves into lungs down pressure gradient

Question 317

Explain how humans breathe out

Answer 317

1. Diaphragm muscles relax and the diaphragm therefore moves upwards
2. External intercostal muscles relax, and the internal intercostal muscles contract, so the ribcage moves down and in
3. This decreases volume and increases pressure in thoracic cavity
4. Air moves out of lungs down pressure gradient

Question 318

Suggest why expiration is normally passive at rest

Answer 318

Internal intercostal muscles do not normally need to contract, and expiration is aided by elastic recoil in alveoli

Question 319

Suggest how different lung diseases reduce the rate of gas exchange

Answer 319

• Thickened alveolar tissue (eg. fibrosis) → increases diffusion distance
• Alveolar wall breakdown → reduces surface area
• Reduced lung elasticity → lungs expand / recoil less → reduces concentration gradients of O2 / CO2

Question 320

Suggest how different lung diseases affect ventilation

Answer 320

• Some lung diseases reduce lung elasticity (eg. fibrosis which is a build-up of scar tissue) → lungs expand / recoil less. This reduces volume of air in each breath (tidal volume) and also reduces the maximum volume of air breathed out in one breath (forced vital capacity)
• Some lung diseases narrow airways or reduce airflow in & out of lungs (e.g. asthma, which is inflamed bronchi) Therefore, reduces maximum volume of air breathed out in 1 second (forced expiratory volume)
• Some lung diseases reduce rate of gas exchange, resulting in increased ventilation rate to compensate for reduced oxygen in blood

Question 321

Suggest why people with lung disease experience fatigue

Answer 321

Cells receive less oxygen, so the rate of aerobic respiration is reduced and less ATP made

Question 322

Explain what happens in digestion

Answer 322

Large insoluble biological molecules are hydrolysed into smaller soluble molecules that are small enough be absorbed across cell membranes into the blood

Question 323

Describe the digestion of starch in mammals

Answer 323

• Amylase (produced by salivary glands and pancreas) hydrolyses starch into maltose
• Membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose
• Hydrolysis of glycosidic bond

Question 324

Describe the digestion of lipids in mammals

Answer 324

• Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets
• This increases surface area of lipids for increased hydrolysis
• Lipase (made in pancreas) hydrolyses lipids into monoglycerides and fatty acids by hydrolysing the ester bonds

Question 325

Describe the digestion of proteins by a mammal

Answer 325

• Endopeptidases hydrolyse internal peptide bonds within the polypeptide chain. This results in the polypeptide chains being broken into shorter chains, so there are therefore more terminal ends for exopeptidases
• Exopeptidases hydrolyse peptides bonds at the terminal ends of the polypeptide chains, releasing singular amino acids
• Membrane bound dipeptidases hydrolyse the peptide bond between dipeptides, releasing 2 amino acids

Question 326

Why are membrane-bound enzymes are important in digestion?

Answer 326

Membrane bound enzymes are located on the cell membrane of the epithelial cells lining the ileum. By hydrolysing molecules at the site of absorption, they maintain concentration gradients for higher absorption rates

Question 327

Describe the absorption of amino acids and monosaccharides in mammals

Answer 327

1. Sodium ions are actively transported from the epithelial cells lining ileum to blood, establishing a concentration gradient of Sodium ions, as the concentration is now higher in lumen than the epithelial cell
2. Sodium ions enter the epithelial cell down its concentration gradient with co-transported glucose or amino acids. This occurs via a co-transporter protein. This establishes a concentration gradient of Glucose or Amino Acids, as the concentration is higher in the epithelial cell than in the blood
3. The Glucose or Amino Acids move down the concentration gradient into blood via facilitated diffusion

Question 328

Describe the absorption of lipids by a mammal

Answer 328

• Micelles carry bile salts, monoglycerides and fatty acids to the epithelial cells of the ilium and breakdown, allowing monoglycerides and fatty acids to diffuse across membrane because they are lipid-soluble.
• The monoglycerides and fatty acids are transported to the Endoplasmic Reticulum where they recombine to form triglycerides again.
• Inside the Golgi, the triglycerides bind with cholesterol and proteins to form chylomicrons.
• Chylomicrons travel in a vesicle to the cell membrane and leave the epithelial cell via exocytosis.
• The Chylomicrons enter lymphatic capillaries called lacteals which transport them away from the small intestine to tissues around the body, where the triglycerides can be hydrolysed and the fatty acids are used by the tissues.

Question 329

Describe the role of red blood cells and haemoglobin in oxygen transport

Answer 329

• Red blood cells contain lots of haemoglobin, which binds with Oxygen at gas exchange surfaces where partial pressure of Oxygen is high
• This forms oxyhaemoglobin which transports Oxygen (each haemoglobin protein can carry 4 oxygen molecules as it has 4 haem groups
• The haemoglobin unloads the oxygen near cells and tissues where the partial pressure of oxygen is low, so that the cells and tissues can respire

Question 330

Describe the structure of haemoglobin

Answer 330

Haemoglobin is a protein with a quaternary structure. It is made up of 4 polypeptide chains, and each chain contains a Haem group containing an iron ion

Question 331

Explain how the cooperative nature of oxygen binding results in an
Sigmoid oxyhaemoglobin dissociation curve

Answer 331

When each Oxygen molecule binds, the tertiary structure of the Haemoglobin changes, exposing more Haem group binding sites, meaning it is easier for Oxygen to bind to the Haemoglobin

Question 332

Describe evidence for the cooperative nature of oxygen binding

Answer 332

• At lower partial pressures of oxygen, as oxygen increases there is a slow increase in % saturation of Haemoglobin with oxygen
• At higher partial pressures of oxygen, as oxygen partial pressure increases, there is a faster increase in % oxygen saturation of Haemoglobin, showing it has become easier for oxygen molecules to bind

Question 333

What factors affect haemoglobin’s affinity for oxygen?

Answer 333

• Partial pressure of Oxygen
• Haemoglobin saturation
• Partial pressure of Carbon Dioxide

Question 334

Explain effect of CO2 concentration on the dissociation of oxyhaemoglobin

Answer 334

An increase in blood carbon dioxide levels due to increased rate of respiration results in the formation of Carbonic Acid, lowering the pH of the blood. This changes the tertiary structure of the Haemoglobin, meaning the Oxygen is unloaded more easily at any given partial pressure of oxygen.

Question 335

Explain the advantage of the Bohr effect during exercise

Answer 335

Oxygen is unloaded more quickly, meaning that the respiring tissues have access to more oxygen so they can aerobically respire at a faster rate. Therefore, more ATP is produced.

Question 336

Explain why different types of haemoglobin can have different oxygen transport properties

Answer 336

Different types of Haemoglobin are made up of polypeptide chains with different amino acid sequences, resulting in different tertiary structures, and therefore different affinities for oxygen

Question 337

Explain how organisms can be adapted to their environment by having different types of haemoglobin with different oxygen transport properties

Answer 337

• Left shift - Oxygen with bind with Haemoglobin at a lower partial pressure. For example, organisms in high altitude or underground environments and foetuses
• Right shift - Oxygen will dissociate from haemoglobin more easily. For example, organisms with high metabolic rate such as small mammals with a larger SA:V ratio, so will lose heat more easily)

Question 338

What type of circulatory system do mammals have?

Answer 338

Closed double circulatory system

Question 339

Describe the general pattern of blood circulation in a mammal

Answer 339

• Deoxygenated blood in right side of heart is pumped to the lungs, and the oxygenated blood returns to left side
• Oxygenated blood in left side of heart pumped to rest of body and the deoxygenated blood returns to right side

Question 340

Suggest the importance of a double circulatory system

Answer 340

• Prevents mixing of oxygenated / deoxygenated blood, so blood pumped around the body is fully saturated with oxygen for aerobic respiration
• Blood can be pumped to body at a higher pressure, so oxygen can reach the body cells more efficiently

Question 341

Name the blood vessels entering and leaving the heart and lungs
and state their function

Answer 341

• Vena cava – Transports deoxygenated blood from the respiring body cells to the heart
• Pulmonary artery - Transports deoxygenated blood from the heart to the lungs
• Pulmonary vein – Transports oxygenated blood from the lungs to the heart
• Aorta – Transports oxygenated blood from heart to the respiring body cells

Question 342

Which blood vessels provide the heart with oxygenated blood?

Answer 342

The coronary arteries

Question 343

Label a diagram to show the gross structure of the human heart

Answer 343

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Question 344

Suggest why the wall of the left ventricle is thicker than that of the right

Answer 344

The left side has thicker muscle to contract with greater force, so it can generate higher pressure to pump blood around entire body

Question 345

Explain the pressure & volume changes and associated valve movements
during the cardiac cycle that maintain a unidirectional flow of blood

Answer 345

• Atrial systole - The atria contract, so the volume decreases, increasing the pressure. The atrioventricular valves open when the pressure in the atria exceeds the pressure in the ventricles, so the blood is pushed into the ventricles
• Ventricular systole - The ventricles contract, so the volume decreases, increasing the pressure. The semilunar valves open when the pressure in the ventricles exceed the pressure in the arteries, so blood is pushed into the arteries
• Diastole - The atria and ventricles relax, so the volume increases, so the pressure decreases. The semi-lunar valves shut when the pressure in the arteries exceeds the pressure in the ventricles, so the blood enters the atria.

Question 346

At what points do the cardiac valves open and close?

Answer 346

• Semi-lunar valves close when the pressure in the artery is higher than the pressure in the ventricles
• Semi-lunar valves open when the pressure in the ventricle is higher than the pressure in the artery
• Atrioventricular valves close when the pressure in the ventricle is higher than the pressure in the atrium
• Atrioventricular valves open when the pressure in the atrium is higher than the pressure in the ventricle

Question 347

What is the equation for cardiac output?

Answer 347

Cardiac output (volume of blood pumped out of heart per minute) = stroke volume (volume of blood pumped in each heart beat) x heart rate (number of beats per minute)

Question 348

How can heart rate be calculated from cardiac cycle data?

Answer 348

Heart rate (beats per minute) = 60 (seconds) / length of one cardiac cycle (seconds)

Question 349

Explain how the structure of arteries and arterioles relates to their function

Answer 349

Their function is to carry blood away from the heart at high pressures. Adaptations include:
* Thick muscular walls - can withstand high pressure, muscles can contract and relax to control blood flow
* Narrow lumen - maintains high pressure

Question 350

Explain how the structure of veins relates to their function

Answer 350

Their function is to carry blood towards the heart at lower pressures. Adaptations include:
* Wide lumen - Maintains blood flow
* Valves - To prevent backflow of blood

Question 351

Explain how the structure of capillaries relates to their function

Answer 351

Their function is to allow efficient exchange of substances between blood and tissue fluid . Adaptations include:
* Walls are one cell thick - reduces diffusion distance
* Capillary bed is a large network - increases surface area for diffusion

Question 352

Explain the formation of tissue fluid

Answer 352

At the arteriole end of capillaries, there is a higher hydrostatic pressure inside the capillaries than outside the capillaries. This forces water and dissolved substances out of the capillaries into the surrounding tissues, and large plasma proteins remain in the capillary

Question 353

Explain the return of tissue fluid to the circulatory system

Answer 353

The water potential outside the capillary is higher than the water potential inside the capillary. Therefore, water enters the capillary via osmosis down a water potential gradient. Excess fluid enters the lymphatic system to become lymph, containing lymphocytes

Question 354

Suggest and explain causes of excess tissue fluid accumulation

Answer 354

• Low concentration of protein in blood plasma or high salt concentration will result in higher water potential, so the water potential gradient is reduced. This will mean more tissue fluid will be formed at arteriole end and less water absorbed at venule end by osmosis
• High blood pressure will result in high hydrostatic pressure. Therefore more tissue fluid will form at arteriole end and less water will be absorbed at venule end by osmosis

Question 355

What is a risk factor, and give an example for cardiovascular disease.

Answer 355

A risk factor is an aspect of a person’s lifestyle, body or environment which has been shown to increase their likelihood of getting a certain disease. Examples for cardiovascular disease include age, salt and fat concentration in diet, smoking, lack of exercise

Question 356

What is the function of xylem tissue?

Answer 356

Transports water and mineral ions up the stem to the leaves

Question 357

How is xylem tissue adapted for its function?

Answer 357

• Cells are joined together with no end walls, forming a long continuous tube, so water can flow as a continuous column
• Made up of dead cells which contain no cytoplasm or nucleus, allowing for easier water flow
• Thick walls made of lignin which provides support and allows the xylem to withstand tension
• Pits in side walls → allow lateral flow of water

Question 358

Explain the cohesion-tension theory of water transport in the xylem

Answer 358

Water evaporates from the mesophyll due to heat from the sun (Transpiration). This results in the cells having a negative water potential, causing more water to diffuse in through osmosis. This increase in water tension pulls more water into the leaf (transpiration pull). Water molecules are cohesive due to the fact they form Hydrogen bonds, so when some are pulled into the leaf, others follow. This along with adhesion of the water molecules to the xylem pulls the whole column of water in the xylem up from the roots to the mesophyll tissue, and water enters the stem through the roots via osmosis.

Question 359

Evidence to support cohesion-tension theory

Answer 359

• If a trunk or stem is damaged and a xylem cell is broken, water does not leak out. Once air enters, the tree can no longer draw up water because the continuous column of water has been broken
• The trunks of trees reduce in diameter during the daytime when transpiration is at its greatest rate. This is because adhesion of water molecules to the walls of the xylem results in tension, pulling the xylem walls in.

Question 360

Name and explain the 4 factors affecting transpiration rate

Answer 360

• Light Intensity - As light intensity increases, the rate of photosynthesis increases. This means that the stomata open to allow CO2 for photosynthesis to enter. This also results in more water being lost.
• Temperature - As temperature increases, the kinetic energy of the water molecules increases. This results in the water evaporating at a faster rate
• Wind intensity - As wind intensity increases, wind blows water molecules away from the stomata, increasing the water potential gradient, so water evaporates at a faster rate
• Humidity - At higher humidity levels, there is more water in the air, so there is a lower water potential gradient between the leaf and the air, so water evaporates at a slower rate

Question 361

What is the function of the phloem?

Answer 361

Transports products of photosynthesis in plants

Question 362

How is the phloem tissue adapted to its function

Answer 362

The sieve tubes have no nucleus and very few organelles, to maximise space for easier flow of substances. The end walls between the phloem cells are perforated to allow substances to pass between the individual cells.

The companion cells contain many mitochondria, to allow a high rate of respiration to make ATP for active transport of substances

Question 363

What is translocation?

Answer 363

The movement of solutes such as sucrose from sources to sinks

Question 364

Explain the mass flow hypothesis for translocation in plants

Answer 364

At the source, sucrose is actively transported into the phloem cells by the companion cells. This lowers water potential in the phloem cells, so water enters laterally from the xylem via osmosis. This increases the hydrostatic pressure in phloem, creating a hydrostatic pressure gradient. So mass flow occurs, as the solute moves from the source to the sink. At the sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs

Question 365

How does the active loading of sucrose into the companion cell occur at the source?

Answer 365

Hydrogen ions are actively transported out of the companion cell into the cells of the source tissue using the hydrolysis of ATP. This creates a hydrogen concentration gradient across the companion cell membrane. This means that the Hydrogen ions diffuse down the gradient through co-transporters, and they bring a co-transported sucrose molecule with them. This increases the concentration of sucrose in the companion cells, so the sucrose diffuses into the phloem cell

Question 366

What evidence is there to support the mass flow hypothesis?

Answer 366

When sieve tubes are cut, sap is released. This demonstrates that the sap is under pressure within the phloem

Question 367

What evidence is there that goes against the mass flow hypothesis?

Answer 367

Sucrose travels to many different sinks and does not always travel to the one with the highest water potential first, which it should according to the mass flow hypothesis

Question 368

Describe fish ventilation

Answer 368

TBA