Biology AS level Flashcards

Question

What is the function of a cell surface membrane?

Answer 1

- Barrier between cytoplasm and external environment - Cell signalling - Cell recognition (surface antigens) - Cell-to-cell adhesion - Site for enzyme catalysed reactions - Anchoring the cytoskeleton - Selection of substances that enter/leave the cell - Formation of Hydrogen bonds with water for stability

Answer 2

Nucleus is the largest organelle surrounded by the double membraned nuclear envelope and is continuous with rough endoplasmic reticulum. It is surrounded by a double membrane called the envelope containing pores which enable molecules to enter and leave the nucleus, the nucleus also contains chromatin and a nucleolus which is the site of ribosome production.

Answer 3

Nuclear pore are gaps in the nuclear envelope that allow exchange between the nucleus and cytoplasm. - Substances leaving: mRNA and ribosomes for protein synthesis. - Substances entering: protein to help make ribosome, nucleotide, ATP, & some hormones

Answer 4

Chromosomes contain the hereditary material DNA that is organised into genes which controls the activities of the cell and inheritance.

Answer 5

Rough endoplasmic reticulum (RER) is an extensive membrane system with 80S ribosomes are sites for protein synthesis which produce the rough appearance. RER folds and processes proteins made on the ribosomes

Answer 6

- The R.E.R provides a pathway for transport of materials through cell. - Forms a complex 3D system of sheet like membranes and tubes enclosing fluid-filled sacs. - Proteins made by ribosomes on RER enter sacs and move through them. - Transport vesicles bud off from the RER and join forming the Golgi body.

Answer 7

Smooth Endoplasmic reticulum is a system of membrane bound sacs these are a site for lipid synthesis and steroids e.g. cholesterol and reproductive hormones. the system is a meshwork of tubular membrane vesicles with fluid filled sacs that have no ribosome on its surface

Answer 8

- Golgi body/complex/apparatus is a stack of flattened membranes enclosing hollow sacs (cisternae) formed by transport vesicles which bud off of the RER, and are broken down to form Golgi vesicles. - It collects, processes, modifies and sorts molecules that are ready for transport in Golgi vesicles to other parts of the cell or out of the cell by secretion/exocytosis - Makes lysosomes, glycoproteins and functioning proteins.

Answer 9

Golgi apparatus is a series of fluid filled, flattened & curved sacs with vesicles surrounding the edges. The Golgi apparatus processes and packages proteins and lipids. It also produces lysosomes.

Answer 10

secretion/exocytosis is the fusion of vesicle with plasma membrane to release content.

Answer 11

Mitochondria are usually oval shaped, bound by a double membrane called the envelope. The inner membrane is folded to form projections called cristae with matrix on the inside containing all the enzymes needed for respiration

Answer 12

- Has a matrix that contains 70S ribosomes and circular DNA which is used to make some of the mitochondrion’s own proteins. - Cristae: folding of inner membrane that projects into interior solution, matrix. - Intermembrane space: space between the two membranes. - Porin: transport protein in outer membrane, forms wide aqueous channel allowing water-soluble molecules from cytoplasm to intermembrane space. - Inner membrane: selective barrier controlling entrance of ions and molecules into the matrix.

Answer 13

Mitochondria surrounded by mitochondrial envelope; provides energy for aerobic respiration, synthesizes lipids and is more in areas that require maximal energy.

Answer 14

Ribosomes are the site at which mRNA (transcribed from the nucleus) is translated into polypeptides with the help of tRNA, therefore help with protein synthesis. They are not membrane bound and are made up of rRNA and rProteins. Ribosomes are composed of two sub units and are the site of protein production

Answer 15

- Small subunit: site of translation (mRNA binds here and is read by this unit) - Large subunit: joins amino acids to form polypeptide.

Answer 16

- 80S ribosomes: in the cytoplasm and R.E.R - 70S ribosomes: in chloroplast and mitochondria.

Answer 17

- Lysosomes are a single membrane with no internal structure in animal cells. They contain digestive (hydrolytic) enzymes bound by a single membrane that’s kept separate from rest of cell to prevent damage. - They are responsible for breakdown of unwanted structures e.g. old organelles or whole cells, in WBC to digest bacteria.

Answer 18

Microtubules are long hollow tubes that make up the cytoskeleton which helps determine cell shape. .

Answer 19

- Made up of alpha and beta tubulin that combine to form dimers. - Dimers join end to end to form protofilaments (polymerisation). - 13 protofilaments line up alongside each other in a ring to form a cylinder with a hollow center i.e. microtubule. - Forms an intracellular transport system by moving along secretary vesicles, organelles and cell components on its outer surface

Answer 20

- Centriole are formed by 9 triplets of microtubules. - --- Microtubules extend from centriole and attach themselves to kinetochore of chromosomes, forming spindle fibres. - Centrioles duplicate, and a pair of centrioles then move to opposite poles of the cell (2 centrosome regions), thus separating sister chromatids during nuclear division. - Centrioles at bases of cilia and flagella (basal bodies) act as MTOCs. Microtubules extending from basal bodies into cilia and flagella help with their beating movements.

Answer 21

The vacuole is a fluid-filled sac present in plant cells, surrounded by a membrane called the tonoplast. It contains mineral salts, sugars, amino acids, waste substances and pigments. Its role is to colour the cell to attract pollinating insects, act as a temporary food store and provide support through turgidity

Answer 22

- Cell surface membrane - Nucleus - Endoplasmic reticulum - Mitochondria - Ribosomes - Lysosomes - Microtubules - Centrosome - Centriole

Answer 23

‎- Cell surface membrane - Nucleus - Endoplasmic reticulum - Mitochondria - Ribosomes - Lysosomes - Microtubules - Centrosome - Centriole - Chloroplasts - Cell wall - Plasmodesmata - Large vacuole and tonoplast

Answer 24

- Cell membrane - Nucleus - Cytoplasm - Mitochondria - Golgi apparatus

Answer 25

Animal cells look spherical while plant cells look rectangular

Answer 26

They both make ATP, which the energy currency of the cell

Answer 27

- The DNA in prokaryotes is circular and lies free in the cytoplasm. It is naked - The DNA in eukaryotes is non circular, contains a nucleus - In prokaryotes there are very few organelles none which are membrane bound - In eukaryotes there are many organelles, some single bound, some double-bound and some have no membrane - In eukaryotes ER is present and maybe attached to ribosomes - In prokaryotes the cell wall contains murein - In eukaryotes the cell wall when present contains cellulose or lignin in plants and chitin in fungi - In prokaryotes their are no ER - In prokaryotes the Ribosomes are 70s - In eukaryotes the ribosomes are 80s

Answer 28

Equal volume of sample being tested and Benedict’s solution are mixed and heated in a water bath up to 95C. Positive: green → yellow → orange → brick red Negative: blue

Answer 29

Benedict’s solution can be used to test for the presence of reducing sugars. Reducing sugars include all monosaccharides and some disaccharides. Therefore it can be used to test for glucose, fructose and maltose. It does not test for sucrose, however the test involves heating the sugar with Benedict’s solution – if the colour changes from blue to brick red then glucose is present.

Answer 30

If there is no colour change after caring out benedict's solution it could be that there is a non-reducing sugar present (e.g. sucrose). You can break the glyosidic bonds by acid hydrolysis. 1. Add dilute Hydrochloric acid (catalyst) to the test solution in the ratio of 1:2 and heat in a water bath. . respectively approximately 2 minutes. 2. A pinch of sodium hydroxide is added to make the solution alkaline therefore neutralising it 3. Heat with Benedict’s solution

Answer 31

- You can do this by measuring the time it takes for the colour change to happen. The different times can help estimate the concentration. - You could also use the different colour changes and work out the concentration of glucose using colorimetry. - Higher glucose concentration means lower absorbance of the solution 1. Do Benedict’s test 2. Calibrate colorimeter with plain water and use this as your control 3. Remove precipitates from each test tube by using a centrifuge 4. Measure the absorbance using a colorimeter 5. Create a calibration curve of concentration of glucose vs absorbance (this can be used to find glucose concentrations of different unknowns)

Answer 32

Biuret’s test used to detect the presence of proteins: Equal amounts of the sample and Biuret’s solution are added together, giving purple colour over several minutes in the presence of proteins, and blue in its absence.

Answer 33

You use biuret test

Answer 34

Emulsion test for lipids: The sample is added to 2cm3 of ethanol and mixed well until it dissolves (lipids are soluble in ethanol). This mixture is then placed into a test tube containing the same amount of water. A milky white emulsion will appear if lipids are present and remain clear if not.

Answer 35

You use the emulsion test

Answer 36

You use the benedict's test

Answer 37

Iodine test for the presence of starch: Iodine solution is orange-brown. Add a drop of iodine solution to the solid or liquid substance to be tested. A blue-black colour is quickly produced if starch is present.

Answer 38

You use the iodine test

Answer 39

Carbohydrates are molecules which consist only of carbon, hydrogen and oxygen and they are long chains of sugar units called saccharides

Answer 40

There are three types of saccharides monosaccharides, disaccharides and polysaccharides.

Answer 41

Glucose is a monosaccharide containing six carbon atoms in each molecule, it is the main substrate for respiration and the energy source which is broken down during respiration therefore it is of great importance. It has two isomers – α- glucose and β - glucose and their difference lies between the position of an –OH group in their ring structures.

Answer 42

Glucose has the molecular formula C6H12O6.

Answer 43

Glucose is the monomer from which Starch and Cellulose are made

Answer 44

A monomer simple molecule which is used as a basic building block for the synthesis of a polymer; many monomers are joined together to make the polymer, usually by condensation reactions e.g. monosaccharides, amino acids, nucleotide.

Answer 45

A polymer: is a giant molecule made from monomers e.g. polysaccharides, proteins, nucleic acids

Answer 46

A macromolecule are large and complex molecules that are formed due to polymerisation of smaller monomers e.g. polysaccharides, nucleic acids

Answer 47

Monosaccharide is a molecule consisting a single sugar unit, the simplest form of carbohydrate and cannot be hydrolysed further. It has a general formula of (CH2O)n.

Answer 48

Monosaccharides are small organic molecules used as building blocks of complex carbohydrates. Monosaccharides have a varying number of carbon atoms, for instance: - Glyceraldehyde is a triose used in metabolic reactions - Ribose is a pentose sugar which is a component of nucleic acid - Glucose is a monosaccharide which is the substrate for respiration

Answer 49

Disaccharide is a sugar molecule consisting of two monosaccharides joined together by a glycosidic bond.

Answer 50

Disaccharides are formed in a condensation reaction between two monosaccharides. - Maltose is a disaccharide formed by condensation of two glucose molecules - Sucrose is a disaccharide formed by condensation of glucose & fructose - Lactose is a disaccharide formed by condensation of glucose & galactose

Answer 51

Polysaccharide is a polymer whose subunits are monosaccharides joined together by glycosidic bonds.

Answer 52

Polysaccharides include: - Glycogen and starch which are both formed by the condensation of alpha glucose - Cellulose formed by the condensation of beta glucose

Answer 53

Glyosidic bonds are covalent bonds that occur between constituent monomers and are formed due to a condensation reaction which involves the removal of a water molecule in order to form polysaccharides and disaccharides such as sucrose.

Answer 54

Constituent molecules in glyosidic bonds can also be separated by hydrolysis which breaks the glycosidic bond between monomers e.g.: Acid hydrolysis of non-reducing sugars (sucrose) breaks glycosidic bond in order to retrieve constituent monomers

Answer 55

Starch is a macromolecule that is found in plant cells. Starch is highly compact and stores energy.

Answer 56

Starch is made up of two components known as amylose and amylopectin. These components are polysaccharides that are made from a glucose molecules and contain 1,4 glycosidic bonds

Answer 57

Amylose is an unbranched chain of glucose molecules joined by α 1,4 glycosidic bonds, as a result of that amylose is coiled and thus it is a very compact molecule meaning it can store a lot of energy in a small space.

Answer 58

Amylopectin is branched and is made up of glucose molecules joined by α 1,4 and 1,6 glycosidic bonds, due to the presence of many side branches it is rapidly digested by enzymes therefore energy is released quickly.

Answer 59

Glycogen is a macromolecule that is the main energy storage molecule in animals and is also made from α glucose molecules. The structure of glycogen is very similar to that of amylopectin; however, it is more branched and therefore contains more α 1,6 glycosidic bonds and therefore energy can be released quickly. Moreover, it is a relatively large but compact molecule thus maximising the amount of energy it can store

Answer 60

Cellulose is found in the cell wall of plant cells and is made from long unbranched chains of β-glucose units that form β-1,4 glycosidic bonds. Alternate β- glucose molecules are rotated 180 degrees in order to form these bonds.

Answer 61

- Hydrogen bonds are also formed between parallel cellulose molecules. 60 and 70 cellulose molecules become tightly cross-linked to form bundles called microfibrils. Microfibrils are in turn held together in bundles called fibres by hydrogen bonding. - Fibres increase tensile strength to withstand osmotic pressure, making the plant rigid and determine cell shape. They’re also freely permeable...

Answer 62

Triglyceride are forms by the condensation of 3 fatty acid chains and 1 glycerol molecule, joined by an ester bond formed in condensation. Triglycerides are non-polar and hydrophobic molecules. Triglycerides are used as energy reserves in plant and animal cells.

Answer 63

Fatty acid chains are long hydrocarbon chains with a carboxylic head.

Answer 64

Glycerol is an alcohol containing 3 OH groups.

Answer 65

Glycerol is an alcohol containing 3 OH groups.

Answer 66

Glycerol is an alcohol containing 3 OH groups.

Answer 67

-Unsaturated fatty acids contain c=c bonds that are easier to break and melt easily. More than one c=c is a polyunsaturated fatty acid.

Answer 68

Saturated fatty acids contain c-c bonds that are solids at room temperature.

Answer 69

- Better energy reserves than carbohydrates as more CH bonds - Acts as an insulator and provides buoyancy - A metabolic source of water as gives CO2 and H20 on oxidation in respiration

Answer 70

In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group

Answer 71

In a phospholipid the hydrophilic head contains a phosphate group and glycerol while the hydrophobic tail contains 2 fatty acid chains. This is due to the partial negative charge on the phosphate group that gets attracted to the partial positive charge on the hydrogen atom of the water molecule.

Answer 72

Phosphate heads are hydrophilic and the tails are hydrophobic and as a result phospholipids form micelles when they are in contact with water as heads are on the outside as they are attracted to water and tails are on the inside as they move away from water.

Answer 73

The hydrophobic/hydrophilic nature of phospholipids is what makes cell membranes selectively membrane

Answer 74

Proteins are made of amino acids which only differ in the R- groups/ variable side chains and will always contain an amine group (basic), carboxyl group (acidic) and a hydrogen atom attached to the central carbon atom.

Answer 75

Amino acids are joined by peptide bonds formed in condensation reactions.

Answer 76

Amino acids are the monomers from which proteins are made. Amino acids contain an amino group –NH2, carboxylic acid group and a variable R group which is a carbon-containing chain. There are 20 different amino acids with different R groups.

Answer 77

A dipeptide contains two amino acids and polypeptides contain three or more amino acids.

Answer 78

- A peptide bond is formed by condensation between 2 amino acids, forming a dipeptide. Many amino acids that join together by peptide bonds form a polypeptide.‎ - Peptide bonds are broken when hydrolysed into amino acids, often occurring in the small intestine and stomach.

Answer 79

The structure of proteins is determined by the order and number of amino acids, bonding present and the shape of the protein.

Answer 80

Primary structure is a sequence of amino acids in a polypeptide/protein. A slight change in the sequence of amino acids can affect the protein’s structure and function. It has a unique sequence for each protein.

Answer 81

Secondary structure is the structure of a protein molecule resulting from the regular coiling or folding of the chain of amino acids. The secondary structure is the shape that the chain of amino acids chains – either alpha helix or beta pleated sheet. The type of bond determines the shape

Answer 82

Hydrogen bonding - weak bonds between a slightly positively-charged hydrogen atom and another slightly negatively-charged atom (usually nitrogen, oxygen or fluorine).

Answer 83

Ionic bond - attraction between oppositely charged R groups

Answer 84

Disulphide bridges - when 2 cysteine amino acids come into close contact and the sulfur in each cysteine forms a bond.

Answer 85

α- helix: the polypeptide chain twists into a regular spiral and is maintained by hydrogen bonds between the (-NH) group of one amino acid and the (CO-) group of another amino acid 4 spaces later in the polypeptide chain.

Answer 86

β- pleated sheet: the chain is not tightly coiled and lies in a looser, straighter shape.

Answer 87

Tertiary structure is the compact structure of a protein molecule resulting from the three-dimensional coiling of the already-folded chain of amino acids. It can be globular or fibrous.‎

Answer 88

- Hydrogen bonds between wide varieties of R- groups (can be broken by PH and temperature changes) - Disulphide bridges between two cysteine molecules (can be broken by reducing agents) - Ionic bonds between R groups containing amine and carboxyl groups. (Can be broken by PH changes.) - Hydrophobic interactions between non polar R groups.

Answer 89

Globular proteins curl up into a spherical shape with their hydrophobic regions pointing into the centre of the molecule and hydrophilic regions pointing outwards. They are soluble in water e.g. enzymes and haemoglobin.

Answer 90

Fibrous proteins: form long strands, are insoluble in water, and have structural roles e.g. collagen, hair, nails.

Answer 91

Quaternary structure is the three-dimensional arrangement of two or more polypeptides, or of a polypeptide and a non-protein component such as haem, in a protein molecule closely packed together. The polypeptide chains are held together by bonds in the tertiary structure.

Answer 92

Haemoglobin is a globular protein that has a quaternary structure with 4 polypeptide chains, 2 α-globin and 2 β-globin chains. Each chain has one prosthetic (non-protein) group - haem (Fe2+). containing an iron atom that reversibly binds to an oxygen molecule. Oxyhaemoglobin is bright red, when the haem group is combined with oxygen, otherwise it’s purplish.

Answer 93

Sickle cell anaemia is a genetic condition in which a polar amino acid, glutamic acid is substituted by non-polar valine on the surface of the β chain in haemoglobin, making it less soluble.

Answer 94

Collagen is a fibrous protein that is present in the skin, bones, teeth, cartilage and walls of blood vessels. It is an important structural protein.

Answer 95

- A collagen molecule has 3 polypeptide chains that are coiled in the shape of a stretched-out helix. - Compact structure and almost every 3rd amino acid is glycine, the smallest amino acid which can form H-bonds. - 3 polypeptide strands are held together by hydrogen and covalent bonds. - Many of these collagen molecules lie side by side, linked to each other by covalent cross-links between the side chains of amino acids, forming fibrils, and many fibrils make up a fibre.‎

Answer 96

- A collagen molecule has 3 polypeptide chains that are coiled in the shape of a stretched-out helix. - Compact structure and almost every 3rd amino acid is glycine, the smallest amino acid which can form H-bonds. - 3 polypeptide strands are held together by hydrogen and covalent bonds. - Many of these collagen molecules lie side by side, linked to each other by covalent cross-links between the side chains of amino acids, forming fibrils, and many fibrils make up a fibre.‎

Answer 97

A water molecule contains two hydrogen atoms and one oxygen atom held together by hydrogen bonds.

Answer 98

Water is an effective solvent because of its polarity and so can form electrostatic interactions with other polar molecules and ions. Thus it’s a transport medium and reagent for metabolic and other reactions in the cells of plants and animals.

Answer 99

It is a metabolite in metabolic reactions such as condensation and hydrolysis which are used in forming and breaking of chemical bonds

Answer 100

High surface tension and cohesion: cohesion refers to the attraction of one water molecule to the other. Water molecules have strong cohesive forces due to hydrogen bonds, thus having high surface tension. As a result of strong cohesion the surface tension at the water-air boundary is high

Answer 101

Specific heat capacity is the amount of heat energy required to raise the temperature of 1 kg of water by 1 °C.

Answer 102

Water has high specific heat capacity due to its hydrogen bonds. Temperature within organisms remains constant compared to external temperature, and water bodies also have a slow change in temperature, providing stable aquatic habitats. A lot of energy is required to warm water up therefore minimising temperature fluctuations in living things therefore it acts as a buffer.

Answer 103

latent heat of vaporization is the measure of the heat energy needed to vaporise a liquid.

Answer 104

Water has a high latent heat of vaporization due to its high specific heat capacity as H bonds need to be broken before water can be vaporised, cooling the surrounding environment. Sweating is a good cooling mechanism. However, a large amount of energy can be lost for little amount of water, thus dehydration is prevented e.g. in transpiration.

Answer 105

Ice is less dense than water and floats on it, insulating water and preventing it from freezing, preserving aquatic life underneath it. Changes in the density of water with temperature cause currents, which help to maintain the circulation of nutrients in the oceans.

Answer 106

In beta glucose the OH molecule is above the ring and with alpha glucose the OH molecule is below the ring

Answer 107

In beta glucose the OH molecule is above the ring and with alpha glucose the OH molecule is below the ring

Answer 108

In beta glucose the OH molecule is above the ring and with alpha glucose the OH molecule is below the ring

Answer 109

An enzyme is a biological catalyst that accelerates metabolic reactions. Enzymes are globular proteins as they have a roughly spherical shape and are water soluble. They increase the rate of reaction by lowering the activation energy of the reaction they catalyse. Each enzyme has a specific shape that must be complementary to the substrate

Answer 110

An enzyme is a biological catalyst that accelerates metabolic reactions. Enzymes are globular proteins as they have a roughly spherical shape and are water soluble. Enzymes increase the rate of reaction by lowering the activation energy. Each enzyme has a specific shape that must be complementary to the substrate

Answer 111

An enzyme is a biological catalyst that accelerates metabolic reactions. Enzymes are globular proteins as they have a roughly spherical shape and are water soluble. Enzymes functioning inside a cell are intracellular, but those that are secreted by cells and catalyse reactions outside cells are described as extracellular.

Answer 112

The active site is the area of the enzyme where the reaction with the substrate takes place. Each enzyme has a specific shape that must be complementary to the substrate, meaning that only one type of substrate fits into the active site of each enzyme. Enzymes have specific active sites that are complementary to the shape of the substrate.

Answer 113

The substrate is held in place at the active site by weak hydrogen and ionic bonds. The combined structure is called the enzyme-substrate complex.

Answer 114

When the enzyme and substrate form a complex, the structure of the enzyme is altered so that the active site of the enzyme fits around the substrate. This is called the induced fit model.

Answer 115

Proposed by Fischer in 1894 - Active site and substrate have complementary shapes prior to binding - The enzyme binds with substrate forming an enzyme-substrate complex - Products are released from active site and enzyme can be reused - Only one substrate can fit each active site - The enzyme-substrate complexes formed enable the reaction to take place more easily.

Answer 116

Activation energy is the energy required in any chemical reaction to break the bonds in reactant molecules so that new bonds are formed to make the product. An enzyme lowers the activation energy required for the reaction. However, overall energy released during reaction is maintained

Answer 117

Proposed by Koshland in 1958 - Enzyme has active site that is not initially an exact fit to the substrate molecule. - Enzyme is moulded around substrate as it enters to become complementary forming an enzyme-substrate complex - Bonds form between oppositely charged groups on substrate and R groups to induce a better fit. This puts a strain on the substrate molecule so reactions occur more easily.

Answer 118

- By measuring the amount of product accumulated over a period of time. Rate of reaction = volume of product produced / time. E.g. enzyme catalase breaking hydrogen peroxide to H2O + o2 - By measuring the rate at which the reactants disappear from the reaction mixture, the effect of the enzyme on the rate of reaction can be determined. E.g.: measuring the rate at which starch disappears when the enzyme amylase is added.

Answer 119

- Catalyses the breakdown of hydrogen peroxide - Products: oxygen and water - Measure the rate of oxygen produced over a period of time - You can plot a graph of time vs volume of oxygen produced

Answer 120

- Catalyses the breakdown of starch - Products: maltose - Amylase is added to the starch samples - At regular timed intervals take samples - Use iodine/KI solution to test for the presence of starch (colour change to orange-brown when starch breakdown is complete) - Measure the absorbance in a colorimeter (make sure colorimeter is calibrated and sample has been mixed properly with the iodine) - The darker the colour the higher the starch concentration hence a higher absorbance - Plot a graph of time vs absorbance

Answer 121

- Catalyses the breakdown of starch - Products: maltose - Amylase is added to the starch samples - At regular timed intervals take samples - Use iodine/KI solution to test for the presence of starch (colour change to orange-brown when starch breakdown is complete) - Measure the absorbance in a colorimeter (make sure colorimeter is calibrated and sample has been mixed properly with the iodine) - The darker the colour the higher the starch concentration hence a higher absorbance - Plot a graph of time vs absorbance

Answer 122

Initially, there’s a large number of substrates and every enzyme has a substrate in its active site. The rate at which the reaction occurs depends only on how many enzymes there are and the speed at which the enzyme can convert the substrate into product, release it, and then bind with another substrate. However, overtime, there are fewer substrates to bind with enzymes; the reaction gets slower, until it eventually stops.

Answer 123

- Enzyme concentration - Substrate concentration - Temperature - pH - Concentration of competitive reversible inhibitors - Concentration of non-competitive reversible inhibitors

Answer 124

As the temperature increases, the kinetic energy and the enzyme activity increases as there’s more collisions until optimal temperature is reached (usually 40C). At optimal temperature, maximum rate of reaction is achieved. If the temperature continues to increase beyond optimal temperature, the rate of the reaction begins to decrease as more kinetic energy breaks the hydrogen bonds in the secondary and tertiary structure of enzyme. This changes the shape of the enzyme and its active site and causes the substrate to no longer fit. The enzyme is denatured.

Answer 125

Any change in the pH value of the medium around the enzyme will cause ionic and hydrogen bonds to be damaged, this will change the 3-D shape of the enzyme and deform the active site. The substrate will therefore not be able to fit into active site so the reaction slows down or stops. The effects of pH is reversible within certain limits but if the pH is far from optimal value, the enzyme gets denatured.

Answer 126

As the concentration of enzymes is increased, there are more available active sites for substrates to fit into. More enzyme-substrate complexes are formed, more products are formed and the rate of reaction is increased. The limiting factor is the enzyme concentration. Once all substrates have formed enzyme-substrate complexes, a further increase in concentration will have no effect on the rate of reaction. At this point, the limiting factor is the substrate concentration. During comparison, look at initial rate to ensure differences in reaction rate are caused only by differences in enzyme concentration.

Answer 127

As the concentration of the substrates increases, there are greater chances of collision with enzyme. More enzyme-substrate complexes are formed, more products are formed and the rate of reaction is increased. The limiting factor is the substrate concentration. Once all enzymes are occupied and working at maximum rate (vmax), a further increase in substrate concentration will have no effect on the rate of reaction. At this point, the limiting factor is the enzyme concentration.

Answer 128

Inhibitors are substances which stop the enzyme from binding to its substrate. Inhibitors interfere with enzyme activity and reduce the rate of an enzyme catalysed reaction. Therefore, as the concentration of inhibitors increases, the rate of reaction decreases.

Answer 129

Reversible competitive inhibitor have a similar shape to the substrate and fits into the active site. This reduces the number of enzyme-substrate complexes formed and the rate of reaction decreases. It is said to be reversible because it can be reversed by increasing the concentration of the substrate.

Answer 130

The reversible non-competitive inhibitor have a different shape to the substrate and fits into a site other than the active site. While the non-competitive inhibitor is bound, the tertiary structure of the entire enzyme is distorted, preventing the formation of enzyme-substrate complexes and decreasing the rate of reaction regardless of substrate concentration

Answer 131

End-product inhibition is used to control metabolic reactions via non-competitive reversible inhibitors. As the enzyme converts substrate to product, it is slowed down because the end product binds to another part of the enzyme and prevents more substrate binding. However, the end-product can lose its attachment to the enzyme and go on to be used elsewhere, allowing the enzyme to reform into its active state.

Answer 132

Feedback inhibition occurs when the end product binds to the enzyme at the start of the reaction/pathway and this stops the pathway until the concentration of the end product decreases

Answer 133

Theoretical maximum rate velocity (Vmax) is the reaction rate is measured at different substrate concentrations while keeping the enzyme concentration constant. As substrate concentration is increased, reaction rate rises until the reaction reaches its maximum rate.

Answer 134

Michaelis-Menten equation can be used to calculate the maximum rate of reaction (Vmax) by relating the velocity of enzyme reactions (V) to concentration of a substrate [S]. Vmax represents the maximum rate of reaction achieved by the system at maximum substrate concentration.

Answer 135

- The enzyme is mixed with a solution of sodium alginate. - Little droplets of this mixture are then added to a solution of calcium chloride. - The sodium alginate and calcium chloride instantly react to form jelly, which turns each droplet into a little bead. The jelly bead contains the enzyme. - Can reuse the enzyme as it is not mixed with the solution, and can keep the product enzyme free, thus preventing contamination. - More tolerant to PH changes as the enzyme molecules are held firmly in shape by the alginate beads, thus don’t denature easily. - More tolerant to temperature changes as parts of the molecules embedded in the beads are not fully exposed to temperature or pH changes. - Active site may be distorted by immobilizing - Substrate passes through matrix when immobilized - Some product is retained within matrix

Answer 136

All cells and organelles are surrounded by a partially permeable membrane composed of a sea of phospholipids with protein molecules between the phospholipid molecules. The main function of the membrane is controlling the movement of substances in and out of the cell/organelle. However, it also contains receptors for other molecules to allow signalling between cells.

Answer 137

Fluid mosaic model is a model of the individual phospholipid and protein molecules and how they move around within their own monolayer. The word ‘mosaic’ describes the pattern produced by scattered protein molecule when the surface of the membrane is viewed from above. The fluidity of the phospholipid membrane and the mosaic arrangement of the protein give the structure of the membrane its name – fluid mosaic mode.

Answer 138

Phospholipid bilayer provides the basic structure of membranes; it is selectively permeable and acts as a barrier to most water-soluble substances.

Answer 139

Phospholipids have non-polar tails and hydrophilic heads, thus forming a barrier to most water soluble substances.

Answer 140

- The more unsaturated the tails, the more fluid the membrane as unsaturated fatty acid tails are bent and therefore fit together more loosely. - The longer the tail, the less fluid the membrane.

Answer 141

Micelle are phospholipid molecules that arrange themselves in a spherical form in aqueous solutions.

Answer 142

Cholesterol regulates the fluidity of a membranes. Its hydrophobic region prevents polar molecules from passing through the membrane e.g. in myelin sheath They also helps mechanical control the stability of membrane.

Answer 143

- At low temperatures: cholesterol increases the fluidity of the membrane, preventing it from becoming too rigid. - At higher temperatures: helps stabilize cells when the membrane could otherwise become too fluid.

Answer 144

- Glycolipids are carbohydrate chains that are attached to phospholipids (glycolipid) project out into the watery fluids surrounding the cell where they form hydrogen bonds to stabilize the membrane structure. - Glycolipids are short carbohydrate chains that help make membranes stable by forming hydrogen bonds with H2O. Help cells attach to one another.

Answer 145

- Glycoproteins- recognition sites, act as antigens - Glycoproteins are carbohydrate chains that are attached to membrane protein (glycoprotein) and project out into the watery fluids surrounding the cell where they form hydrogen bonds to stabilize the membrane structure. Carbohydrate chains act as receptors.

Answer 146

Signalling receptors are the receptors recognise messenger molecules like hormones and neurotransmitters. When the messenger molecule binds to the receptor, a series of chemical reactions is triggered inside the cell.

Answer 147

Endocytosis are groups of receptors bind to molecules that are to be engulfed by the cell surface membrane.

Answer 148

Cell adhesion is binding cells to other cells in tissues and organs. Some glycolipids and glycoproteins act as antigens, allowing cell–cell recognition.

Answer 149

Cell surface receptors are present in membranes and bind with particular substances, e.g.: hormones which are chemical messengers which circulate in the blood but only bind to specific target cells.

Answer 150

- Heat - Ethanol - pH

Answer 151

Transport proteins provide hydrophilic channels for ions and polar molecules. Enzymes catalyse the hydrolysis of molecules. Cytoskeleton made of protein filaments help maintain the shape of the cell.

Answer 152

Intrinsic proteins are proteins that are found embedded within the membrane. They may be found in the inner layer, the outer layer or, most commonly, spanning the whole membrane, known as transmembrane proteins. Some intrinsic proteins form channels or carriers for water soluble molecules.

Answer 153

Extrinsic proteins are found on the inner or outer surface of the membrane. Many are bound to intrinsic proteins or to phospholipids. Some extrinsic proteins act as enzymes

Answer 154

Channel proteins are water-filled pores that allow charged substances, usually ions, to diffuse through the membrane. They have a fixed shape and can be gated to control ion exchange. This does not use ATP and is in facilitated diffusion. Channel proteins are a hydrophobic channel where diffusion of polar molecules and ions happens

Answer 155

Carrier proteins can flip between two shapes, it allows active and passive transport. Carrier proteins are used mainly in active transport where it uses ATP to change shape and carry ions/molecules up the concentration gradient. It is also involved in passive transport (facilitated diffusion) down the concentration gradient without the use of energy.

Answer 156

Cell surface antigen act as cell identity markers. Each type of cell has its own antigen. This enables cells to recognise other cells and behave in an organised way

Answer 157

- A signal arrives at a specific protein receptor in a cell surface membrane that recognises the signal. - The signal brings about a conformational change in the shape of the receptor, spanning the membrane, and the message is passed to the inside of the cell (signal transduction). - Changing the shape of the receptor allows it to interact with the G protein, which brings about the release of a ‘second messenger’ (a small molecule which diffuses through the cell relaying the message). - The second messenger activates a cascade of enzyme catalysed reactions which brings about the required change. - This is an active process involving ATP use.

Answer 158

Specific ligands are released from the cell which are transported to the target cell where they bind to specific receptors on the cell surface membrane. This produces a response which may cause a cascade of more reactions.

Answer 159

Diffusion is the passive net movement of molecules or ions from a region of high concentration to a region of low concentration. It is a passive process (molecules have natural kinetic energy). As a result of diffusion, molecules reach equilibrium. The molecules move directly through the phospholipid bilayer

Answer 160

Steeper concentration gradient, higher temperature and increased surface areas all increase rate of diffusion.

Answer 161

- Non polar molecules can pass directly through the membrane e.g. steroid hormones - Gases can diffuse through the membrane directly - Water can diffuse through directly as it is a small molecule despite being polar. - Small, non-polar lipid soluble molecules such as carbon dioxide and oxygen

Answer 162

Facilitated diffusion is the movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient. The movement is passive; however, molecules go through transport proteins instead of passing through phospholipids. This allows for the passage of large polar ions and molecules e.g. glucose, amino acids, Na+, Cl-

Answer 163

Facilitated diffusion requires a channel protein in the cell membrane to transport polar molecules, charged and water soluble molecules across the membrane.

Answer 164

Osmosis is the diffusion of water molecules from a region of higher water potential (ψ) (less negative) to a region of lower ψ (more negative) through a selectively permeable membrane..

Answer 165

Negative ψ means that solution has more solute than solvent, therefore solute potential (ψs) reduces ψ

Answer 166

In plant cells: ψ = ψs + ψp - ψ is the tendency of water to move out of a solution; pressure potential (ψp) on liquid increases ψ

Answer 167

Water potential is the pressure exerted by water molecules that are free to move in a system. It is measured in kPa. Pure water has a water potential of 0 pKa, the higher the water potential the larger the number of water molecules that are free to move

Answer 168

A solution’s water potential falls as solutes are added as water molecules cluster around the solute. The contribution of solute to the water potential is called the solute potential

Answer 169

Protoplast is he living part of the cell inside the cell wall

Answer 170

In pure water, water enters the cell by osmosis, and the cell wall pushes back against the expanding protoplast, building up pressure rapidly, becoming turgid. This is the ψp, and it increases the ψ of the cell until equilibrium is reached.

Answer 171

In a concentrated solution, water will leave the cell by osmosis. The protoplast gradually shrinks until it is exerting no pressure on the cell wall. The ψp = 0, so ψ = ψs. The protoplast continues to shrink and pulls away from the cell wall, so the cell is plasmolysed. The point at which ψp has just reached 0 and plasmolysis is about to occur is referred to as incipient plasmolysis. ‎

Answer 172

Active transport is the movement of substances from a region of low concentration to a region of high concentration against a concentration gradient. This occurs via specific carrier proteins for specific ions/molecules that use energy from ATP. Active transport can transport all types of molecules through carrier proteins.

Answer 173

Exocytosis is the movement of substances out of the cell. A secretory vesicle from Golgi body moves towards the plasma membrane with the help of cytoskeleton, using energy from ATP. The vesicles fuse with the cell surface membrane releasing the contents outside.

Answer 174

Endocytosis involves the engulfing of the material by fusing with the plasma membrane to form an endocytic vacuole in the form of phagocytosis (bulk uptake of solids) or pinocytosis (bulk uptake of liquids) using ATP.

Answer 175

- As the surface area of the surface increases - As the diffusion distance decreases - As the diffusion gradient becomes more steep

Answer 176

- permit continued replication - prevent loss of genes - protect ends of chromosomes from being, degraded

Answer 177

Telomeres are the repeated short base sequence at end of chromosome (by telomerase).

Answer 178

Chromosomes is a threadlike structure containing DNA and genes. Chromosome condensed, so DNA is tightly packed therefore it is easier to separate chromatids at centromere into daughter cells.

Answer 179

Chromatin is a combination of DNA wound around histones proteins (basic).

Answer 180

Kinetochore is a protein structure found on centromere of chromatid to which microtubules attach. Two kinetochore at centromere on each chromatid during metaphase. Microtubules extend from kinetochore to the poles of spindle.

Answer 181

Centrosome are poles of spindle and act as microtubule-organizing centre with a pair of centriole that are surrounded by proteins which make microtubules.

Answer 182

Cancer is a mutation that occurs in genes that control cell division, an oncogene, that results in uncontrolled mitosis. Cancerous cells divide repeatedly and form a tumour, which is an irregular mass of cells.

Answer 183

A malignant tumour iare tumours that spread through the body, invade other tissues and destroy them. These cells break off from the tumour and form secondary growth, known as metastasis.

Answer 184

Cells produced by mitosis are undifferentiated (those are called stem cells) which can be made into specialised cells via differentiation. Stem cells repeatedly undergo cell division and are used for cell replacement and tissue repair. Once the cell becomes specialised for a specific function it stops dividing.

Answer 185

However if cell division is uncontrolled this can lead to the formation of a mass of cells called a tumour, which can cause cancer

Answer 186

Totipotent is the cells that can divide repeatedly to form any other cell in the body, e.g.: zygote

Answer 187

Pluripotent are embryotic stem cells that lead to development of the embryo and later the adult. They are not specialized into placenta.

Answer 188

The role of mitosis and the cell cycle is to produce identical daughter cells for growth and asexual reproduction of cells.

Answer 189

All the cells produced by mitosis are genetically identical therefore mitosis does not give rise to genetic variation.

Answer 190

Multipotent is an adult stem cells that are only able to produce a few types of cells e.g. stem cells in bone marrow. Their telomeres don’t shorten as telomerase is present and active. Its activity is diminished after birth, except in reproductive and adult stem cells.

Answer 191

- replacing dead or damaged cells - repair of tissue - growth / increase in cell numbers - asexual reproduction / vegetative - maintains / same, number of chromosomes - genetically identical to parents - It produces daughter cells that are genetically identical - Cloning of B and T-lymphocytes during immune response

Answer 192

3 phases: Interphase → nuclear division/mitosis → cell division.

Answer 193

In interphase a cell grows to its normal size then prepares to divide – chromosomes and some organelles are replicated, chromosomes also begin to condense. Interphase consists of the G1, G2 and S phases

Answer 194

Growth 1 phase is a gap after cell division and before S phase. 46 chromosomes and chromatids at this phase. The cell receives a signal committing the cell to replicate DNA by producing RNA, proteins and enzymes, the cell grows and prepares to enter the S phase

Answer 195

S phase is the synthesis of DNA so each chromosome consists of two identical chromatids. 46 chromosomes and 92 chromatids at this stage. Chromatin also replicates along with DNA so histones are replicated for M phase

Answer 196

Growth 2 phase is the gap after S phase and before nuclear division. G2 prepares for mitosis. New DNA checked, and errors are repaired. There is a sharp increase in the production of tubulin to make microtubules for the formation of mitotic spindle and the nuclear envelope envelopes nucleus.

Answer 197

Mitosis is a form of cell division that produces identical cells, there are four stages of mitosis: prophase, metaphase, anaphase and telophase.

Answer 198

Mitosis is a form of cell division that produces identical cells, there are four stages of mitosis: prophase, metaphase, anaphase and telophase. Mitosis is also called the M phasse of the cell cycle

Answer 199

During cytokinesis the parent and replicated organelles move to opposite sides of the cell and the cytoplasm divides thus producing two daughter cells. It is the last stage of cell division

Answer 200

During cytokinesis the parent and replicated organelles move to opposite sides of the cell and the cytoplasm divides thus producing two daughter cells. It is the last stage of cell division

Answer 201

1. Chromosomes condense and become visible 2. Spindle fibres emerge from the centrosomes 3. Nuclear envelope breaks down 4. Nucleus disappears

Answer 202

1. Chromosomes continue to condense 2. Kinetochores appear at the centrosomes 3. Mitotic spindles microtubules attach themselves to kinetochores 4. Centrosomes move toward opposite poles

Answer 203

1. Mitotic spindle is fully developed centrosomes are at opposite poles of the cell 2. Chromosomes are lined up at the metaphase plate 3. Each sister chromatid is attached to a spindle fibre originating from opposite poles

Answer 204

1. Cohesion proteins binding the sister chromatids together break down 2. Sister chromatids (now called chromosomes) are pulled toward opposite poles 3. Non-kinetochore spindle fibres lengthen, elongating the cell

Answer 205

1. Chromosomes arrive at opposite poles and begin to decondense 2. Nuclear envelope material surrounds each set of chromosomes 3. the mitotic sipindle breaks down

Answer 206

1. Chromosomes arrive at opposite poles and begin to decondense 2. Nuclear envelope material surrounds each set of chromosomes 3. the mitotic spindle breaks down

Answer 207

Nucleotides are the basic building block of nucleic acids, such as DNA and RNA. It is an organic compound made up of nitrogenous base, a pentose sugar, and a phosphate group.

Answer 208

Nucleotides consist of pentose which is a 5 carbon sugar, a nitrogen containing organic base and a phosphate group. Nucleotides join together by phosphodiester bonds formed in condensation reactions.

Answer 209

The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. Adenine and guanine both have double ring structure and are classified as purine bases.

Answer 210

The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. Thymine, uracil and cytosine all have single ring structure and are classified as pyrimidines.

Answer 211

- A double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases. - In DNA the 2 strands lie antiparallel and complementary base pairing takes place between the 5’ to 3’ strand and the 3’ to 5’ strand - A purine always joins to a pyrimidine base - Depending on the bases a different number of hydrogen bonds are formed. - Adenine and Thymine join together by 2 hydrogen bonds - Cytosine and guanine join together by 3 hydrogen bonds. - Nucleotides are joined together by phosphodiester bonds

Answer 212

- RNA is a relatively short polynucleotide chain. - An RNA nucleotide consists of ribose instead of deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine and uracil (instead of thymine).

Answer 213

Purines are nitrogenous bases with double ring structures (Guanine and adenine).

Answer 214

Pyrimidines are nitrogenous bases with single ring structures (Thymine, uracil and cytosine).

Answer 215

- The double helix unwinds and the hydrogen bonds between the complementary bases break using DNA helicase thus separating the two strands of DNA - One of the strands is used as the template and complementary base pairing occurs between the template strand and free nucleotides - Adjacent nucleotides are joined by phosphodiester bonds formed in condensation reactions using DNA polymerase

Answer 216

DNA polymerase only works in the 5’ to 3’ direction. This means that DNA polymerase is only able to add nucleotides starting from the 3’ end of the new strand.

Answer 217

- The leading strand is replicated continuously in the 3’ to 5’ direction. - The second strand which is called the lagging strand is replicated discontinuously in the 5’ to 3’ direction. This means it is replicated in short sections forming Okazaki fragments. - The Okazaki fragments are joined together with DNA ligase.

Answer 218

RNA is a single stranded polynucleotide chain present in the nucleus, cytoplasm and ribosome. It contains a pentose sugar (ribose) and has 4 nitrogenous bases Adenine, uracil, guanine and cytosine

Answer 219

mRNA (messenger RNA) carries the genetic information in the form of a template from the nucleus to the ribosome for translation.

Answer 220

tRNA (transfer RNA) has a specific amino acid at one end and an anticodon at the other end. It fits onto the mRNA at ribosomes at complementary mRNA codon for protein synthesis.

Answer 221

A gene is a length of DNA which codes for a specific polypeptide or amino acid chain

Answer 222

A codon is a sequence of three nucleotide bases which code for a specific amino acid.

Answer 223

- redundant or degenerate - multiple codon codes for the same amino acid. - universal - all organisms use the same code - Has start and stop codons to mark the beginning and end of the gene for protein synthesis.

Answer 224

Proteins are polypeptide chains, coded for by a gene.

Answer 225

- The genetic code is universal and the sequence of bases determines which protein the gene is coding for. - The triplet code is the sequence of 3 nucleotides which code for either an amino acid, start codon or stop codon.

Answer 226

- Transcription occurs in the nucleus and involves DNA and mRNA and during transcription, DNA strand is transcribed into mRNA - Translation occurs in the nucleus which involves mRNA, tRNA and ribosomes and translation is the process during which the amino acids are assembled together to form a polypeptide chain/protein.

Answer 227

- DNA unwinds to form two strands (template and non-template) and the template strand acts as a template. - Free activated RNA nucleotides line up with their complementary base and forms H-bonds. - RNA polymerase catalyses the synthesis of Phosphodiester bonds to form sugar-phosphate backbone. - Hydrogen bonds between the DNA and mRNA strand are then broken - DNA is reformed - mRNA detaches from DNA then moves out of the nucleus through a pore and attaches to a ribosome in the cytoplasm which is the site of next stage of protein synthesis called translation

Answer 228

- In eukaryotic cells, the RNA molecule formed from transcription is called the primary transcript. - Removal of non-coding sequences called introns - Joining together coding sequences called exons - This forms mRNA

Answer 229

- In eukaryotic cells, the RNA molecule formed from transcription is called the primary transcript. - Removal of non-coding sequences called introns - Joining together coding sequences called exons - This forms mRNA

Answer 230

Eukaryotic genes are made of introns and exons. Exons are coding sequences. Introns are non-coding sequence which is not translated. Both introns and exons are transcribed but only exons will be translated into amino acids by RNA splicing.

Answer 231

RNA splicing is the removal of introns from primary transcript Primary Transcript is the original molecule of mRNA before splicing.

Answer 232

- Small ribosomal subunit attaches to mRNA - tRNA enters the ribosome and attaches to the mRNA - A codon on the mRNA attaches to a specific anticodon on the tRNA - Only 2 tRNA molecules can fit in the ribosome at the same time - Each tRNA carries a specific amino acid - A peptide bond is formed between the amino acids of 2 adjacent tRNA molecules with the help of peptidyl transferase - Ribosome moves along the mRNA, reading the next codon. A third tRNA molecule brings a third amino acid, which joins to the second one. The first tRNA leaves and is reused. - The polypeptide chain continues to grow until a ‘stop’ codon

Answer 233

mature mRNA are exons are joined together to form continuous strand.

Answer 234

A gene mutation occurs when the base sequence of DNA is altered. If the DNA sequence is altered, this change is replicated in the mRNA chain and thus can result in an altered polypeptide chain.

Answer 235

Mutation is a change in the nucleotide sequence of a gene, which may then result in an altered polypeptide. Mutations can be harmful as it changes the amino acid sequence which changes the mRNA created, tRNA attaching and polypeptide formation.

Answer 236

Substitution mutation is when a nucleotide base is replaced by a different nucleotide Eg: Sickle Cell Anaemia where the base Thymine is replaced with Adenine (CTT → CAT) so Glutamine becomes Valine.

Answer 237

Frame-shift mutation is the insertion or deletion of one or more nucleotides which cause a change in frame and hence affects the codons from the mutation onwards.

Answer 238

Elimination mutation is the removal of one or more than nucleotide and is not replaced

Answer 239

Insertion is when one or more nucleotides are inserted into the DNA strand

Answer 240

- Nonsense - a mutation resulting in a stop codon hence no polypeptide chain will be formed - Missense - a mutation resulting in a different amino acid being coded for hence changing the polypeptide chain - Silent - a mutation resulting in a different codon however it still codes for the same amino acid meaning the polypeptide chain produced is the same

Answer 241

Xylem vessels are dead cells which from a long, narrow and hollow tube to increase capillarity. They transport water unidirectionally (from roots to leaves).

Answer 242

- Hollow / no cell contents and no end walls: little resistance to flow of water - Wide lumen: large amounts of water can be transported - Lignified cell walls: prevents collapse of vessels used as mechanical support, impermeable to water - Cellulose cell wall: allows adhesion of water molecules to xylem walls as walls are hydrophilic - Pits: allow lateral movement of water and connect to all parts of plant - Narrow diameter: for adhesion and prevent air locks

Answer 243

- Sieve tube elements do not contain many organelles (no vacuole and nucleus) to decrease resistance to flow of phloem sap. - Companion cells contains higher numbers of mitochondria and ribosomes as they are metabolically active cells.

Answer 244

- Sieve pores: allow easy flow from one sieve tube element to the next. - Sieve plates: prevents sieve tube from bursting - Little cell contents: little resistance to flow of sap - Plasmodesmata: Allows flow to/from companion cells for loading and unloading of sucrose - Thin walls: Rapid entry of water at source to build up hydrostatic pressure

Answer 245

- They transport water and minerals, and also serve to provide structural support - They are long cylinders made of dead tissue with open ends, therefore they can form a continuous column. - Xylem vessels also contain pits which enable water to move sideways between the vessels. - They are thickened with a tough substance called lignin, which is deposited in spiral patterns to enable the plant to remain flexible

Answer 246

- They’re tubes made of living cells - Involved in translocation which is the movement of nutrients to storage organs and growing parts of the plant - Consist of sieve tube elements and companion cells - Sieve tube elements form a tube to transport sugars such as sucrose, in the dissolved form of sap - Companion cells are involved in ATP production for active processes such as loading sucrose into sieve tubes - Cytoplasm of sieve tube elements and companion cells is linked through structures known as plasmodesmata which are gaps between cell walls which allow communication and flow of substances such as minerals between the cells

Answer 247

- Xylem and phloem are components of the vascular bundle, which serves to enable transport of substances as well as for structural support. - The xylem vessels are arranged in an X shape in the centre of the vascular bundle. This enables the plant to withstand various mechanical forces such as pulling. - The X shape arrangement of xylem vessels is surrounded by endodermis, which is an outer layer of cells which supply xylem vessels with water. - An inner layer of meristem cells known as the pericycle

Answer 248

- Xylem is located on the inside in non-wooded plants to provide support and flexibility to the stem - Phloem is found on the outside of the vascular bundle - There is a layer of cambium in between xylem and phloem, that is meristem cells which are involved in production of new xylem and phloem tissue

Answer 249

- Xylem is located on the inside in non-wooded plants to provide support and flexibility to the stem - Phloem is found on the outside of the vascular bundle - There is a layer of cambium in between xylem and phloem, that is meristem cells which are involved in production of new xylem and phloem tissue

Answer 250

- The vascular bundles form the midrib and veins of a leaf - Dicotyledonous leaves have a network of veins, starting at the midrib and spreading outwards which are involved in transport and support

Answer 251

Transpiration is the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves. It is an inevitable consequence of gaseous exchange and thus photosynthesis, as stomata open and this allows water vapour to diffuse.

Answer 252

- Water vapour in air spaces diffuses out of the leaf through stomata down the water potential gradient. - Water evaporates from cell walls of spongy mesophyll cells into the intercellular air spaces. This causes water to move from the cell's cytoplasm into the cell wall. - Water from neighbouring xylem vessels (through pits) move into mesophyll cells by osmosis, down a water potential gradient. - A cohesion-tension and transpiration pull is created at the top of the plant due to the evaporation of water, as the hydrostatic pressure at the top of the xylem is reduced. This causes water to move up the xylem from roots to leaves.

Answer 253

The rate of transpiration can be investigated with the help of a potometer where water lost by the leaf is replaced by water in the capillary tube. Therefore, measuring the movement of the meniscus can be used to determine the rate of transpiration.

Answer 254

Factors which affect the rate of transpiration include number of leaves, number/size or position of stomata, presence of waxy cuticle, the amount of light present, the temperature, humidity, air movement and water availability.

Answer 255

Cohesion is when hydrogen bonds are formed between individual water molecules. Therefore, as one water molecule moves up the xylem, it pulls the other molecule along with it. This allows for water molecules to move up as a continuous stream.

Answer 256

Adhesion is when hydrogen bonds are formed between a water molecule and cellulose cell walls of xylem temporarily. This allows water molecules to continue moving upwards against gravity.

Answer 257

Hydrogen bonding in water causes cohesion and adhesion. Water then moves from root hairs to xylem in the roots by passing through cortical parenchyma cells.

Answer 258

Apoplastic pathway involves the movement of water through cell walls and intercellular spaces/ dead material by mass flow. Water can pass via apoplast pathway until it reaches the Casparian strip, as it is impermeable to water due to a suberin deposition in the cell wall. The apoplastic pathway has little resistance to water, allowing water to flow faster. Water moving through the xylem takes the apoplastic route as xylem vessels are dead and contain no cytoplasm.

Answer 259

Symplastic pathway involves the movement of water through the cytoplasm/ living material. Firstly, water passes through the partially permeable membrane. Then, the water passes through cytoplasm and vacuole before moving from one cell to another through the plasmodesmata.

Answer 260

- Rolled leaf increases humidity around stomata, reducing the water potential gradient - Thick waxy cuticle increases distance for diffusion, acting as a barrier for transpiration. Shiny surface reflects heat, lowering temperature. - Hairs / trichomes on surface trap moisture to reduce water potential gradient - Sunken stomata/ stomata in pits the moist air trapped in pits reduces water loss. - No stomata on upper surface Not exposed to sunlight, reducing evaporation rate - Small leaves, reduced to spines this reduce surface area for transpiration

Answer 261

Xerophytes are plants adapted to living in dry conditions. They are able to survive in such conditions because of various adaptations which serve to minimise the water loss.

Answer 262

- Water is taken up by the root hairs from the soil. The soil has a relatively high-water potential while the cytoplasm of the root hair cell has a relatively low water potential, as it contains many more inorganic and organic substances. Therefore, water moves into the root hair cells by osmosis. The large number of root hairs increases the surface area, allowing more water and ions to be taken up by the cells. - Ions may be taken up along with water by osmosis, or separately through facilitated diffusion or active transport.

Answer 263

- It can either occur via the symplast pathway where water enters the cytoplasm through the plasma membrane and passes from one cell to the next through plasmodesmata, the channels which connect the cytoplasm of one cell to the next. - The other pathway is the apoplast pathway where the water moves through the water filled spaces between cellulose molecules in the cell walls. In this pathway, water doesn’t pass through any plasma membranes therefore it can carry dissolved mineral ions and salts. - When the water reaches a part of the root called the endodermis, it encounters a layer of suberin which is known as the Casparian strip, which cannot be penetrated by water. - Therefore, in order for the water to cross the endodermis, the water that has been moving through the cell walls must now enter the symplast pathway - Once it has moved across the endodermis, the water continues down the water potential gradient from cell to cell until it reaches a pit in the xylem vessel which is the entry point of water.

Answer 264

Translocation is transport of soluble organic substances within a plant. These are called assimilates and they include sucrose and amino acids. Sugars are transported as sucrose instead of glucose as glucose interferes with the water potential of cells. Translocation is an energy requiring process

Answer 265

Sucrose is loaded into the phloem tubes by companion cells at the source (leaves). This is made possible due to the transport proteins present in the cell surface membrane of companion cells.

Answer 266

Sucrose is loaded into the phloem tubes by companion cells at the source (leaves). This is made possible due to the transport proteins present in the cell surface membrane of companion cells.

Answer 267

- The proton pump driven by ATP, pumps H+ ions (protons) out of the companion cell’s cytoplasm into the cell wall (apoplast pathway), creating a high concentration of H+ ions in the cell wall. - The H+ ion-sucrose co-transporter then drives the movement of H+ ions from a region of high concentration (cell wall of companion cells) to a region of low concentration (cytoplasm of companion cell) via facilitated diffusion, along with sucrose, against its concentration gradient via secondary active transport/co-transport.

Answer 268

- After sucrose is in the cytoplasm of the companion cell, it diffuses into the phloem sieve tubes through plasmodesmata, down a concentration gradient. - When sucrose enters phloem sieve tubes, the water potential of the cells decreases, due to an increase in solute. This causes water to move in from xylem vessels, causing an increase in hydrostatic pressure. - Unloading occurs at the sink (roots, tubers), where sucrose is used for metabolism or storage. - Assimilates are transported in large quantities by mass flow and from a region of high hydrostatic pressure to a region of low hydrostatic pressure from the source to the sink.

Answer 269

Root pressure is pressure obtained by increasing the pressure difference between the top and bottom by raising the water pressure at the base of the vessels.

Answer 270

Pressure is raised by active secretion of solutes into the xylem vessels in the roots, lowering the water potential and thus drawing water from surrounding root cells by osmosis.

Answer 271

- Water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient. The push of water upwards is aided by the root pressure which is where the action of the endodermis moving minerals into the xylem by active transport, drives water into the xylem by osmosis, thus pushing it upwards. - The flow of water is also maintained with the help of surface tension of water and the attractive forces (hydrogen bonding) between water molecules, cohesion. .

Answer 272

Cohesion-tension theory, states the flow of water is maintained with the help of surface tension of water and the attractive forces (hydrogen bonding) between water molecules known as cohesion. The capillary action where the forces involved in cohesion cause the water molecule to adhere to the cellulose in the cell wall of the xylem, thus pulling water up.

Answer 273

The mammalian circulatory system is a closed double circulation. This is because blood passes through the heart twice in one circulation of the body (pulmonary circuit and systemic circuit), contained inside blood vessels

Answer 274

Circulatory systems can either be open, for instance in insects, or closed, like in fish and mammals where the blood is confined to blood vessels only.

Answer 275

Closed circulatory systems come in two forms, either a single form which consists of a heart with two chambers meaning the blood passes through the heart once for every circuit of the body, or double, where the heart has four chambers and blood passes through the heart twice for every circuit of the body.

Answer 276

Arteries and veins contain three layers of tissues: - Tunica interna: endothelium, containing squamous epithelial cells - Tunica media: containing mainly smooth muscle and elastic fibres with some collagen - Tunica externa: containing mainly collagen, with some elastin fibres.

Answer 277

Arteries are blood vessels transport oxygenated blood swiftly to the tissues at high pressures. They have thick walls to withstand this high pressure. The exception to this is the pulmonary artery, carrying deoxygenated blood to the lungs. As arteries reach tissues, they branch into smaller vessels called arterioles.

Answer 278

- Elastin fibres: recoil and contract, squeezing the blood and so moving it along in a continuous flow. They allow the walls to stretch, as pulses of blood surge through. - Smooth muscles: contracts, reducing blood flow in arterioles. This controls volume of blood flowing into a tissue. - Collagen: give arteries strength, structure and flexibility.

Answer 279

Arterioles branch off arteries, have thinner and less muscular walls, their role is to feed blood into capillaries

Answer 280

Arterioles continue to branch into capillaries. These are the smallest blood vessels and they take the blood as close as possible to the cells. This allows for rapid transfer of substances between cells and the blood. Due to the very small diameter of these blood vessels, blood travels very slowly. This increases the opportunity for diffusion to occur.

Answer 281

Walls of capillaries are made of a single layer of endothelial cells with pores between individual cells present to allow some components of blood to pass through into the cells and tissues of the body

Answer 282

Many capillaries join to form venules which then join to form veins. These blood vessels carry deoxygenated blood back to the heart. The exception is pulmonary vein, carrying oxygenated blood from the lungs to the heart.

Answer 283

- Tunica media is thinner in veins when compared to arteries, as the blood in veins is at a lower pressure. - Valves are also present to prevent backflow of blood. - To keep blood flowing upwards from legs, veins are usually near muscles, so pressure is increased when muscles contract. - Veins also have a much larger lumen than arteries

Answer 284

Venules are larger than capillaries but smaller than veins

Answer 285

- Plasma: This is the liquid part of the blood. It is a dilute solution of salts, glucose, amino acids, vitamins, urea, protein and fats. - Leukocytes (white blood cells): Involved in the immune system. - Platelets: Involved in blood clotting. - Erythrocytes (red blood cells): Involved in carrying oxygen

Answer 286

Tissue fluid is fluid surrounds all the cells. Substances move from the blood to the tissue fluid and from the tissue fluid they diffuse into the cells. Tissue fluid has almost the same components as plasma, but lacks large plasma proteins which are too large to diffuse through pores in capillaries. Osmotic pressure causes tissue fluid to move into and out of capillaries.

Answer 287

About 90% of fluid that leaks from capillaries at the arterial end into tissue spaces eventually returns to the capillaries at the venous end. The remaining 10% is returned to the lymphatic system. Fluid inside the lymphatic vessels is called lymph. Lymph is very similar to tissue fluid but has a different name as it is in a different place. Lymph contains more large proteins and white blood cells than tissue fluid.

Answer 288

- The pCO2 is high and the pO2 is low. - CO2 from the cells diffuses into the plasma. - CO2 combines with -NH2 terminal of Hb to form carbaminohaemoglobin. (10% carried this way) - Most CO2 combines with water (catalysed by carbonic anhydrase) to form carbonic acid which then dissociates into H+ and HCO3- ions.

Answer 289

The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen which is a measure of oxygen concentration. The greater the concentration of dissolved oxygen in cells the greater the partial pressure. Therefore, as partial pressure increases, the affinity of haemoglobin for oxygen increases, that is oxygen binds to haemoglobin tightly. This occurs in the lungs in the process known as loading

Answer 290

H+ ions combine with Hb to form Haemoglobinic acid (HHb).

Answer 291

- The pO2 is high and the pCO2 low. - CO2 in plasma diffuses from the blood into the alveoli and oxygen diffuses into the blood from the alveoli. - Carbaminohaemoglobin dissociates to form CO2 and Hb - Hb then picks up O2, and HHb (haemoglobinic acid) dissociates to form H+ and Hb. - The H+ ions combine with HCO3- to form carbonic acid, which dissociates to form CO2 and water (catalysed by carbonic anhydrase). - CO2 diffuses into alveoli.

Answer 292

The presence of a high pCO2 causes Hb to release oxygen. This is called the Bohr Effect. High pCO2 are found in actively respiring tissues which need oxygen. This causes Hb to release oxygen even more readily than it would otherwise.

Answer 293

In high altitudes the pO2 is low, causing altitude sickness. In order to increase oxygen intake, populations that live-in high-altitude areas have adapted by developing higher haemoglobin count, larger lung capacity, increased red blood cell count and greater number of mitochondria to increase the efficiency of oxygen transport from lung to tissue. The muscular wall of the right ventricle also thickens (∴ larger heart), so more blood can be oxygenated.

Answer 294

- Biconcave shape increases surface area: volume ratio, for more efficient diffusion of oxygen - Contain haemoglobin to transport oxygen - Nucleus, mitochondrion, ER absent: more room for Hb, thus more o2 carrying capacity. - Thin outer membrane to allow oxygen to diffuse easily - Very small (7μm): no haemoglobin molecule within the cell is very far from the cell surface membrane, thus oxygen is exchanged quickly with outer fluid. Capillaries are 7μm allowing RBC to squeeze through. - Flexible: capillaries narrow than 7μm cause RBC to deform (as they have a specialized cytoskeleton) and return back to shape in venules

Answer 295

Blood vessels that leave the heart are the Aortic arch and pulmonary artery. Blood vessels that enter the heart are the superior vena cava, the inferior vena cava and the pulmonary vein. The left and right side of the heart are separated by the septum.

Answer 296

The human hearts four chambers: - Atria: 2 upper chamber are known as atria. They are thin walled and receive blood at low pressure. - Ventricles: 2 lower chambers are known as ventricles. They are thick walled, receive blood from atria and pump it out through arteries. - The left ventricle has a thicker muscular wall, as it has to pump blood into the systemic circuit which has a higher resistance to blood flow than the pulmonary circuit. The systemic circuit is also longer and requires more pressure

Answer 297

- Aorta - connects to the left ventricle and carries oxygenated blood all around the body except the lungs - Pulmonary artery - connects to the right ventricle and carries deoxygenated blood to the lungs where it is oxygenated. - Pulmonary vein - connects to the left atrium and takes oxygenated blood back to the lungs - Vena cava - connects to the right atrium and brings deoxygenated blood back from the tissues except the lungs

Answer 298

- Atrial systole: This occurs when muscles in the atrial walls contract and blood passes on to the ventricles. 70% of the blood flows passively down to the ventricles. The bicuspid and tricuspid valves open while the semilunar valves are closed. - Ventricular systole: 0.1 seconds after the atria contracts, the ventricle walls contract as well increasing the blood pressure and pushing it out of the heart. The blood passes through the aorta and pulmonary arteries. The semilunar valves open and the bicuspid and tricuspid valves are closed. - Ventricular diastole: This lasts for about 0.3 seconds; the ventricles relax and the pressure falls below that in the arteries. The higher pressure in the arteries pushes against the semilunar valves, shutting them. - Diastole: All muscles of the heart relax and the pressure inside ventricles gets lower than in the atria. When this happens most of the blood starts to flow from the atria to the ventricles even though the atria is not contracting. However, the atria contracts towards the end to push out the last bit of blood into the ventricles and the cycle begins all over again

Answer 299

The cardiac cycle begins in the right atrium. There is a specialised patch of muscles in the wall of the right atrium known as the Sino-Atrial node. Cells at the SAN set the rhythm for all of the cardiac muscle cells to beat as they send out electrical impulses to the rest of the atria. This causes atrial contraction.

Answer 300

The electrical impulses do not pass down to the ventricles. Instead, a second node Atrio-ventricular node picks up the electrical impulses from the atria. The AV node causes the impulse to be delayed by 0.1 seconds, so the atria contract before the ventricles, giving the ventricles time to fill up with blood.

Answer 301

The impulse swiftly moves down to the septum of the heart, along fibres called Purkyne tissue. Once the impulse arrive at the base of the ventricles, it moves outwards and upwards through ventricular walls. This causes the ventricle to contract. This is important as the ventricles must push blood upwards into the Aorta and the pulmonary artery

Answer 302

Due to the heart’s ability to initiate its own contraction, it is referred to as myogenic.

Answer 303

In the wall of the right atrium there is a region of specialised fibres called the sinoatrial node which is the pacemaker of the heart, as it initiates a wave of electrical stimulation which causes the atria to contract at roughly the same time

Answer 304

- The chloride shift helps maintain the cell’s electrical balance. - When blood reaches the lung tissue it has a relatively low carbon dioxide concentration - Carbonic anhydrase catalyses the reaction breaking down carbonic acid into water and carbon dioxide - Bicarbonate diffuses into the red blood cells and reacts with the hydrogen ions. This forms carbonic acid. - When carbonic acid is broken down by carbonic anhydrase free carbon dioxide is released. This diffuses from the blood into the lungs. - Chloride ions then move from the red blood cells into the plasma - down an electrochemical gradient

Answer 305

Lungs are in chest cavity surrounded by the pleural membranes, which enclose an airtight space. This space contains a small quantity of fluid to allow friction-free movement as the lungs are ventilated by the movement of the diaphragm and ribs.

Answer 306

The lungs replace alveolar air with air from outside the body. This helps maintain the diffusion gradient.

Answer 307

Bronchi branch to form smaller bronchioles. Bronchioles are surrounded by smooth muscle, which can contract or relax to adjust the diameter of these tiny airways due to the absence of cartilage. During exercise, the muscles relax to allow a greater flow of air to the alveoli. Only the larger bronchioles contain cartilage. The smallest of bronchioles have alveoli clusters at the ends

Answer 308

Cartilage is a connective tissue. It keeps airways in trachea and bronchi open and air resistance low, and prevents them from collapsing or bursting as the air pressure changes during breathing. Cartilage present in irregular plates and incomplete rings in bronchus, and as incomplete rings only in trachea.

Answer 309

Air flows down lungs through trachea and bronchi which are lined by cells adapted to remove particles from air before it reaches the lungs. These cells make up a tissue called epithelium

Answer 310

Air flows down lungs through trachea and bronchi which are lined by cells adapted to remove particles from air before it reaches the lungs. These cells make up a tissue called epithelium. These cells are ciliated cells and goblet cells.

Answer 311

Epithelium tissue lined with tiny cytoplasmic extensions known as cilia. They are responsible for the continual beating of mucus towards the larynx.

Answer 312

Goblet cells are found in between ciliated cells in large amounts. The upper part of a goblet cell is swollen with mucin droplets that are secreted by the cell. The mucous secreted by goblet cells traps pathogens which are then moved out with the help of the cilia.

Answer 313

Mucus is a slimy solution of mucin, which is composed of glycoproteins with many carbohydrate chains that make them sticky and able to trap inhaled particles. It is also made by mucous glands (multicellular) beneath the epithelium. SO2 and NO2 can dissolve in mucus to form an acidic solution that irritates the lining of the airways

Answer 314

Elastic fibres allow the alveoli to stretch during inhalation and recoil during expiration. During maximum expansion, surface area for diffusion increases and the air is expelled efficiently during recoil.

Answer 315

- Large number of alveoli are present to increase surface area. - The wall of alveolus is very thin (single layer of squamous epithelium) to decrease the diffusion distance for efficient gas exchange. - Outside the alveolus are capillaries also one cell thick. The steep concentration gradient of CO2 and O2 is maintained by blood circulation and breathing

Answer 316

- The air enters through the nose and passes along the trachea, bronchi and bronchioles, which are structures well adapted to their role in enabling passage of air into the lungs. - The airways are held open with the help of rings of cartilage, incomplete in the trachea to allow passage of food down the oesophagus behind the trachea. - The gaseous exchange takes place in the walls of alveoli, which are tiny sacs filled with air and surrounded by capillaries. Capillaries have a constant flow of blood which moves oxygenated blood away from the area of diffusion to maintain the concentration gradient. - The oxygen that is inhaled moves from the alveoli into the blood. At the same time carbon dioxide is also removed from the capillaries to the alveoli which again maintains a steep concentration gradient. The alveoli also have a thin layer of surfactant which keeps them from collapsing - keeping them inflated.

Answer 317

Disease is ill-health or sickness causes reduced effectiveness of functions, has a set of symptoms, can be poor physical, mental, social well-being. Diseases can be infectious or non-infectious.

Answer 318

- Infectious diseases are caused by pathogens and can spread between organisms by physical contact, through the air, or through vectors such as mosquitos. - Non-infectious diseases, such as sickle cell anaemia and lung cancer, are not caused by pathogens and are not spread between organisms.

Answer 319

- Cholera pathogen - Vibrio cholerae - Type of pathogen - Bacteria

Answer 320

- Transmission - Faecal-oral route - Site of action Walls of small intestine - Method of action - Bacteria secretes choleragen toxin which binds to complementary receptor on intestinal cells via endocytosis, loss of Na+ and Cl- from cells, water moves out of blood

Answer 321

- Symptoms - Severe diarrhoea, severe dehydration, loss of water and salts, fatigue - Treatment -Oral-rehydration therapy (ORT) - Prevention - Proper sewage treatment, chlorinate water, drink bottled waters, no effective vaccine due to mutation and several strains Global Distribution - Outbreaks follow natural disasters due to lack of proper sanitation, poor hygiene and living conditions.

Answer 322

- Pathogen - Plasmodium falciparum Plasmodium vivax Plasmodium ovale Plasmodium malariae - Type of pathogen - Protoctist

Answer 323

- Transmission - Vector female Anopheles mosquito. Mosquito takes a blood meal, parasite enters host - Site of action - RBCs, liver, brain - Method of action - Parasite enters blood stream and matures in liver cells, enters RBCs, divide in RBCs causing it to lyse, infect other RBCs

Answer 324

- Symptoms - Fever, anaemia, enlarged spleen - Diagnosis - Dipstick test, microscopical analysis of blood - Treatment - Prophylactic antimalarial drugs; Artemisinin-based combination therapy (ACT)

Answer 325

- Prevention - Use preventative drugs, Reduce no. of mosquito: - Spray insecticides, Spread oil over water surface to prevent breeding fish that feed on larvae, Spray Bacillus thuringiensis bacteria to kill larvae - Prevention of bites: Use mosquito nets soaked in insecticides Repellents No vaccination available - Global Distribution - Endemic in tropical and sub-tropical areas: - vector breeds in hot and humid and needs stagnant water to reproduce - Plasmodium reproduce within mosquito at >20C - Other Problems - Drug resistant Plasmodium, insecticide DDT resistant mosquito, difficulty developing vaccine, migration

Answer 326

- Pathogen - Human immunodeficiency virus (HIV) - Type of pathogen - Virus (RNA retrovirus)

Answer 327

- Transmission - Direct exchange of bodily fluids through semen/vaginal fluids during sexual intercourse, blood transfusions, mother-to-baby across placenta/breast milk - Site of action - T-helper lymphocytes - Method of action - Slow infection. Viral RNA and reverse transcriptase (RT) enters T-helper cells, coverts RNA → DNA, viral DNA incorporated into host DNA, cell expresses viral proteins, T-helper cells cannot function

Answer 328

- Symptoms - AIDS (acquired immunodeficiency syndrome) and opportunistic infections eg. TB, malaria, oral thrush, cancers - Diagnosis - Blood/saliva/urine test - Treatment - No vaccination, no cure. Drugs to slow down onset of AIDS: Zidovudine inhibits RT and viral enzymes. Combination therapy - Prevention - Use condoms, femidoms, dental dams. Provide HIV testing centres and contact tracing Control mother child transmission using drugs. High income HIV+ women should avoid breastfeeding to reduce transmission. Low income HIV+ women should breastfeed for passive immunity.

Answer 329

- Pathogen - Mycobacterium tuberculosis Mycobacterium bovis - Type of pathogen - Bacteria

Answer 330

- Transmission - M. tuberculosis: aerosol infection. Infected person sneezes and the airborne droplets are breathed in by the uninfected person. M. bovis: from infected cattle eg. unpasteurized milk - Site of action - Lungs - Method of action - Slow infection. Disease can stay dormant and become active later when immune system weakened eg. by HIV

Answer 331

- Symptoms - Racking cough, shortness of breath, coughing blood - Diagnosis - Chest X-ray, microscopical examination of sputum - Treatment - Combination therapy of several antibiotics to prevent resistance. Through DOTS (direct observation treatment, short course)

Answer 332

- Prevention - BCG vaccine, contact tracing, quarantine, pasteurise milk - Global Distribution - Endemic due to high rate of migration, HIV/AIDS prevalence, overcrowded living spaces - Other Problems - MDR-TB and XDR-TB, requires longer treatment with more expensive antibiotics

Answer 333

- Bacteria and viruses are the main disease-causing pathogens in humans. - Bacteria are prokaryotic cells meaning that they do not have a nucleus – their genetic information is stored in the form of a circular strand of DNA whereas viruses consist of just nucleic acid enclosed in a protein coat and their genetic material can take the form of DNA or RNA - Bacteria do not require a host to survive whereas viruses are entirely dependent on their hosts and cannot survive without them - Viruses are significantly smaller than bacteria - Bacteria have a cell membrane, cell wall and cytoplasm as well as other organelles such as ribosomes, plasmids, flagellum and pili, whereas viruses possess no such structures.

Answer 334

- To prevent disease, social, economic and biological factors must be considered. - Diseases generally spread faster in densely populated and poorly sanitized areas. - Countries with healthcare systems and good education often have less disease as people are educated on how to prevent the spread of pathogens and can access treatments and vaccines.

Answer 335

- To prevent disease, social, economic and biological factors must be considered. - Diseases generally spread faster in densely populated and poorly sanitized areas. - Countries with healthcare systems and good education often have less disease as people are educated on how to prevent the spread of pathogens and can access treatments and vaccines.

Answer 336

Antibiotics are drugs that kills or stops the growth of bacteria, without harming the cells of the infected organism. There are two types of antibiotics Bactericidal antibiotics and Bacteriostatic antibiotics

Answer 337

- Bactericidal antibiotics kill bacteria by destroying their cell wall thus causing them to burst e.g. Penicillin - Bacteriostatic antibiotics which inhibit the growth of bacteria by stopping protein synthesis and production of nucleic acids so the bacteria can’t grow and divide

Answer 338

Antibiotics interfere with some aspect of growth or metabolism of the target bacterium including: - Synthesis of bacterial cell walls - Activity of proteins in the cell surface membrane - Enzyme action - DNA and protein synthesis

Answer 339

- Penicillin inhibits transpeptidase enzyme - Stops formation of cross-links between peptidoglycan polymer in cell wall - Autolysin continues making holes but cross-links not formed as penicillin inhibits transpeptidase enzyme - These little holes allow the wall to stretch so that new peptidoglycan chains can link together. - The cell wall therefore becomes progressively weaker and it bursts due to osmotic pressure. Bacteria lyses.

Answer 340

- Virus do not have peptidoglycan cell wall, they have protein coat - Virus do not have their own metabolism - Virus have very few organelle so very few sites of action - Virus live inside host cells, out of reach of antibiotics.

Answer 341

DNA mutation is an existing gene within the bacterial genome changes spontaneously to give rise to a nucleotide sequence that codes for a slightly different protein that is not affected by the antibiotic.

Answer 342

DNA mutation is an existing gene within the bacterial genome changes spontaneously to give rise to a nucleotide sequence that codes for a slightly different protein that is not affected by the antibiotic.

Answer 343

- Overuse of antibiotics or misuse of antibiotics eg. taking antibiotics for viral infections, overprescribing for bacterial infections - Not completing an antibiotic course - Reservoir of bacteria remains - Antibiotic acts as a selection pressure and induces a mutation in bacteria which makes it resistant - Any detail of mutation eg. protein produced as a changed binding site - Bacteria with resistance survive or only bacteria sensitive to antibiotic are killed (antibiotic acts as a selection pressure) - Bacteria reproduce and pass on gene/allele for resistance to offspring by: -asexual reproduction (vertical transmission) or -conjugation which transfers resistance genes to plasmid (horizontal transmission) - Frequency of resistance alleles/gene increase in bacterial population

Answer 344

Selection pressure is any phenomena which alters the behavior and fitness of living organisms within a given environment. It is the driving force of evolution and natural selection e.g.- natural disaster, predators, diseases, etc. - Increasing misuse of antibiotics increases selection pressure exerted on bacteria to evolve resistance to them. - Areas with widespread use of antibiotics (farms, hospitals), increases spread of resistance between bacteria. - Resistance may first appear in non-pathogenic bacterium (soil), then to pathogenic

Answer 345

Multiple resistance is multiple resistant genes in plasmid. E.g.- methicillin-resistant Staphylococcus aureus (MRSA) was controlled by vancomycin. Another bacteria common in hospitals, Enterococcus faecalis developed resistance to vancomycin and passed its resistance to MRSA.

Answer 346

- Using antibiotics only when appropriate and necessary; not prescribing them for viral infections - Making sure that patients complete their course of medication - Medicines should only be given if prescribed narrow spectrum drugs should be used rather than wide spectrum drugs - Reduce the use of antibiotics in drugs in agriculture - Use of many antibiotics to destroy the pathogen to make sure it doesn’t mutate

Answer 347

Resistance to antibiotics results in antibiotic resistant bacterial infections in hospitals such as MRSA. Antibiotics do not work in killing viruses because they don’t have the same structure and antibiotics are made to specifically target bacteria. As well as this viruses insert themselves into the cell and replicate rapidly.

Answer 348

Immune response is the body’s immune system responding to non-self-antigens by lymphocytes and phagocytes. It involves the production of antibodies and the killing of cells that have become infected by pathogens.

Answer 349

Phagocytosis is a process in which specialised white blood cells engulf pathogens thus destroying them by fusing a pathogen such as bacteria enclosed in a phagocytic vacuole with a lysosome. The phagosome and lysosome combine and the enzymes from the lysosomes destroy the pathogen. The main phagocytes are macrophages and neutrophils.

Answer 350

Neutrophils make up 60% of WBC. They circulate the blood and leave through capillary walls to enter tissues during an infection. They are short lived and often die after ingesting and destroying bacteria, forming pus.‎‎

Answer 351

Monocytes → (matured) Macrophages: - made in bone marrow - are long lived and larger than neutrophils. - monocytes in blood; macrophage found in lungs, liver, spleen, kidney & lymph nodes where they engulf foreign particles and microorganism. - Also known as antigen presenting cells (APCs) as they ingest antigens and display them on their surface, allowing T-lymphocytes to bind to the antigen and stimulate immune response. - They cut up pathogen using lysozymes.

Answer 352

Phagocytes is produced in and stored by the bone marrow throughout life before being distributed around the body by blood. They remove dead cells as well as invasive microorganisms.

Answer 353

Lymphocytes are WBC produced before birth in the bone marrow. They are smaller than phagocytes and have a nucleus that fills most of the cell.‎

Answer 354

- B-lymphocytes are produced and mature in the bone marrow, concentrating in lymph nodes and the spleen. When mature, each B cell produces one type of antibody molecule. - Part of the antibody molecules form a glycoprotein receptor that combines specifically with one type of antigen. - Once combined, the B cell will undergo mitosis repeatedly to form clone cells. - As only the B cells with antibodies complementary to antigens divide like this, it is known as clonal selection followed by clonal expansion. - Some differentiate into short lived plasma cells and others become long term memory cells (have glycoprotein receptor on cell membrane)

Answer 355

There is an extensive network of RER in the cytoplasm for production of antibody molecules, which plasma cells secrete into blood or lymph by exocytosis. The mitochondria provide ATP for protein synthesis and the movement of secretory vesicles.

Answer 356

T-lymphocytes are cells that are produced at the bone marrow but collect in the thymus till maturity. Mature T cells have receptors that are complementary to antigens of pathogens. T cells only respond when they encounter this antigen on the cell surface membrane of a host cell such as a macrophage (APC). A particular T cell with complementary receptor will bind to antigen found on the surface of APC. Clonal selection and expansion also take place, and differentiate into T helper cells or T killer cells/ cytotoxic T cells.

Answer 357

T helper cells are cells that secrete hormones called cytokines which stimulate B and T killer cells to divide, and macrophages to carry out phagocytosis more vigorously.

Answer 358

T killer cells/ cytotoxic T cells are cells that destroy the cell to which they are bound. They search the body for cells that have become invaded by pathogens and are displaying the pathogen’s antigen on their plasma membranes. When T-killer cells recognise the antigens, they attach themselves to the surface of infected cells and secrete toxic substances that kill the cells and pathogens within them.

Answer 359

- Neutrophils during bacterial infection and whenever tissues become inflamed and die. - Lymphocytes in the blood increases in viral infections and in TB. - Most of the lymphocytes that circulate in the blood are T cells. HIV invades helper T cells and causes their destruction, so blood tests for people who are HIV+ record the numbers of specific T cells.

Answer 360

Memory cells are cells which replicate themselves when exposed to an invading pathogen and remain in the lymph nodes searching for the same antigen thus resulting in a much faster immune response. This allows long term immunity.

Answer 361

In secondary response there are many more antibodies produced as many memory cells divide quickly and differentiate into plasma cells.

Answer 362

- Y-shaped glycoproteins - Bind to specific antigens to trigger an immune response - 2 long identical polypeptide chains and 2 shorter identical chains - The chains are held in place by disulfide bridges which also helps them maintain their shape - Antibodies bind to the antigen via a ‘lock and key’ mechanism similar to enzymes - 2 antigen binding sites allowing antibody to bind to 2 antigens

Answer 363

Antibodies are globular glycoproteins with quaternary structure, forming plasma proteins called immunoglobulins.

Answer 364

- upper part of the ‘Y’ - provides 2 identical antigen binding sites - specific for binding antigen as it is complementary shape to the antigen - R groups at antigen binding site forms H-bonds and ionic bonds with specific antigen - primary structure at variant region is different for each type of antibody

Answer 365

- formed by heavy and light chains - lower part of the ‘Y’ - when circulating in the blood, they bind to receptors on phagocytes - divides up antibody into classes eg. igM, igG, igA, igE

Answer 366

- held by disulfide bridges - gives flexibility when binding to antigen

Answer 367

The sequence of amino acids in this region make the specific 3-D shape which binds to just one type of antigen. R group interactions with the antigen gives it the specific shape.

Answer 368

- Combine with viruses and bacterial toxins preventing them entering or damaging cells. - Antibodies that combine with toxins and neutralize them (antitoxins). - Attach to the flagellum of the bacterium making them less active and easier for phagocytes to engulf.‎‎ - Agglutination (clumping together) of bacteria reducing the chances of spread throughout the body.‎‎ - Punch holes in the cell wall of bacteria, causing them to burst when they absorb water by osmosis.‎‎ - Antibodies coat bacteria, making it easier for phagocytes to ingest them; phagocytes have receptor proteins for the heavy polypeptide chains of antibodies.‎

Answer 369

Monoclonal antibodies are highly specific and identical antibodies made by identical B cell clones.

Answer 370

- A mouse is injected with relevant antigen, stimulating immune response. - Plasma cells specific to antigen are extracted from the spleen and fused with cancerous cells forming hybridoma cells. - Hybridoma cells that produce the required antibody are cloned.

Answer 371

- Radioactive chemicals are attached to each antibody that binds to fibrin. Radioactivity emitted by these antibodies can be detected by gamma rays camera, thus finding the position of a clot. - The same method can be used to locate cancer cells and identify the exact strain of a virus or bacterium during an infection.

Answer 372

- Mabs from rats into humans trigger an immune response as they are non-self. This is overcome by altering genes that code for polypeptide chains of antibodies into human sequences and the type/position of sugar groups into human antibodies. - Used in cancer therapy by marking cancerous cells for their destruction or binding to protein produced by T cells that reduces immune response. - Controls over/inappropriate production of B cells, preventing leukaemia and autoimmune diseases.

Answer 373

Immunity can either be active or passive; active immunity results from the production of antibodies by the immune system in response to the presence of an antigen whereas passive immunity results from the introduction of antibodies from another person or animal. There are also two subtypes of immunity; natural or artificial.

Answer 374

Natural active immunity arises from being exposed to an antigen/getting the disease whereas natural passive immunity is the result of crossing of mother’s antibodies through the placenta and their presence in breast milk.

Answer 375

Active artificial immunity is acquired through vaccinations which stimulate the immune system and lead to production of antibodies whereas passive artificial immunity is where antibodies are injected into the body

Answer 376

Vaccination is an antigenic material, which could be a live, dead or attenuated micro-organism, or perhaps a harmless form of a toxic (toxoid) or simply surface antigens. This allows our immune system to produce the requisite B and T cells without actually suffering the disease, mimicking natural immunity.

Answer 377

Less effective vaccines do not mimic an infection as they are dead bacteria or viruses that do not replicate inside the body. They need booster injections to stimulate secondary responses that give enhanced protection

Answer 378

Poor response due to: - Defective immune system, so doesn’t produce necessary B and T cell clones. - Suffer from malnutrition and don’t have enough protein to make antibodies.

Answer 379

- pathogens mutate rapidly and form different strain with different strains - memory cells unable to recognize pathogens hence vaccine is ineffective - plasmodium is eukaryotic and has many more genes and antigens on its surface

Answer 380

pathogens escape from immune system by living inside host cells or suppressing immune system beyond reach of antibodies

Answer 381

- No effective vaccine - Protoctists are eukaryotes so many more genes and antigens than bacteria and virus - Display different antigens on its cell surface for different strains or different stages of its life cycle - shows antigenic variation - Parasite changes antigens during infection - Plasmodium hides in liver and RBC - antigenic concealment - Needs more than one type of vaccine which targets common antigen present in all stages of life cycle - Needs more funding and research

Answer 382

- chloride presence outside cell causes water potential (outside cells) to, decrease chloride leaving cell causes water potential (in cell) to increase - water potential gradient, present (between inside and outside of cell) ; - water leaves cell by osmosis (to be taken up by mucin to form mucus) ; - from higher to lower water potential