Mixed

Question 1

Endopeptidases and exopeptidases are involved in the hydrolysis of proteins.
Name the other type of enzyme required for the complete hydrolysis of proteins to amino acids.

Answer 1

Dipeptidase

Question 2

Suggest and explain why the combined actions of endopeptidases and exopeptidases are more efficient than exopeptidases on their own.

Answer 2

Endopeptidase hydrolyse the peptide bonds in the middle of the protein.
Exopeptidase hydrolyse the peptide bonds at the end of the protein.
More efficient because their are more ends for the exopeptidase to hydrolyse.

Question 3

figure 1 shows the co-transport mechanism for the absorption of amino acids into the blood by a cell lining the ileum. The addition of a respiratory inhibitor stops the absorption of amino acids.
Use Figure 1 to explain why.

Answer 3

A respiratory inhibitor will lead to less ATP produced this means no active transport. Therefore the sodium ions cannot be uptaken or leave the cell. This will lead to their being no concentration gradient for the sodium ions to be uptaken with amino acid. Therefore stopping absorption of amino acid.

Question 4

The pieces of leaf tissue examined were very thin.

Explain why this was important.

Answer 4

The leaf tissue being very thin means there are less layers of cell therefore light can pass through

Question 5

Give two reasons why it was important that the student counted the number of stomata in several parts of each piece of leaf tissue.

Answer 5

1. This means the sample is representative

2. This means the mean of the results will be more reliable for comparison

Question 6

What are the three ways a pathogen can damage hosts cells?

Answer 6

1) rupturing them to release nutrients inside them
2) breaking down nutrients inside the cell for their own use. This starves and eventually kills the cell.
3) replicating inside the cells and bursting them when they’re released

Question 7

What are the two ways pathogens cause disease?

Answer 7

1) production of toxins

2) cell damage

Question 8

What are the three surfaces of contact- where pathogens enter our body? and how do they get in?

Answer 8

1) gas-exchange system = if you breathe in air that contains pathogens, most of them will be trapped in mucus lining the lung epithelium. Some pathogens are able to reach the alveoli where they can invade cells and cause damage.
2) Skin = if you damage your skin, pathogens on the surface can enter your bloodstream. blood clots prevent pathogens from entering.
3) Digestive system = if you eat or drink food that contains pathogens. Some will survive from the acidic conditions of the stomach, and invade cells of the gut wall and cause disease.

Question 9

Explain the process of phagocytosis.

Answer 9

1) A phagocyte recognises the antigens on a pathogen
2) The cytoplasm of the phagocyte moves round the pathogen, engulfing it.
3) The pathogen is now contained in a vacuole or a vesicle in the cytoplasm of the phagocyte.
4) A lysosome fuses with the phagocytic vacuole and the lytic enzymes break down with the pathogen
5) The phagocyte presents the pathogens antigens, it sticks the antigens on its surface to activate other immune system cells.

Question 10

What is the cellular and humoral response?

Answer 10

Cellular = The T-cells and other immune system cells that they interact with e.g phagocytes , form the cellular response

Humoral - B cells and the production of antibodies form the humoral response.

Question 11

Explain what is meant by a primary response.

Answer 11

The primary response is slow because there aren’t many B-cells that can make the antibody needed to bind to it.

The infected person will show symptoms of the disease while the body produces enough of the right antibody to overcome the infection.

T-cells and B-cells produce memory cells. Memory T-cells remember the specific antigen and will recognise it second time round. Memory B-cells record the specific antibodies needed to bind the antigen.

The body is now immune.

Question 12

Explain what is meant by the secondary response.

Answer 12

If the same pathogen enters the body again, the immune system will produce a quicker, stronger immune response.

Memory B-cells divide into plasma cells that produce the right antibody to the antigen. Memory T-cells divide into the correct type of T cells to kill the cell carrying the antigen.

The secondary response often gets rid of the pathogen before you begin to show any symptoms.

Question 13

Explain how monoclonal antibodies target cancer

Answer 13

1) Cancer cells have antigens called tumour markers that are not found on body cells.
2) Monoclonal antibodies can be made that will bind to the tumour markers.
3) You can also attach anti-cancer drugs to the antibodies
4) Antibodies come into contact with the cancer cells and bind to the tumour markers
5) This means the drug will acuumulate in the body where there are cancer cells.
6) side effects of an antibody based drug are lower than other drugs because they accumulate near specific cells

Question 14

how do monoclonal cells work in pregnancy tests?

Answer 14

1) The application area contains antibodies for hCG bound to a coloured bead
2) when urine is applied to the application area any hCG will bind to the antibody on the beads, forming an antigen-antibody complex.
3) the urine moves up the stick to the test strip, carrying any beads with it
4) the test strip contains antibodies to hCG that are stuck in place.
5) if there is hCG present the test strip turns blue because the immobilised antibody binds to any hCG. if no hCG is present, the beads will pass through the test area without binding to anything so it won’t go blue.

Question 15

How do vaccines protect individuals and populations against disease?

Answer 15

Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. This means you become immune without getting the symptoms.

Vaccines protect individuals because they reduce the occurance of the disease. Those not vaccinated are less likely to catch the diease because there are fewer people to catch it from - this is called herd immunity.

Question 16

Explain the primary, secondary, tertiary and quaternary structure of a protein.

Answer 16

Primary structure = A sequence of amino acids in the polypeptide chain

Secondary structure = Hydrogen bonds form between the amino acids. This makes it coil or fold.

Tertiary structure = The coiled or folded chain is coiled or folded furthur. More bonds form between different parts of the polypeptide chain. For proteins made from a single polypeptide chain, It forms their final 3D structure

Quaternary structure = the way the polypeptide chains are assembled together. Several different polypeptide chains held together by bonds. For proteins made from more than one polypeptide chain this is their final 3D structure

Question 17

Describe the test for proteins.

Answer 17

1) add sodium hydroxide solution
2) add a few drops of copper (II) sulfate solution
3) if theres protein = it will go purple
if theres no protein = it will stay blue

Question 18

Describe three functions of proteins:

Answer 18

REMEMBER EAST:
Enzymes = break down large food molecules or help to synthesise large molecules

Antibodies = involved in the immune response, they have variable regions.

Structural proteins = physically strong. They consist of long polypeptide chains lying parallel to each other with cross links between them.

Transport proteins = transport ions and molecules across membranes

Question 19

Describe the cause and symptoms of lactose intolerance

Answer 19

When you don’t have enough of the enzyme lactase, you can’t break down lactose in milk properly.

undigested lactose is fermented by bacteria and can cause a whole host of intestinal complaints such as stomach cramps, excessive flatulence and diarrhoea

Question 20

Describe the test for reducing sugars

Answer 20

Add benedicts and heat

if the sample contains reducing sugars = will turn red

Question 21

Describe the test for non-reducing sugars

Answer 21

1) boil with hydrocholric acid and neutralise with sodium hydrogencarbonate.
2) carry out the benedicts test as if you would for the reducing sugars test.

Question 22

Describe the test for starch

Answer 22

1) Add iodine dissolved in potassium iodide solution

2) if there is starch = will turn a blue-black colour

Question 23

Explain the effect of temperature on enzymes.

Answer 23

1) The rise in temperature makes the enzymes molecules vibrate more
2) If the temperature goes above a certain level, this vibration breaks some of the bonds that hold the enzyme in shape
3) The active site changes shape and the enzyme and subtrate no longer fit together
4) At this point, the enzyme in denatured

Question 24

Explain the effect of PH on enzymes

Answer 24

1) All enzymes have an optimum PH value
2) Above and below the optimum PH, the H + and OH - ions can mess up the ionic and hydrogen bonds that hold the enzymes tertiary structure in place.
3) The active site changes shape, so the enzyme is denatured

Question 25

Explain the effect of substrate concentration on enzymes.

Answer 25

1) the higher the substrate concentration, the faster the reaction.
2) More substrate molecules means a collision between substrate and enzyme is more likely
3) This only happens until the saturation point, when there are too many substrates, all active sites are full and adding more makes no difference

Question 26

Explain What a competitive inhibitor is

Answer 26

1) Competitive inhibitors compete with the substrate to bind to the active site but no reaction takes place.
2) Instead they block the active site so that no subtrate molecule can fit in it
3) If theres a high concentration of the inhibitor, it’ll take up nearly all the active sites and hardly any of the substrate will get to the enzyme.

Question 27

Explain what a non-competitive inhibitor is.

Answer 27

1) non-competitive inhibitor molecules bind to the enzyme away from its active site
2) This causes the active site to change shape so the substrate molecules can no longer bind to it.
3) They dont compete with the substrate molecules to bind to the active site because they are a different shape.
4) increasing the concentration wont make any difference - enzyme activity will still be inhibited.

Question 28

Explain the function of the:

1) plasma membrane
2) nucleus
3) lysosome
4) ribosome

Answer 28

1) regulates the movement of substances in and out of the cell
2) The pores allow substances to move between the nucleus and cytoplasm, the nucleolus makes ribosomes.
3) Contains digestive enzymes. These are kept seperate from the cytoplasm by the surrounding membrane and can be used to digest invading cells or to break down worn out components of cell.
4) The site where proteins are made.

Question 29

Explain the functions of:

1) endoplasmic recticlum
2) golgi apparatus
3) microvilli
4) mitochondrion

Answer 29

1) synthesises and processes lipids
2) Processes and packages lipids and proteins. it also makes lysosomes
3) They increase the surface area of the plasma membrane - found on cells involved in processes such as absorption
4) the site of aerobic respiration

Question 30

Explain the process of cell fractionation

Answer 30

1) homogenisation = breaking up the cells = by vibrating the cells or grinding the cells, this breaks up the plasma membrane and releases the organelles into solution
2) filtration = getting rid of the big bits = by filtering through a gauze to seperate any large debris or tissue debris.
3) ultracentrifuguation = seperating the organelles = to seperate a particular organelle from all the others.

Question 31

Explain the process of ultracentrifugation.

Answer 31

1) Cell fragments are poured into a tube. The tube is put in a centrifuge and is spun at low speed. The heaviest organelles get flung to the bottom of the tube by the centrifuge.
2) the supernatent (fluid above sediment) is drained off, poured into another tube and spun in the centrifuge at a higher speed. the heaviest organelles form a pellet at the bottom, the supernatant is drained and spun at a higher speed.
3) The process in repeated at higher speeds until all the organelles are seperated.

Question 32

Explain what order organelles are seperated in ultracentrifugation

Answer 32

1) nuclei
2) mitochondria
3) lysosome
4) endoplasmic recticulum
5) ribosomes

Question 33

Explain the two types of microscope - light and electron

Answer 33

light =
they have a lower resolution than electron microscopes
they use light

Electron microscopes =
They use electrons
they have a higher resolution so give a more detailed image

Question 34

Explain the strengths and weaknesses of the transmission scanning microscope and the scanning electron microscope

Answer 34

TEM = good because they give high resolution images, but bad because they can only use thin specimens

SEM = good because they can be used on thick specimens but bad because they give a lower resolution

Question 35

Explain what an aneurysm is

Answer 35

Aneurysm is a balloon-like swelling of the artery. Atheroma plaques damage and weaken arteries, they narrow arteries increasing blood pressure.

When blood travels through a weakened artery at high pressure, it may push inner layers through the outer elastic layer to form an aneurysm.

This may burst - causing a haemorrahage

Question 36

Explain what thrombosis is.

Answer 36

Thrombosis is a formation of a blood clot.
An atheroma plaque can rupture the endothelium damaging the artery wall and leaving a rough surface

platelets and fibrin accumulate at the site of damage and form a blood clot

This blood clot can cause a complete blockage of the artery or it can become dislodged and block a blood vessel elsewhere in the body.

Debris from the rupture can cause another blood clot to form furthur down the artery.

Question 37

Explain what atheroma is.

It blocks the lumen of the artery and restricts blood flow, which causes blood pressure to increase.

Answer 37

If damage occurs to the endothelium, white blood cells and lipids clump together to form fatty streaks.

over time, white blood cells, lipds and connective tissue builds up and hardens to form an atheroma - a fibrous plague.

Question 38

Explain the process of the cardiac cycle.

Answer 38

1) The sino-atrial node (SAN) sends out impulses across atrial walls
2) This causes the right and left atria to contract at the same time / causes atrial systole
3) impulses can not cross to ventricles due to non conducting tissue
4) Waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN)
5) new impulse travels down bundle of His
6) slight delay before the AVN reacts to make sure ventricles contract after atria empty
7) bundle of His conducts impulses to the purkyne fibres.
8) purkyne fibres carry the impulses to the muscular walls of the right and left ventricles causing ventricles to contract from bottom up.

Question 39

Describe what happens in inspiration and expiration in the lungs.

Answer 39

1) the intercostal and diaphragm muscles contract.
2) this causes the ribcage to move upwards and downwards and the diaphragm to flatten increasing the volume of the throax
3) As the volume of the throax increases, the lung pressure decreases.
4) This causes air to flow into the lungs. Inspiration requires energy.Expiration_
1) The intercostal and diaphragm muscles relax
2) the ribcage moves downwards and inwards and the diaphragm becomes curved again
3) the thorax volume decreases, causing the air pressure to increase
4) air is forced out of the lungs. it doesnt require energy_

Question 40

Explain what happens when someone gets cholera

Answer 40

1) The toxin causes chloride ion protein channels in the plasma membranes of the small intestine epithelial cells to open.
2) chloride ions move into the small intestine lumen. The build up of chloride ions lowers the water potential of the lumen.
3) water moves out of the blood across the epithelial cells and into the small intestine lumen by osmosis.
4) the increase in water secretion in intestine leads to diarrhoea causing the body to become dehydrated.

Question 41

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers.

Give two named examples of polymers and their associated monomers to illustrate your answer (5 marks)

Answer 41

1. A condensation reaction joins monomers together and forms a (chemical) bond and releases water
2. A hydrolysis reaction breaks the (chemical) bond between monomers and uses water
3. A suitable example of polymers and monomers from which they are made
4. A second suitable example of polymers and the monomers from which they are made
5. Reference to a correct bond within a named polymer

Suitable Example =

Amino Acids –> Polypeptide/Protein/Enzyme/Antibody

Nucleotides –> Polynucleotide/DNA/RNA

Alpha Glucose –> Starch/Glycogen

Beta Glucose –> Cellulose

Question 42

Describe how the structures of starch and cellulose molecules are related to their functions. (5 marks)

Answer 42

Starch (max 3)

1. Helical/spiral shape so compact;
2. Molecule is insoluble so osmotically inactive (does not affect water potential)
3. Branched so glucose is easily accessible by enzymes to break down for respiration;
4. Large molecule so cannot leave cell/cross cell-surface

membrane

Cellulose (max 3)

5. Long, straight & unbranched chains of β glucose;
6. Joined by hydrogen bonding, to form (micro/macro) fibrils
7. These provide rigidity/strength;

Question 43

Describe the structure of a cellulose molecule and explain how cellulose is adapted for its function
in cells (6 marks)

Answer 43

1. made from β-glucose;
2. joined by condensation to form glycosidic bond;
3. 1 : 4 link described;
4. “flipping over” of alternate molecules;
5. hydrogen bonds linking long straight chains;
6. cellulose makes cell walls strong;
7. can resist turgor pressure/osmotic pressure;
8. bond difficult to break;
9. resists action of enzymes

Question 44

Describe the structure of proteins (5 marks)

Answer 44

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. That are formed by condensation;
4. Primary structure is the order of amino acids;
5. Secondary structure is folding of polypeptide chain due to hydrogen bonding;
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide
   bonds;
7. Quaternary structure is two or more polypeptide chains

Question 45

ATP is useful in many biological processes. Explain why (4 marks)

Answer 45

1. Releases energy in small & easily manageable amounts;
2. Broken down in a one step reaction which makes sure energy is available rapidly;
3. Phosphorylates substances to make them more reactive
4. Reformed/made again rapidly

Question 46

Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample (5 marks)
LIPID

Answer 46

1. Add ethanol/alcohol then add water and shake/mix
2. White/Milky emulsion

NON-REDUCING SUGAR

1. Do Benedict’s Test and stays blue/negative
2. Boil with acid then neutralises with alkali
3. Heat with Benedict’s and becomes red/orange (precipitate)

AMYLASE

1. Add biuret (reagent) and becomes purple/vioelt/mauve/lilac
2. Add starch, (leave for time), test for reducing sugar/absence of starch

Question 47

Haemoglobins are chemically similar molecules found in many different species. Differences
in the primary structure of haemoglobin molecules can provide evidence of phylogenetic
(evolutionary) relationships between species.
Explain how. (5 marks)

Answer 47

1. Mutations change base / nucleotide (sequence);
2. (Causing) change in amino acid sequence;
3. Mutations build up over time;

4.More mutations / more differences (in amino acid/ base / nucleotide sequence / primary
structure) between distantly related species;
OR
Few(er) mutations / differences (in amino acid / base / nucleotide sequence / primary structure) in
closely related species;

5.Distantly related species have earlier common ancestor;
OR
Closely related species have recent common ancestor;

Question 48

Describe the roles of iron ions, sodium ions and phosphate ions in cells (5 marks)

Answer 48

IRON IONS
1. Haemoglobin binds/associates with oxygen

SODIUM IONS

1. Co-transport of glucose/amino acids (into cells)
2. (Because) sodium moved out by active transport/sodium potassium pump
3. Creates a sodium ion concentration/diffusion gradient
4. Affects osmosis/water potential

PHOSPHATE IONS

1. Affects osmosis/water potential
2. Joins nucleotides/in phosphodiester bond/in backbone of DNA/RNA/in nucleotides
3. Used in/to produce ATP
4. Phosphorylates other compounds (usually) making them more reactive
5. Hydrophilic/water soluble part of phospholipid bilayer/membrane

Question 49

Explain five properties that make water important for organisms (5 marks)

Answer 49

1. A metabolite in condensation/hydrolysis/photosynthesis/respiration
2. A solvent so (metabolic) reactions can occur
3. High heat capacity so buffers changes in temperature
4. Large latent heat of vaporisation so provides a cooling effect (through evaporation)
5. Cohesion (between water molecules) so supports columns of water (in plants)
6. Cohesion (between water molecules) so produces surface tensions supporting (small) organisms

Question 50

Describe the induced fit model of enzyme action. (2 marks)

Answer 50

Active site of enzyme is not complementary;

Active site is flexible & can mould around the substrate;

Change in enzyme allows substrate to fit and form an E-S complex

Question 51

Explain why maltase:
• only breaks down maltose
• allows this reaction to take place at normal body temperature.
(5 marks)

Answer 51

1. Specific Tertiary structure enzyme (means)
2. Active site is only complementary to maltose
3. Description of induced fit;
4. Enzyme is a catalyst which lowers the activation energy required for the reaction
5. By forming an enzyme-substrate complex;

Question 52

Describe competitive and non-competitive inhibition of an enzyme (5 marks)

Answer 52

1. Inhibitors reduce binding of enzyme to substrate & prevent the formation of E-S complexes

(Competitive inhibition),

2. Inhibitor has a similar shape to substrate;
3. It binds to the active site (of enzyme);
4. Inhibition can be overcome by adding more substrate;

(Non-competitive inhibition),

5. Inhibitor binds to a site on enzyme other than active site;
6. This changes the shape of the active site
7. Inhibition cannot be overcome by adding more substrate

Question 53

Describe the role of the enzymes of the digestive system in the complete breakdown of starch
(5 marks)

Answer 53

Amylase (mouth);
Breaks down starch to maltose

Maltase (Small Intestine);
Breaks down maltose to glucose
Hydrolysis of starch involves breaking of glycosidic bond

Question 54

Describe the processes involved in the absorption of the products of starch digestion (5
marks)

Answer 54

Sodium removed from epithelial cell by active transport/sodium- potassium pump;

Into blood;

Maintaining low concentration of sodium in the epithelial cell compared to the
lumen;

Glucose moves in to the epithelial cell with sodium
Via carrier/channel protein
Glucose moves into blood;

By (facilitated) diffusion

Question 55

Describe how proteins are digested in the human gut (4 marks)

Answer 55

1. Hydrolysis of peptide bonds;
2. Endopeptidases break polypeptides into smaller peptide chains;
3. Exopeptidases remove terminal amino acids;
4. Dipeptidases hydrolyse dipeptides into amino acids.

Question 56

The epithelial cells that line the small intestine are adapted for the absorption of glucose.
Explain how. (6 marks)

Answer 56

1. Microvilli provide a large surface area;
2. Many mitochondria produce ATP for active transport;
3. Carrier proteins present for active transport;
4. Channel / carrier proteins for facilitated diffusion;
5. Co-transport of sodium and glucose achieved through carrier protein for sodium (ions) and glucose;
6. Membrane-bound enzymes digest disaccharides to produce glucose;

Question 57

The movement of substances across cell membranes is affected by membrane structure. Describe how. (5 marks)

Answer 57

1. Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances
2. Phospholipid (bilayer) prevents movement/diffusion of polar/lipid-insoluble substances
3. Carrier proteins allow active transport
4. Channel/carrier proteins allow facilitated diffusion/co-transport
5. Shape/Charge of channel/carrier determines which substances move
6. Number of channels/carriers determines how much movement
7. Membrane surface area determines how much diffusion/movement
8. Cholesterol affects fluidity/rigidity/permeability

Question 58

Explain how the structure of DNA is related to its functions (6 marks)

Answer 58

1. Sugar-phosphate (backbone) is double stranded into a helix so provides strength &
   stability (protects bases);
2. Long / large molecule so can store lots of information;
3. Helix / coiled so compact;
4. Base sequence allows information to be stored (protein formation);
5. Double stranded so replication can occur semi-conservatively as existing
   strands can act as templates via complementary base pairing
6. Weak hydrogen bonds for replication and strand separation
   OR
   many hydrogen bonds so stable/strong;

Question 59

Describe and explain how the structure of DNA results in accurate replication (4 marks)

Answer 59

1. Two strands therefore semi-conservative replication;
2. base pairing held together by hydrogen bonds
3. hydrogen bonds weak so easily broken, which allows strands to separate;
4. bases exposed and act as a template;
5. A with T, C with G;
6. DNA made has one parent strand and one new strand

Question 60

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is
produced in the nucleus of a cell. (5 marks)

Answer 60

1. DNA Helicase;
2. Breaks hydrogen bonds between base pairs, exposing them;
3. Only one DNA strand acts as a template;
4. RNA nucleotides attracted to exposed bases;
5. (Attraction) according to base pairing rule (A-U & C-G);
6. RNA polymerase joins the RNA nucleotides together, to form pre-mRNA;
7. Pre-mRNA is spliced to remove introns, forming mRNA

Question 61

Starting with mRNA in the nucleus of a cell, describe how a molecule of protein is synthesised.
(6 marks)

Answer 61

1. mRNA leaves the nucleus through nuclear pore;
2. Enters the ribosome;
3. tRNA molecules bring amino acids to the ribosome;
4. A specific tRNA molecule exists for a specific amino acid;
5. Anticodon of tRNA complementary to codon on mRNA;
6. Peptide bonds form between adjacent amino acids;
7. tRNA detaches and leaves to collect another amino acid;
8. Ribosome moves along mRNA

Question 62

Explain how a mutation can result in the production of a non-functional protein receptor (4 marks)

Answer 62

1. Change in DNA base sequence;
2. Change in amino acid sequence;
3. This alters position of hydrogen/ionic/disulfide bonds;
4. And causes a change in the tertiary structure (of receptor);

Question 63

Describe the behaviour of chromosomes during mitosis and explain how this results in the production of
two genetically identical cells. (7 marks)

Answer 63

1. chromosomes shorten & thicken;
2. chromosomes made from two identical chromatids, due to replication in interphase;
3. chromosomes move to equator of the cell;
4. Chromosomes attach to individual spindle fibres;
5. Spindle fibres contract & the centromeres divide;
6. Sister chromatids move to opposite poles;
7. Each pole receives all the genetic information;
8. Nuclear envelope re-forms around each group of chromosomes

Question 64

Describe what happens to chromosomes in meiosis (6 marks)

Answer 64

1. Chromosomes shorten & thicken;
2. Chromosomes associate into their homologous pairs;
3. Crossing-over occurs between chromosomes, through the formation of a chiasma;
4. Chromosomes join to spindle fibres, via there centromeres
5. Whilst at the equator
6. Homologous chromosomes move to opposite poles
7. Pairs of chromatids separated in 2 nd division;

Question 65

Meiosis results in genetic variation in the gametes which leads to variation in the offspring formed
by sexual reproduction. Describe how meiosis causes this variation and explain the advantage of
variation to the species. (5 marks)

Answer 65

1. Crossing-over;
2. Independent assortment (segregation of homologous chromosomes) in meiosis I;
3. Independent assortment (segregation of chromatids) in meiosis II;

+ Any three from:
4. Causes individuals to have different adaptations making some better adapted;

5. Better adapted survive;
6. To reproduce;
7. These pass on the gene / allele;
8. Allows for changing environment / different environment

Question 66

Describe and explain how selection will have affected the genetic diversity of a species (2 marks)

Answer 66

1. Diversity reduced as fewer different alleles present creating a smaller gene pool;
2. As alleles have been chosen or rejected;

Question 67

Describe how you could make a temporary mount of a piece of plant tissue to observe the position
of starch grains in the cells when using an optical (light) microscope. (4 marks)

Answer 67

1. Add drop of water to glass slide;
2. Obtain thin section of plant tissue and place on slide;
3. Stain with iodine in potassium iodide.
4. Lower cover slip using mounted needle

Question 68

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate
mitochondria from a suspension of animal cells. (5 marks)

Answer 68

1. Cell homogenisation to break open cells;
2. Filter to remove large debris / whole cells;
3. Use isotonic solution to prevent damage to mitochondria/organelles;
4. Keep cold to reduce damage by enzymes/ use buffer to prevent enzyme denaturation;
5. Centrifuge at lower speed to separate nuclei / cell fragments / heavy organelles;
6. Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get
   mitochondria at bottom.

Question 69

Describe the principles and the limitations of using a transmission electron microscope to
investigate cell structure. (5 marks)

Answer 69

Principles:

1. Electrons pass through thin specimen;
2. Denser parts absorb more electrons;
3. So denser parts appear darker;
4. Electrons have short wavelength so give high resolution;

Limitations:

5. Cannot look at living material / Must be in a vacuum;
6. Specimen must be very thin;
7. Artefacts present;
8. Complex staining method / complex / long preparation time;
9. Image not in 3D / only 2D images produced

Question 70

Scientists use optical microscopes and transmission electron microscopes (TEMs) to investigate cell structure.
Explain the advantages and the limitations of using a TEM to investigate cell structure. (5 marks)

Answer 70

Advantages:
1 Small objects can be seen;
2 TEM has high resolution as wavelength of electrons shorter;

Limitations:
3 Cannot look at living cells as cells must be in a vacuum / must cut section / thin specimen;
4 Preparation may create artefact
5 Does not produce colour image;

Question 71

Many different substances enter and leave a cell by crossing its cell surface membrane. Describe
how substances can cross a cell surface membrane.(5 marks)

Answer 71

1. Simple / facilitated diffusion from high to low concentration;
2. Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
   OR
   Large / polar / water-soluble molecules go through proteins;
3. Water moves by osmosis from high water potential to low water potential;
4. Active transport against concentration gradient;
5. Active transport/facilitated diffusion involves proteins /carriers;
6. Active transport requires energy / ATP;
7. Ref. to Na + / glucose co-transport

Question 72

Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of
chloride ions involves a membrane protein. The diffusion of oxygen does not involve a
membrane protein.
Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen
does not. (5 marks)

Answer 72

1. Chloride ions water soluble/charged/polar;
2. Cannot cross lipid bilayer;
3. Chloride ions transported by facilitated diffusion using
   channel/carrier protein;
4. Oxygen not charged/non-polar;
5. Oxygen soluble in/can diffuse across lipid bilayer;

Question 73

The epithelial cells that line the small intestine are adapted for the absorption of glucose. Explain
how. (6 marks)

Answer 73

1. Microvilli provide an increased surface area;
2. Many mitochondria produce ATP / release or provide energy for active
   transport;
3. Carrier proteins for active transport;
4. Channel/carrier proteins for facilitated diffusion;
5. Co-transport of sodium ions and glucose using a symport / carrier protein;
6. Membrane-bound enzymes digest disaccharides to produce glucose;

Question 74

What is an antigen? (2 marks)

Answer 74

Protein on surface/membrane (of cell);

causes an immune response

Question 75

Describe the difference between active and passive immunity. (5 marks)

Answer 75

1. Active involves memory cells, passive does not;
2. Active involves production of antibody by plasma cells / memory cells;
3. Passive involves antibody introduced into body from outside;
4. Active long term, because antibody produced in response to antigen;
5. Passive short term, because antibody given is broken down;
6. Active can take time to develop / work, passive fast acting.

Question 76

When a pathogen enters the body it may be destroyed by phagocytosis. Describe how. (4 marks)

Answer 76

1. Phagocyte recognises foreign antigen;
2. Pathogen engulfed / ingested;
3. Enclosed in vacuole, forming a phagosome;
4. Vacuole fuses with lysosome;
5. Lysosome contains enzymes that are emptied into the vacuole;
6. Pathogen digested / molecules hydrolysed;

Question 77

Describe how B-lymphocytes respond when they are stimulated by antigens. (4 marks)

Answer 77

Divide by mitosis to form clones;

Produce plasma cells;

Plasma cells make antibodies;

Plasma cells produce memory cells

Question 78

An antigen in a vaccine leads to the production of antibodies. Describe the part played by B lymphocytes in this
process. (4 marks)

Answer 78

1 Macrophages present antigens to B lymphocytes;

2 antigen binds to complementary receptors on B lymphocyte;

3 lymphocyte becomes activated;

4 (B) lymphocytes reproduce by mitosis;

5 plasma cells secrete antibodies;

Question 79

What is a monoclonal antibody? (2 marks)

Answer 79

Antibodies from the same plasma cell;

Specific to/complementary to only one antigen;

Question 80

What is vaccination? (2 marks)

Answer 80

Injection of antigens from attenuated microorganism;

Stimulates the formation of memory cells;

Question 81

When a vaccine is given to a person, it leads to the production of antibodies against a disease-causing organism.
Describe how. (5 marks)

Answer 81

1. Vaccine contains antigen from pathogen;
2. Antigen is displayed on antigen-presenting cells (macrophages);
3. Helper T cell with complementary receptor protein binds to antigen;
4. Helper T cell stimulates B cell;
5. With the complementary antibody on its surface;
6. B cell secretes large amounts of antibody;
7. B cell divides to form clone which produces the same antibody

Question 82

Vaccines protect people against disease. Explain how. (5 marks)

Answer 82

1. Vaccines contain antigens from a dead/weakened pathogens are injected;
2. Memory cells made;
3. On second exposure memory cells produce become active and produce antibodies;
4. Rapidly produce antibodies/ produces more antibodies;
5. Antibodies destroy pathogens;

Question 83

Scientists use an antibody to detect an antigen on the bacterium that causes stomach ulcers. Explain why the
antibody will only detect this antigen. (3 marks)

Answer 83

1. Antibody has a variable region with a specific amino acid sequence / primary structure;
2. The shape of tertiary structure of the binding site;
3. Complementary to the antigens;
4. Forms complex between antigen and antibody;

Question 84

Describe how HIV is replicated after it has entered a human cell. (4 marks)

Answer 84

Reverse transcriptase;

Enzyme uses HIV RNA to make DNA copy;

DNA joined to host cell’s DNA/chromosome;

DNA used to make HIV RNA copies;

And HIV capsid proteins/enzymes;

Made at host ribosomes;

Assembly of new virus particles;

Budding off from membrane of host cell;

Question 85

Explain how the heart muscle and the heart valves maintain a one-way flow of blood from
the left atrium to the aorta. (5 marks)

Answer 85

1. Atrium has higher pressure than ventricle due to filling / contraction. This causes the
   atrioventricular valves to open;
2. Ventricle now has higher pressure than atrium (due to filling / contraction).
   This causes atrioventricular valves to close;
3. Ventricle has higher pressure than aorta causing semilunar valve to open;
4. This leads to a higher pressure in the aorta than the ventricle (as heart
   relaxes) causing semilunar valve to close;
5. (Muscle / atrial / ventricular) contraction causes increase in pressure;

Question 86

Describe and explain four ways in which the structure of a capillary adapts it for the
exchange of substances between blood and the surrounding tissue (4 marks)

Answer 86

1. Permeable capillary membrane;
2. Single cell thick walls - reduces diffusion distance;
3. Flattened (endothelial) cells - reduces diffusion distance;
4. Fenestrations - allows large molecules through;
5. Small diameter/ narrow - gives a large surface area to volume / short
   diffusion distance;
6. Narrow lumen - reduces flow rate giving more time for diffusion;
7. Red blood cells in contact with wall / pass singly - gives short diffusion
   distance / more time for diffusion

Question 87

Explain how tissue fluid is formed and how it may be returned to the circulatory
system. (6 marks)

Answer 87

1. Hydrostatic pressure of blood is high at arterial end;
2. Fluid & soluble molecules pass out;
3. Proteins & large molecules remain behind;
4. This lowers the water potential;
5. Water moves back into venous end of capillary
   by osmosis;
6. Lymph system collects any excess tissue fluid which returns to blood and
   returns this tissue fluid to the veins;

Question 88

Describe and explain how fish maintain a flow of water over their gills. (4 marks)

Answer 88

1. mouth opens, operculum/opercular valve shuts;
2. floor of mouth is lowered;
3. water enters due to a decreased pressure & an
   increased volume;
4. mouth closes, operculum/opercular valve opens;
5. floor raised results in an increased pressure & a decreased volume;
6. Increased pressure forces water over gills;

Question 89

Explain how the ventilation mechanism of a fish and the structure of its gills result in the
efficient uptake of oxygen from water (6 marks)

Answer 89

filaments / lamellae –> large surface area;

gill plates or secondary lamellae;

large number of capillaries –> to remove oxygen / to maintain a gradient;

thin epithelium –> short diffusion pathway;

pressure changes –> to bring in more water / to maintain gradient;

countercurrent flow (or description) –>Exchange occurs along the whole length as a concentration gradient is maintained & an equilibrium is not achieved (blood always meets water with higher oxygen concentration);

Question 90

Describe and explain how the structure of the mammalian breathing system enables
efficient uptake of oxygen into the blood. (6 marks)

Answer 90

1. Alveoli provide a large surface area;
2. Walls of alveoli thin to provide a short diffusion pathway;
3. Walls of capillary are thin between the alveoli so provides a short diffusion pathway;
4. Walls of capillaries/alveoli have flattened cells;
5. Cell membrane permeable to gases;
6. Many blood capillaries provide a large surface area;
7. Intercostal muscles & diaphragm muscles used to ventilate lungs to maintain a diffusion
   gradient;
8. Wide trachea & branching of bronchi/bronchioles for efficient flow of air;
9. Cartilage rings used to keep airways open

Question 91

From the root, water is transported upwards through the stem. Explain how evaporation
from the leaves can cause the water to move upwards (4 marks)

Answer 91

1. Water potential in leaf cells decreases / becomes more negative;
2. Therefore water moves out of xylem (into surrounding tissues)
3. by osmosis; this creates a tension on the water in xylem;
4. which is in a continuous column as water molecules are cohesive;
5. The cohesion is due to H bonding;
6. The column of water doesn’t break because of adhesion with xylem walls;

Question 92

Describe the processes involved in the transport of sugars in plant stems (5 marks)

Answer 92

1. At source sucrose is actively transported into the phloem/sieve tube;
2. By companion cells;
3. Lowers water potential in phloem/sieve element/tube and
   water enters by osmosis;
4. This produces a high hydrostatic pressure;
5. Mass flow/transport towards sink/roots/storage tissue
   occurs;
6. At sink/roots sugars are removed/unloaded;

Question 93

Use your knowledge of the cohesion-tension theory of water movement
through a plant, to explain why the diameter of the trunk is smallest at midday (6 marks)

Answer 93

1. Diameter of trunk is minimal at warmest/brightest time of day;
2. Stomata are open in light → so more water loss;
3. Water evaporates more when warm as there is more heat energy for water
   evaporation;
4. Hydrogen-bonding between water molecules causes cohesion between
   water molecules;
5. Adhesion occurs between water molecules and walls of the xylem vessels;
6. The xylem is pulled inwards by faster flow of water/tension;

Question 94

Describe and explain three ways in which the leaves of xerophytic plants may be
adapted to reduce water loss (3 marks)

Answer 94

Think cuticle / wax layer waterproof –> impermeable;

Sunken stomata –>saturated layer of still air outside;

Hairy –> saturated layer of still air outside;

Leaves small / reduced to spines / needles –> reduced surface area for water loss;

Leaves roll up in dry weather –> Less surface area for water loss / stomata covered / saturated region of still air;

Reduced number of stomata –> Reduced surface area for water loss;

Question 95

Different cells in the body have different functions

The epithelial cells that line the small intestine are adapted for the absorption of glucose. Explain how. (6)

Answer 95

1. Microvilli provide a large / increased surface area;
2. Many mitochondria produce ATP / release or provide energy (for active transport);
3. Carrier proteins for active transport;
4. Channel / carrier proteins for facilitated diffusion;
5. Co-transport of sodium (ions) and glucose or symport / carrier protein for sodium (ions)
   and glucose;
6. Membrane-bound enzymes digest disaccharides / produce glucose;

Question 96

Describe and explain how the lungs are adapted to allow rapid exchange of oxygen between air in the
alveoli and blood in the capillaries around them. (5)

Answer 96

1. Many alveoli / alveoli walls folded provide a large surface area;
2. Many capillaries provide a large surface area;
3. (So) fast diffusion;
4. Alveoli or capillary walls / epithelium / lining are thin / short distance between alveoli
   and blood;
5. Flattened / squamous epithelium;
   Accept: endothelial
6. (So) short diffusion distance / pathway;
7. (So) fast diffusion;
8. Ventilation / circulation;
   Accept: descriptions for ventilation / circulation
9. Maintains a diffusion / concentration gradient;
10. (So) fast diffusion;

Question 97

Breathing out as hard as you can is called forced expiration.
Describe and explain the mechanism that causes forced expiration. (4)

Answer 97

1. Contraction of internal intercostal muscles;
2. Relaxation of diaphragm muscles / of external intercostal muscles;
3. Causes decrease in volume of chest / thoracic cavity;
4. Air pushed down pressure gradient.

Question 98

Forced expiration volume (FEV) is the volume of air a person can breathe out in1
second. Explain how an asthma attack can cause a drop in the mean FEV

Answer 98

1. Muscle walls of bronchi / bronchioles contract;
2. Walls of bronchi / bronchioles secrete more mucus;
3. Diameter of airways reduced;
4. (Therefore) flow of air reduced.

Question 99

Describe and explain how the structure of the mammalian breathing system
enables efficient uptake of oxygen into the blood.

Answer 99

1. alveoli provide a large surface area;
2. walls of alveoli thin to provide a short diffusion pathway;
3. walls of capillary thin / close to alveoli provides a short diffusion pathway;
4. walls (of capillaries / alveoli) have flattened cells;
5. cell membrane permeable to gases;
6. many blood capillaries provide a large surface area;
7. intercostal / chest muscles / diaphragm muscles / to ventilate lungs / maintain a diffusion /
   concentration gradient;
8. wide trachea / branching of bronchi / bronchioles for efficient flow of air;
9. cartilage rings keep airways open;
   (reject moist and thin membranes)

Question 100

Explain how the ventilation mechanism of a fish and the structure of its gills result in the efficient
uptake of oxygen from water. (6)

Answer 100

(explanation must be linked to structures to gain second mark for each linked
pair)

filaments / lamellae –> large SA;

large number of capillaries; –> to remove oxygen / to maintain a gradient;

thin epithelium; –> short diffusion pathway;

pressure changes; –> to bring in more water / to maintain gradient;

countercurrent flow (or description); –> exchange / diffusion along whole length / concentration gradient maintained / equilibrium not achieved / blood always meets water with higher oxygen concentration

Question 101

Explain the movement of oxygen into the gas exchange system of an insect when it is at
rest. (3)

Answer 101

1. Oxygen used in (aerobic) respiration;
2. (so) oxygen (concentration) gradient (established);
   Accept description of gradient
   Ignore: ‘along gradient idea’ unless direction is made clear
   Ignore: movement through gas/water
   Reject: gradient in wrong direction
3. (so) oxygen diffuses in; 2 and 3.
   Accept: oxygen moves down a diffusion gradient for 2 marks

Question 102

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the
leaf. (3)

Answer 102

1. (Carbon dioxide enters) via stomata;
Reject stroma
2. (Stomata opened by) guard cells;
3. Diffuses through air spaces;
Allow concentration gradient. Reject along gradient unless direction made clear
4. Down diffusion gradient;

Question 103

Describe the part played by the diaphragm in causing air to enter the lungs during
breathing.(3)

Answer 103

Diaphragm (muscle) contracts;
Flattens / Increases volume of chest;
Reduced pressure allows air to enter;

Question 104

What’s the difference between alpha and beta glucose?

Answer 104

The OH group in alpha is on the same level on both sides but in beta glucose it is on opposite levels on both sides.
Alpha -H OH
Beta -OH H

Question 105

What is a condensation reaction?

Answer 105

When two monomers bond through the loss of a water molecule.

Question 106

Describe amylose

Answer 106

Long, UNbranded chain of alpha glucose
Coiled structure because of the angle of glycosidic bonds
Almost cylindrical

THEREFORE, it is good for storage

Question 107

Describe amylopectin

Answer 107

A long, branched chain of alpha glucose.
Side branches increase SA for enzymes to break down the molecule {EXOPEPTIDASE}

THEREFORE, glucose is released quickly

Question 108

Does starch affect water potential?

Answer 108

No, doesn’t cause water to enter cells by osmosis which can make cells swell

THEREFORE, it is good for storage

Question 109

How do animals store glucose?

Answer 109

Glycogen which is a polysaccharide of alpha glucose

Question 110

Difference and similarities between starch and glycogen

Answer 110

Difference=Glycogen is highly branched whereas starch has a few branches

Therefore, it releases glucose quickly

Similarities=They are both good for storage as they’re compact

Question 111

How does cellulose form?

Answer 111

When beta glucose molecules bond, they form straight cellulose chains
The cellulose chains are linked together by hydrogen bonds to form microfibrils
This provides structural support for cells

Question 112

What is the structure of a triglyceride?

Answer 112

one glycerol and three fatty acids

Question 113

Describe the fatty acid molecule

Answer 113

Has a long tail
Made of hydrocarbons
Tails are hydrophobic (repel water) —> insoluble in water

Question 114

State the structure of a fatty acid

Answer 114

Oxygen=Carbon Atom - R group
|
Hydroxyl group

A carbon attached to an oxygen,hydroxyl and a R group

Question 115

What bond forms in the triglyceride?

Answer 115

Ester bond (—O—)
Formed when the -OH hydroxyl group is replaced by an -O- alkyl group

Question 116

State the difference between saturated and unsaturated fatty acids

Answer 116

Unsaturated fatty acids have 1 or more double bonds

Question 117

What are phospholipids?

Answer 117

a lipid containing a phosphate group in its molecule found in the cell membrane

Question 118

How are phospholipids and triglycerides different

Answer 118

1 fatty acid is replaced by a phosphate group

Question 119

What part of the phospholipid hydrophilic?

Answer 119

The phosphate group

Question 120

Structure of a phospholipid

Answer 120

1 glycerol, 2 fatty acids and 1 phosphate
Phosphate forms a hydrophillic head
Fatty acids form hydrophobic tails
forms a phospholipid bilayer
basic structure of membranes

Question 121

How does the structure of triglycerides relate to its function?

Answer 121

Triglycerides = energy storage molecules

Long hydrocarbon tail containing lots of chemical energy

• > lots of energy released when they’re broken down
• > lipids contain 2x the energy per gram of carbohydrates

They have a hydrophobic tail

• > They’re insoluble and don’t effect water potential or water to enter by osmosis
• > triglycerides clump as insoluble droplets which shields themselves from water with their glycerol heads

Question 122

How does the structure of phospholipids relate to its function?

Answer 122

Phospholipids = cell membranes/bilayer
Cell membranes control the movement of substances in a cell

Hydrophilic heads
-> form double layer with their heads facing out towards the water on either side

Center of bilayer is hydrophobic

• > water soluble substances can’t easily pass through the membrane
• > acts as a barrier

Question 123

Describe the structure of an amino acid

has an:

Answer 123

• amino group
• carboxyl group
• H atom
• and an R group
• all bonded to a central C atom

Question 124

How is glycine different from other amino acaids?

Answer 124

Only amino acid without a C in its R group

It has 1 H in the R group

Question 125

How are polypeptides formed?

Answer 125

the condensation of many amino acids causing the release of a water molecule and peptide bonds to form in the (di/poly)peptide chain

Question 126

How is a disulphide bridge formed?

Answer 126

When sulphur atom in one cysteine bonds to the other sulphur in the other cysteine when the 2 molecules get close.

Question 127

Name an example of a quaternary protein

Answer 127

Haemoglobin
Insulin
Collagen

Question 128

Features of an enzyme

Answer 128

Roughly spherical in shape –> tight folding of polypeptide chains
Soluble
Have a role in metabolism (can break or synthesise things)

Question 129

Features of an antibody

Answer 129

Involved in immune response
Made of 4 polypeptide chains (2 light + 2 heavy) [quaternary structure]
Have variable regions-amino acid sequence varies here

Question 130

Features of a structural protein

Answer 130

Physically strong
Parallel, long polypeptide chains that have cross links
E.g Keratin or Collagen

Question 131

Features of a transport protein using an example

Answer 131

Channel proteins
present in cell membrane
contain hydrophobic and hydrophilic amino acids- causing protein to fold up into channels
transport molecules and ions across membranes

Question 132

Enzymes act as…

Answer 132

Biological Catalysts

as they speed up chemical reactions

Question 133

What is activation energy often in the form of?

Answer 133

Heat

Question 134

What is activation energy?

Answer 134

energy needed to start a reaction

Question 135

Why are enzymes highly specific?

Answer 135

due to their tertiary structure, only one complementary substrate will fit into the active site

Question 136

What is an enzyme-substrate complex?

Answer 136

The physical interaction between enzyme and substrate… duh

Question 137

How is activation energy reduced?

Answer 137

Enzyme-substrate complexes hold substances close together, reducing repulsion which allows bonds to form more easily

Enzyme-substrate co flexes can strain bonds so substrate molecule breaks easily.

Question 138

How do enzymes help a reaction?

Answer 138

They lower the activation energy required allowing reaction to happen at a lower temperature

Question 139

Describe the lock and key model

Answer 139

The enzyme and the substrate have specific complementary geometric shapes that fit exactly into one another

Question 140

Problems with the lock and key method

Answer 140

New evidence shows enzyme-substrate complex changes the shape slightly of the enzymes

Question 141

Describe the induced fit model

Answer 141

~ Substrate collides with active site

~ The tertiary structure of the active site is able to combine with complementary substrates

~ The active site changes to fit more closely around the substrate - held in position by oppositely charged R-groups

~ Enzyme-substrate complex forms

~ Change in shape puts strain on the bonds, weakening them, so lowers the activation energy

~ Enzyme-product complex forms and the substrate no longer fits so is released

Question 142

State the factors affecting the enzyme activity

Increases the rate of reaction to a point

Answer 142

Temperature
pH
Substrate Concentration
Enzyme Concentration

Question 143

How does temperature affect enzyme activity?

Answer 143

Temperature increases enzyme activity.

More heat = more kinetic energy so molecules move faster

Makes enzymes more likely to collide with substrate molecules –> more successful collisions as energy of each collision also increases

If the temperature gets too high, the reaction stops

Question 144

How does temperature stop a reaction?

Answer 144

Rise in temperature makes enzyme molecules vibrate more

The excessive vibration breaks some bonds holding the tertiary structure

Active site changes shape so substrate no longer fits

Enzyme is denatured and can’t function as a catalyst

Question 145

How does pH affect enzyme activity?

Answer 145

Increases the rate of reaction to a point

H+ and OH- ions found in acids and alkalis can distrupt the ionic bonds and hydrogen bonds that hold the enzymes tertiary structures in place making the active site change shape

Question 146

Optimum pH values of Pepsin (an endopeptidase)

Answer 146

Pepsin = 2

Pepsin found in stomach where HCl is present

Question 147

How does enzyme concentration affect enzyme activity?

Answer 147

The increased number of enzymes, increases the chance of successful collisions

This increases the rate of reaction to a point

Question 148

What is the limiting factor for enzyme concentration?

Answer 148

Substrate concentration

There’s a point where there is no further effect of adding more enzymes if all substrates occupy an enzyme

Question 149

How does substrate concentration affect enzyme activity?

Answer 149

Increases rate of reaction to a point

Higher the substrate concentration, the faster the concentration

more substrate molecules = more successful collisions

Question 150

Function of RNA

Answer 150

transfer genetic info from DNA to ribosomes

Question 151

What do ribosomes use RNA for?

Answer 151

Read RNA to make polypeptides (proteins) in a process called translation
Ribosomes are made of RNA and proteins

Question 152

How are polynucleotides formed?

Answer 152

• Nucleotides join by condensation reactions
• Phosphodiester covalent bonds form [2 ester bonds and a phosphate group]
• Phosphate group combines to OH group of pentose sugar of adjacent nucleotide
• chain of phosphate and sugars are called a sugar-phosphate backbone

Question 153

How do 2 DNA strands form a double helix?

Answer 153

Hydrogen bonding between bases

Question 154

How many hydrogen bonds are between A and T?

Answer 154

2 bonds

Question 155

How many hydrogen bonds are between C and G?

Answer 155

3 bonds

Question 156

What do hydrogen bonds do?

Answer 156

hold the nitrogen bases together

keeps strands coiled together

Question 157

How is the double helix formed?

Answer 157

2 antiparallel polynucleotide strands twist to form double helix structure

Question 158

How does DNA replicate?

Answer 158

semi conservatively

exact daughter DNA consists half of parental DNA and half of new DNA

Question 159

Describe the process of DNA semi conservative replication

Answer 159

• DNA Helicase breaks hydrogen bonds between bases on the 2 nucleotides causing helix to unwind forming 2 single strands
• Each original strand acts as a template for new strand to be formed
• Complimentary base pairing occurs where free floating DNA nucleotides are attracted to the exposed complimentary base
• DNA polymerase catalyses a condensation reaction between the nucleotides
• Hydrogen bonds form
• each new DNA molecule contains one strand from the original DNA and one new strand

Question 160

Advantages of semi conservative replication

Answer 160

daughter DNA consists half of parental DNA which allows genetic continuity

Question 161

Which way does DNA polymerase move?

Answer 161

DNA moves down template strand in 3’ to 5’ direction

Question 162

How did Meselson and Stahl confirm the semiconservative mechanism of DNA replication?

Answer 162

• 2 samples of bacteria grown:
• in nutrient broth containing light nitrogen isotope
• in nutrient broth containing heavy nitrogen isotope

-bacteria reproduces whilst taking nitrogen from broth to form nucleotides for new DNA. Gradually the different types on nitrogen become part of DNA

• Sample of DNA taken from each batch and is spun in a centrifuge:
• Heavier Nitrogen containing DNA settles lower than the light one
• Bacteria grown in heavier nitrogen taken out of centrifuge tube and placed in light nitrogen nutrient broth only for it to replicate for 1 round
• DNA sample is taken and spun in centrifuge

Expected observation to prove conservative replication –> original heavy DNA settles at the bottom | new lighter DNA settles at the top

Actual result that proves semi conservative mechanism –> new DNA contains strand of old and new nitrogen strands so should settle in the middle

Question 163

Structure of water

Answer 163

2 slightly positive hydrogen atoms, 1 slightly negative oxygen atom

Polar molecule
Intermolecular force = van der Waals and hydrogen bonding

Question 164

Name 4 properties of water

Answer 164

Polar
High specific latent heat of vaporisation
High specific heat capacity
Cohesive

Question 165

Name 5 functions of water

Answer 165

Transport in plants
Buffer to changes in temperature
Metabolite
Reduced water loss 
solvent