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Biology A Level AQA > Common 4/5/6 mark questions > Flashcards

Common 4/5/6 mark questions Flashcards

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1
Q

Describe how substances can cross a cell surface membrane. (5)

A

1 (simple / facilitated) diffusion from high to low conc / down conc grad;
2 Small / non-polar / lipid-soluble molecules go through proteins;
OR
Large / polar / water-soluble molecules go through proteins;
3 Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;
4 Active transport is movement from low to high conc / against conc grad;
5 Active transport / facilitated diffusion involved proteins/carriers;
6 Active transport requires energy / ATP;
7 Ref. to Na+ / glucose co-transport;

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2
Q

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)

A

1 Many alveoli / alveoli walls folded provide a large surface area;
2 Many capillaries provide a large SA;
3 (So) fast diffusion;
———————————
4 Alveoli or capillary walls / epithelium / lining are thin / short distance between alveoli and blod;
5 Flattened / squamous epithelium;
6 (So) short diffusion distance / pathway;
7 (So) fast diffusion;
———————————-
8 Ventilation / circulation;
9 Maintains a diffusion / concentration gradient;
10 (So) fast diffusion;

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3
Q

Scientists believe that it may be possible to develop vaccines that make use of microfold cells. Explain how this sort of vaccine would lead to a person developing immunity to the pathogen. (5)

A

1 B lymphocytes produce antibodies/involved in humoral response;
2 T lymphocytes involved in cell mediated immunity;
3 Macrophages present antigens;
4 (specific) B lymphocytes recognise/bine to antigen;
5 increase in numbers by mitosis;
6 produce plasma cells (which make antibodies);
7 Antibodies bind to and clump/ agglutinate virus;
8 memory cells produced by 1st exposure/ cloned on 2nd exposure;
9 T lymphocytes (helpers) produce
10 lymphokines/chemicals;
11 which aid B lymphocyte cloning;
12 encourages phagocytes to engulf clumped virus;
13 killer T cells kill virus infected cells;

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4
Q

Explain how water enters a plant root from the soil and travels through the epidermis.

A

1 Water enters root hair cells;
2 By osmosis;
3 Because active uptake of mineral ions has created a water potential gradient;
4 Water moves through cortex;
5 Down water potential gradient;
6 Through cell vacuoles and cytoplasms (symplastic pathway);
7 And through apoplastic pathway (cell walls);

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5
Q

Root pressure is a force that is partly responsible for the movement of water through xylem in stems. Explain how the active transport of mineral ions into the xylem vessels in the roots results in water entering these vessels and then being moved up the xylem tissue. (5)

A
  1. Entry of ions leads to a reduced water potential;
  2. Water potential established between xylem and surrounding cells;
  3. Plasma membranes of surrounding cells are partially permeable;
  4. Water enters xylem by osmosis;
  5. Volume of water in xylem increases;
  6. Cannot move back due to gradient;
  7. Pressure in xylem increases and forces water upwards.
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6
Q

The presence of an air bubble in a xylem vessel in the stem blocks the movement through that vessel. Use the cohesion-tension theory to explain why. (4)

A
  1. Evaporation from leaves (transportation);
  2. Water in xylem under tension (pulled up);
  3. Because water molecules cohere (stick together) (because of hydrogen bonds);
  4. So water travels in a single column;
  5. A water bubble would break the column;
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7
Q

In daylight, most of the water evaporates from the leaves but some is used by the plant. Describe the ways in which this water could be used by the plant. (6)

A
  1. (water is used in) the light-dependent reactions of photosynthesis;
  2. electrons from water enable ATP production / H+ are used to reduce NADP / produces O2;
  3. (water can be used in) hydrolysis reactions within the plant;
  4. to create turgor;
  5. as a solvent for transport;
  6. as a medium for chemical reactions;
  7. component of cells/ cytoplasm;
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8
Q

Describe two features you would expect in the leaves of a tree adapted to a dry environment. Explain how each feature helps the tree’s survival. (6)

A

Sunken stomata;
water evaporation into pit creates local humidity; increased humidity reduces gradient for water evaporation;
close arrangement of stomata;
diffusion of shells of individual stomata overlap;
interferes with water diffusion and slows evaporation;
restriction of stomata to lower side of leaf;
rate of air movement below leaf less/ heating effect of sun less;
gradient for water evaporation reduced/ water molecules have less kinetic energy;

thick cuticle/wax/suberin (on upper surface); (wax/suberin) waterproof;
water unable to diffuse onto surface to evaporate, presence of trichomes/hairs;
surface traps water close to leaf surface;
increased humidity reduces gradient for water evaporation;
reduced leaves/spines/small surface area to volume;
less surface area for evaporation;
more distance across leaf for water to diffuse;
rolled leaves;
stomata enclosed in localised humidity;
increased humidity reduces gradient for water evaporation;

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9
Q

Xylem transports water through a plant. Describe and explain how the cells of xylem are adapted for this function. (5)

A

Thick cell walls;
Withstand tension / negative pressure;
Lignin in cell walls;
Walls waterproof / withstand tension / negative pressure;
Xylem cells have no end walls / tubular (not hollow);
So a continuous column of water;
Xylem vessels are stacked on top of each other;
So a continuous column of water;
Have no cytoplasm / hollow;
Reduces resistance to flow of water / so a continuous column of water;
Xylem cells have pores / pits (in side walls);
Enable sideways flow / by-pass blockages / allows entry or exit of water;
Narrow tubes;
Allows capillarity / increased surface area for adhesion;
(Molecules in) cell walls;
Allows adhesion

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10
Q

Explain how the structure of the endodermis affects the passage of water by this (apoplast) pathway. (4)

A
  1. Casparian bands;
  2. Which are waterproof (impermeable);
  3. Lower water potential of endodermis cell;
  4. Water enters symplastic pathway;
  5. By osmosis;
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11
Q

Describe the mass flow hypothesis for the mechanism of translocation in plants. (4)

A
  1. In the leaf sugars are actively transported into phloem;
  2. By companion cells;
  3. Lowers water potential of sieve tubes and water enters by osmosis;
  4. Increase in pressure causes mass movement (towards roots);
  5. Sugars used (converted) in root for respiration/for storage;
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12
Q

A laboratory has oat plants containing the resistance gene and a supply of plasmids. Describe how bacteria may be produced which have the resistance gene in their plasmids. (6)

A
  1. Cut desired gene (from DNA) of oat plant;
  2. Using restriction endonuclease (restriction enzyme);
  3. Cut plasmid open;
  4. With (same) restriction endonuclease;
  5. Sticky ends/unpaired bases attached;
  6. Use (DNA) ligase to join;
  7. Return plasmid to (bacterial) cells;
  8. Use of Ca2+/calcium salts/electric shock;
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13
Q

The polymerase chain reaction (PCR) can be used to produce large quantities of DNA. Describe how the PCR is carried out. (6)

A
  1. DNA heated to 90-95°C;
  2. Strands separate;
  3. Cooled to temperature below 70°C;
  4. Primers bind;
  5. Nucleotides attach;
  6. By complementary base pairing;
  7. Temperature 70-75°C;
  8. DNA Polymerase joins nucleotides together;
  9. Cycle repeated;
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14
Q

Describe how an action potential is produced in an axon. (6)

A
  1. A stimulus causes a temporary reversal of charge and inside of axon becomes positive (depolarisation);
  2. Energy of stimulus causes some sodium channels to open;
  3. So sodium diffuses into axon;
  4. Once an action potential of +40mV has been established, channels close and there is no more influx of sodium;
  5. Potassium channels open and potassium ions diffuse out, causing repolarisation of axon;
  6. Axon becomes more negative than usual, so potassium gates close and sodium-potassium pump again actively transports sodium ions out and potassium ions in;
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15
Q

Describe the sequence of events which allows information to pass from one neurone to the next neurone across a cholinergic synapse. (6)

A
  1. Impulse causes calcium ions to enter axon;
    2 Vesicles move to/fuse with membrane;
  2. Acetylcholine is released;
  3. Diffuses across synaptic cleft/synapse;
  4. Binds with receptors on postsynaptic neurone;
  5. Sodium ions enter postsynaptic neurone;
  6. Depolarisation of postsynaptic membrane;
  7. If above threshold, nerve impulse/action potential is produced;
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16
Q

How does temperature affect the speed of a nerve impulse? (5)

A
  1. The higher the temperature the faster the nerve impulse;
  2. Sodium-potassium pump uses active transport;
  3. Which requires energy from ATP from respiration;
  4. Respiration is enzyme-controlled;
  5. More kinetic energy so more enzyme-substrate complexes formed and faster diffusion
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17
Q

Explain how nervous control in a human can cause increased cardiac output during exercise. (4)

A
  1. Coordination via medulla (of brain) / cardiac centre;
  2. (Increased) impulses along sympathetic (/ cardiac accelerator) nerve;
  3. To S.A. node / pacemaker;
  4. Release of noradrenalin;
  5. More impulses sent from / increased rate of discharge of S.A. node / pacemaker;
  6. Increased heart rate / increased stroke volume;
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18
Q

Describe the structure of a cell membrane. (5)

A
  1. Double layer of phospholipid molecules;
  2. Detail of arrangement of phospholipids;
  3. Intrinsic proteins/protein molecules passing right through;
  4. Some with channels/pores;
  5. Extrinsic proteins/proteins only in one layer/on surface;
  6. Molecules can move in membrane/dynamic/membrane contains cholesterol;
  7. Glycocalyx/carbohydrates attached to lipids/proteins;
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19
Q

Describe the part played by cell surface membranes in regulating the movement of substances into and out of cells. (6)

A
  1. Non-polar/lipid soluble molecules move through phospholipid layer/bilayer;
  2. Small molecules/water/gases move through phospholipid layer/bilayer;
  3. Ions/water soluble substances move through channels in proteins;
  4. Some proteins are gated;
  5. Reference to diffusion;
  6. Carriers identified as proteins;
  7. Carriers associated with facilitated diffusion;
  8. Carriers associated with active transport/transport with ATP/pumps;
  9. Different cells have different proteins;
  10. Correct reference to cytosis;
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20
Q

Explain how amino acid molecules may be linked to form a polypeptide chain which is folded into a specific tertiary shape. (6)

A
  1. Condensation;
  2. removal of water molecule;
  3. from amino and carboxyl groups;
  4. forming peptide bonds;
  5. same amino acids in same sequence;
  6. bonds form between R-groups/side chains;
  7. e.g. sulphur-containing amino acids / ionic bonds / hydrogen bonds;
  8. bonds form in same place;
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21
Q

Describe how molecular shape is important in explaining the way in which enzymes may be affected by inhibitors. (6)

A

1 Active site (of enzyme) has particular shape;
2 (Into which) substrate molecule fits / binds;
3 Appropriate reference linking induced fit and shape;
4 (Competitive inhibitor) has similar shape to substrate;
5 Also fits active sites;
6 Prevents substrate access;
7 (Non-competitive inhibitor) fits at site other than active site;
8 Distorting shape of active site / enzyme;
6 Prevents substrate access; (award once only)
9 Two types identified as competitive and non-competitive;

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22
Q

The bacteria in the intestine are prokaryotic cells. The epithelial cells which line the small intestine are eukaryotic cells. Describe the ways in which prokaryotic cells and eukaryotic cells differ. (6)

A

1 Prokaryotic cells do not have a nucleus / have genetic material
in cytoplasm;
2 DNA in loop / ring;
3 Not associated with proteins / do not have chromosomes /
chromatin / do not divide by mitosis;
4 Smaller ribosomes;
5 No membrane-bound organelles;
6 Such as mitochondria / lysosomes / endoplasmic reticulum /
Golgi / chloroplasts;
7 Prokaryotic cells may have mesosomes;
8 Prokaryotic cells smaller;
9 May be enclosed by capsule;

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23
Q

Describe how proteins are arranged in a plasma membrane and the part they play in transporting substances into and out of cells. (6)

A

1 Some proteins pass right through membrane;
2 Some proteins associated with one layer;
3 Involved in facilitated diffusion;
4 Involved in active transport;
5 Proteins act as carriers;
6 Carrier changes shape / position;
7 Proteins form channels / pores;
8 Protein allows passage of water soluble molecules / charged particles / correct named example;

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24
Q

Skin cells may be studied with a transmission electron microscope or an optical microscope. Explain the advantages and limitations of using a transmission electron microscope to study cells. (6)

A

1 TEM uses (beam of) electrons;
2 These have short wavelength;
3 Allow high resolution/greater resolution/Allow more detail to be seen/greater useful magnification;
4 Electrons scattered (by molecules in air);
5 Vacuum established;
6 Cannot examine living cells;
7 Lots of preparation/procedures used in preparing specimens/ fixing/staining/sectioning;
8 May alter appearance/result in artefacts;

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25
Q

Explain how oxygen is loaded, transported and unloaded in the blood (6)

A
  1. Haemoglobin carries oxygen / has a high affinity for oxygen / oxyhaemoglobin;
  2. In red blood cells;
  3. Loading / uptake/association in lungs;
    at high p.O2;
  4. Unloads / dissociates / releases to respiring cells / tissues;
  5. at low p.O2;
  6. Unloading linked to higher carbon dioxide (concentration);
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26
Q

Describe how the regular contraction of the atria and ventricles is initiated and coordinated by the heart itself. (5)

A
  1. (cardiac) muscle is myogenic;
  2. sinoatrial node/SAN;
  3. wave of depolarisation/ impulses /electrical activity (across atria);
  4. initiates contraction of atria
    atrioventricular node/AVN;
  5. bundle of His/purkyne tissue spreads impulse across ventricles;
  6. ventricles contract after atria/time delay enables ventricles to fill;
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27
Q

What is atheroma and how may it cause myocardial infarction? (5)

A
  1. Cholesterol/ plaque / lipoprotein / LDL / fatty material / cells;
  2. In artery wall / under lining / endothelium of artery / blood vessel;
  3. Atheroma linked with blood clotting / thrombosis;
  4. (Blocks) coronary artery / artery supporting heart muscle / tissue / cells;
  5. Reduces oxygen / glucose supply (to heart muscle / tissues / cells);
  6. (Heart muscle / tissue / cells) unable to respire / dies;
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28
Q

The diet of a person can increase the risk of coronary heart disease. Explain how. (5)

A
  1. Too much saturated fat / cholesterol in diet;
  2. Increase in LDL / cholesterol in blood;
  3. Atheroma / fatty deposits / plaques in artery walls;
  4. Reduces diameter of / blocks coronary arteries;
  5. Less oxygen / glucose to heart muscles / tissues / cells;
  6. Increase in blood pressure;
  7. (Increased risk of) clot / thrombosis / embolism / aneurysm.
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29
Q

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

A

Starch (max 3)
1. Helical/ spiral shape so compact;
2. Large (molecule)/insoluble so osmotically inactive;
3. Branched so glucose is (easily) released 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;
7. To form (micro/macro)fibrils;
8. Provides rigidity/strength;

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30
Q

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

A
  1. (At source) sucrose is actively (transported) into the phloem/sieve element/tube;
  2. By companion/transfer cells;
  3. Lowers water potential in phloem/sieve element/tube and water enters by osmosis;
  4. (Produces) high (hydrostatic) pressure;
  5. Mass flow/transport towards sink/roots/storage tissue;
  6. At sink/roots sugars are removed/unloaded;
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31
Q

Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. (5)

A
  1. Chloride ions water soluble/charged/polar;
  2. Cannot cross (lipid) bilayer (of membrane);
  3. Chloride ions transported by facilitated diffusion OR diffusion involving channel/carrier protein;
  4. Oxygen not charged/non-polar;
  5. (Oxygen) soluble in/can diffuse across (lipid) bilayer;
32
Q

Glucose is absorbed from the lumen of the small intestine into epithelial cells. Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells. (5)

A
  1. Na+ ions leave epithelial cell and enter blood;
  2. (Transport out is by) active transport / pump / via carrier protein using ATP;
  3. So, Na+ conc. in cell is lower than in lumen (of gut);
  4. Sodium/Na+ ions enter by facilitated diffusion;
  5. Glucose absorbed with Na+ ions against their concentration/diffusion gradient / glucose absorbed down an electrochemical gradient;
33
Q

Blood leaving the kidney eventually returns to the kidney.
Describe the pattern of blood circulation in a mammal that causes blood to return to the kidney. (6)

A
  1. (blood flows from kidney along) renal vein to vena cava;
  2. (along) vena cava to right atrium/side of heart;
  3. (along) pulmonary artery to lungs;
  4. (along) capillaries to pulmonary vein;
  5. (along) pulmonary vein to left atrium/side of heart;
  6. (along) aorta to renal artery (to kidney);
  7. Blood may pass through several complete circuits before returning to kidney;
34
Q

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

A
  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 prevent/reduce damage by enzymes / use buffer to prevent protein/enzyme denaturation;
  5. Centrifuge (at lower speed/1000 g) to separate nuclei/cell fragments/ heavy organelles;
  6. Re-spin (supernatant / after nuclei/pellet removed) at higher speed to get mitochondria in pellet/at bottom;
35
Q

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

A

Principles:
1. Electrons pass through/enter (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;

36
Q

Use your knowledge of protein structure to explain why enzymes are specific and may be affected by non-competitive inhibitors. (5)

A

1 each enzyme / protein has specific primary structure / amino acid sequence;
2 folds in a particular way / has particular tertiary structure giving an active site with a unique structure;
3 shape of active site complementary to / will only fit that of substrate; maximum of three marks for inhibition, points 5 - 8
4 inhibitor fits at site on the enzyme other than active site;
5 distorts active site;
6 so substrate will no longer fit / form enzyme-substrate complex

37
Q

Skin cells may be studied with a transmission electron microscope or an optical microscope. Explain the advantages and limitations of using a transmission electron
microscope to study cells. (6)

A

Advantages:
1 Small objects can be seen;
2 TEM has high resolution as wavelength of electrons shorter;
Accept better
Limitations:
3 Cannot look at living cells as cells must be in a vacuum;
4 must cut section / thin specimen;
5 Preparation may create artefact
6 Does not produce colour image;

38
Q

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

A

Amylase;
(Starch) to maltose:
Maltase;
Maltose to glucose;
Hydrolysis;
(Of) glycosidic bond;

39
Q

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

A

Glucose moves in with sodium (into epithelial cell);
Via (carrier / channel) protein / symport;
Sodium removed (from epithelial cell) by active transport / sodium- potassium pump;
Into blood;
Maintaining low concentration of sodium (in epithelial cell) / maintaining sodium
concentration gradient (between lumen and epithelial cell);
Glucose moves into blood;
By (facilitated) diffusion;

40
Q

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

A
  1. made from β-glucose;
  2. joined by condensation / removing molecule of water / glycosidic bond; 3. 1 : 4 link specified or described;
  3. “flipping over” of alternate molecules;
  4. hydrogen bonds linking chains / long straight chains;
  5. cellulose makes cell walls strong / cellulose fibres are strong; 7. can resist turgor pressure / osmotic pressure / pulling forces; 8. bond difficult to break;
  6. resists digestion / action of microorganisms / enzymes;
41
Q

In humans, the enzyme maltase breaks down maltose to glucose. This takes place at normal body temperature.
Explain why maltase:
-only breaks down maltose
-allows this reaction to take place at normal body temperature. (5)

A
  1. Tertiary structure / 3D shape of enzyme (means);
  2. Active site complementary to maltose / substrate / maltose fits into active site / active site and substrate fit like a lock and key;
  3. Description of induced fit;
  4. Enzyme is a catalyst;
  5. Lowers activation energy / energy required for reaction;
  6. By forming enzyme-substrate complex;
42
Q

Scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase.
Describe competitive and non-competitive inhibition of an enzyme. (5)

A
  1. Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;
    (Competitive inhibition),
  2. Inhibitor similar shape (idea) to substrate;
  3. (binds) in to active site (of enzyme);
  4. (Inhibition) can be overcome by more substrate;
    (Non-competitive inhibition),
  5. Inhibitor binds to site on enzyme other than active site;
  6. Prevents formation of active site / changes (shape of) active site;
  7. Cannot be overcome by adding more substrate;
43
Q

Scientists believe that it may be possible to develop vaccines that make use of microfold cells (lines 9 -10). Explain how this sort of vaccine would lead to a person developing immunity to a pathogen. (5)

A
  1. (Vaccine contains) antigen/attenuated/dead pathogen;
  2. Microfold cells take up/bind and present/transport antigen (to immune system/lymphocytes/T- cells);
  3. T-cells activate B-cells;
  4. B-cells divide/form clone/undergo mitosis;
  5. B-cells produce antibodies;
  6. Memory cells produced;
  7. More antibodies/antibodies produced faster in secondary response/on reinfection;
44
Q

Describe how a heartbeat is initiated and coordinated. (5)

A
  1. SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
  2. Non-conducting tissue prevents immediate contraction of ventricles/prevents impulses reaching the ventricles;
  3. AVN delays (impulse) whilst blood leaves atria/ventricles fill;
  4. (AVN) sends wave of electrical activity / impulses down Bundle of His;
  5. Causing ventricles to contract from base up;
45
Q

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

A
  1. Atrium has higher pressure than ventricle (due to filling/contraction);
  2. Atrioventricular valve opens;
  3. Ventricle has higher pressure than atrium (due to filling/contraction);
  4. Atrioventricular valve closes;
  5. Ventricle has higher pressure than aorta;
  6. Semilunar valve opens;
  7. Higher pressure in aorta than ventricle (as heart relaxes);
  8. Semilunar valve closes;
  9. (Muscle/atrial/ventricular) contraction causes increase in pressure;
46
Q

A mutation can lead to the production of a non-functional enzyme. Explain how. (6)

A
  1. Change/mutation in base/nucleotide
    sequence (of DNA/gene);
  2. Change in amino acid
    sequence/primary structure (of
    enzyme);
  3. Change in hydrogen/ionic/disulfide
    bonds;
  4. Change in the tertiary
    structure/shape;
  5. Change in active site;
  6. Substrate not complementary/cannot
    bind (to enzyme/active site) / no
    enzyme-substrate complexes form;
47
Q

The events that take place during interphase and mitosis lead to the production of two genetically identical cells. Explain how. (4)

A
  1. DNA replicated;
  2. (Involving)
    specific/accurate/complementary
    base-pairing;
  3. (Ref to) two identical/sister
    chromatids;
  4. Each chromatid/ moves/is separated
    to(opposite) poles/ends of cell;
48
Q

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

A
  1. Sugar-phosphate (backbone)/double
    stranded/helix so provides strength/stability
    /protects bases/protects hydrogen bonds;
  2. Long/large molecule so can store lots of
    information;
  3. Helix/coiled so compact;
  4. Base sequence allows information to be
    stored/ base sequence codes for amino
    acids/protein;
  5. Double stranded so replication can occur
    semi-conservatively/ strands can act as
    templates;
  6. Complementary base pairing / A-T and G-C
    so accurate replication/identical copies can
    be made;
  7. (Weak) hydrogen bonds for replication/
    unzipping/strand separation;
  8. Many hydrogen bonds so stable/strong;
49
Q

Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer.
Describe other ways by which substances cross this membrane. (5)

A

By osmosis (no mark)
1. From a high water potential to a low water potential/down a water potential gradient;
2. Through aquaporins/water channels;
By facilitated diffusion (no mark)
3. Channel/carrier protein;
4. Down concentration gradient;
By active transport (no mark)
5. Carrier protein/protein pumps;
6. Against concentration gradient;
7. Using ATP/energy (from respiration);
By phagocytosis/endocytosis (no mark)
8. Engulfing by cell surface membrane to form vesicle/vacuole;
By exocytosis/role of Golgi vesicles (no mark)
9. Fusion of vesicle with cell surface membrane;

50
Q

Atheroma formation increases a person’s risk of dying. Explain how. (5)

A
  1. Atheroma is fatty material/cholesterol/foam cells/plaque/calcium deposits/LDL;
  2. In wall of artery;
  3. (Higher risk of) aneurysm/described;
  4. (Higher risk of) thrombus formation/blood clot;
  5. Blocks coronary artery;
  6. Less oxygen/glucose to heart muscle/cells/tissue;
  7. Reduces/prevents respiration;
  8. Causing myocardial infarction/heart attack;
  9. Blocks artery to brain;
  10. Causes stroke/stroke described;
51
Q

Describe how DNA is replicated. (6)

A
  1. Strands separate / H-bonds break;
  2. DNA helicase (involved);
  3. Both strands/each strand act(s) as (a) template(s);
  4. (Free) nucleotides attach;
  5. Complementary/specific base pairing / AT and GC;
  6. DNA polymerase joins nucleotides (on new strand);
  7. H-bonds reform;
  8. Semi-conservative replication / new DNA molecules contain one old strand and one new strand;
52
Q

Describe how tissue fluid is formed and how it is returned to the circulatory system. (6)

A

Formation
1. High blood / hydrostatic pressure /
pressure filtration;
2. Forces water / fluid out;
3. Large proteins remain in capillary;
Return
4. Low water potential in capillary /
blood;
5. Due to (plasma) proteins;
6. Water enters capillary / blood;
7. (By) osmosis;
8. Correct reference to lymph;

53
Q

Blood leaving the kidney eventually returns to the kidney.
Describe the pattern of blood circulation in a mammal that causes blood to return to the kidney. (6 marks)

A
  1. (blood flows from kidney along) renal vein to vena cava;
  2. (along) vena cava to RIGHT atrium/side of heart;
  3. (along) pulmonary artery to lungs;
  4. (along) capillaries to pulmonary vein;
  5. (along) pulmonary vein to RIGHT atrium/side of heart;
  6. (along) aorta to renal artery (to kidney);
  7. Blood may pass through several complete circuits before returning to kidney;
54
Q

There are nine subspecies of giraffe. These subspecies evolved when populations of giraffe were separated for long time periods. Each subspecies has distinct coloured skin markings. Some biologists have suggested that up to six of these subspecies should be classified as different species.
Explain how different subspecies of giraffe may have evolved from a common ancestor. Use information from the passage in your answer. (5 marks)

A
  1. No interbreeding / gene pools are separate / geographic(al) isolation;
  2. Mutation linked to (different) markings/colours;
  3. Selection/survival linked to (different) markings/colours;
  4. Adapted organisms breed / differential reproductive success
  5. Change/increase in allele frequency/frequencies;
55
Q

Crops use light energy to produce photosynthetic products.
Describe how crop plants use light energy during the light-dependent reaction. (5 marks)

A
  1. Excites electrons / electrons removed (from chlorophyll);
  2. Electrons move along carriers/electron transfer chain releasing energy;
  3. Energy use to join AFP and Pi to form ATP;
  4. Photolysis of water produces protons, electrons and oxygen;
  5. NADP reduced by electrons / electrons and protons / hydrogen;
56
Q

Describe the advantages and disadvantages of using chemical pesticides to control pests of crops. (5 marks)

A

(Advantages)
1. Acts quickly;
2. Can apply to a particular area;
3. Kills all/most/wide variety of pests;
(Disadvantages)
4. Needs to be re-applied;
5. Not specific;
6. Pests can develop resistance;
7. (Bio)accumulation;

57
Q

After harvesting, the remains of crop plants are often ploughed into the soil.
Explain how microorganisms in the soil produce a source of nitrates from these remains. (5 marks)

A
  1. Protein/amino acids/DNA into ammonium compounds / ammonia;
  2. By saprobionts;
  3. Ammonium/ammonia into nitrite;
  4. Nitrite into nitrate;
  5. By nitrifiying bacteria/microorganisms;
58
Q

Large areas of tropical forest are still found on some Caribbean islands. The concentration of carbon dioxide in the air of these forests changes over a period of 24 hours and at different heights above ground.
Use your knowledge of photosynthesis and respiration to describe and explain how the concentration of carbon dioxide in the air changes:
- over a period of 24 hours
- at different heights above ground. (5 marks)

A
  1. High concentration of/increase in carbon dioxide linked with respiration at night/in darkness;
  2. No photosynthesis in dark/night / photosynthesis ONLY in light/day;
  3. In light net uptake of carbon dioxide / use more carbon dioxide than produced / (rate of) photosynthesis greater than rate of respiration;
  4. Decrease in carbon dioxide concentration with height;
  5. (At ground level) less photosynthesis / less photosynthesising tissue / more respiration / more micro-organisms / micro-organisms produce carbon dioxide;
59
Q

In daylight, most of the water evaporates from the leaves but some is used by the plant. Describe the ways in which this water could be used by the plant. (6)

A
  1. (water is used in) the light-dependent reactions of photosynthesis;
  2. electrons from water enable ATP production / H+ are used to reduce NADP / produces O2;
  3. (water can be used in) hydrolysis reactions within the plant;
  4. to create turgor;
  5. as a solvent for transport;
  6. as a medium for chemical reactions;
  7. component of cells / cytoplasm;
60
Q

Describe two features you would expect in the leaves of a tree adapted to a dry environment. Explain how each feature helps the tree’s survival. (6)

A

Sunken stomata;
water evaporation into pit creates local humidity;
increased humidity reduces gradient for water evaporation;
close arrangement of stomata;
diffusion shells of individual stomata overlap;
interferes with water diffusion and slows evaporation;
restriction of stomata to lower side of leaf;
rate of air movement below leaf less/ heating effect of sun less;
gradient for water evaporation reduced/ water molecules have less
kinetic energy;

thick cuticle/wax/suberin (on upper surface);
(wax/suberin )waterproof;
water unable to diffuse onto surface to evaporate,
presence of trichomes/ hairs;
surface traps water close to leaf surface;
increased humidity reduces gradient for water evaporation;
reduced leaves/spines/small surface area to volume;
less surface area for evaporation;
more distance across leaf for water to diffuse;
rolled leaves;
stomata enclosed in localised humidity;
increased humidity reduces gradient for water evaporation;

61
Q

A laboratory has oat plants containing the resistance gene and a supply of plasmids. Describe how bacteria may be produced which have the resistance gene in their plasmids. (6)

A
  1. Cut desired gene (from DNA) of oat plant;
  2. Using restriction endonuclease (restriction enzyme);
  3. Cut plasmid open;
  4. With (same) restriction endonuclease;
  5. Sticky ends/unpaired bases attached;
  6. Use (DNA) ligase to join;
  7. Return plasmid to (bacterial) cells;
  8. Use of Ca2+/calcium salts/electric shock;
62
Q

The polymerase chain reaction (PCR) can be used to produce large quantities of DNA. Describe how the PCR is carried out. (6)

A
  1. DNA heated to 90-95°C;
  2. Strands separate;
  3. Cooled to temperature below 70°C;
  4. Primers bind;
  5. Nucleotides attach;
  6. By complementary base pairing;
  7. Temperature 70-75°C;
  8. DNA Polymerase joins nucleotides together;
  9. Cycle repeated;
63
Q

Describe how an action potential is produced in an axon. (6)

A
  1. A stimulus causes a temporary reversal of charge and inside of axon becomes positive (depolarisation);
  2. Energy of stimulus causes some sodium channels to open;
  3. So sodium diffuses into axon;
  4. Once an action potential of +40mV has been established, channels close and there is no more influx of sodium;
  5. Potassium channels open and potassium ions diffuse out, causing repolarisation of axon;
  6. Axon becomes more negative than usual, so potassium gates close and sodium-potassium pump again actively transports sodium ions out and potassium ions in;
64
Q

Describe the sequence of events which allows information to pass from one neurone to the next neurone across a cholinergic synapse. (6)

A
  1. Impulse causes calcium ions to enter axon;
    2 Vesicles move to/fuse with membrane;
  2. Acetylcholine is released;
  3. Diffuses across synaptic cleft/synapse;
  4. Binds with receptors on postsynaptic neurone;
  5. Sodium ions enter postsynaptic neurone;
  6. Depolarisation of postsynaptic membrane;
  7. If above threshold, nerve impulse/action potential is produced;
65
Q

How does temperature affect the speed of a nerve impulse? (5)

A
  1. The higher the temperature the faster the nerve impulse;
  2. Sodium-potassium pump uses active transport;
  3. Which requires energy from ATP from respiration;
  4. Respiration is enzyme-controlled;
  5. More kinetic energy so more enzyme-substrate complexes formed and faster diffusion
66
Q

Explain how nervous control in a human can cause increased cardiac output during exercise. (4)

A
  1. Coordination via medulla (of brain) / cardiac centre;
  2. (Increased) impulses along sympathetic (/ cardiac accelerator) nerve;
  3. To S.A. node / pacemaker;
  4. Release of noradrenalin;
  5. More impulses sent from / increased rate of discharge of S.A. node / pacemaker;
  6. Increased heart rate / increased stroke volume;
67
Q

Describe how the regular contraction of the atria and ventricles is initiated and coordinated by the heart itself. (5)

A
  1. (cardiac) muscle is myogenic;
  2. sinoatrial node/SAN;
  3. wave of depolarisation/ impulses /electrical activity (across atria);
  4. initiates contraction of atria
    atrioventricular node/AVN;
  5. bundle of His/purkyne tissue spreads impulse across ventricles;
  6. ventricles contract after atria/time delay enables ventricles to fill;
68
Q

What is atheroma and how may it cause myocardial infarction? (5)

A
  1. Cholesterol/ plaque / lipoprotein / LDL / fatty material / cells;
  2. In artery wall / under lining / endothelium of artery / blood vessel;
  3. Atheroma linked with blood clotting / thrombosis;
  4. (Blocks) coronary artery / artery supporting heart muscle / tissue / cells;
  5. Reduces oxygen / glucose supply (to heart muscle / tissues / cells);
  6. (Heart muscle / tissue / cells) unable to respire / dies;
69
Q

The diet of a person can increase the risk of coronary heart disease. Explain how. (5)

A
  1. Too much saturated fat / cholesterol in diet;
  2. Increase in LDL / cholesterol in blood;
  3. Atheroma / fatty deposits / plaques in artery walls;
  4. Reduces diameter of / blocks coronary arteries;
  5. Less oxygen / glucose to heart muscles / tissues / cells;
  6. Increase in blood pressure;
  7. (Increased risk of) clot / thrombosis / embolism / aneurysm.
70
Q

Blood leaving the kidney eventually returns to the kidney.
Describe the pattern of blood circulation in a mammal that causes blood to return to the kidney. (6)

A
  1. (blood flows from kidney along) renal vein to vena cava;
  2. (along) vena cava to right atrium/side of heart;
  3. (along) pulmonary artery to lungs;
  4. (along) capillaries to pulmonary vein;
  5. (along) pulmonary vein to left atrium/side of heart;
  6. (along) aorta to renal artery (to kidney);
  7. Blood may pass through several complete circuits before returning to kidney;
71
Q

Describe how a heartbeat is initiated and coordinated. (5)

A
  1. SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
  2. Non-conducting tissue prevents immediate contraction of ventricles/prevents impulses reaching the ventricles;
  3. AVN delays (impulse) whilst blood leaves atria/ventricles fill;
  4. (AVN) sends wave of electrical activity / impulses down Bundle of His;
  5. Causing ventricles to contract from base up;
72
Q

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

A
  1. Atrium has higher pressure than ventricle (due to filling/contraction);
  2. Atrioventricular valve opens;
  3. Ventricle has higher pressure than atrium (due to filling/contraction);
  4. Atrioventricular valve closes;
  5. Ventricle has higher pressure than aorta;
  6. Semilunar valve opens;
  7. Higher pressure in aorta than ventricle (as heart relaxes);
  8. Semilunar valve closes;
  9. (Muscle/atrial/ventricular) contraction causes increase in pressure;
73
Q

A mutation can lead to the production of a non-functional enzyme. Explain how. (6)

A
  1. Change/mutation in base/nucleotide
    sequence (of DNA/gene);
  2. Change in amino acid
    sequence/primary structure (of
    enzyme);
  3. Change in hydrogen/ionic/disulfide
    bonds;
  4. Change in the tertiary
    structure/shape;
  5. Change in active site;
  6. Substrate not complementary/cannot
    bind (to enzyme/active site) / no
    enzyme-substrate complexes form;
74
Q

Atheroma formation increases a person’s risk of dying. Explain how. (5)

A
  1. Atheroma is fatty material/cholesterol/foam cells/plaque/calcium deposits/LDL;
  2. In wall of artery;
  3. (Higher risk of) aneurysm/described;
  4. (Higher risk of) thrombus formation/blood clot;
  5. Blocks coronary artery;
  6. Less oxygen/glucose to heart muscle/cells/tissue;
  7. Reduces/prevents respiration;
  8. Causing myocardial infarction/heart attack;
  9. Blocks artery to brain;
  10. Causes stroke/stroke described;
75
Q

Describe how tissue fluid is formed and how it is returned to the circulatory system. (6)

A

Formation
1. High blood / hydrostatic pressure /
pressure filtration;
2. Forces water / fluid out;
3. Large proteins remain in capillary;
Return
4. Low water potential in capillary /
blood;
5. Due to (plasma) proteins;
6. Water enters capillary / blood;
7. (By) osmosis;
8. Correct reference to lymph;

76
Q

Some desert mammals have long loops of Henle and secrete large amounts of antidiuretic hormone (ADH). Explain how these two features are adaptions to living in desert conditions. (6)

A

For general principle, applied to either example:

  1. More water (from filtrate) reabsorbed / returned to blood / less lost in urine;
  2. By osmosis;
  3. From collecting duct / from end of second convoluted tubule;
  4. Due to longer loop of Henle;

For loop of Henle, maximum 2 marks:

  1. Sodium / chloride ions absorbed from filtrate in ascending limb;
  2. Gradient established in medulla / concentration of ions increases down medulla;

For ADH, maximum 2 marks:

  1. Acts on collecting duct / distal convoluted tubule / second convoluted tubule;
  2. Makes cells more permeable / inserts aquaporins in plasma membranes;

Note: to score full marks, candidates must make one specific statement about

Loop of Henle and one about ADH.

Biology A Level AQA (13 decks)

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