AQA Biology A-Level Markscheme Flashcards

Explain how glycogen's structure relates to its function (4 marks).

1. Insoluble so does not draw water into cells (by osmosis);

2. (Insoluble so) does not diffuse out of cells;

3. Compact so a lot can be stored in a small space:

4. (More) highly branched (than starch) so more rapidly broken down to form monomers of glucose;

Describe how the structure of a protein depends on the amino acids it contains (5 marks).

1. Structure is determined by (relative) position of amino acid/R group/interactions;

(accept for 'interactions', hydrogen bonds / disulfide bridges / ionic bonds / hydrophobic hydrophilic interactions)

2. Primary structure is sequence/order of amino acids;

3. Secondary structure formed by hydrogen bonding (between amino acids);

(Accept alpha helix/β-pleated sheet for 'secondary structure')

4. Tertiary structure formed by interactions (between R groups);

5. Creates active site in enzymes

OR

Creates complementary/specific shapes in antibodies/carrier proteins/receptor (molecules);

6. Quaternary structure contains >1 polypeptide chain

OR

Quaternary structure formed by interactions/bonds between polypeptides;

(accept prosthetic (group))

Explain how the active site of an enzyme causes a high rate of reaction (3 marks).

1. Lowers activation energy;

2. Induced fit causes active site (of enzyme) to change shape;

3. (So) enzyme-substrate complex causes bonds to form/break;

(accept: description, of induced fit)

(accept: enzyme-substrate complex causes stress/strain on bonds)

Describe how a non-competitive inhibitor can reduce the rate of an enzyme-controlled reaction (3 marks).

1. Attaches to the enzyme at a site other than the active site;

(accept 'attaches to allosteric/inhibitor site')

2. Changes (shape of) the active site

OR

Changes tertiary structure (of enzyme);

3. (So active site and substrate) no longer complementary so less/no substrate can fit/bind;

(accept 'no longer complementary so less/no enzyme- substrate complexes form')

(accept abbreviations of enzyme-substrate complex.)

Describe how a competitive inhibitor can reduce the rate of an enzyme-controlled reaction (3 marks).

1. Attaches to the active site;

2. Changes (shape of) the active site

OR

Changes tertiary structure (of enzyme);

3. (So active site and substrate) no longer complementary so less/no substrate can fit/bind;

(accept 'no longer complementary so less/no enzyme- substrate complexes form')

(accept abbreviations of enzyme-substrate complex.)

Describe the structure of DNA (5 marks).

1. Polymer of nucleotides;

(accept 'Polynucleotide')

2. Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base;

3. Phosphodiester bonds (between nucleotides);

4. Double helix/2 strands held by hydrogen bonds;

5. (Hydrogen bonds/pairing) between adenine, thymine and cytosine, guanine;

Describe the process of semi-conservative replication (5 marks).

1. DNA helicase breaks hydrogen bonds between complementary bases (on the DNA strand);

2. Each exposed polynucleotide strand then acts like a template;

3. Free nucleotides that have been activated by ATP bind to their complementary bases;

4. DNA polymerase joins the nucleotides together by making phosphodiester bonds;

5. (Semi-conservative because) the new DNA molecules contain half of the original DNA and half of the new DNA;

Describe how an ATP molecule is formed from its component molecules (4 marks).

1. and 2. Accept for 2 marks correct names of three components adenine, ribose/pentose, three phosphates;;

(accept for 1 mark, correct name of two components)

(accept for 1 mark, ADP and phosphate/Pi)

(ignore adenosine)

(accept suitably labelled diagram)

3. Condensation (reaction);

(ignore phosphodiester)

4. ATP synthase;

(reject ATPase)

ATP is formed via a condensation reaction between adenine and three phosphates, catalysed by ATP synthase

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

1. A metabolite in condensation/hydrolysis/ photosynthesis/respiration;

2. A solvent so (metabolic) reactions can occur

OR

A solvent so allowing transport of substances;

3. High (specific) heat capacity so buffers changes in temperature;

(for 'buffer' accept 'resist')

4. Large latent heat of vaporisation so provides a cooling effect (through evaporation);

(reject latent heat of evaporation)

5. Cohesion (between water molecules) so supports columns of water (in plants);

(for 'columns of water' accept 'transpiration stream')

(do not credit 'transpiration' alone but accept description of 'stream')

(for 'columns of water' accept 'cohesion-tension (theory)')

6. Cohesion (between water molecules) so produces surface tension supporting (small) organisms;

(for cohesion accept hydrogen bonding)

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

(must have MP1 for 5 max)

(3 max for sodium and 3 max for phosphate)

IRON IONS:

1. Haemoglobin binds/associates with oxygen

OR

Haemoglobin transports/loads oxygen;

(ignore reference to 2+ or 3+ in Fe2+ or Fe3+)

SODIUM IONS:

2. Co-transport of glucose/amino acids (into cells);

3. (Because) sodium moved out by active transport/Na - K pump;

4. Creates a sodium concentration/diffusion gradient;

5. Affects osmosis/water potential;

PHOSPHATE IONS:

6. Affects osmosis/water potential;

(accept 5. OR 6. - not both)

7. Joins nucleotides/in phosphodiester bond/in backbone of

DNA/RNA/in nucleotides;

8. Used in/to produce ATP;

(reject 'energy produced')

9. Phosphorylates other compounds (usually) making them more reactive;

10. Hydrophilic/water soluble part of phospholipid bilayer/membrane;

Describe how a sample of chloroplasts could be isolated from leaves (4 marks).

1. Break open cells/tissue and filter

OR

Grind/blend cells/tissue/leaves and filter;

(accept homogenise and filter)

2. In cold, same water potential/concentration, pH controlled solution;

(accept for 'same water potential/ concentration', isotonic)

(accept for 'pH controlled', buffered)

3. Centrifuge/spin and remove nuclei/cell debris;

4. (Centrifuge/spin) at high(er) speed, chloroplasts settle out;

A biologist separated cell components to investigate organelle activity. She prepared a suspension of the organelles in a solution that prevented damage to the organelles.

Describe three properties of this solution and explain how each property prevented damage to the organelles (3 marks).

1. (Ice) cold to prevent/reduce enzyme activity;

2. Buffered to prevent denaturing of enzyme/protein;

(accept description of buffer)

(accept: prevent change of tertiary structure)

3. Same water potential/ Ψ to prevent lysis/bursting (of organelle);

(accept: isotonic for same water potential)

(reject: references to turgor or plasmolysis or crenation)

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells (6 marks).

1. TEM uses electrons and optical light:

2. TEM allows a greater resolution;

3. (So with TEM) smaller organelles / named cell structure can be observed

OR

greater detail in organelles / named cell structure can be

observed;

('clearer' is not equivalent to 'detail')

4. TEM view only dead / dehydrated specimens and optical (can) view live specimens;

(accept 'Only optical can view live specimens')

5. TEM does not show colour and optical (can);

(accept 'Only optical can show colour')

6. TEM requires thinner specimens;

7. TEM requires a more complex/time consuming preparation;

(accept 'TEM requires a more difficult preparation')

8. TEM focuses using magnets and optical uses (glass) lenses;

(ignore references to artefacts)

Describe binary fission in bacteria (3 marks).

1. Replication of (circular) DNA;

(accept nucleoid)

(reject chromosome)

(reject mitosis)

2. Replication of plasmids;

3. Division of cytoplasm (to produce daughter cells);

(ignore genetically identical)

Describe the appearance and behaviour of chromosomes during mitosis (5 marks).

(During prophase)

1. Chromosomes coil / condense / shorten / thicken / become visible;

2. (Chromosomes) appear as (two sister) chromatids joined at the centromere;

(During metaphase)

3. Chromosomes line up on the equator / centre of the cell;

4. (Chromosomes) attached to spindle fibres;

5. By their centromere;

(During anaphase)

6. The centromere splits / divides;

7. (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell / separate;

(During telophase)

8. Chromatids / chromosomesuncoil / unwind / become longer / thinner.

Name and describe five ways substances can move across the

cell-surface membrane into a cell (5 marks).

1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;

2. Facilitated diffusion down a concentration gradient via protein carrier/channel;

3. Osmosis of water down a water potential gradient;

4. Active transport against a concentration gradient via protein carrier using ATP;

5. Co-transport of 2 different substances using a carrier protein;

(for any answer accept a correct example)

(for 'carrier protein' accept symport OR cotransport protein)

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

1. Phospholipid (bilayer) allows movement/diffusion of non- polar/lipid-soluble substances;

2. Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances

OR

(Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;

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;

Describe how HIV is replicated (4 marks).

1. Attachment proteins attach to receptors on helper T cell/lymphocyte;

2. Nucleic acid/RNA enters cell;

3. Reverse transcriptase converts RNA to DNA;

4. Viral protein/capsid/enzymes produced;

5. Virus (particles) assembled and released (from cell);

Describe how a phagocyte destroys a pathogen present in the blood (3 marks).

1. Engulfs;

(accept endocytosis)

2. Forming vesicle/phagosome and fuses with lysosome;

3. Enzymes digest/hydrolyse;

(accept lysozymes for 'enzymes')

Explain how HIV affects the production of antibodies when AIDS develops in a person (3 marks).

1. Less/no antibody produced;

2. (Because HIV) destroys helper T cells;

(accept 'reduces number' for 'destroys')

3. (So) few/no B cells activated / stimulated

OR

(So) few/no B cells undergo mitosis/differentiate/form plasma cells;

Describe how B lymphocytes would respond to vaccination (3 marks).

1. B cell (antibody) binds to (viral) specific/complementary receptor/antigen;

(accept B cell forms antigen-antibody complex)

2. B cell clones

OR

B cell divides by mitosis;

3. Plasma cells release/produce (monoclonal) antibodies (against the virus);

4. (B/plasma cells produce/develop) memory cells;

(accept B cell undergoes clonal selection/expansion)

Describe the role of antibodies in producing a positive result in an ELISA test (4 marks).

1. (First) antibody binds/attaches /complementary (in shape) to antigen;

2. (Second) antibody with enzyme attached is added;

3. (Second) antibody attaches to antigen;

(accept (second) antibody attaches to (first) antibody (indirect ELISA test))

4. (Substrate/solution added) and colour changes;

(only award if enzyme mentioned)

During vaccination, each animal is initially injected with a small volume of venom. Two weeks later, it is injected with a larger volume of venom.

Use your knowledge of the humoral immune response to explain this vaccination programme (3 marks).

1. B cells specific to the venom reproduce by mitosis;

2. (B cells produce) plasma cells and memory cells;

3. The second dose produces antibodies (in secondary immune response) in higher concentration and quickly

OR

The first dose must be small so the animal is not killed;

Describe how vaccination can lead to protection against viruses (6 marks).

1. Antigen / bacterium binds to surface protein / surface receptor on a (specific / single) B cell.

2. (Activated) B cell divides by mitosis / produces clone;

3. (Division) stimulated by cytokines / by T cells;

4. B cells / plasma cells release antibodies;

5. (Some) B cells become memory cells;

6. Memory cells produce plasma / antibodies faster;

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

1. Active involves production of antibody by plasma cells / memory cells;

2. Active involves memory cells, passive does not;

3. Passive involves antibody introduced into body from outside/named source;

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;

Describe and explain the advantage of the counter-current principle in gas exchange across a fish gill (3 marks).

1. Water and blood flow in opposite directions;

2. Maintains diffusion/concentration gradient of oxygen;

(accept oxygen concentration always higher (in water))

3. (Diffusion) along length of lamellae/filament/gill/capillary;

Describe the gross structure of the human gas exchange system and how we breathe in and out (6 marks).

1. Named structures - trachea, bronchi, bronchioles, alveoli;

2. Above structures named in correct order

OR

Above structures labelled in correct positions on a diagram;

3. Breathing in - diaphragm contracts and external intercostal muscles contract;

4. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in);

5. Breathing out - Diaphragm relaxes and internal intercostal muscles contract;

6. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out);

Explain three ways in which an insect's tracheal system is adapted for efficient gas exchange (3 marks).

1. Tracheoles have thin walls so short diffusion distance to cells;

2. Highly branched / large number of tracheoles so short diffusion distance to cells;

3. Highly branched / large number of tracheoles so large surface area (for gas exchange);

4. Tracheae provide tubes full of air so fast diffusion (into insect tissues);

5. Fluid in the end of the tracheoles that moves out (into tissues) during exercise so faster diffusion through the air to the gas exchange surface;

OR

Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange);

6. Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide;

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels (5 marks).

1. Micelles contain bile salts and fatty acids/monoglycerides;

2. Make fatty acids/monoglycerides (more) soluble (in water)

OR

Bring/release/carry fatty acids/monoglycerides to cell/lining (of the iluem)

OR

Maintain high(er) concentration of fatty acids/monoglycerides to cell/lining (of the ileum);

(accept lipid/fat for fatty acid/ monoglyceride)

4. Fatty acids/monoglycerides absorbed by diffusion;

(reject if absorbed by facilitated diffusion)

(ignore if micelles themselves are being absorbed)

5. Triglycerides (re)formed (in cells);

(accept chylomicrons form)

6. Vesicles move to cell membrane;

(accept exocytosis for 'vesicles move')

Describe the role of enzymes in the digestion of proteins in a mammal (4 marks).

1. (Reference to) hydrolysis of peptide bonds;

2. Endopeptidase act in the middle of protein/polypeptide

OR

Endopeptidase produces short(er) polypeptides/ increase number of ends;

3. Exopeptidases act at end of protein/polypeptide

OR

Exopeptidase produces dipeptides/amino acids;

4. Dipeptidase acts on dipeptide/between two amino acids OR

Dipeptidase produces (single) amino acids;

Describe the process of the cardiac cycle (6 marks).

1. Diastole - blood flows into the atria;

2. AV valves open and blood flows into ventricles;

3. Pressure within ventricles is lower than that of the arteries so SL valves close ('dub' sound);

4. Atrial systole - atria contract, forcing any remaining blood into ventricles;

5. Ventricular systole - ventricles contract;

6. Pressure increases so AV valves close ('lub' sound);

7. (Once pressure exceeds that of the arteries) blood flows into arteries;

Explain how water from tissue fluid is returned to the circulatory system (4 marks).

1. (Plasma) proteins remain;

2. (Creates) water potential gradient

OR

Reduces water potential (of blood);

3. Water moves (to blood) by osmosis;

4. Returns (to blood) by lymphatic system;

Describe the cohesion-tension theory of water transport in the xylem (5 marks).

1. Water lost from leaf because of transpiration / evaporation of water (molecules) / diffusion from mesophyll / leaf cells;

OR

Transpiration / evaporation / diffusion of water (molecules) through stomata / from leaves;

2. Lowers water potential of mesophyll / leaf cells;

3. Water pulled up xylem (creating tension);

4. Water molecules cohere / 'stick' together by hydrogen bonds;

5. (forming continuous) water column;

6. Adhesion of water (molecules) to walls of xylem;

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

1. In source / leaf sugars actively transported into phloem;

osmosis;

2. By companion cells;

3. Lowers water potential of sieve cell / tube and water enters by osmosis;

4. Increase in pressure causes mass movement (towards sink /

root);

5. Sugars used / converted in root for respiration for storage.

Describe how a gene is a code for the production of a polypeptide (3 marks).

1. (Because) base/nucleotide sequence;

2. (In) triplet(s);

3. (Determines) order/sequence of amino acid sequence/primary structure (in polypeptide);

Because base sequences in triplets determines the order of the amino acid sequence in polypeptides

Compare and contrast the DNA in a eukaryotic cell with the DNA in the prokaryotic cell (5 marks).

Eukaryote v prokaryote

COMPARISONS:

1. Nucleotide structure is identical;

2. Nucleotides joined by phosphodiester bond;

OR

Deoxyribose joined to phosphate (in sugar, phosphate backbone);

3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes;

CONTRASTS:

4. (Associated with) histones/proteins v no histones/proteins;

5. Linear v circular;

6. No plasmids v plasmids;

7. Introns v no introns;

8. Long(er) v short(er);

Describe how mRNA is formed by transcription in eukaryotes (5 marks).

1. Hydrogen bonds (between DNA bases) break;

2. (Only) one DNA strand acts as a template;

3. (Free) RNA nucleotides align by complementary base pairing;

4. (In RNA) Uracil base pairs with adenine (on DNA)

OR

(In RNA) Uracil is used in place of thymine;

5. RNA polymerase joins (adjacent RNA) nucleotides;

6. (By) phosphodiester bonds (between adjacent nucleotides);

7. Pre-mRNA is spliced (to form mRNA)

OR

Introns are removed (to form mRNA);

Describe how a polypeptide is formed by translation of mRNA (6 marks).

1. (mRNA attaches) to ribosomes

OR

(mRNA attaches) to rough endoplasmic reticulum;

2. (tRNA) anticodons (bind to) complementary (mRNA) codons;

3. tRNA brings a specific amino acid;

4. Amino acids join by peptide bonds;

5. (Amino acids join together) with the use of ATP;

6. tRNA released (after amino acid joined to polypeptide);

7. The ribosome moves along the mRNA to form the polypeptide;

Define 'gene mutation' and explain how a gene mutation can have:

no effect on an individual

a positive effect on an individual (4 marks).

(Definition of gene mutation)

1. Change in the base/nucleotide (sequence of chromosomes/DNA);

For 4 marks at least one mark must be scored in each section of the answer.

2. Results in the formation of new allele;

(Has no effect because)

3. Genetic code is degenerate (so amino acid sequence may not change);

OR

Mutation is in an intron (so amino acid sequence may not change);

(accept description of 'degenerate', eg some amino acids have more than one triplet/codon)

4. Does change amino acid but no effect on tertiary structure;

5. (New allele) is recessive so does not influence phenotype;

(Has positive effect because)

6. Results in change in polypeptide that positively changes the

properties (of the protein)

OR

Results in change in polypeptide that positively changes a named protein;

7. May result in increased reproductive success

OR

May result in increased survival (chances);

Describe the process of crossing over and explain how it increases genetic diversity (4 marks).

1. Homologous pairs of chromosomes associate / form a bivalent;

2. Chiasma(ta) form;

3. (Equal) lengths of (non-sister) chromatids / alleles are exchanged;

4. Producing new combinations of alleles;

Bivalent: a pair of homologous chromosomes

Chaisma(ta): X-shaped points where two non-sister chromatids of homologous chromosomes are physically connected

REMEMBER🙆🏽‍♀️’Chaii’

Describe the process of photosynthesis (6 marks).

1. (LDR) in thylakoid membrane;

2. (Photolysis) - water converted to oxygen, 2e- and 2H+;

3. (Photoionisation) - electrons are excited and leave chlorophyll;

4. energy (from photoionisation) used in chemiosmosis;

5. (chemiosmosis) - protons pass through ATP synthase to form ATP

6. protons combine with the co-enzyme NADP to form NADPH;

7. (LIR) in stroma;

8. (Calvin cycle) - CO2 + RuBP combine with the enzyme Rubisco to make 2x GP;

9. GP reduced by ATP and NADPH to 2x triose phosphate;

Describe the process of respiration (6 marks).

1. (Glycolysis) phosphorylation (and splitting) of glucose using ATP;

2. Oxidation of triose phosphate to pyruvate;

3. Net gain of 2x ATP and NAD reduced;

4. (Link reaction) pyruvate is oxidised to form acetate and CO2 (NAD is reduced);

5. Acetate combines with co-enzyme A to form acetyl CoA;

6. (Krebs cycle) acetyl CoA combines with a 4-carbon molecule to form citrate;

7. Citrate loses CO2 and H2 to form a 4-carbon molecule that can combine with acetyl CoA;

8. (Oxidative phosphorylation) NADH and FADH reduce carrier proteins by donating their electrons;

9. Electrons travel along the ETC releasing energy;

10. This energy is used to pump protons through ATP synthase channel to produce ATP;

11. Oxygen is the final electron acceptor in the chain;

Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration (2 marks).

1. Regenerates/produces NAD

OR

oxidises reduced NAD;

2. (So) glycolysis continues;

Describe the advantage of the Bohr effect during intense exercise (2 marks).

1. Increases dissociation of oxygen;

(Accept unloading/release/reduced affinity for dissociation)

2. For aerobic respiration at the tissues/muscles/cells

OR

Anaerobic respiration delayed at the tissues/muscles/cells

OR

Less lactate at the tissues/muscles/cells;

Describe the nitrogen cycle (6 marks).

1. (Ammonification) saprobionts feed on proteins / amino acids / urea;

2. And release ammonia into the soil;

3. (Nitrification) oxidation of ammonia to nitrite ions;

4. oxidation of nitrite ions to nitrate ions;

5. (Denitrification) anaerobic denitrifying bacteria convert soil nitrates into gaseous nitrogen;

6. (Nitrogen fixation) nitrogen-fixing bacteria convert gaseous nitrogen into nitrogen-containing compounds;

Describe the phosphorus cycle (5 marks).

1. Weathering and erosion of sedimentary rocks dissolves phosphate ions and plants absorb them;

2. Phosphate ions pass into animals that feed on the plants;

3. Excess phosphate ions are excreted by animals;

4. (on the death of plants and animals) decomposer break down remains releasing phosphate ions into soil

5. phosphate ions are transported by streams into lakes and oceans where they form sedimentary rocks;

Describe and explain the effects of eutrophication (6 marks).

1. (In lakes and rivers) nitrate ions are a limiting factor for plant and algal growth;

2. Leaching increases nitrate ion concentration so algae and plants grow;

3. Algal bloom;

4. Light becomes a limiting factor for the growth of plants at lower depths so they die;

5. Lack of dead organisms no longer a limiting factor for saprobionts so they grow;

6. Saprobionts need oxygen for respiration, so O2 concentration decreases;

7. O2 becomes a limiting factor for the population of aerobic organisms so they die;

8. Less competition for anaerobic respiration so their population grows;

9. Further decomposition of dead material, releasing toxic water such as H2S, which makes the water putrid;

Exercise causes an increase in heart rate. Describe the role of receptors and of the nervous system in this process (4 marks).

1. Chemoreceptors detect rise in CO2 / H+ / acidity / carbonic acid / fall in pH

OR

Baro / pressure receptors detect rise in blood pressure;

2. Send impulses to cardiac centre / medulla;

3. More impulses to SAN;

4. By sympathetic (nervous system for chemoreceptors / CO2)

OR

By parasympathetic (nervous system for baro / pressure receptors / blood pressure);

Explain the sequence of events that control the heart rate (5 marks).

1. SAN releases a wave of depolarisation across the atria, causing it to contract;

2. AVN releases another wave of depolarisation (when the first reaches it);

3. Depolarisation only through Bundle of His / AVN;

4. Wave of electrical activity passes over / through both ventricles at the same time

5. As a result, the ventricles contract

6. (Short delay before this) To allow enough time for the atria to pump all the blood into the ventricles

Explain how cone cells allow us to see in detail (3 marks).

1. High (visual) acuity;

2. (Each) cone is connected to a single neurone;

(accept no retinal convergence)

(accept 'bipolar/nerve cell' for neurone)

3. (Cones send) separate (sets of) impulses to brain;

(accept 'optic nerve' for brain)

(reject 'signals', 'messages' for 'impulses')

(accept 'action potential')

Explain how rod cells allow us to see in the dark (3 marks).

1. High (visual) sensitivity;

(accept retinal convergence)

2. Several rods connected to a single neurone;

(accept 'bipolar/nerve cell' for neurone)

3. Enough (neuro)transmitter to reach/overcome threshold

OR

Spatial summation to reach/overcome threshold; more for 'several';

Explain how applying pressure to the Pacinian corpuscle produces the changes in membrane potential (3 marks).

1. (Pressure causes) membrane / lamellae to become deformed /stretched;

2. Sodium ion channels in membrane open and sodium ions move in;

3. Greater pressure more channels open / sodium ions enter;

Explain the effects of IAA on growth of plants (3 marks).

1. Tip produces IAA;

2. IAA diffuses (into shoot);

3. (More) elongation of cells on one side (than other);

(accept (more) elongation of cells on shaded side).

Explain how a resting potential is maintained across the axon membrane in a neurone (3 marks).

1. Higher concentration of potassium ions inside and higher concentration of sodium ions outside (the neurone)

OR

potassium ions diffuse out

OR

sodium ions diffuse in;

2. (Membrane) more permeable to potassium ions (leaving than sodium ions entering)

OR

(Membrane) less permeable to sodium ions (entering than potassium ions leaving);

(accept for 'less permeable to sodium ions' is 'impermeable to sodium ions' or 'sodium gates/channels are closed')

3. Sodium ions (actively) transported out and potassium ions in;

Explain how an action potential is generated (4 marks).

1. A stimulus provides the energy that causes Na+ voltage-gated ion channels to open and Na+ ions start diffusing into the axon

2. If the stimulus is large enough to cause enough of the Na+ ion channels to open, the threshold is met and an action potential is generated;

3. Voltage-gated Na+ ion channels close, yet K+ ions still moving out, so there is a decrease in voltage (repolarisation);

4. More K+ ion channels open and eventually so many positive ions diffuse out that it overshoots the resting potential (hyperpolarisation);

Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon (3 marks).

1. Myelination provides (electrical) insulation;

2. (In myelinated) saltatory (conduction)

OR

(In myelinated) depolarisation at nodes (of Ranvier);

3. In non-myelinated depolarisation occurs along whole/length (of axon);

(accept action potentials for depolarisation)

Describe the sequence of events involved in transmission across a cholinergic synapse (5 marks).

1. Depolarisation of presynaptic membrane;

(accept action potential for depolarisation)

2. Calcium channels open and calcium ions enter (synaptic knob);

3. (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter;

4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft);

5. (Acetylcholine attaches) to receptors on the postsynaptic membrane;

6. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Explain the effects of inhibitory synapses (3 marks).

1. (Inside of postsynaptic) neurone becomes more negative/hyperpolarisation/inhibitory postsynaptic potential;

2. More sodium ions required (to reach threshold)

OR

Not enough sodium ions enter (to reach threshold);

3. For depolarisation/action potential;

Describe the roles of calcium ions and ATP in the contraction of a myofibril (5 marks).

1. Calcium ions diffuse into myofibrils from sarcoplasmic reticulum;

2. This causes tropomyosin to move to reveal the actin binding sites;

3. Myosin heads attach to binding sites on actin;

4. Hydrolysis of ATP causes myosin to bend, pulling on the actin molecules;

5. Attachment of another ATP molecule causes the myosin head to be released;

6. Phosphocreatine assists this process by providing Pi to bind with ADP;

Explain the banding pattern of a sarcomere (3 marks).

1. Light/I band only actin;

2. H zone/band only myosin;

3. Darkest/overlapping region actin and myosin;

Describe the action of insulin when blood glucose levels are too high (3 marks).

1. Attaches to receptors on the surfaces of target cells, changing the tertiary structure of the channel proteins resulting in more glucose being absorbed by facilitated diffusion;

2. More protein carriers are incorporated into cell membranes so that more glucose is absorbed from the blood into cells;

3. Activates enzymes involved in the conversion of glucose to glycogen;

Describe the role of glucagon when blood glucose levels are too low (3 marks).

1. Attaches to receptors on the surfaces of liver cells;

2. Causes adenyl cyclase to be activated which causes ATP to be converted into cyclic AMP (cAMP). This activates protein kinase to hydrolyse glycogen into glucose (glycogenolysis);

3. Activates enzymes involved in the conversion of glycerol and fatty acids into glucose (gluconeogenesis);

Describe the mechanism of the second-messenger model (5 marks).

1. Adrenaline binds to a transmembrane protein receptor within the cell-surface membrane of a liver cell;

2. (Binding) causes the protein to change shape on the inside of the membrane;

3. (This leads to the) activation of adenyl cyclase which converts ATP to cyclic AMP;

4. The cAMP acts as a second messenger that binds to protein kinase;

5. (Binding) causes the protein to change shape on the inside of the membrane;

6. (This leads to the) activation of protein kinase which converts glycogen to glucose;

7. Glucose moves out of the liver cell by facilitated diffusion and into the blood (through channel proteins);

Using your knowledge of the kidney, explain why glucose is found in the urine of a person with untreated diabetes (3 marks).

1. High concentration of glucose in blood/filtrate;

2. Not all the glucose is (re)absorbed at the proximal convoluted tubule;

(reject no glucose is (re)absorbed)

3. Carrier/co-transport proteins are working at maximum rate

OR

Carrier/co-transport proteins/ are saturated;

(accept all carrier/co-transport proteins are 'in use' but reject all carriers are 'used up')

Describe how ultrafiltration occurs in a glomerulus (3 marks).

1. High blood/hydrostatic pressure;

(ignore 'increasing/higher blood pressure' as does not necessarily mean high)

2. Two named small substances pass out eg water, glucose, ions, urea;

3. (Through small) gaps/pores/fenestrations in (capillary) endothelium;

(accept epithelium for endothelium)

4. (And) through (capillary) basement membrane;

5. And form the glomerular filtrate;

Describe the process of selective reabsorption by the nephron.

1. Na+ ions are actively transported out of the cells lining the proximal convoluted tubule and into the blood;

2. Due to the concentration gradient, Na+ ions diffuse from the lumen of the proximal convoluted tubule into the cells that line it, by carrier proteins that also carry glucose;

3. Glucose then diffuses from the proximal convoluted tubule epithelium into the blood;

4. Mitochondria in the walls of the cells provide energy to actively transport Na+ ions out of the ascending limb (impermeable) of the loop of Henle;

5. This creates a low water potential in the interstitial region so water enters by osmosis and is then reabsorbed into the blood;

6. At the base of the ascending limb, Na+ ions diffuse out of the filtrate so its water potential increases;

7. The filtrate moves into the distal convoluted tubule and collecting duct, where there is a very low water potential, so water leaves the filtrate;

8. What filtrate remains goes on to form the urine;

Describe how the body responds to a fall in blood water potential (6 marks).

1. Osmoreceptors in the hypothalamus detect the decrease in water potential;

2. Osmoreceptors shrink, causing the hypothalamus to produce ADH;

3. Causes more aquaporins to be inserted into the walls of the collecting duct/distal convoluted tubule;

4. This causes the permeability of membrane / cells (to water) is increased;

5. More water absorbed from / leaves distal tubule / collecting

duct;

6. Smaller volume of urine;

7. Urine becomes more concentrated;

Define gene (1 mark).

1. A base sequence of DNA that codes for the amino acid sequence of a polypeptide

Define allele (1 mark).

1. An alternative form of a gene.

Define locus (1 mark).

1. Position of a gene on a molecule of DNA

Define autosomal linkage (1 mark).

1. Two or more genes carried on the same autosome (non-sex chromosome)

Define epistasis (1 mark).

1. When the allele of one gene affects or masks the expression of another in the phenotype

Define gene pool (1 mark).

1. Total number of all the alleles of all the genes of all the individuals within a particular population at a given time

Define allelic frequency (1 mark).

1. The number of times an allele occurs within the gene pool

The flowering time of two species are different. Suggest how these two species arose by sympatric speciation (5 marks).

1. Occurs in the same habitat / environment / population;

(accept not geographically isolated)

2. Mutation/s cause different flowering times;

3. Reproductive separation / isolation

OR

No gene flow

OR

Gene pools remain separate;

4. Different allele/s passed on / selected

OR

Change in frequency of allele/s

5. Disruptive (natural) selection;

6. Eventually different species cannot (inter)breed to produce fertile offspring;

Define ecosystem (1 mark).

1. A community (biotic) and the abiotic components of its environment;

Define population (1 mark).

1. A group of organisms of the same species occupying a particular space at a particular time and can potentially interbreed;

Define community (1 mark).

1. All the populations of different species living and interacting in a particular place at the same time;

Define habitat (1 mark).

1. A habitat is the place where an organism normally lives and is characterised by physical conditions and the other organisms present;

Define niche (1 mark).

1. Describes how an organism fits into its environment - it describes what a species is like, where it occurs, how it behaves, its interactions with other species and how it responds to its environment;

The mark-release-recapture method can be used to estimate the size of a fish population. Explain how (4 marks).

1. Capture/collect sample, mark and release;

2. Ensure marking is not harmful (to fish)

OR

Ensure marking does not affect survival (of fish);

(accept examples e.g., marking should not be toxic)

3. Allow (time for) fish to (randomly) distribute before collecting a second sample;

4. (Population =) number in first sample × number in second sample divided by number of marked fish in second sample/number recaptured;

Succession occurs in natural ecosystems. Describe and explain how succession occurs (4 marks).

1. (Colonisation by) pioneer species;

2. Pioneers/species/organisms change the environment/habitat/conditions/factors;

(accept example of change e.g. forms soil/organic matter/nutrients)

3. (Environment becomes) less hostile for other/new species

OR

(Environment becomes) more suitable for other/new species

OR

(Environment becomes) less suitable for previous species;

(accept previous species out-competed)

4. Change/increase in diversity/biodiversity;

5. (To) climax community;

Define conservation (1 mark).

1. Management of the Earth's natural resources by humans in such a way that maximum use of them can be made in the future;

Explain how a single base substitution causes a change in the structure of a polypeptide (3 marks)

1. Change in (sequence of) amino acid(s)/primary structure;

2. Change in hydrogen/ionic/disulfide bonds;

3. Alters tertiary/30 structure;

Describe the different type of stem cells (4 marks).

1. Totipotent - can differentiate into any cell;

2. Pluripotent - can differentiate into almost any cells;

3. Multipotent - can differentiate into particular cells e.g. blood cells;

4. Unipotent - can only differentiate into one type of cell;

Describe and explain how oestrogen can initiate transcription (3 marks).

1. Oestrogen binds to the receptor site on the transcriptional factor;

2. (Binding) causes the transcriptional factor to change shape which makes it complementary;

3. (This makes it able to) bind to the DNA to initiate transcription;

Describe how RNAi regulates translation (4 marks).

1. An enzyme cuts the mRNA into siRNA;

2. One strand of the siRNA then combines with another enzyme;

3. This siRNA-enzyme complex will bind via complementary base pairing to another mRNA molecule;

4. Once bound, the enzyme will cut up the mRNA so it cannot be translated;

Define epigenetics (1 mark).

1. Environmental factors that can cause heritable changes in gene function without changing the base sequence of DNA

Define epigenome (1 mark).

1. Layer of chemical tags that cover DNA and histones

Explain how methylation inhibits the transcription of genes (2 marks).

1. Prevents the binding of transcriptional factors to the DNA;

2. Attracts proteins that condense the DNA-histone complex making the DNA inaccessible to transcription factors

Explain how oncogenes can cause cancer (3 marks).

1. Mutation of proto-oncogenes, which stimulate a cell to divide when growth factors attach to a protein receptor on its cell surface membrane;

2. This receptor protein can be permanently activated, so that cell division is switched on even with the absence of growth factors;

3. The oncogene could code for a growth factor that is then produced in excessive amounts, stimulating excessive cell division;

Describe how alterations to tumour suppressor genes can lead to the development of tumours (3 marks).

1. (Increased) methylation (of tumour suppressor genes);

(accept abnormal methylation or hypermethylation)

2. Mutation (in tumour suppressor genes);

3. Tumour suppressor genes are not transcribed/expressed

OR

Amino acid sequence/primary structure altered;

4. (Results in) rapid/uncontrollable cell division;

(accept cell division cannot be regulated)

(ignore growth)

Explain how decreased acetylation could lead to cancer (3 marks).

1. Acetyl groups (could be) removed from associated histone proteins on DNA;

2. Histones become positive and are attracted more to the phosphate group on DNA;

3. This makes the DNA and histones more strongly associated and harder for the transcription factors to bind;

Describe how reverse transcriptase converts mRNA to cDNA (3 marks).

1. A cell that naturally produces the protein of interest and lots of mRNA that codes for it is selected;

2. Reverse transcriptase joins the DNA nucleotides with complementary bases to the mRNA sequence;

3. Single stranded DNA is made (cDNA);

4. To make it double stranded, DNA polymerase is used;

Describe how restriction endonucleases are used to cut up DNA containing desired gene (2 marks).

1. Active site is complementary to a specific DNA base sequence (recognition sequences) where it cuts the DNA;

2. To create sticky ends and exposed DNA bases;

Describe how the gene machine is used to create a desired gene (3 marks).

1. Examine the protein of interest to identify the amino acid sequence in order to find the mRNA and DNA sequence;

2. The DNA sequence is entered into a computer, which checks for biosafety and biosecurity to ensure the DNA being created is safe and ethical;

3. The computer creates small sections of overlapping single strands of nucleotides that make up the gene (oligonucleotides);

4. The oligonucleotides can then be joined to created the DNA for the entire gene;

Describe how enzymes could be used to insert the GH gene into a plasmid (2 marks).

1. Cut the plasmid with a restriction endonuclease;

2. (So that) both have complementary / sticky ends;

3. (Mix together) and add ligase to join the complementary / sticky ends;

Describe and explain how the polymerase chain reaction (PCR) is used to amplify a DNA fragment (4 marks)

1. (Requires DNA fragment) DNA polymerase, (DNA) nucleotides and primers;

2. Heat to 95 °C to break hydrogen bonds (and separate strands);

(accept temperature in range 90 to 95 °C)

3. Reduce temperature so primers bind to DNA/strands;

(accept temperature in range 40 to 65 °C)

4. Increase temperature, DNA polymerase joins nucleotides (and repeat method);

(accept Taq polymerase for DNA polymerase)

(accept temperature in range 70 to 75 °C)

Explain how DNA probes work (3 marks).

1. Probes are single stranded / have a specific base sequence;

2. Complementary base sequence on (specific) spacer

OR

Complementary/specific to (particular) spacer;

3. (In white squares probe) binds (to single-

stranded spacer) and glows/produces light/fluoresce;

(accept converse for dark squares)

The scientists used a radioactively labelled DNA probe to show that the cells of tobacco plant leaves contained the SUT1 gene.

Describe how they would do this (4 marks).

1. Extract DNA and add restriction endonucleases/restriction enzymes;

2. Separate fragments using electrophoresis;

3. (Treat DNA to) form single strands

OR

(Treat DNA to) expose bases;

4. The probe will bind to/hybridise/base pair with the SUT1/gene;

5. Use autoradiography (to show the bound probe);

(accept use photographic or X ray film (to show the bound probe))