Biology Past Paper Long Answer Questions - AQA

May split questions into smaller parts or may generalise so that it is applicable to similar questions

June 2016 - AS Paper 2

Describe how the structure of starch and cellulose molecules are related to their functions [5 marks]

Points for starch (max. 3 out of 4)

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

June 2016 - AS Paper 2

Describe how the structure of starch and cellulose molecules are related to their functions [5 marks]

Points for cellulose (max. 3 out of 4)

5. Long, straight/unbranched chains of β glucose

6. Joined by hydrogen bonding

7. To form (micro/macro)fibrils

8. Provides rigidity/strength

June 2016 - AS Paper 2

Describe the processes involved in the transport of sugars in plant cells [5 marks]

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;

June 2017 - AS Paper 1

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

June 2017 - AS Paper 2
Describe how a student could use an eyepiece graticule to determine the mean… (diameter of stomata) [3 marks]

1. Measure using eyepiece graticule

2. Calibrate eyepiece graticule against stage micrometer / ruler / graph paper

3. Take a number of measurements (to calculate a mean)

June 2017 - AS Paper 2
Explain why the [name] antibody only damages these cells. [4 marks]

1. antibody has a (specific) tertiary structure

2. Has binding site / variable region that only binds to / complementary to one antigen

3. Antigen to this antibody (only) found on these nerve cells

4. So, antibody (only) binds to / forms antigen-antibody complex with these nerve cells (causing damage);

June 2017 - AS Paper 2

Describe the cohesion-tension theory of water transport in the xylem. [5 marks]

1. Water lost from leaf because of transpiration

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

June 2017 - AS Paper 2

Describe how mRNA is produced in a [plant] cell. [5 marks]

1. The DNA strands separate by breaking the H bonds OR H bonds broken between (complementary) (DNA) bases

2. (Only) one of the strands/template strand is used (to make mRNA/is transcribed)

3. (Complementary) base pairing so A→U, T→A, C→G, G→C

4. (RNA) nucleotides joined by RNA polymerase

5. pre-mRNA formed

6. Splicing / introns removed to form mRNA

June 2018 - AS Paper 1

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA. [2 marks]

1. Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip

2. Two strands, so both can act as templates

3. Complementary base pairing allows accurate replication

June 2018 - AS Paper 1

Give two structural differences between a molecule of messenger RNA (mRNA) and a molecule of transfer RNA (tRNA). [2 marks]

1. mRNA does not have hydrogen bonds / base pairing, tRNA does OR mRNA is linear / straight chain, tRNA is cloverleaf

2. mRNA does not have an amino acid binding site, tRNA does

3. mRNA has more nucleotides

4. (Different) mRNAs have different lengths, all tRNAs are similar / same length

5. mRNA has codons, tRNA has an anticodon

June 2018 - AS Paper 1

Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.

Do not include descriptions of transcription and splicing in your answer. [5 marks]

1. mRNA associates with a ribosome / ribosome attaches to mRNA

2. Ribosome moves to / finds the start codon / AUG

3. tRNA brings / carries (appropriate / specific) amino acid

4. Anticodon (on tRNA complementary) to codon (on mRNA)

5. Ribosome moves along to next codon; OR Ribosome ‘fits’ around two codons / can fit two tRNAs

6. (Process repeated and) amino acids join by peptide bonds / condensation reaction (to form polypeptide); OR (Process repeated and) amino acids joined using (energy from) ATP (to form polypeptide);

June 2018 - AS Paper 2
Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. [5 marks]

‘Compare’ part

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

June 2018 - AS Paper 2
Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. [5 marks]

‘Contrast’ part

4. Eukaryotic DNA is longer

5. Eukaryotic DNA contain introns, prokaryotic DNA does not

6. Eukaryotic DNA is linear, prokaryotic DNA is circular

7. Eukaryotic DNA is associated with / bound to protein / histones, prokaryotic DNA is not

June 2018 - AS Paper 2

Haemoglobins are chemically similar molecules found box 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]

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 vice versa)

5. Distantly related species have earlier common ancestor (or vice versa)

June 2019 - AS Paper 1

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

4. (Substrate/solution added) and colour changes

June 2019 - AS Paper 1

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

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

June 2019 - AS Paper 1

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

June 2019 - AS Paper 2

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

June 2019 - AS Paper 2
Describe the roles of iron ions, sodium ions, and phosphate ions in cells. [5 marks]

Answer for iron

1. Haemoglobin binds/associates with oxygen OR Haemoglobin transports/loads oxygen

June 2019 - AS Paper 2
Describe the roles of iron ions, sodium ions, and phosphate ions in cells. [5 marks]

Answer for sodium

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

June 2019 - AS Paper 2
Describe the roles of iron ions, sodium ions, and phosphate ions in cells. [5 marks]

Answer for phosphate

6. Affects osmosis/water potential

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

8. Used in/to produce ATP

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

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

June 2019 - AS Paper 2

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

1. Phospholipid (bilayer) allows movement/diffusion of nonpolar/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

8. Cholesterol affects fluidity/rigidity/permeability

June 2017 - A-level Paper 1

Describe how phagocytosis of a virus leads to presentation of its antigens. [3 marks]

1. Phagosome/vesicle fuses with lysosome

2. (Virus) destroyed by lysozymes/hydrolytic enzymes

3. Peptides/antigen (from virus) are displayed on the cell membrane

June 2017 - A-level Paper 1

Describe how presentation of a virus antigen leads to the secretion of an antibody against this virus antigen. [3 marks]

1. Helper T cell/TH cell binds to the antigen (on the antigen-presenting cell/phagocyte)

2. This helper T/TH cell stimulates a specific B cell

3. B cell clones OR B cell divides by mitosis

4. (Forms) plasma cells that release antibodies

June 2017 - A-level Paper 1
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

June 2017 - A-level Paper 1

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 use electrons and optical use 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

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

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

6. TEM requires thinner specimens

7. TEM requires a more complex/time consuming preparation

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

June 2017 - A-level Paper 2

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)

Describe how the mark-release-recapture method could be used to determine the population of [mosquitoes] at the start of the investigation. [3 marks]

1. Capture/collect/sample, mark and release

2. Leave time for mosquitoes/Aedes to disperse before second sampling/collection

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

June 2017 - A-level Paper 2

Info - 2 species have different flowering times

Using this information, suggest how these two species of palm tree arose by sympatric speciation. [5 marks]

1. Occurs in the same habitat/environment/population

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

June 2018 - A-level Paper 1

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)

June 2018 - A-level Paper 1

Compare and contrast the structure and properties of triglycerides and phospholipids. [5 marks]

1. Both contain ester bonds (between glycerol and fatty acid)

2. Both contain glycerol

3. Fatty acids on both may be saturated or unsaturated

4. Both are insoluble in water

5. Both contain C, H and O but phospholipids also contain P

6. Triglyceride has three fatty acids and phospholipid has two fatty acids plus phosphate group

7. Triglycerides are hydrophobic/non-polar and phospholipids have hydrophilic and hydrophobic region

8. Phospholipids form monolayer (on surface)/micelle/bilayer (in water) but triglycerides don’t

June 2019 - A-level Paper 1
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

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

June 2019 - A-level Paper 1
Describe the structure of the human immunodeficiency virus (HIV). [4 marks]

1. RNA (as genetic material)

2. Reverse transcriptase

3. (Protein) capsomeres/capsid

4. (Phospho)lipid (viral) envelope OR Envelope made of membrane

5. Attachment proteins

June 2019 - A-level Paper 1
The scientists determined the percentage of heart cells undergoing DNA replication by using a chemical called BrdU. Cells use BrdU instead of nucleotides containing thymine during DNA replication.
Describe how BrdU would be incorporated into new DNA during semi-conservative replication. [5 marks]

1. DNA helicase

2. Breaks hydrogen bonds (between 2 DNA strands)

3. BrdU complementary to adenine (on template strand) OR BrdU forms hydrogen bonds with adenine (on template strand)

4. DNA polymerase joins (adjacent) nucleotides (to incorporate BrdU into the new DNA strand)

5. Phosphodiester bonds form (between nucleotides)

June 2019 - A-level Paper 1
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 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 tension supporting (small) organisms

June 2019 - A-level Paper 1
Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample. [5 marks]
Lipid Part

1. Add ethanol/alcohol then add water and shake/mix OR Add ethanol/alcohol and shake/mix then pour into/add water

2. White/milky emulsion OR emulsion test turns white/milky

June 2019 - A-level Paper 1
Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample. [5 marks]
Non-reducing Sugar Part

1. Do Benedict’s test and stays blue/negative

2. Boil with acid then neutralise with alkali

3. Heat with Benedict’s and becomes red/orange (precipitate)

June 2019 - A-level Paper 1
Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample. [5 marks]
Amylase Part

1. Add biuret (reagent) and becomes purple/violet/mauve/lilac

2. Add starch, (leave for a time), test for reducing sugar/absence of starch

June 2019 - A-level Paper 1
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]

1. A condensation reaction joins monomers together and forms a (chemical) bond and releases water

2. A hydrolysis reaction breaks a (chemical) bond between monomers and uses water

3. e.g.: amino acid and polypeptide

4. e.g.: nucleotide and polynucleotide

5. e.g.: peptide bond

June 2019 - A-level Paper 2
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

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

4. Change/increase in diversity/biodiversity

5. (To) climax community

June 2019 - A-level Paper 2
Describe the role of glucagon in gluconeogenesis. Do not include in your answer details on the second messenger model of glucagon action. [2 marks]

1. (Attaches to receptors on target cells and) activates/stimulates enzymes

2. Glycerol/amino acids/fatty acids into glucose

June 2019 - A-level Paper 2
Describe how you could estimate the size of a population of sundews in a small marsh. [5 marks]

1. Use a grid OR Divide area into squares/sections

2. Method of obtaining random coordinates/numbers e.g. calculator/computer/random numbers table/generator

3. Count number/frequency in a quadrat/section

4. Large sample and calculate mean/average number (per quadrat/section)

5. Valid method of calculating total number of sundews, e.g. mean number of plants per quadrat/section/m2 multiplied by number of quadrats/sections/m² in marsh

June 2018 - A-level Paper 2
Describe how alterations to tumour suppressor genes can lead to the development of tumours. [3 marks]

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

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

June 2018 - A-level Paper 2

Describe how you would determine the mean percentage cover for beach grass on a sand dune. [3 marks]

1. Method of randomly determining position (of quadrats) e.g. random numbers table/generator

2. Large number/sample of quadrats

3. Divide total percentage by number of quadrats/samples/readings

June 2019 - A-level Paper 2

Dopamine is a neurotransmitter released in some synapses in the brain. The transmission of dopamine is similar to that of acetylcholine.

Dopamine stimulates the production of nerve impulses in postsynaptic neurones.

Describe how.

Do not include in your answer the events leading to the release of dopamine and the events following production of nerve impulses at postsynaptic neurones. [3 marks]

1. (Dopamine) diffuses across (synapse)

2. Attaches to receptors on postsynaptic membrane

3. Stimulates entry of sodium ions and depolarisation/action potential

June 2018 - A-level Paper 3
Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind. Explain why. [2 marks]

1. Binding of first oxygen changes tertiary / quaternary (structure) of haemoglobin

2. Creates / leads to / uncovers second / another binding site OR Uncovers another iron / Fe / haem group to bind to