AQA A-Level Biology long answer Qs

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 marks) [Unit 1, June 16]

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;

Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein.

Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. (5 marks) [Unit 1, June 16]

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;

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) [Unit 2, June 16]

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;

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) [Unit 4, June 16]

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;

Crops use light energy to produce photosynthetic products.

Describe how crop plants use light energy during the light-dependent reaction. (5 marks) [Unit 4, June 16]

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;

Describe the advantages and disadvantages of using chemical pesticides to control pests of crops. (5 marks) [Unit 4, June 16]

(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;

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) [Unit 4, June 16]

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;

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells. (5 marks) [Unit 1, June 2015]

Any FIVE from:

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;

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure. (5 marks) [Unit 1, June 2015]

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. Imagine not in 3D / only 2D images produced;

The events that take place during interphase and mitosis lead to the production of two genetically identical cells. Explain how. (4 marks) [Unit 2, June 2015]

1. DNA replicated;

2. (Involving) specific/accurate/complemetary base-pairing;

3. (Ref to) two identical/sister CHROMATIDS;

4. Each chromatid/ moves/is separated to(opposite) poles/ends of cell;

A mutation can lead to the production of a non-functional enzyme. Explain how. (6 marks) [Unit 2, June 2015]

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 struture/shape;

5. Change in ACTIVE SITE;

6. Substrate not complementary/cannot bind (to enzyme/active site) / no enzyme-substrate complexes form;

On islands in the Caribbean, there are almost 150 species of lizards belonging to the genus /Anolis/. Scientists believe that these species evolved from two species found on mainland USA. Explain how the Caribbean species could have evolved. (6 marks) [Unit 4, June 15]

1. GEOGRAPHIC(AL) isolation;

2. Separate gene pools / no interbreeding/gene flow (between populations);

3. Variation due to mutation;

4. Different selection pressures / different abiotic/biotic conditions/environments/habitats;

5. Different(ial) reproductive success / selected organisms (survive and) reproduce;

6. Leads to change/increase in ALLELE frequency;

/Anolis sagrei/ is a species of lizard that is found on some of the smallest Caribbean islands. Describe how you could use the mark-release-recapture method to estimate the number of /Anolis sagrei/ on one of these islands. (4 marks) [Unit 4, June 15]

1. Capture/collect sample, mark AND release;

2. Method of marking does not harm lizard/make it more visible to predators;

3. Leave sufficient time for lizards to (randomly) distribute (on island) before collecting a second sample;

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

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) [Unit, June 15]

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;

Describe the differences between active and passive immunity. (Spec 3.2, Cells)

1. Active involves memory cells, passive does not;

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

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

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. (Spec 3.2, Cells)

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);

[Co-transport subsumed into mark scheme for active transport and facilitated diffusion]

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;

Compare the structure of a prokaryotic cell with a eukaryotic cell. (Spec 3.2, Cells)

1. Pro(karyotes) has no membrane bound organells (motochondria, Gogli/ER etc.);

2. Pro do not have DNA inside a nucles/DNA floats free in the cytoplasm/DNA is in a loop rather than linear;

3. Pro has smaller ribosomes (70S, not 80S);

4. Pro has a capsule/flagellum/peptidoglycan cell wall/plasmids;

When a vaccine is given to a person, it leads to the production of antibodies against a disease-causing organism. Describe how. How are they then protected in the long term? (Spec 3.2, Cells)

1. Vaccine contains antigen from pathogen (dead/weakened);

2. Macrophage presents antigen on its surface;

3. T cell with complementary receptor protein binds to antigen;

4. T cell stimulates B cell;

5. (With) complementary antibody on its surface;

6. B cell secretes large amounts of anitbody;

7. B cell divides to form clone all secreting / producing same antibody;

8. Memory cells made;

9. On second exposure memory cells produce antibodies / become active / recognise pathogens;

10. Memory cells repsond;

11. Rapidly produce antibodies / produces more anitbodies;

12. Antibodies destroy pathogens;

Describe the appearance and behaviour of chromosomes during mitosis and explain how this results in the production of two genetically identical cells. (Spec 3.2, Cells)

1. Chromosomes shorten / thicken / supercoiling; during prophase;

2. Chromosomes appear as two identical chromatids (sister chromatids) joined by centromere / copies (due to replication);

3. Chromosomes /chromatids move to equator / middle of the spindle / cell (line up on equator) (metaphase)

4. Attach to individual spindle fibres by their centromere;

5. Spindle fibres contract / centromeres divide / repel;

6. (sister) Chromatids / chromosomes (separate)/anaphase / move to opposite poles / ens of the spindle;

7. Each pole / end receives all genetic information / identical copies of each chromosomes;

8. Nuclear envelope forms around each group of chromosomes / chromatids / at each pole; Each chromatid uncoils and unwinds;

How does phagocytosis destroy pathogens? (Spec 3.2, Cells)

1. Phagocyte attracted by a substance / recognises (foregin) antigen;

2. (Pathogen) engulfed / ingested;

3. Enclosed in vacuole / vesicle / phagosome;

4. (Vacuole) fused / joins with lysosome;

5. Lysosome contains enzymes;

6. Pathogen digested / molecules hydrolysed;

Describe and explain how the structure of DNA results in accurate replication. (Spec 3.1, Biological Molecules)

1. Two strands therefore semi-conservative replication (possible);

2. Base pairing / hydrogen bonds hold strands together;

3. Hydrogen bonds weak / easily broken, allows strands to separate;

4. Base (sequence) (exposed so) acts as template / can be copied;

5. A with T, C with G / complementary copy;

6. DNA now made of one parent strand, one new strand;

Describe the biochemical test you would use to identify each of the following:

- Reducing sugars

- Non-reducing sugars

- Starch

- Proteins

- Lipids (Spec 3.1, Biological Molecules)

Reducing sugar

1. Heat with Benedicts reagent (blue);

2. Brick red (orange/green/yellow) precipitate formed as positive result;

Non-reducing sugar

3. Boil with acid (break glycosidic bonds);

4. Heat with Benedicts reagent (blue);

5. Brick red (orange/green/yellow) precipitate formed as positive result;

Starch

6. Add iodine / potassium iodide solution (orange/brown);

7. Blue/black colour as positive result;

Proteins

8. Add Biuret reagent (blue);

9. Lilac/purple colour change as positive result;

Lipids

10. Mix / crush / grind; with ethanol / alcohol;

11. Then add water;

12. Forms emulsion / goes white / cloudy as positive result;

Explain how each of the following properties of water are of benefit to living organisms:

- When water freezes to form ice it becomes less dense

- A large amount of heat energy is required to evaporate water

- Water has a high specific heat capacity (Spec 3.1, Biological Molecules)

1. So ice floats so lakes & ponds remain liquid to support life (otherwise they would freeze solid without ice acting as insulator);

2. This is called "latent heat of vapourisation" e.g. evaporation of sweat is a very effective way of cooling animals;

3. Enables it to exist as a liquid at Earth temperatures which is essential for life (as the large no. of H bonds means it is not easily vapourised). AND enables it to act as a buffer against sudden temperature variations so aquatic environments are stable (and also enables terrestrial organisms to survive temp. fluctuations);

Explain how the structure of DNA is related to its functions. (Spec 3.1, Biological Molecules)

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. Heliz / coiled so compact;

4. Base sequence allows information to be stored / base sequence codes for amino acids / proteins;

5. Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and C-G so accurate replication / identical copies can be made;

6. (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;

Describe competitive and non-competitive inhibition of an enzyme. (Spec 3.1, Biological Molecules)

1. Inhibitors reduce binding of enzyme to substrate / prevent formation of E-S complexes;

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;

Describe the structure of proteins. (Spec 3.1, Biological Molecules)

1. Polymer of amino acids;

2. Joined by peptide bonds;

3. Formed by condensation;

4. Primary structure is order of amino acids;

5. Secondary structure is folding of polypeptide chain due to hydrogen bonding; Accept alpha helix/ beta-pleated sheet;

6. Tertiary structure is 3-D folding due to hydrogen bonding AND ionic / disulfide bonds;

7. Quaternary sturcure is two or more polypeptide chains;

Describe how proteins are digested in the human gut. (Spec 3.1, Biological Molecules)

1. Hydrolysis of peptide bonds;

2. Endopeptidases break polypeptides into smaller peptide chains;

3. Exopeptidases remove terminal amino acids;

4. Dipeptidases hydrolyse / break down dipeptides into amino acids;

Describe the processes invovled in the absorption of the products of starch digestion. (Spec 3.1, Biological Molecules)

1. Glucose moves in with sodium (into epithelial cell);

2. Via (carrier / channel) proetin / symport;

3. Sodium removed (from epithelial cell) by active transport / sodium-potassium pump; into blood;

4. Maintaing low concentration of sodium (in epithelial cell) / maintaining sodoum concentration gradient (between lumen and epithelial cell);

5. Glucose moves into blood;

6. By (facilitated) diffusion;

Describe what courtship behaviour is and the importance of it. (Spec 3.4, Genetic information, variation and relationships between organisms)

1. Courtship behaviour is species specific;

2. Only members of the same species will do and respond to that courtship behaviour;

3. This prevents interbreeding;

4. Means reproduction is more successful (fertile offspring is produced);

5. Because it is so specific it gives us a way to classify organisms - more closely related, more similar their courtship;

6. Courtship allows identification of a mate that is capable of breeding;

7. Can result in the formation of a pair bond;

8. Correctly synchronises mating so it takes place when there is a maximum probability that the sperm and egg will meet;

Why would one area (e.g. Wild flower meadow) have a higher species diversity than another (e.g. potato field)? (Spec 3.4, Genetic information, variation and relationships between organisms)

1. There would be more plant species;

2. Hence more habitats;

3. More niches;

4. More food sources;

5. The area is able to support more biomass;

Describe how altered DNA may lead to cancer. (Spec 3.4, Genetic information, variation and relationships between organisms)

1. (DNA altered by) mutation;

2. (mutation) changes base sequence;

3. Of gene controlling cell growth / oncogene / that monitors cell division;

4. Of tumour suppressor gene;

5. Change protein structure / non-functional protein / protein not formed;

6. (Tumour suppressor genes) produce proteins that inhibit cell division;

7. Mitosis;

8. Uncontrolled / rapid / abnormal (cell division);

9. Malignant tumour;

Describe aseptic techniques and justify the chosen method. (Spec 3.4, Genetic information, variation and relationships between organisms)

1. Keep lid on Petri dish / open lid of Petri dish as little as possible;

2. To prevent unwanted bacteria contaminating the dish or avoid bacteria getting out;

3. Wear gloves / mask and wash hands;

4. Or prevent spread of bacteria outside the lab;

5. Use sterile pipette or Flame the loop or the neck of the container of the culture;

6. To maintain a pure culture of bacteria;

7. Use Bunsen Burner to maintain upward movement of air;

8. Helps to prevent contamination of culture when lid is removed;

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell. (Spec 3.4, Genetic information, variation and relationships between organisms)

1. Helicase;

2. Breaks hydrogen bonds;

3. Only one DNA strands acts as template;

4. RNA nucleotides attracted to exposed base;

5. (Attraction) according to base pairing rule;

6. RNA polymerase joins (RNA) nucleotides together;

7. Pre-mRNA spliced to remove introns;

How do the structures of mRNA and tRNA compare? (Spec 3.4, Genetic information, variation and relationships between organisms)

1. Has more nucleotides than tRNA;

2. mRNA is a straight molecule but tRNA is a folded molecule/clover-leaf shaped molecule;

3. mRNA contains no paired bases/hydrogen bonds;

4. But tRNA has some paired bases/hydroen bonds;

Meiosis results in genetic variation in the gametes which leads to variation in the offspring formed by sexual reproduction. Describe how meiosis causes this variation and explain the advantage of variation to the species. (Spec 3.4, Genetic information, variation and relationships between organisms)

1. Crossing-over involves exchange of alleles between homologous chromosomes;

2. Independent segregation;

3. Maternal and paternal chromosomes are re-shuffled in any combination/line up in meiosis randomly with their homologous partner; Produces gametes which have a random combination of maternal and paternal alleles;

+ ANY 3 FROM:

4. Different adaptions / some better adapted;

5. Some survive / example described;

6. To reproduce;

7. Pass on gene / allele;

8. Allows for coping in a changing environment/ different environment / example described;

Describe what happens to chromosomes in meiosis. (Spec 3.4, Genetic information, variation and relationships between organisms)

1. Chromosomes shorten / thicken / condense;

2. Chromosomes associate in homologous / (described) pairs / formation of bivalents;

3. Crossing-over / chiasma formation;

4. Join to spindle (fibres) / moved by spindle;

5. (At) equator / middle of cell;

6. (join via) centromere / kinetochore;

7. (Homologous) chromosomes move to opposite poles / chromosomes separate / move apart; (ALLOW 'are pulled apart')

8. (Pairs of) chromatids separated in 2nd division;

Describe the mass flow hypothesis for the mechanism of translocation in plants. (Spec 3.3, Organisms and exchange in their environment)

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

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;

A fish uses its gills to absorb oxygen from water. Explain how the gills of a fish are adapted for efficient gas exchange. (Spec 3.3, Organisms and exchange in their environment)

1. Large surface area provided by lamellae / filaments increases diffusion / makes diffusion efficient;

2. Thin epithelium / distance between water and blood;

3. Water and blood flow in opposite directions / counter current;

4. Maintains concentration gradient (along gill) / equilibrium not reached / as water always next to blood with lower concentrations of oxygen;

5. Circulation replaces blood saturated with oxygen;

6. Ventilation replaces water (as oxygen removed);

Explain how the structures of the walls of arteries and arterioles are related to their functions. (Spec 3.3, Organisms and exchange in their environment)

Elastic tissue

1. Elastic tissue stretches under pressure / when heart beats then recoils / springs back;

2. Evens out pressure / flow;

Muscle

3. Muscle contracts to reduce diameter of lumen / vasoconstriction / constricts vessel;

4. Changes flow / pressure;

Epithelium

5. Epithelium smooth;

6. Reduces friction / blood clots / less resistance;

Explain how tissue fluid is formed and how it may be returned to the circulatory system. (Spec 3.3, Organisms and exchange in their environment)

1. (hydrostatic) pressure of blood high at arterial end;

2. Fluid / water / soluble molecules pass out (reject plasma);

3. Proteins / large molecules remain;

4. This lowers the water potential / water potential becomes more negative;

5. Water moves back into venous end of capillary (reject tissue fluid) by osmosis / diffusion;

6. Lymph system collects any excess tissue fluid which returns to blood / circulatory system / link with veina cava / returns tissue fluid to vein;

Describe how haemoglobin loads and unloads oxygen in the body.

EXTRA: The oxygen dissociation curve of the fetus is to the left of that for its mother. Explain the advantage of this for the foetus. (Spec 3.3, Organisms and exchange in their environment)

1. Loading / uptake / association of oxygen at high p.p. of O2;

2. In lungs (haemoglobin) is (almost) fully saturated / in lungs haemoglobin has a high affinity for oxygen;

3. Unloads / releases / dissociates oxygen at low p.p. of O2;

4. Unloading linked to higher carbon dioxide concentration; Bohr Shift;

1. Higher affinity / loads MORE oxygen at low / same / high PARTIAL PRESSURE / P.P. O2;

2. (Therefore) oxygen moves from mother / to foetus;

Describe and explain ways in which the structure of a capillary adapts it for the exchange of substances between blood and the surrounding tissue. (Spec 3.3, Organisms and exchange in their environment)

1. Permeable capillary wall / membrane;

2. Single cell thick / thin walls, reduces diffusion distance;

3. Flattened (endothelial) cells, reduces diffusion distance;

4. Fenestrations, allows large molecules through;

5. Small diameter / narrow, gives a large surface area to volume / short diffusion distance;

6. Narrow lumen, reduces flow rate giving more time for diffusion;

7. Red blood cells in contact with wall / pass singly, gives short diffusion distance / more time for diffusion;

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. (Spec 3.3, Organisms and exchange in their environment)

1. Many alveoli / alveoli WALLS folded provide a large surface area;

2. Many capillaries provide a large surface area;

3. (So) fast DIFFUSION;

4. Alveoli or capillary walls / epithelium / lining are thin / short distance between alveoli and blood;

5. Flattened / squamos epithelium;

6. (So) short DIFFUSION distance / pathway;

7. (So) fast DIFFUSION;

8. Ventilation / circulation;

9. Maintains a diffusion / concentration gradient;

10. (So) fast DIFFUSION;

Explain how water enters the xylem from the endodermis in the root and is then transported to the leaves. (Spec 3.3, Organisms and exchange in their environment)

In the root

1. Casparian strip blocks apoplast pathway / only allows symplast pathway;

2. Active transport by ENDODERMIS;

3. (Of) ions / salts into xylem;

4. Lower water potential in xylem / water enters xylem by osmosis / down a water potential gradient;

Xylem to leaf

5. Evaporation / transpiration (from leaves creates) cohesion / tension / H-bonding between water molecules / negative pressure;

6. Adhesion / water molecules bind to xylem; (creates continuous) water column;