Paper 4 style practice paper: Structured questions (Extended) — mark scheme

Chloroplast; absorbs light energy for photosynthesis.

• chloroplast [1]
• absorbs light for photosynthesis [1]

Any two of: cell wall, cell membrane, cytoplasm, ribosomes.

• two structures named [1]
• [1]

Actual size = image size ÷ magnification = 12 ÷ 600 = 0.02 mm.

• 12 ÷ 600 seen [1]
• 0.02 mm with unit [1]

A tissue.

• group of similar cells doing one function [1]

Changed (independent variable): pH. Controlled: temperature and enzyme concentration (also: starch concentration, volume of solutions, time).

• pH is the independent variable [1]
• temperature controlled [1]
• enzyme concentration controlled [1]

Samples of the mixture are taken at intervals and mixed with iodine on a spotting tile; iodine turns blue-black while starch remains; the time until samples stop changing colour shows when all the starch has been digested — a shorter time means faster amylase activity.

• iodine tests samples at intervals [1]
• blue-black = starch present [1]
• time to no colour change measures rate [1]

Activity at pH 2 is much lower or zero. Amylase has an optimum pH near neutral; at pH 2 the ion concentrations interfere with the bonds holding the enzyme’s shape, so the active site changes shape (denatures) and the substrate no longer fits.

• activity lower/zero at pH 2 [1]
• optimum pH concept [1]
• active site shape changed [1]
• denatured / substrate no longer fits [1]

Water lost by transpiration from the leaves is replaced by water drawn up the xylem, so uptake approximately equals loss — though some water is retained in the tissues, so it is only an estimate.

• loss replaced by uptake through xylem [1]
• approximately / not exact because some water is retained [1]

Water uptake falls. Higher humidity raises the water potential of the air around the leaves, reducing the water-potential gradient between leaf and air, so evaporation from the mesophyll and loss through the stomata slow down.

• uptake decreases [1]
• water-potential gradient reduced [1]
• less evaporation/stomatal loss [1]

Any two of: temperature, air movement (wind), light intensity, the shoot used (surface area/species).

• two variables [1]
• [1]

Volume = 45 × 2.0 = 90 mm³; rate = 90 ÷ 5 = 18 mm³ per minute.

• 45 × 2.0 = 90 mm³ [1]
• divide by 5 minutes [1]
• 18 mm³ per minute [1]

The left ventricle pumps oxygenated blood at high pressure around the whole body; the right ventricle only pumps blood a short distance to the lungs at lower pressure, so it needs less muscle.

• left pumps to whole body at high pressure [1]
• right only to lungs / shorter distance [1]
• more muscle needed for systemic pressure [1]

Vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.

• right atrium → right ventricle [1]
• pulmonary artery → lungs → pulmonary vein [1]
• left atrium → left ventricle [1]
• aorta [1]

Platelets are cell fragments involved in clotting. When a vessel is damaged, platelets and damaged tissue trigger a cascade that converts soluble fibrinogen into insoluble fibrin; fibrin fibres form a mesh across the wound, trapping red cells and platelets; the mesh dries into a scab that prevents blood loss and blocks the entry of pathogens.

• platelets for clotting [1]
• fibrinogen converted to fibrin [1]
• fibrin mesh traps cells [1]
• dries to scab [1]
• prevents blood loss / pathogen entry [1]

Gametes B and b from each parent; offspring BB, Bb, Bb, bb.

• gametes identified [1]
• BB, Bb, Bb, bb [1]

Genotypes BB : Bb : bb = 1 : 2 : 1; phenotypes black : brown = 3 : 1.

• 1:2:1 [1]
• 3:1 [1]

The black parent was heterozygous (Bb). Brown offspring are bb, so they must have received b from both parents — including a b from the black parent, which therefore carried both B and b.

• black parent is Bb [1]
• brown offspring are bb [1]
• so black parent supplied b [1]

Females are XX and males XY; the egg always carries X, while sperm carry X or Y; fertilisation gives XX or XY in equal proportions — a 1:1 ratio.

• XX female, XY male [1]
• egg X, sperm X or Y [1]
• 1:1 [1]

Black is dominant, so both BB and Bb mice look black — the dominant phenotype hides the recessive allele; only genetic crosses (or testing) reveal the hidden allele.

• dominant allele masks recessive [1]
• BB and Bb share a phenotype [1]

Any two of: very large total surface area; walls one cell thick (short diffusion distance); good blood supply maintaining gradients; good ventilation.

• two features [1]
• [1]

Expired air has less oxygen (about 16% vs 21%), more carbon dioxide (about 4% vs 0.04%) and more water vapour. Limewater turns milky with expired air and stays clear with inspired air.

• less oxygen / more CO₂ [1]
• more water vapour [1]
• limewater milky with expired air [1]

During the sprint her muscles respired anaerobically, building up lactic acid and creating an oxygen debt; her heart rate stays fast, transporting lactic acid in the blood from the muscles to the liver; her breathing stays deep and fast, supplying the oxygen needed; the liver respires the lactic acid aerobically, removing it; once the debt is repaid, breathing and pulse return to resting rates.

• anaerobic respiration → lactic acid / oxygen debt [1]
• fast heart transports lactic acid to liver [1]
• deep fast breathing supplies oxygen [1]
• liver respires lactic acid aerobically [1]
• debt repaid → rates return to normal [1]

Population: a group of organisms of one species living in the same area at the same time. Community: all of the populations of different species in an ecosystem.

• population [1]
• community [1]

e.g. algae → water flea → small fish → heron. Algae: producer (level 1); water flea: primary consumer (level 2); small fish: secondary consumer (level 3); heron: tertiary consumer (level 4).

• chain begins with producer [1]
• arrows point eater-ward [1]
• four organisms [1]
• trophic levels named [1]

Fertiliser increases nitrate and other ions in the water; algae grow rapidly (algal bloom); the algae die and decomposers multiply; the decomposers respire aerobically, reducing dissolved oxygen; organisms needing dissolved oxygen, including fish, die.

• nutrients increase → algal bloom [1]
• algae die → decomposition rises [1]
• aerobic respiration by decomposers → oxygen falls [1]
• fish die from lack of dissolved oxygen [1]

Any two of: rapid reproduction rate; ability to make complex molecules; few ethical concerns; presence of plasmids.

• two reasons [1]
• [1]

Restriction enzymes cut the human insulin gene, leaving sticky ends; the same restriction enzymes cut the bacterial plasmid, forming complementary sticky ends; DNA ligase joins the human DNA into the plasmid, forming a recombinant plasmid.

• restriction enzymes cut gene — sticky ends [1]
• same enzyme cuts plasmid — complementary sticky ends [1]
• DNA ligase joins them [1]
• recombinant plasmid formed [1]

The bacteria are multiplied in fermenters; they express the human insulin gene, producing insulin that is harvested and purified.

• multiplied in fermenters [1]
• express the gene → insulin harvested [1]