Mark Scheme

Section A: 1. A; 2. B; 3. C; 4. D; 5. C; 6. C. (6 marks)

7 (a) Meiosis (1); four cells (1). (b) Any two: tail/flagellum to swim; many mitochondria for energy; enzymes in the head to penetrate the egg (2). (c) Resolution is the ability to distinguish two separate points (1); higher resolution shows finer detail of sub-cellular structures (1). (d) magnification = image ÷ real = 40 ÷ 0.02 = ×2000 (1 for method, 1 for answer). (8)

8 (a) Most alpha particles passed straight through → atom is mostly empty space (1); a few were deflected/bounced back → small dense positive nucleus (1); so mass and positive charge are concentrated in the nucleus (1). (b) (35×75 + 37×25)/100 = (2625 + 925)/100 = 35.5 (1 method, 1 answer). (c) They have the same number of electrons / same electron arrangement (1); chemical reactions depend on outer electrons (1). (7)

9 (a) Muscles need more energy/oxygen (1); to supply more oxygen and glucose and remove CO₂ (1); faster/deeper breathing and faster heart rate increase the supply (1). (b) Lactic acid (1); oxygen debt = the extra oxygen needed after exercise (1) to break down the lactic acid that built up (1). (c) stimulus → receptor (1) → sensory neurone (1) → relay neurone in CNS/spinal cord (1) → motor neurone → effector → response (1). (10)

10 (a) Contamination = radioactive atoms get onto/into an object (1); irradiation = exposure to radiation without becoming radioactive (1). (b) 15 h = 3 half-lives; 800 → 400 → 200 → 100 counts/min (1 for 3 half-lives, 1 for answer). (c) Any two: shielding (lead); greater distance; less time of exposure; protective clothing (2). (6)

11 (a) Correct Punnett square BB, Bb, Bb, bb (1); 3 dominant : 1 recessive (1); stated as a ratio 3:1 (1). (b) Mutation produces a new allele/characteristic (1); if it gives a survival advantage (1); those individuals survive and reproduce (1); passing the allele on, so it becomes more common over generations / natural selection (1). (c) Any two: fossils; antibiotic-resistant bacteria; (extinction) (2). (9)

12 (a) The Sun's short-wave radiation is absorbed by the Earth (1); the Earth re-emits longer-wavelength infrared (1); greenhouse gases absorb and re-radiate this back to the surface, warming it (1). (b) Any two: burning fossil fuels; deforestation; cattle farming; landfill (methane) (2). (c) The climate system is very complex / hard to model / many interacting variables (1). (6)

13 — Levels (6): Indicative: removing/adding a species changes the food available to others; predators of that species decline; prey of that species increase; knock-on effects through the web; interdependence. With a worked example (e.g. fewer rabbits → fewer foxes; more of the rabbits' food plant).

• L3 (5–6): Clear chain of effects through the web with a correct example.
• L2 (3–4): Some linked effects, example may be incomplete.
• L1 (1–2): Simple statement of one effect.

14 — Levels (6): Indicative: place pondweed in water with sodium hydrogencarbonate; vary distance of a lamp (light intensity); measure bubbles of oxygen per minute / volume of gas; control temperature (water bath/heat shield), CO₂ concentration, same piece of pondweed; repeat for a mean.

• L3 (5–6): A valid, ordered method with the measured variable, the changed variable and two controls.
• L2 (3–4): Method present but missing a control or the measured variable.
• L1 (1–2): Fragmented points.

Sample Answers with Examiner Commentary

Question 13 — Sample Answers

Grade 9 answer "In a food web the organisms are interdependent, so a change in one population affects others. For example, if a disease reduced the number of rabbits, the foxes that eat them would have less food, so the fox population would also fall. At the same time, the grass that the rabbits feed on would be eaten less, so the grass would increase. Other animals that also eat grass might then increase too. This shows how removing or reducing one species has knock-on effects throughout the whole community." Mark: 6/6. Examiner commentary: A clear chain of cause and effect in both directions (up and down the web), supported by a relevant, correct example and the key term "interdependent". Top band.

Grade 6 answer "If one animal dies out then the animal that eats it has less food so it dies too. For example if there were no rabbits the foxes would have nothing to eat." Mark: 3/6. Examiner commentary: A correct single chain (prey → predator) with an example, but it does not consider the effect on the species the changed population feeds on, or wider knock-on effects, which would be needed for the top band.

Grade 3 answer "All the animals need each other. If one dies the others are sad and the food chain breaks." Mark: 1/6. Examiner commentary: Shows a vague idea of interdependence but uses no scientific reasoning. "The food chain breaks" is too vague, and there is no worked example or chain of effects.

Question 8(a) — Sample Answers

Grade 9 answer "In the experiment, positively charged alpha particles were fired at thin gold foil. Most passed straight through, which showed the atom is mostly empty space. A small number were deflected through large angles or bounced straight back, which showed there must be a small, dense, positively charged nucleus at the centre that repelled them. This replaced the plum pudding model with the nuclear model." Mark: 3/3. Examiner commentary: All three observations are correctly linked to the conclusions about empty space and a dense positive nucleus.

Grade 6 answer "They fired alpha particles at gold foil. Most went through but some bounced back, which showed there was a nucleus in the middle." Mark: 2/3. Examiner commentary: Correct on "most went through" (empty space) and "some bounced back" (a nucleus), but does not state that the nucleus is positively charged and dense, which is the third marking point.