Metabolic Processes — Making Food, Burning It, and Life in the Dark

Total: 136 marks — Investigation 1: 28 · Investigation 2: 25 · Investigation 3: 34 · Investigation 4: 23 · Investigation 5: 26.

Investigation 1 — the starch tests

• Q1 (3): Leaves store starch from earlier photosynthesis; it must be used up first (1), or starch found later might have been made before the experiment (1). Leaf (a) shows the de-starching worked — there was no starch at the start, so any starch found later was made during the experiment (1).
• Q2 (2): Boiling water kills the cells, stops all reactions and softens the leaf so the spirits and iodine can get in (1). Chlorophyll's green colour would hide the iodine colour change, so it is removed to see the result clearly (1).
• Q3 (2): Methylated spirits are highly flammable and boil at a lower temperature than water (1); the hot water warms them safely without any flame or hot element that could ignite the vapour (1).
• Q4 (6): (b) Starch was made in the light — the leaf photosynthesised (1). (c) Only the uncovered parts made starch: light is necessary for photosynthesis (1). (d) Only the green parts made starch: chlorophyll is necessary (1). Foil leaf: independent variable — whether the leaf received light (light vs no light) (1); the control — the uncovered part of the same leaf (or leaf b), which had light and was treated the same way in every other respect (2).
• Q5 (4): Two outline drawings of the same leaf (1); "before" showing green and white areas, labelled (1); "after" showing blue-black exactly where it was green and yellow-brown where it was white, labelled (1); a caption: no chlorophyll, so no photosynthesis, so no starch (1).
• Q6 (3): Soda lime removed the carbon dioxide from bag A, so plant A could not photosynthesise or make starch (1); plant B had carbon dioxide and made starch, so carbon dioxide is necessary (1). Bag B is the control: it must be identical — same plant type, light, time, a similar dish — so that the presence of carbon dioxide is the only difference (1).
• Q7 (2): Gases are less soluble in hot water, so boiling drives the dissolved carbon dioxide out, and it does not return quickly on cooling (1). With no carbon dioxide the discs could not photosynthesise, make oxygen and float — carbon dioxide is necessary (1).
• Q8 (4): Carbon dioxide + water → glucose + oxygen, with light energy and chlorophyll (1). 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (1). In the chloroplast (1). The light phase (in the grana) needs light — it splits water, releasing oxygen; the dark phase (light-independent, in the stroma) does not need light directly — it uses carbon dioxide to make glucose (1). No biochemical detail is required.
• Q9 (2): Glucose made in photosynthesis is converted to insoluble starch for storage (1), so starch builds up only where glucose has been made — it is a stored record of photosynthesis (1).

Investigation 2 — oxygen and the rate

• Q10 (2): 10 cm: 43.3; 20 cm: 22.0; 30 cm: 11.0; 40 cm: 6.3 bubbles per minute (2 for all four; 1 for two or three).
• Q11 (4): Axes labelled with units (1); points accurate (1); a smooth curve (1); the rate falls as the lamp moves further away, steeply at first, then more gently (1).
• Q12 (5): Intensities: 20 cm → 25; 30 cm → 11; 40 cm → 6.25 (1). Graph plotted correctly (1). At low intensity the rate rises in proportion — 6.25 → 6.3, 11 → 11, 25 → 22 (1); between 25 and 100 the rate only doubles while the light quadruples, so the curve levels off (1). At high light intensity light is no longer the limiting factor; something else — carbon dioxide or temperature — limits the rate (1).
• Q13 (2): Without dissolved carbon dioxide the rate fell from about 43 to about 3 bubbles per minute, even in bright light (1) — carbon dioxide is a raw material, so its supply sets how fast photosynthesis can go (1).
• Q14 (4): Graph plotted (1). The optimum is about 30 °C (1). Photosynthesis is controlled by enzymes; above the optimum they begin to denature (1); at 45 °C they were denatured — their shape permanently changed — so the rate collapsed and could not recover on cooling (1).
• Q15 (2): Temperature also changes the rate, so it must be kept constant for light to be the only variable (1); by checking with the thermometer and standing a jar of water between the lamp and the plant to absorb heat (or using a cool LED lamp) (1).
• Q16 (3): Any two: bubbles are not all the same size; some oxygen dissolves in the water instead of forming bubbles; the plant uses some of its oxygen in respiration; bubbles may contain other gases; counting fast bubbles is error-prone (2). Better: collect the gas in a measuring tube or a syringe and measure its volume per minute (1).
• Q17 (1): A glowing splint relights in oxygen — the gas given off in photosynthesis is oxygen.
• Q18 (2): Light is absorbed as it passes down through the water, so the rate of photosynthesis falls with depth (1); below about 15–20 m in coastal water too little light reaches the kelp for it to make more food than it uses, and the light that remains is mostly blue (1).

Investigation 3 — respiration in a jar

• Q19 (4): The germinating seeds respired and gave off carbon dioxide (1), which dissolved in the indicator as a weak acid, so it turned pink (1). Jar B: dead seeds do not respire, so it rules out the seeds' material itself causing the change — it was living seeds (1). Jar C: with nothing in it there was no change, so it rules out the jar, the air or the indicator changing by themselves (1).
• Q20 (1): In the dark the seedlings cannot photosynthesise, which would use up the carbon dioxide and hide the respiration.
• Q21 (3): The flame lasted 4 s in jar A against 13–14 s in B and C (1). The germinating seeds had used up some of the oxygen in jar A (1). A flame needs oxygen; with less oxygen left it went out sooner (1).
• Q22 (3): The seeds take in oxygen and give out carbon dioxide (1); the soda lime absorbs the carbon dioxide, so the air in the tube shrinks by the volume of oxygen used, pulling the drop towards the seeds (1). Without soda lime the carbon dioxide given out would replace the oxygen used, and the drop would hardly move (1).
• Q23 (2): 20 °C: 18 mm; 30 °C: 31.3 mm (1). Respiration is controlled by enzymes, which work faster at the higher temperature (up to their optimum), so oxygen was used faster (1).
• Q24 (4): Independent: temperature (1). Dependent: the distance the drop moved in 10 minutes — the oxygen used (1). Controlled — any two: the number or mass of seeds; the type and age of seeds; the amount of soda lime; the size of the tube; the time; the same start position of the drop (2).
• Q25 (3): Anaerobic respiration (alcoholic fermentation) — the balloon filled with carbon dioxide (1). Glucose → ethanol (alcohol) + carbon dioxide + a little energy (1). Without sugar the yeast had no glucose to break down, so no carbon dioxide was produced (1).
• Q26 (6): Diagram (4): glucose → glycolysis (cytoplasm, 2 ATP) → pyruvic acid → Krebs cycle (mitochondrion matrix, CO₂ released, energised H atoms) → oxidative phosphorylation (cristae, about 36 ATP) → H combines with O₂ to form H₂O; arrows and annotations. Explanation (2): oxygen collects the depleted hydrogen at the end of the chain to form water (1); without oxygen the hydrogen cannot be removed, so the Krebs cycle and oxidative phosphorylation stop and only the 2 ATP of glycolysis are made (1).
• Q27 (4): Any two, each explained (2 each): anaerobic respiration releases only about 2 ATP per glucose instead of about 38, so the muscles quickly run short of energy or use up glucose stores fast; lactic acid builds up in the muscles, which is harmful and causes fatigue; the lactic acid must be broken down with oxygen afterwards, so the animal must spend time at the surface recovering (an oxygen debt) before it can dive again.
• Q28 (4): A cycle: photosynthesis takes in carbon dioxide and water and gives out glucose and oxygen (1); respiration takes in glucose and oxygen and gives out carbon dioxide and water (1); light energy enters at photosynthesis (1); energy (ATP) is released in respiration for life processes, and lost as heat (1).

Investigation 4 — the compensation depth

• Q29 (2): Respiration only — no light, so no photosynthesis (1). The plankton and bacteria in the bottle use oxygen to respire, so the oxygen falls (1).
• Q30 (3): 0 m: 5.2 · 5 m: 4.0 · 10 m: 2.6 · 15 m: 1.5 · 20 m: 0.7 · 30 m: 0.2 mg/L per 24 h (3 for all six; 2 for four or five; 1 for two or three).
• Q31 (5): Depth on the vertical axis, increasing downwards (1); oxygen change on the horizontal axis with zero marked (1); both lines plotted accurately (2); a key (1).
• Q32 (3): The light-bottle line crosses zero at about 19 m (accept 18–20 m) (1). About 1 % of surface light reaches it (between 2.4 % at 15 m and 0.7 % at 20 m) (1) — matching CAPS's "approximately 1 % of surface light" (1).
• Q33 (2): Below the compensation depth the seaweed uses more food and oxygen in respiration over 24 hours than it makes in photosynthesis (1), so it runs down its stores and cannot grow or survive (1).
• Q34 (3): Secchi depth ≈ 19 ÷ 2.7 ≈ 7 m (1). After a flood the water would be muddier, light would be attenuated faster (1), so the compensation depth would become shallower (1).
• Q35 (5): Euphotic zone from the surface to the compensation depth, about 150–200 m in clear open ocean (1); compensation depth marked at its base, about 1 % of surface light (1); disphotic zone below it, to about 1 000 m (1); aphotic zone below 1 000 m, no light (1); photic shown as the lit layer — either the euphotic zone alone (booklet) or euphotic plus disphotic (accept either if labelled consistently) (1).

Investigation 5 — life without sunlight

• Q36 (5): Energy source: sunlight / chemical energy from oxidising hydrogen sulphide (1). Raw materials: CO₂ + H₂O / CO₂ + H₂S + O₂ (1). Products: glucose + oxygen / carbohydrate + sulphate (1). Organisms: plants, seaweeds, phytoplankton, cyanobacteria / bacteria and archaea (1). Where: the euphotic zone / hydrothermal vents, sea-floor sediments and the crust (1).
• Q37 (3): Any three, one mark each: hyperthermophiles or thermophiles — heat; piezophiles (barophiles) — pressure; halophiles — salt; acidophiles — acid; psychrophiles — cold.
• Q38 (3): Chemosynthetic bacteria living inside its body make its food (1). The red plume's haemoglobin collects oxygen and hydrogen sulphide from the water (1) and carries them to the bacteria, which need both to make carbohydrates (1).
• Q39 (3): Left: C 1, O 4, H 2, S 1. Right: C 1, O 5, H 2, S 1 — the oxygen does not balance (1). With H₂O added on the left: C 1, O 5, H 4, S 1 (1). With H₂SO₄ on the right: C 1, O 5, H 4, S 1 — balanced: CO₂ + H₂S + O₂ + H₂O → CH₂O + H₂SO₄ (1).
• Q40 (2): Before the 1990s chemosynthesis was known, but thought to happen only in a few special places such as vents (1). In the 1990s drilling showed living microbes hundreds of metres down in sea-floor sediments and crust all over the ocean, so chemosynthesis was found to be widespread and significant (1).
• Q41 (2): Most vents are on mid-ocean ridges (and at other spreading centres), not trenches (1). There the plates move apart, magma lies close below the sea floor, and seawater seeps into the cracks, is heated and comes back out loaded with dissolved minerals and hydrogen sulphide (1).
• Q42 (2): On the abyssal plain the only food is the little that sinks from the sunlit surface (1); at a vent, chemosynthetic bacteria produce food on the spot from the chemicals in the vent water, so a whole food chain can be supported (1).
• Q43 (6): (a) photic or euphotic zone · (b) anaerobic · (c) extremophiles (3). (d) Chemosynthesis — a process in which certain bacteria use the energy in locally abundant compounds to make energy-rich compounds for their own use and storage (1). (e) Compensation depth — the depth at which the CO₂ used and O₂ produced by a photosynthetic organism by day equals the CO₂ produced and O₂ used at night (1). (f) Glycolysis — the breakdown of a glucose molecule, releasing two ATP and energised hydrogen atoms, and passing the residue (pyruvic acid) to a mitochondrion (1).

Marking note. CAPS allows experiments or data for every photosynthesis and respiration investigation; this sheet gives both, and marks only the reasoning, so a learner who used the supplied panels loses nothing. Q12 is the hardest question: the levelling-off is a limiting factor, and the learner should name what might be limiting it. Q39–Q41 check CAPS and the booklet against the evidence — the chemosynthesis equation as printed does not balance, the "early 1990s" claim needs card 5 to make sense, and vents are on ridges, not in trenches.