Lesson 2: Cellular Energetics: Photosynthesis, Respiration and RegulationAP Deep Dive

Energetics questions at the top of the mark scheme are rarely about reciting the stages of respiration. They hand you strange apparatus and stranger chemicals — inhibitors, uncouplers, artificial electron acceptors — and ask you to predict what happens to oxygen consumption and ATP production. This lesson builds the reasoning system that makes those predictions mechanical.

The inhibitor matrix: reasoning through the electron transport chain

Hold the chain as a map: NADH feeds electrons in at complex I; succinate (via FADH₂ from the Krebs cycle) feeds in at complex II; electrons pass to complex III, then IV, where oxygen is the final acceptor. Proton pumping at I, III and IV builds the gradient; ATP synthase spends it. Now perturb the map:

Worked example 1. Isolated mitochondria are given succinate and ADP. A student predicts rotenone will stop oxygen consumption "because the electron transport chain is blocked." Evaluate. The prediction fails: succinate donates electrons through complex II, *downstream* of the rotenone block at complex I, so electrons still reach oxygen and ATP is still made — roughly 1.5 ATP per succinate-derived FADH₂. Add cyanide instead and everything stops, because complex IV is the only exit to oxygen. The exam skill is positional: locate the block, ask what is upstream (backs up, reduced) and what is downstream (starved, oxidised).

Worked example 2 — the Jagendorf experiment. Thylakoids are soaked in pH 4 buffer until their internal spaces equilibrate at pH 4, then transferred in the dark to pH 8 medium containing ADP and Pi. ATP is produced — briefly, until the gradient dissipates. No light, no electron flow, yet ATP appears. This is the cleanest possible evidence that the proton gradient itself, not electron transport directly, drives ATP synthase. If an FRQ asks "what conclusion is supported," the answer is chemiosmosis; if it asks for a control, it is thylakoids transferred pH 4 → pH 4 (no gradient, no ATP). A companion classic: leaves supplied with C¹⁸O₂ produce unlabelled O₂, while H₂¹⁸O produces labelled ¹⁸O₂ — proving the oxygen released in photosynthesis comes from water, not carbon dioxide.

Respirometry with real numbers

Worked example 3. Germinating seeds (10 g) sit in a respirometer with KOH, which absorbs all CO₂ released. The manometer fluid moves 0.80 mL in 10 minutes — that movement equals oxygen uptake alone. Rate = 0.80 mL / 10 min = 4.8 mL per hour, or 0.48 mL g⁻¹ h⁻¹. A second respirometer, identical but *without* KOH, moves only 0.20 mL in 10 minutes: there, CO₂ release pushes gas back, so net movement = O₂ taken up − CO₂ given out, giving CO₂ = 0.80 − 0.20 = 0.60 mL. The respiratory quotient RQ = CO₂/O₂ = 0.60/0.80 = 0.75 — below the carbohydrate value of 1.0, indicating the seeds are respiring a lipid-rich store (oils run ≈ 0.7). Three scored ideas in one apparatus: what KOH does, why the two readings differ, and what the ratio means.

Regulation: the control points examiners probe

Energetics is regulated at a few choke points, and questions target them precisely. Phosphofructokinase (glycolysis) is inhibited by ATP and citrate and activated by ADP/AMP — when energy is plentiful, the pathway throttles itself; citrate also signals "Krebs intermediates are abundant, slow down." This is feedback inhibition one level up from the classic end-product example (isoleucine inhibiting the first enzyme of its own synthesis from threonine). In the Calvin cycle, light indirectly activates rubisco and other enzymes: electron flow raises stromal pH and Mg²⁺ concentration and reduces regulatory disulfide bonds — which is why the cycle effectively switches off in the dark even though its enzymes are still present. Fermentation's purpose, stated precisely, is regenerating NAD⁺ so glycolysis can continue; it produces no ATP itself beyond the net 2 from glycolysis, a sentence whose absence costs a point every year.

C4 and CAM: the quantitative trade-off

C3 leaves lose water every moment their stomata open to admit CO₂, and rubisco wastes roughly a fifth to a third of fixed carbon to photorespiration in hot, bright conditions (oxygen competes at the active site, and the problem worsens with temperature). C4 plants pay about 2 extra ATP per CO₂ to run the CO₂ pump into bundle-sheath cells, buying near-zero photorespiration and the ability to keep stomata narrower. CAM plants open stomata only at night, fixing CO₂ into malate, and release it internally by day — water loss per carbon gained can be an order of magnitude lower than C3, at the cost of slow growth because vacuole storage limits nightly uptake. Exam questions present a habitat (hot/dry/bright, cool/moist, desert) and ask you to match the pathway and justify with these trade-offs — cost paid, benefit bought.

Common mistakes at the top band

• "Fermentation produces ATP." It yields none directly; it recycles NAD⁺ so that glycolysis's net 2 ATP can keep coming.
• "The oxygen from photosynthesis comes from CO₂." Isotope evidence says water.
• "Uncouplers increase ATP because respiration speeds up." Oxygen consumption rises; ATP collapses. Rate of the chain and yield per glucose move in opposite directions here.
• Counting Krebs-cycle ATP as the main output. The cycle's real harvest is NADH and FADH₂; oxidative phosphorylation makes the bulk of the ~30–32 ATP per glucose.
• Saying inhibitors "stop respiration" without locating the block. With alternative entry points (succinate at complex II), position is everything.

AP Exam Experience

1. DNP (2,4-dinitrophenol) makes the inner mitochondrial membrane leaky to protons. In its presence, mitochondria supplied with glucose and oxygen will show

• (A) decreased oxygen consumption and decreased ATP production
• (B) decreased oxygen consumption and increased ATP production
• (C) increased oxygen consumption and decreased ATP production
• (D) increased oxygen consumption and increased ATP production

Answer: (C). Uncoupling lets protons bypass ATP synthase, so the gradient is dissipated as heat and ATP synthesis falls. The electron transport chain accelerates to rebuild the gradient, consuming oxygen faster. This is why DNP was a dangerous (fatal) diet drug in the 1930s: energy burned as heat.

2. Mitochondria are provided with succinate as the only substrate. Which addition will stop oxygen consumption completely?

• (A) Rotenone, a complex I inhibitor
• (B) Cyanide, a complex IV inhibitor
• (C) Oligomycin, an ATP synthase inhibitor
• (D) Extra ADP

Answer: (B). Succinate feeds electrons in at complex II, bypassing the complex I block, so (A) fails; oligomycin slows the chain by back-pressure but does not seal it, and in any case the question asks for complete stoppage. Complex IV is the sole route from the chain to oxygen, so cyanide ends all consumption.

3. A plant is grown in water labelled with ¹⁸O (H₂¹⁸O) and unlabelled CO₂. The oxygen gas it releases will be

• (A) entirely unlabelled, because O₂ comes from CO₂
• (B) labelled with ¹⁸O, because O₂ is a product of splitting water
• (C) half labelled, because O₂ comes equally from water and CO₂
• (D) unlabelled, because isotopes cannot enter gas products

Answer: (B). Photolysis of water at photosystem II supplies the electrons and releases the O₂; the isotope experiments of the 1940s settled that the gas tracks water, not CO₂.

Free-response question. A student investigates photosynthesis in an aquatic plant. Identical sprigs are placed in test tubes of pond water under a lamp at different distances. The number of oxygen bubbles produced per minute is counted. Results: at 10 cm, 62 bubbles/min; at 20 cm, 31 bubbles/min; at 40 cm, 9 bubbles/min; at 80 cm, 2 bubbles/min.

(a) Identify the independent and dependent variables, and state two variables that must be controlled for the comparison to be valid. (b) The student claims the data prove light intensity is inversely proportional to bubble rate. Evaluate this claim using the data. (c) Predict and justify the effect on the results of adding sodium hydrogen carbonate (a source of CO₂) to the tubes at the 40 cm distance. (d) Describe how the student could modify the investigation to measure the rate of respiration of the same plant material, and state what result would locate the light compensation point.

Scoring guideline.

• (a) 2 points. 1 point for independent = distance from lamp (light intensity) and dependent = bubbles per minute (rate of oxygen production). 1 point for any two valid controls: same mass/length of plant, same water volume and temperature, same CO₂ concentration initially, same tube size, equilibration/acclimatisation time. Listing "the plant" generically earns nothing; controls must be variables that could otherwise change the rate.
• (b) 2 points. 1 point for testing the claim: doubling distance from 10 → 20 cm halves the rate (62 → 31), but 20 → 40 cm cuts it to 9, roughly a third/fourth — not a constant halving pattern, and 40 → 80 gives 9 → 2. 1 point for the verdict: the data show rate decreases as distance increases (and physically light intensity falls with the square of distance), but they do not support a simple inverse proportionality between rate and distance; also bubble *size* may vary, so counts are a crude proxy for volume. A bare "the claim is wrong" without data earns neither point.
• (c) 2 points. 1 point for the prediction: bubble rate at 40 cm increases. 1 point for the justification: at low light the plant can still use more CO₂ — CO₂ concentration is a limiting factor here (or becomes limiting as light reactions supply ATP/NADPH faster than the Calvin cycle can consume them); adding hydrogen carbonate raises CO₂ availability so the rate rises toward the light-imposed ceiling. "More respiration" or "CO₂ is a reactant of the light reactions" earns nothing.
• (d) 2 points. 1 point for a workable respiration measurement: keep identical tubes in darkness (foil-wrapped) and measure oxygen *uptake* — e.g. with a respirometer or oxygen sensor — since photosynthesis is off but respiration continues. 1 point for the compensation point: the light intensity (distance) at which measured net oxygen exchange is zero, where photosynthetic O₂ production exactly balances respiratory O₂ consumption. Naming the term without the zero-exchange criterion earns the first point only.