Photosynthesis in Higher Plants — the Z-Scheme, Calvin Cycle and C₄ Advantage
Photosynthesis is two separate processes that students merge into one and then cannot answer questions about. The light reactions happen on the thylakoid membrane, need light, and produce ATP, NADPH and oxygen — nothing else. The Calvin cycle happens in the stroma, needs no light directly, and spends that ATP and NADPH to fix CO₂ into sugar. Once you keep the two apart, most questions become bookkeeping: which product comes from which reaction, and where. The chapter's deepest idea is that RuBisCO is a flawed enzyme — it cannot reliably tell CO₂ from O₂ — and that C₄ plants exist essentially as an engineering fix for that flaw. Read the C₄ pathway as a solution and it stops being a second set of names to memorise.
The Calvin Cycle and Its Cost
| Phase | What happens | Cost per CO₂ |
|---|---|---|
| Carboxylation | RuBP (5C) + CO₂ → two molecules of 3-PGA, catalysed by RuBisCO — the most crucial step | No ATP or NADPH |
| Reduction | 3-PGA is reduced to sugar | 2 ATP + 2 NADPH |
| Regeneration | RuBP is rebuilt so the cycle can continue | 1 ATP |
| Totals | For one glucose, six turns of the cycle are needed | 18 ATP + 12 NADPH per glucose; 3 ATP + 2 NADPH per CO₂ |
🎯 Photorespiration: why the most abundant enzyme on Earth is also a bad one
RuBisCO's active site can bind either CO₂ or O₂, and although its affinity for CO₂ is much greater, what decides the outcome is the relative concentration of the two gases. When CO₂ is low and O₂ high — a hot, bright day with stomata closing to save water — RuBisCO increasingly binds oxygen, and instead of two molecules of 3-PGA you get one 3-PGA and one two-carbon phosphoglycolate. That is photorespiration, and it is pure loss: no sugar is made, no ATP is made, no NADPH is made, and CO₂ is released. Only ATP is consumed. So a C₃ plant on a hot afternoon can be running a substantial fraction of its RuBisCO backwards, which is why crop yields fall in heat even with water available. Note the exam habit here: questions ask "which of the following is produced in photorespiration" with sugar, ATP and NADPH among the options, and the answer is none of them — the only products are phosphoglycolate and, downstream, CO₂.| Feature | C₃ plants | C₄ plants |
|---|---|---|
| First CO₂ acceptor | RuBP (5 carbons) | PEP (3 carbons), by PEP carboxylase |
| First stable product | 3-PGA (3 carbons) | OAA (4 carbons) — hence the name |
| Anatomy | Ordinary mesophyll; no special sheath | Kranz anatomy — large, chloroplast-rich bundle sheath cells |
| Where RuBisCO acts | In the mesophyll, exposed to atmospheric O₂ | In the bundle sheath, where CO₂ is concentrated |
| Photorespiration | Significant, and worse in heat | Absent — that is the whole point of the pathway |
| Examples and optimum | Rice, wheat, most temperate crops | Maize, sugarcane, sorghum, *Amaranthus* — higher temperature optimum, higher productivity |
🧠 What C₄ actually buys, and what it costs
The C₄ pathway does not replace the Calvin cycle — it feeds it. Mesophyll cells fix CO₂ with PEP carboxylase, an enzyme with no affinity for oxygen at all, into a four-carbon acid; that acid travels to the bundle sheath and is decarboxylated there, releasing CO₂ right next to RuBisCO. The effect is to raise the local CO₂ concentration so far above atmospheric that RuBisCO never sees an unfavourable O₂ : CO₂ ratio, and photorespiration simply does not occur. The cost is extra ATP — roughly 5 ATP per CO₂ against 3 in a C₃ plant — which is why C₄ is not universally better: in cool, moist conditions the C₃ plant's lower running cost wins, while in hot bright conditions the C₄ plant's freedom from photorespiration wins easily. Note the two anatomical requirements a question may test: mesophyll cells of a C₄ plant lack RuBisCO, and bundle sheath cells lack PEP carboxylase. The division of labour is strict, and it is enforced by which enzyme sits where.ℹ️ Limiting factors — and why light is almost never the problem
Blackman's law of limiting factors says that when several factors affect a process, the rate is set by the factor nearest its minimum value. Applying it to a field gives a result that surprises students: light saturates at about 10% of full sunlight, so on a clear day light is not limiting for a leaf in the open — shading and mutual leaf overlap aside. The factor that actually limits photosynthesis in nature is CO₂, at only about 0.03–0.04% of air, and this is why raising CO₂ in a greenhouse increases yield, an effect called CO₂ fertilisation. Temperature matters through the enzymes, so C₃ and C₄ plants have different optima; water matters mostly indirectly, because a water-stressed plant closes its stomata, which starves it of CO₂ and simultaneously raises the internal O₂ ratio, driving photorespiration. That chain — drought → closed stomata → less CO₂ → more photorespiration → lower yield — is worth being able to state, because it links three sections of this chapter into one answer.Key takeaways
- Light reactions on the thylakoid give ATP, NADPH and O₂ only; the Calvin cycle in the stroma spends them to fix CO₂.
- Van Niel showed O₂ comes from water, not CO₂; Engelmann gave the first action spectrum, peaking in blue and red.
- Chlorophyll a is the reaction centre; the rest are accessory pigments that widen the spectrum and protect it. P680 and P700 are both chlorophyll a.
- Cyclic photophosphorylation uses PS I only, in stroma lamellae, and makes ATP alone — no NADPH, no O₂.
- Protons accumulate in the thylakoid lumen and flow out through ATP synthase, making ATP in the stroma.
- Calvin cycle costs 3 ATP + 2 NADPH per CO₂, hence 18 ATP + 12 NADPH per glucose.
- Photorespiration makes no sugar, no ATP and no NADPH — it only consumes ATP and releases CO₂.
- C₄ concentrates CO₂ at RuBisCO via PEP carboxylase and Kranz anatomy, abolishing photorespiration at a cost of ~5 ATP per CO₂. Light saturates at ~10% of sunlight; CO₂ is the real limiting factor.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Which of these is produced during photorespiration?
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Answer: A — None of sugar, ATP or NADPH — only phosphoglycolate, and CO₂ downstream
Photorespiration is pure loss: RuBisCO binds O₂ instead of CO₂, giving one 3-PGA and one two-carbon phosphoglycolate, with no sugar, no ATP and no NADPH produced and ATP actually consumed. Every distractor here offers one of the three products that the process specifically fails to make.Cyclic photophosphorylation differs from the non-cyclic form in that it:
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Answer: A — Involves PS I only and produces ATP without NADPH or oxygen
It runs on PS I alone in the stroma lamellae, which lack PS II and NADP reductase, so no water is split and no NADPH forms — only ATP. Producing oxygen requires PS II and the photolysis of water, which is exactly what the cyclic pathway does not include.The first stable product of CO₂ fixation in a C₄ plant such as maize is:
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Answer: A — Oxaloacetic acid, a four-carbon acid
PEP carboxylase adds CO₂ to the three-carbon PEP to give the four-carbon OAA, which is where the name C₄ comes from. 3-PGA is the C₃ answer, RuBP is the acceptor rather than a product, and phosphoglycolate belongs to photorespiration — which C₄ plants avoid altogether.Van Niel's work with purple and green sulphur bacteria established that:
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Answer: A — The oxygen released in photosynthesis comes from water, not from CO₂
Those bacteria use H₂S as the hydrogen donor and deposit sulphur instead of releasing oxygen, showing that the donor determines the by-product — so in plants, where water is the donor, the O₂ must come from water. Light being essential and only green parts working were Ingenhousz's conclusions, and starch was von Sachs'.How many ATP and NADPH molecules does the Calvin cycle consume to produce one molecule of glucose?
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Answer: A — 18 ATP and 12 NADPH
Each CO₂ costs 3 ATP and 2 NADPH, and six CO₂ are needed for one glucose, giving 18 and 12. The third option is the per-CO₂ figure rather than the per-glucose one, and the second simply swaps the two numbers — both are traps for a half-remembered ratio.In a C₄ plant, the mesophyll cells characteristically lack:
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Answer: A — RuBisCO
The division of labour is strict: mesophyll cells carry PEP carboxylase and no RuBisCO, while bundle sheath cells carry RuBisCO and no PEP carboxylase. That is precisely what keeps RuBisCO away from atmospheric oxygen and prevents photorespiration, so reversing the two enzymes destroys the whole mechanism.During photophosphorylation, protons accumulate in the:
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Answer: A — Thylakoid lumen, making it acidic
Water is split on the inner face of the thylakoid membrane and the carriers pump H⁺ inward, so the lumen turns acidic and the stroma alkaline; protons then return through ATP synthase, making ATP in the stroma where the Calvin cycle needs it. Option two reverses the gradient, which would put the ATP in the wrong compartment.For a leaf in the open on a clear day, the factor most likely to be limiting photosynthesis is:
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Answer: A — Carbon dioxide concentration
Light saturates at roughly 10% of full sunlight, so on a clear day it is well above the limiting level, whereas CO₂ is only about 0.03–0.04% of air — which is why enriching CO₂ in a greenhouse raises yield. By Blackman's law the rate is set by whichever factor is nearest its minimum, and that is almost always CO₂ in the open.