Photosynthesis in Higher Plants

1Where the two halves run

Photosynthesis in higher plants happens in chloroplasts, and the chloroplast is split into two workplaces. The light reactions sit in the thylakoid membranes: that is where the photosystems, the electron carriers and the ATP synthase live. The Calvin cycle — still often called the dark reactions, though it runs in the light whenever ATP and NADPH are on hand — sits in the stroma, the fluid outside the thylakoids.

Confusing the two locations is a frequent stem. Photolysis of water, O₂ release and the Z-scheme are membrane events. CO₂ fixation onto RuBP is a stromal event. Grana are stacks of thylakoids; stroma lamellae link them — useful anatomy words, but the examinable cut is membrane against stroma.

Figure. Outer box = stroma. Inner bands = thylakoid membranes where the light reactions run. Not a micrograph — no stacked granum outlines.

2The overall equation

The balanced summary of oxygenic photosynthesis is 6\mathrm{CO}_2 + 12\mathrm{H}_2\mathrm{O} \xrightarrow{\text{light}} \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 + 6\mathrm{H}_2\mathrm{O}. Twelve waters appear on the left because the oxygen released as O₂ comes from water, not from CO₂ — a fact settled by van Niel's work on purple sulphur bacteria and by later ^{18}\mathrm{O} labelling. Six waters are rewritten on the product side as the net accounting of the Calvin cycle.

Net, six CO₂ and six H₂O become one glucose and six O₂, but writing 12 H₂O on the left is the form that keeps the source of O₂ honest. A stem that asks 'where does the released oxygen come from' wants water.

Figure. Oxygenic photosynthesis mass balance: twelve waters enter, six appear as product water, six are net consumed; oxygen released equals carbon dioxide fixed.

3Photosystems and the Z-scheme

Two photosystems cooperate in the light reactions. Photosystem II has reaction-centre chlorophyll P680; Photosystem I has P700. Light absorbed at PS II ejects an electron that travels through an electron-transport chain to PS I, and the hole at PS II is filled by electrons from water — photolysis — releasing O₂ and protons. Light at PS I re-excites the electron so it can reduce NADP⁺ to NADPH.

The path of the electron, drawn as the Z-scheme, is what the Emerson enhancement effect demonstrated: far-red light that drives only PS I and red light that drives PS II give more photosynthesis together than the sum of each alone, so both photosystems must act in series.

Figure. Electron flow left to right: water → PS II → chain (ATP) → PS I → NADPH. Drawn as boxes, not as the classic energy Z — heights would invent voltages.

Non-cyclic electron flow

1. PhotolysisPS II splits water: electrons refill P680, O₂ is released, protons enter the lumen.
2. Chain to PS IThe ejected electron falls through carriers between the photosystems; the drop helps build the proton gradient for ATP.
3. Re-excitationPS I (P700) absorbs light and boosts the electron again.
4. NADPHThe electron reduces NADP⁺ to NADPH in the stroma.

4Cyclic and non-cyclic photophosphorylation

Non-cyclic photophosphorylation uses both photosystems. Electrons flow from water through PS II and PS I to NADP⁺, so the products are ATP, NADPH and O₂. Cyclic photophosphorylation uses only PS I: the excited electron cycles back through the cytochrome chain to P700, so the only product is ATP — no NADPH and no O₂.

A cell short of ATP relative to NADPH can lean on the cyclic path to top up ATP without making more NADPH. That is the physiological reason both paths exist, and the distinction the source example asked for.

Figure. Two paths side by side. Cyclic's empty NADPH/O₂ slots are the whole exam point.

5Calvin cycle: fixing one CO₂

In the C3 Calvin cycle, RuBisCO carboxylates ribulose-1,5-bisphosphate (RuBP, 5C) with CO₂ to give two molecules of 3-phosphoglyceric acid (3-PGA, 3C) — the first stable product of C3 photosynthesis. ATP and NADPH from the light reactions then reduce 3-PGA toward triose phosphate, and more ATP regenerates RuBP so the cycle can turn again.

The cofactor price is fixed in the textbooks NEET reads: 3 ATP and 2 NADPH per CO₂ assimilated. Six turns fix six CO₂ into one glucose, so the bill scales by six.

Figure. Sequential circuit: RuBP → 3-PGA (carboxylation), 3-PGA → triose-P (reduction), triose-P → regenerate → RuBP. Return arrow uses the empty centre gap as a vertical, not a diagonal across boxes.

One turn, in outline

1. CarboxylationRuBisCO adds CO₂ to RuBP; the product splits into two 3-PGA.
2. ReductionATP and NADPH convert 3-PGA toward triose phosphate.
3. RegenerationATP regenerates RuBP from the remaining triose so the cycle continues.

6C3, C4 and photorespiration

RuBisCO is an imperfect enzyme: it binds CO₂ (carboxylase, photosynthesis) and O₂ (oxygenase, photorespiration). High O₂ and low CO₂ favour the oxygenase reaction, which wastes energy and releases CO₂ without making sugar — a problem that grows with temperature in C3 plants such as wheat and rice.

C4 plants (maize, sugarcane) use the Hatch–Slack pathway to concentrate CO₂. PEP carboxylase in mesophyll cells fixes CO₂ into oxaloacetic acid (OAA, 4C), the first stable C4 product; a 4C acid then delivers CO₂ to bundle-sheath cells where the Calvin cycle runs behind a high local CO₂ concentration. Photorespiration is suppressed, so net productivity stays high in hot, bright conditions. Kranz anatomy — bundle-sheath cells with chloroplasts ringed by mesophyll — is the anatomical signature.

Figure. C3 lets RuBisCO see leaf air (photorespiration risk); C4 concentrates CO2 at the bundle sheath via PEPcase so RuBisCO runs carboxylase. Kranz anatomy is not drawn.

How C4 suppresses photorespiration

1. Mesophyll fixPEP carboxylase binds CO₂ into OAA; it has no oxygenase activity.
2. ShuttleA 4C acid moves into the bundle-sheath cell.
3. ReleaseCO₂ is released around RuBisCO at high local concentration.
4. CalvinRuBisCO carboxylates RuBP; the oxygenase reaction stays rare.

7Blackman's law of limiting factors

Blackman's law says the rate of photosynthesis is set by the factor in shortest supply. Light intensity, CO₂ concentration and temperature can each be that factor. Raise light while CO₂ is scarce and the rate plateaus — more light cannot help until CO₂ rises. Raise CO₂ at low light and the same plateau appears for the opposite reason.

Exam graphs show rate against light intensity at two CO₂ levels: both curves rise then flatten, and the higher-CO₂ curve flattens higher. The rising limb is light-limited; the flat limb is limited by CO₂ (or by temperature, or by the Calvin machinery). Identifying which limb you are on is the whole skill.

Figure. Rate against light at two CO₂ levels. Both rise then flatten; the higher-CO₂ curve flattens higher. Shape only — no invented numeric rates on the axes.

8What each half delivers

Pull the chapter into one balance sheet. The light reactions, in the thylakoid membrane, deliver ATP, NADPH and O₂. The Calvin cycle, in the stroma, spends ATP and NADPH to fix CO₂ into sugar and regenerates ADP, Pi and NADP⁺ for the light reactions. Non-cyclic flow makes all three light products; cyclic flow adds ATP alone when the stroma needs it.

C3 and C4 differ not in that balance sheet but in how CO₂ reaches RuBisCO. Everything else — Z-scheme, photolysis, Calvin stoichiometry — is shared.

Figure. Light half ships ATP and NADPH to the Calvin half; the Calvin half returns ADP and NADP⁺. O₂ leaves from the light half only.