2.8 Light-dependent Reactions and the Calvin Cycle

A chloroplast has a double envelope around the stroma. Within it, thylakoid membranes form flattened sacs, often stacked as grana and connected by lamellae. Photosystems, electron carriers and ATP synthase occupy the thylakoid membrane; the light-dependent reactions build a proton gradient across it. Enzymes of the Calvin cycle are in the stroma. The small thylakoid lumen permits rapid accumulation of protons.

A photosystem contains a reaction-center chlorophyll a molecule surrounded by accessory pigments and proteins. Antenna pigments absorb photons and transfer excitation energy to the reaction center. In photosystem II, excited electrons leave chlorophyll and enter an electron transport chain. Light energy has therefore been converted first into electron excitation, not directly into glucose.

Photolysis splits water at photosystem II, replacing electrons lost by chlorophyll. It also releases protons into the thylakoid lumen and produces oxygen as a by-product. Electrons move from photosystem II through carriers toward photosystem I. Their energy powers proton movement from the stroma into the lumen, strengthening the electrochemical gradient.

Protons return from the lumen to the stroma through ATP synthase, driving photophosphorylation of ADP. Electrons arriving at photosystem I are excited again by light, pass to an acceptor and reduce NADP with protons to form reduced NADP, usually written NADPH + H⁺. The light-dependent products delivered to the stroma are ATP and reduced NADP; oxygen leaves or is used in respiration.

In the stroma, carbon dioxide combines with the five-carbon acceptor ribulose bisphosphate (RuBP). Rubisco catalyses this carboxylation. The unstable six-carbon product immediately splits into two three-carbon glycerate 3-phosphate molecules (GP). For three carbon dioxide, three RuBP produce six GP, preserving all eighteen carbon atoms.

ATP and reduced NADP from the light-dependent reactions convert GP into triose phosphate (TP). ATP supplies energy and reduced NADP supplies electrons and hydrogen, so GP is reduced. Of the six TP produced for three carbon dioxide, one TP represents the net three-carbon output. Five TP, containing fifteen carbons in total, are rearranged using ATP to regenerate three five-carbon RuBP.

Two net TP molecules can contribute the carbon skeleton for one hexose sugar. TP is also a branching point for synthesis of sucrose, starch, cellulose, lipids and, with mineral nitrogen, amino acids. The Calvin cycle therefore does not eject a ready-made glucose molecule after one turn. It supplies small carbon compounds that the cell uses in multiple anabolic pathways.

The Calvin cycle does not absorb photons directly, so it is described as light-independent. Nevertheless, it normally depends on ATP and reduced NADP supplied by the light-dependent reactions, and several enzymes are regulated by light-linked conditions in the chloroplast. In darkness the cycle soon slows as those supplies and activation states change. Calling it the dark reaction can therefore create a false prediction.

Temperature affects the Calvin cycle because its steps are enzyme-catalysed. Light intensity affects it indirectly by changing ATP and reduced NADP supply. Carbon dioxide affects carboxylation directly. These dependencies explain why the neat division into two stages is useful but incomplete: the stages exchange energy carriers, share redox balance and influence one another continuously.