Photosystems and the Calvin Cycle

How light energy drives electron transport and ATP/NADPH production, then powers carbon fixation in the chloroplast.

Lesson progress

Compare cyclic and non-cyclic photophosphorylationExplain how ATP and NADPH are produced in the light-dependent reactionsDescribe carbon fixation, reduction, and RuBP regeneration in the Calvin cycleRelate chloroplast structure to photosynthesis

Photosynthesis converts light energy into chemical energy. In chloroplasts, the light-dependent reactions generate ATP and reduced NADP, while the Calvin cycle uses those products to build triose phosphate from carbon dioxide.

Light-dependent reactions

Chlorophyll in photosystems absorbs photons and raises electrons to higher energy states. Electron carriers then transfer those electrons through a redox chain. The energy released during transfer contributes to a proton gradient across the thylakoid membrane, which drives ATP synthase.

Key idea

Non-cyclic photophosphorylation uses both photosystem II and photosystem I. Water is split to replace electrons lost from photosystem II, releasing oxygen and protons; electrons ultimately reduce NADP. Cyclic photophosphorylation routes electrons from photosystem I back through carriers and produces extra ATP without producing NADPH or oxygen.

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Cyclic and non-cyclic photophosphorylation

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Cyclic vs non-cyclic photophosphorylation

Feature	Cyclic	Non-cyclic
Photosystems used	Photosystem I	Photosystem II and Photosystem I
Main products	ATP	ATP, NADPH, and O2
Photolysis of water	No	Yes
Electron destination	Returns to photosystem I	Transferred to NADP

The Calvin cycle

The Calvin cycle occurs in the stroma. It does not directly require light, but it depends on ATP and NADPH generated by the light-dependent reactions.

Carbon fixation: rubisco catalyzes the attachment of CO2 to ribulose bisphosphate (RuBP), producing an unstable six-carbon intermediate that splits into two three-carbon molecules.
Reduction: ATP and NADPH convert 3-phosphoglycerate into triose phosphate. Some triose phosphate can leave the cycle and contribute to carbohydrate synthesis.
Regeneration: most triose phosphate is rearranged, using ATP, to regenerate RuBP so the cycle can continue.

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The Calvin cycle: fixation, reduction, and RuBP regeneration

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Chloroplast structure supports function

Thylakoid membranes provide a large membrane surface for photosystems, electron carriers, and ATP synthase. Stacks of thylakoids form grana, while the surrounding stroma contains enzymes used in the Calvin cycle.

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Chloroplast structure

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Limiting factors

Factors that can limit photosynthesis

Factor	Typical effect
Light intensity	Rate rises as more photons are available, then plateaus when another factor becomes limiting.
CO2 concentration	Rate rises while carbon supply limits fixation, then plateaus.
Temperature	Rate increases toward an enzyme-dependent optimum, then falls as enzyme function and membrane processes are disrupted.

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