On the thylakoid membranes, light knocks electrons out of chlorophyll. As those electrons pass along a chain of carriers their energy pumps protons, and the protons flowing back make ATP. Two routes, cyclic and non-cyclic, give different products.
The light-dependent stage happens on the thylakoid membranes by photophosphorylation: making ATP using light energy. Light absorbed by a photosystem causes photoactivation of chlorophyll: electrons are excited to a higher energy level and leave it. In non-cyclic photophosphorylation, PSII and PSI are both involved, the oxygen-evolving complex catalyses the photolysis of water to replace the electrons lost from PSII, and both ATP and reduced NADP are made, with oxygen as a by-product. In cyclic photophosphorylation only PSI is involved; electrons return to PSI and only ATP is made. In both, energetic electrons release energy along the electron transport chain; that energy transfers protons into the thylakoid space, and the protons return to the stroma by facilitated diffusion through ATP synthase, providing the energy for ATP synthesis.
Fig. 1: non-cyclic photophosphorylation on a thylakoid membrane, with proton pumping and ATP synthesis through ATP synthase
Section 01
Two photosystems, one membrane.
Start on non-cyclic. Watch the light (yellow) hit PSII and PSI, the electrons (blue) travel along the carriers, the protons (gold) collect in the thylakoid space and leave through ATP synthase, and oxygen (red) rise from PSII. Then switch to cyclic and compare the three counters: ATP, reduced NADP and O2.
Non-cyclic photophosphorylation
Cyclic photophosphorylation
photosystems
PSII and PSI
PSI only
photoactivation of chlorophyll
yes, at both photosystems
yes, at PSI
photolysis of water
yes, catalysed by the oxygen-evolving complex of PSII
no
path of electrons
water → PSII → carriers → PSI → NADP (a one-way path)
PSI → carriers → back to PSI (a cycle)
products
ATP, reduced NADP and oxygen
ATP only
Photoactivation: light energy absorbed by the pigments is passed to chlorophyll a at the reaction centre; an electron is raised to a higher energy level and is emitted to an electron acceptor.
Photolysis: 2H2O → 4H+ + 4e− + O2. The electrons replace those lost by PSII; the protons add to the gradient and later help reduce NADP; oxygen diffuses out of the leaf or is used in respiration.
Reduced NADP: at the end of the non-cyclic path, electrons from PSI and protons from the stroma reduce NADP.
Why cyclic matters: the Calvin cycle uses more ATP than reduced NADP, so the extra ATP made by cyclic photophosphorylation balances the supply.
Examiner trap
Oxygen comes from water, split by photolysis, not from carbon dioxide. And cyclic photophosphorylation makes no reduced NADP and no oxygen: check the counters in the simulator, where only ATP keeps rising.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the simulator, what happens to the three counters when you switch from non-cyclic to cyclic?
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In non-cyclic mode, where do the red oxygen molecules appear?
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What is the source of the energy that pumps protons into the thylakoid space?
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How do the protons in the simulator get back into the stroma?
Section 02
From electrons to ATP.
Watch the gold protons pile up above the membrane each time an electron passes the carriers, then stream down through ATP synthase. Each proton that passes through turns its head and adds to the ATP count.
Electron transport chain: energetic electrons pass from carrier to carrier in the thylakoid membrane, releasing energy at each step (the names of the carriers are not needed).
Proton pumping: the energy released is used to transfer protons (H+) from the stroma across the thylakoid membrane into the thylakoid space.
Gradient: protons build up in the small thylakoid space, giving a high concentration (low pH) inside and a lower concentration in the stroma.
ATP synthase: protons cannot cross the phospholipid bilayer, so they return to the stroma by facilitated diffusion through ATP synthase. Their movement provides the energy to join ADP and inorganic phosphate to make ATP.
Where the products go: ATP and reduced NADP are released into the stroma, where the Calvin cycle uses them to make complex organic molecules.
Examiner trap
Protons are pumped into the thylakoid space and flow out to the stroma through ATP synthase, so ATP is made on the stroma side, where the Calvin cycle needs it. Do not mix this up with the mitochondrion, where protons are pumped into the intermembrane space.
Stage 2 · Exam
Exam-style questions
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Which row describes non-cyclic photophosphorylation correctly?
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A herbicide stops electrons leaving PSII. What is the most likely effect in a chloroplast in the light?
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Isolated thylakoids are soaked at pH 4 until the thylakoid space is pH 4, then moved to pH 8 in the dark with ADP and phosphate. ATP is made. What does this show?
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Why do chloroplasts carry out cyclic as well as non-cyclic photophosphorylation?
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The light-dependent stage, mastered.
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