The light-dependent reactions of photosynthesis: the diagram explained step by step (photosystem II, photolysis of water, the electron transport chain, photosystem I, NADPH, chemiosmosis and ATP synthase), where they happen (the thylakoid membrane), the inputs and outputs, cyclic against non-cyclic photophosphorylation, how the products feed the Calvin cycle, the Z-scheme, the mistakes, and a printable sheet with practice

"Light reactions of photosynthesis" is searched about 1,100 times a month in the United States, by AP Biology, IB and A-level students who have a diagram in front of them with two photosystems, an electron transport chain and an ATP synthase and need the steps in order. This page walks the diagram step by step, says where everything happens, gives the inputs and outputs, explains the Z-scheme and the two kinds of photophosphorylation, shows how the products feed the Calvin cycle, compares the two stages, lists the mistakes and gives a printable sheet with practice. The primary-school version of photosynthesis (the recipe and the leaf) is on our photosynthesis for kids page.

Download the light reactions sheet with practice (PDF) (A4, the steps in order, the inputs and outputs and the comparison table on page 1, the Z-scheme, cyclic flow, the Calvin link and the practice set with answers on page 2, free).

Where it happens

The chloroplast has an outer and inner membrane, a fluid stroma, and inside the stroma stacks (grana) of flattened membrane sacs, the thylakoids. The light-dependent reactions happen in the thylakoid membrane; the space inside the sac is the thylakoid lumen. Embedded in that membrane, in order along the path of the electrons, are photosystem II, the cytochrome b6f complex, photosystem I and ATP synthase, with mobile carriers (plastoquinone, plastocyanin, ferredoxin) shuttling between them. The Calvin cycle happens in the stroma outside.

A photosystem is a cluster of hundreds of chlorophyll and carotenoid molecules (the antenna or light-harvesting complex) funnelling light energy to a reaction centre: a special pair of chlorophyll a molecules, P680 in photosystem II (absorbing best at 680 nm) and P700 in photosystem I (700 nm). The numbering is historical: photosystem I was discovered first but comes second in the electron path.

The steps in order

1. Light hits photosystem II. Pigments absorb photons and pass the energy to P680, which loses an excited electron to the primary electron acceptor. P680⁺ is now the strongest biological oxidising agent known.
2. Photolysis of water replaces the electron. An enzyme complex on the lumen side splits water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons refill P680; the protons stay in the lumen (building the gradient); the oxygen diffuses out of the leaf. This is the source of atmospheric oxygen.
3. The electron transport chain. The excited electron passes from the primary acceptor to plastoquinone (PQ), then to the cytochrome b6f complex, then to plastocyanin (PC). Each transfer releases energy, and the cytochrome complex uses it to pump protons from the stroma into the lumen.
4. Light hits photosystem I. P700 loses an excited electron to its acceptor and is refilled by the electron arriving from plastocyanin, now at a lower energy.
5. NADPH is made. The re-energised electron passes through ferredoxin (Fd) to the enzyme NADP⁺ reductase, which adds two electrons and a proton to NADP⁺: NADP⁺ + 2e⁻ + H⁺ → NADPH. NADPH is the reducing power carried to the Calvin cycle.
6. A proton gradient builds. Protons from photolysis and from pumping accumulate in the lumen; its pH drops to about 5 while the stroma is about 8, a thousandfold difference in concentration plus a charge difference.
7. Chemiosmosis makes ATP. Protons flow back to the stroma through ATP synthase, and the flow turns the enzyme, which joins ADP + Pi → ATP. Making ATP with light-driven proton flow is photophosphorylation. About 3 protons per ATP.
8. The products go to the stroma. ATP and NADPH drive the Calvin cycle; the ADP, Pi and NADP⁺ it releases return to the thylakoid to be recharged.

Inputs and outputs

For every molecule of oxygen released, two water molecules are split and four electrons travel the chain; for every glucose built, the Calvin cycle needs 18 ATP and 12 NADPH, so the light reactions run many times over.

The Z-scheme

Plot the energy of the electron against its position and the path looks like a Z on its side: a jump up at photosystem II (light energy in), a fall down the electron transport chain (energy out, used to pump protons), a second jump up at photosystem I (light in again), and a short fall to NADPH. Two photosystems are needed because one photon's energy is not enough to lift an electron all the way from water (which holds its electrons very tightly) to NADPH; the two-step lift is the reason the reactions have two light-absorbing complexes.

Non-cyclic against cyclic photophosphorylation

How the products feed the Calvin cycle

In the stroma, RuBisCO fixes CO₂ onto the five-carbon RuBP, giving two three-carbon molecules (3-PGA). ATP phosphorylates them and NADPH reduces them to G3P; some G3P leaves to become glucose, sucrose, starch and cellulose, and the rest is rebuilt into RuBP using more ATP. Without the light reactions there is no ATP or NADPH and the cycle stops within seconds of darkness, which is why the "dark reactions" are better called light-independent: they do not use light, but they cannot run without its products.

The two stages compared

The mistakes that cost marks

• Saying the oxygen comes from carbon dioxide. It comes from water.
• Putting photosystem I before photosystem II in the electron path. The numbers are historical; II comes first.
• Saying ATP is made by the electron transport chain directly. The chain pumps protons; ATP synthase makes ATP from the proton flow (chemiosmosis).
• Placing the reactions in the stroma or "in the chloroplast" without saying thylakoid membrane.
• Writing NADH instead of NADPH (NADH is respiration's carrier).
• Calling the Calvin cycle the "dark reactions" and saying it happens at night. It runs in daylight alongside the light reactions.
• Forgetting that glucose is not made directly in either stage: the Calvin cycle makes G3P.

Where it sits in each curriculum

Practice (answers in the PDF)

1. List the inputs and outputs of the light-dependent reactions and say where in the chloroplast they occur.
2. Write the equation for photolysis and state what happens to each of its three products.
3. Put these in the order the electron meets them: plastocyanin, photosystem I, NADP⁺ reductase, photosystem II, plastoquinone, cytochrome b6f complex, ferredoxin.
4. Explain how a proton gradient leads to ATP synthesis.
5. Why does cyclic photophosphorylation make no oxygen and no NADPH?
6. A plant is moved into the dark. Explain why the Calvin cycle stops within seconds even though it does not use light directly.

Download the sheet and the practice set (PDF)

Where to get help

A 1-on-1 online biology class with a tutor who teaches AP Biology, A-level and IB biology, CBSE Class 11 and 12 and the Australian Year 12 every week: the light reactions taught by drawing the thylakoid membrane together, electron by electron, until the order and the products are second nature. The biology page lists the courses; the free 30-minute first class is a real lesson on this week's topic. Related pages: photosynthesis explained for kids, the four macromolecules and light energy.

Questions parents ask

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