Light-Dependent Reactions of Photosynthesis

The light-dependent reactions of photosynthesis take place in the thylakoid membranes of the chloroplast, where light energy absorbed by photosystems II and I splits water (photolysis), drives electron transport and produces ATP, NADPH and oxygen. In brief: photosystem II (PSII) absorbs light, splits water and releases O₂; electrons then flow through an electron transport chain to photosystem I (PSI), pumping protons that drive ATP synthesis (photophosphorylation); at PSI, light re-energizes the electrons, which reduce NADP⁺ to NADPH. The ATP and NADPH are then used in the Calvin cycle.

This is the stage of photosynthesis that converts light energy into chemical energy. This guide describes the structure of the chloroplast and the photosystems, follows the non-cyclic electron flow (the Z-scheme), explains cyclic photophosphorylation, compares the process with mitochondrial respiration, works through an energy and stoichiometry example and corrects common mistakes.

Where do the light-dependent reactions occur?

A chloroplast has a double outer membrane and an internal system of flattened sacs, the thylakoids, stacked into grana. The thylakoid membrane contains the pigments and the proteins of the electron transport chain, as well as ATP synthase. The space inside the thylakoid is the lumen, and the fluid outside, in which the Calvin cycle occurs, is the stroma. The large surface area of the thylakoid membranes allows many photosystems to fit in one chloroplast.

What are photosystems?

A photosystem is a cluster of pigment molecules and proteins in the thylakoid membrane. It contains an antenna complex, a few hundred molecules of chlorophyll a, chlorophyll b and carotenoids that absorb light and pass the energy to a special pair of chlorophyll a molecules at the reaction center, where an electron is excited and passed to an electron acceptor.

• Photosystem II (PSII): its reaction center chlorophyll absorbs best at 680 nm (P680). It contains the oxygen-evolving complex, which splits water.
• Photosystem I (PSI): its reaction center chlorophyll absorbs best at 700 nm (P700).

The numbering reflects the order of discovery, not the order of electron flow: electrons go from PSII to PSI.

How does non-cyclic electron flow work?

1. Light absorption at PSII: light excites the P680 electrons, which are passed to a primary electron acceptor. P680 becomes a strong oxidizing agent that lacks electrons.
2. Photolysis of water: the oxygen-evolving complex, which contains manganese, splits water to replace the lost electrons: 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen is released as a by-product, and the protons accumulate in the thylakoid lumen.
3. Electron transport chain: the excited electrons pass from the acceptor through plastoquinone, the cytochrome b₆f complex and plastocyanin. As they flow, they lose energy, which is used to pump protons from the stroma into the lumen.
4. Proton gradient and ATP synthesis: the protons from water splitting and from pumping make the lumen acidic. They flow back to the stroma through ATP synthase, which makes ATP from ADP and Pᵢ. This is called photophosphorylation, and the mechanism is chemiosmosis, as in mitochondria.
5. Light absorption at PSI: P700 absorbs light, and its electrons are re-energized, and passed to ferredoxin. The electrons lost from P700 are replaced by those arriving from the chain.
6. NADPH formation: the enzyme NADP⁺ reductase transfers two electrons from ferredoxin to NADP⁺, and with H⁺ from the stroma it makes NADPH.

The zigzag energy diagram of the electron path, rising at each photosystem and falling along the chains, is called the Z-scheme. The overall result of the non-cyclic path: 2H₂O + 2NADP⁺ + about 3 ADP + 3 Pᵢ + light → O₂ + 2NADPH + about 3 ATP (approximately, per 4 electrons passed).

What is cyclic photophosphorylation?

In cyclic flow, electrons from PSI return to the cytochrome b₆f complex instead of going to NADP⁺. They pass back to P700 and are recycled. As they pass through the complex, protons are pumped, which makes ATP, but no NADPH is formed and no water is split, so no oxygen is released. The Calvin cycle needs more ATP than NADPH (a ratio of 3 ATP to 2 NADPH), and the non-cyclic path alone gives somewhat less ATP, so cyclic flow can provide the extra ATP.

Light-dependent reactions and respiration compared

Worked example: photons, electrons and products

Use the stoichiometry to work out the products of the light-dependent reactions for the fixing of six CO₂ molecules into one glucose.

1. Calvin cycle needs: to fix 1 CO₂, 3 ATP and 2 NADPH are needed. For 6 CO₂: 18 ATP and 12 NADPH.
2. Electrons needed for 12 NADPH: each NADPH carries 2 electrons, so 24 electrons are needed.
3. Water to split: each water gives 2 electrons (2H₂O → 4 electrons per O₂), so 12 H₂O are split, producing 6 O₂ molecules. This matches the 6 O₂ in the overall equation for photosynthesis.
4. Photons: each electron passing from water to NADPH is excited twice, once at PSII and once at PSI, so 2 photons are needed per electron, and the minimum number of photons is 24 × 2 = 48. Counted per O₂, that is 8 photons. In practice, 8 to 12 photons are needed per O₂, because of losses.
5. ATP from the proton gradient: non-cyclic flow gives roughly 1.3 ATP for each NADPH made, so 12 NADPH are accompanied by about 12 × 1.3 ≈ 16 ATP. The Calvin cycle needs 18, so the shortfall of about 2 ATP is made up by cyclic photophosphorylation.

The numbers show that the six oxygen molecules in the overall equation come from water, not from carbon dioxide, and that the capture of light is not perfectly efficient. Under ideal conditions, the maximum efficiency of conversion of solar energy to chemical energy is about 4.6 percent for C₃ plants, and in the field most crops reach 1 to 2 percent. The exact ATP counts differ between textbooks, and the ratio of 3 ATP to 2 NADPH is a standard simplification.

How was the mechanism discovered?

Robin Hill showed in 1937 that isolated chloroplasts release oxygen in light when given an artificial electron acceptor (the Hill reaction), proving that oxygen comes from water. Robert Emerson found in 1957 that two wavelengths of light work together, which led to the idea of two photosystems. Hill and Bendall proposed the Z-scheme in 1960. Daniel Arnon discovered photophosphorylation in the 1950s. Peter Mitchell’s chemiosmotic theory was extended to chloroplasts by Andre Jagendorf in the 1960s.

Which mistakes are common?

Mistake 1: Saying that oxygen comes from carbon dioxide

Oxygen is released from the splitting of water in photosystem II.

Mistake 2: Putting PSI before PSII in the electron flow

The numbers reflect the order of discovery. Electrons go from PSII to PSI.

Mistake 3: Calling the Calvin cycle part of the light-dependent stage

The Calvin cycle is the light-independent stage that occurs in the stroma.

Mistake 4: Confusing the locations of protons

In chloroplasts, protons are pumped into the thylakoid lumen. ATP synthase makes ATP in the stroma.