The Light-Dependent Reactions

The first stage of photosynthesis performs one of the hardest chemical tricks in nature: it takes electrons from water, one of the most stable molecules there is, and uses light to push them up to such high energy that they can reduce NADP⁺ to NADPH. Along the way, it releases oxygen and builds a proton gradient that makes ATP. These light-dependent reactions happen in the thylakoid membranes of chloroplasts, and they supply the ATP and NADPH that the Calvin cycle uses to make sugar.

This article assumes you’ve read photosynthesis: the chemistry and chlorophyll.

The challenge in energy terms

Chemists measure how strongly a substance holds its electrons with its reduction potential (E°′). The more positive, the more tightly held.

• O₂/H₂O: E°′ ≈ +0.82 V: water holds its electrons very tightly.
• NADP⁺/NADPH: E°′ ≈ −0.32 V: NADPH gives electrons away readily.

Moving electrons from water to NADP⁺ means pushing them “uphill” by about 1.14 V, the reverse of what happens in the mitochondrial electron transport chain. For two electrons, that requires about +220 kJ mol⁻¹. No single photon of visible light is energetic enough to do this job and power ATP production as well, so plants use two photosystems in series, each absorbing one photon per electron.

The components

The thylakoid membrane contains a chain of protein complexes:

1. Photosystem II (PSII), with its reaction-centre chlorophyll called P680 (it absorbs best at 680 nm) and a water-splitting manganese cluster;
2. plastoquinone (PQ), a small mobile electron and proton carrier in the membrane;
3. the cytochrome b₆f complex, which pumps protons;
4. plastocyanin (PC), a small copper-containing protein that carries electrons along the inside of the membrane;
5. Photosystem I (PSI), with reaction-centre chlorophyll P700;
6. ferredoxin (Fd), an iron–sulfur protein on the outside of the membrane;
7. ferredoxin–NADP⁺ reductase (FNR), the enzyme that makes NADPH;
8. ATP synthase.

(The photosystems are numbered in the order they were discovered, not the order in which electrons pass through them, which is why electrons meet PSII first.)

Step by step: non-cyclic electron flow

1. Light excites Photosystem II

Antenna pigments absorb photons and funnel the energy to P680. Excited P680 (written P680*) gives an electron to a nearby acceptor, becoming P680⁺. This charge separation is the moment light energy becomes chemical energy.

2. Water is split

P680⁺ is the strongest biological oxidising agent known, with a reduction potential estimated at about +1.2 V, strong enough to take electrons from water. It gets them via the oxygen-evolving complex, a cluster of four manganese ions, one calcium ion and five oxygen atoms (Mn₄CaO₅).

The cluster collects oxidising power one photon at a time, stepping through four states. After four photons, it removes four electrons from two water molecules at once:

2H₂O → O₂ + 4H⁺ + 4e⁻

The protons are released inside the thylakoid (the lumen), and the oxygen diffuses away as a by-product. This is where essentially all the oxygen in the atmosphere comes from. The manganese chemistry involved is a classic example of transition metals cycling through oxidation states (see transition metals).

3. Electrons flow to cytochrome b₆f

From PSII, electrons pass to plastoquinone, which picks up two electrons and two protons from the stroma (outside the thylakoid), becoming plastoquinol (PQH₂). PQH₂ moves through the membrane to the cytochrome b₆f complex, which takes the electrons and releases the protons into the lumen. Through a cycle similar to the Q cycle in mitochondria, this complex pumps extra protons inward.

4. Plastocyanin carries electrons to Photosystem I

Cytochrome b₆f passes electrons, one at a time, to plastocyanin, a copper protein (Cu²⁺ ⇌ Cu⁺), which diffuses through the lumen to PSI.

5. Light excites Photosystem I

A second photon excites P700, which gives away an electron and becomes P700⁺. P700⁺ is refilled by the electron from plastocyanin. The electron given away is now at a very high energy, a very negative reduction potential.

6. NADPH is made

The high-energy electron passes through iron–sulfur clusters to ferredoxin, then to ferredoxin–NADP⁺ reductase, which uses two electrons and a proton to reduce NADP⁺:

NADP⁺ + 2e⁻ + H⁺ → NADPH

This happens on the stroma side, where NADPH is needed by the Calvin cycle.

The Z-scheme

If you plot the reduction potential of each carrier against the path of the electrons, the diagram looks like a sideways letter Z:

• the electron starts low (water, +0.82 V),
• jumps up when PSII absorbs light,
• runs downhill through plastoquinone, cytochrome b₆f and plastocyanin (this downhill step pumps protons),
• jumps up again when PSI absorbs light,
• runs downhill to NADP⁺ (−0.32 V).

Two photons are absorbed per electron transferred, so making one O₂ (four electrons) needs at least eight photons.

Making ATP: photophosphorylation

The light reactions build up a large proton gradient across the thylakoid membrane:

• protons released by water splitting in the lumen;
• protons pumped into the lumen by cytochrome b₆f;
• protons removed from the stroma when NADPH is made.

The lumen becomes much more acidic than the stroma, by about 3 pH units in bright light. Protons flow back out through ATP synthase, whose head faces the stroma, driving ATP synthesis just as in mitochondria. This is photophosphorylation, and it follows the same chemiosmotic principle Peter Mitchell proposed (see ATP: the cell’s energy currency).

The acidic lumen is a nice reminder of pH chemistry at work in living cells (see the pH scale explained).

Cyclic electron flow

The Calvin cycle needs ATP and NADPH in a ratio of about 3:2, but non-cyclic flow produces slightly less ATP than that. To make up the difference, chloroplasts can run cyclic electron flow: electrons from PSI’s ferredoxin are sent back to the cytochrome b₆f complex instead of to NADP⁺. They then return to PSI via plastocyanin.

• Only PSI is used.
• No water is split, no oxygen is released and no NADPH is made.
• Protons are still pumped, so ATP is made.

Plants switch between cyclic and non-cyclic flow to balance their ATP and NADPH supply.

Protecting the machinery

The light reactions are powerful but risky. Excess light can over-reduce the electron carriers and create reactive oxygen species that damage proteins. Photosystem II’s D1 protein is damaged so often that plants replace it every few hours in bright light. Carotenoids and special energy-dissipating processes help shed excess energy safely as heat.

Summary of inputs and outputs

Input	Output
Light energy	ATP (in the stroma)
H₂O	NADPH (in the stroma)
ADP + Pᵢ	O₂ (released)
NADP⁺

Key takeaways

• The light-dependent reactions happen in the thylakoid membranes and use light to move electrons from water (+0.82 V) to NADP⁺ (−0.32 V).
• Photosystem II (P680) splits water at the Mn₄CaO₅ cluster, releasing O₂; Photosystem I (P700) boosts electrons to make NADPH.
• Electrons pass through plastoquinone, cytochrome b₆f and plastocyanin, following the Z-scheme, with two photons per electron.
• The resulting proton gradient drives ATP synthase (photophosphorylation).
• Cyclic electron flow through PSI alone makes extra ATP without NADPH or O₂. The products fuel the Calvin cycle.