The Light-Dependent Reaction of Photosynthesis

Core Concepts

Capturing light: photons and pigments

Light arrives as photons, discrete packets of energy. When a photon strikes a pigment such as chlorophyll, it can boost an electron to a higher energy level — the pigment is said to be excited. The excited electron is unstable and quickly returns to its ground state unless its energy is captured; in the antenna complex, dozens of pigment molecules absorb photons and pass the excitation energy from molecule to molecule until it reaches the reaction center, a special pair of chlorophyll molecules. There the energy is used to eject an electron to an acceptor, converting light energy into the chemical energy of separated charges — the first step of the whole process.

Photosystems: PSII and PSI

A photosystem is a protein–pigment complex with two functional parts: the antenna (light-harvesting) complex and the reaction center. Plants use two photosystems in series. Photosystem II is named for its order of discovery, not its position in the pathway — it acts first. Its reaction center, called P680 because it absorbs best at 680 nm, is the strongest biological oxidant known, strong enough to pull electrons from water. Photosystem I's reaction center, P700 (absorbing at 700 nm), is the last stop before NADP⁺. Keeping the two photosystems distinct is a common exam trap: the numbering reflects discovery order, and PSII runs before PSI.

Noncyclic electron flow: from water to NADP⁺

In noncyclic (linear) electron flow, photons excite P680 in PSII, and the ejected electrons travel down an electron transport chain — through plastoquinone, the cytochrome b₆f complex, and plastocyanin — losing energy at each step. That energy pumps protons from the stroma into the thylakoid lumen, building a proton gradient. Meanwhile, PSII replaces its lost electrons by splitting water: 2H₂O → O₂ + 4H⁺ + 4e⁻, which is why oxygen is released. At PSI, another photon excites P700, and these re-energized electrons are transferred to ferredoxin and then to NADP⁺ reductase, which reduces NADP⁺ to NADPH. The overall route — water, PSII, electron transport chain, PSI, NADP⁺ — is often drawn as the Z-scheme, named for the zigzag of energy levels.

Chemiosmosis and ATP synthesis

The proton gradient built across the thylakoid membrane is the same kind of energy store used in mitochondria. Protons flow back into the stroma through ATP synthase, and the enzyme couples that downhill flow to the phosphorylation of ADP, making ATP — photophosphorylation. Note the direction: in chloroplasts, protons are pumped into the thylakoid lumen and ATP is made in the stroma, the reverse of the mitochondrial arrangement. The ATP and NADPH produced here diffuse into the stroma, where the Calvin cycle consumes them.

Cyclic photophosphorylation: ATP without NADPH

Sometimes electrons leaving PSI return through the electron transport chain to PSII's side of the gradient instead of reducing NADP⁺. This cyclic electron flow pumps more protons and makes extra ATP but produces no NADPH and no oxygen. It lets the cell adjust the ATP-to-NADPH ratio to match the Calvin cycle's demand, which needs more ATP than NADPH. Cyclic flow is a useful regulatory option rather than a failure — it fine-tunes the output of the light reactions.

Common Confusions