The light reactions are the first stage of photosynthesis, and the question is how they turn light into chemical energy. In short, light-driven reaction centers in the thylakoid membranes of chloroplasts pull electrons from water and pass them down a chain of carriers to $\ce{NADP+}$, forming NADPH. Along the way, protons are pumped into the thylakoid interior, and their flow back out through ATP synthase makes ATP. The net products are ATP and NADPH, which power the Calvin cycle, and oxygen, which is released as a by-product.

The light reactions need a membrane that divides two compartments. In a chloroplast, flattened sacs called thylakoids enclose an internal space, the lumen. The surrounding fluid is the stroma. Photosystems, electron carriers and ATP synthase are all embedded in the thylakoid membrane, so that electron movement can be coupled to proton movement across it.

Because of this arrangement, the light reactions work like a battery being charged. Light energy drives electrons uphill, and that energy is used to pump protons into the lumen. The resulting gradient is a store of potential energy, and ATP synthase later spends it.

Each photosystem has a large antenna of chlorophyll and carotenoid molecules. When a pigment absorbs a photon, one of its electrons moves to a higher-energy state. That excitation hops from pigment to pigment until it reaches a special chlorophyll pair at the reaction center.

There are two photosystems, named for the order of their discovery rather than their order in the pathway:

• Photosystem II (PSII) has a reaction center called P680, after the wavelength in nanometers that it absorbs best.

• Photosystem I (PSI) has a reaction center called P700.

At the reaction center, the excited chlorophyll hands an electron to a nearby acceptor molecule. This is the key conversion step. Light energy has been turned into a separation of charge, with a strongly reducing electron on one side and a strongly oxidizing chlorophyll on the other.

The diagram traces the path in order. Use the step control to move through it, and read the sections below alongside it.

Photosystem II: splitting water

When P680 absorbs excitation energy, it becomes P680*, which is a very strong electron donor. It passes an electron to a pheophytin molecule, then on to bound quinones, and finally to a mobile carrier called plastoquinone. This leaves the reaction center as P680⁺, one of the strongest biological oxidants known.

P680⁺ is oxidizing enough to take electrons from water. A cluster of manganese and calcium atoms, the oxygen-evolving complex, collects four oxidizing charges one photon at a time. It then oxidizes two water molecules in one go:

$\ce{2H2O -> O2 + 4H+ + 4e-}$

This is the source of all the oxygen released by photosynthesis, and it comes from water, not from carbon dioxide. The four protons are released inside the lumen, which already begins to acidify it.

Plastoquinone: carrying electrons and protons

Plastoquinone picks up two electrons from PSII and, at the same time, two protons from the stroma, becoming plastoquinol. It then diffuses within the membrane to the next complex. Because it takes protons from the stroma side and releases them on the lumen side, this carrier acts as a shuttle for both electrons and protons.

Cytochrome b6f: a proton pump

Plastoquinol gives up its electrons to the cytochrome b6f complex. Through a mechanism known as the Q cycle, the complex passes electrons one at a time to plastocyanin, a small copper protein in the lumen. For each electron delivered, about two protons end up moved into the lumen. This is the main proton-pumping step of the chain, and it contributes more to the gradient than water splitting does.

Photosystem I: a second boost

Electrons carried by plastocyanin arrive at PSI and refill P700⁺, the oxidized reaction center. Meanwhile, a second photon excites P700 to P700*, an even stronger reductant than P680*. Its electron passes through a series of iron-sulfur centers to a small soluble protein, ferredoxin, in the stroma.

The need for two photosystems comes from energy. A single photon cannot move an electron from water, which holds electrons tightly, all the way to $\ce{NADP+}$. PSII lifts electrons from a very oxidizing level to a moderately reducing one. PSI then lifts them again, from a low-energy position after the chain to a strongly reducing one. Plotting the carriers by their energy gives a zigzag shape known as the Z-scheme.

Making NADPH

Ferredoxin carries one electron at a time. The enzyme ferredoxin-NADP⁺ reductase (FNR) collects two of them and uses them, with a proton from the stroma, to reduce $\ce{NADP+}$:

$\ce{NADP+ + H+ + 2e- -> NADPH}$

NADPH is a stable carrier of reducing power. It will deliver electrons to the Calvin cycle, where carbon dioxide is converted to sugar.

The three processes above leave the lumen with more protons than the stroma: water splitting adds protons, the b6f complex pumps them in, and plastoquinone moves them across. Taking protons up in the stroma to make NADPH also lowers the stroma's proton concentration. In illuminated chloroplasts the lumen can reach roughly pH 5 to 6, while the stroma is near pH 8.

In mitochondria, much of the proton-motive force is an electrical voltage. In chloroplasts, most of it is a pH difference, because other ions, such as magnesium and chloride, move across the membrane and cancel most of the charge difference.

Protons can only return through ATP synthase. This enzyme has a membrane-embedded ring that turns as protons pass through it, and the rotation drives conformational changes in the catalytic head, which sits in the stroma. These changes join ADP and inorganic phosphate:

$\ce{ADP + P_i -> ATP}$

The ring in plant chloroplasts has 14 subunits, so a full turn passes 14 protons and makes 3 ATP, or about 4.7 protons per ATP. This process is called photophosphorylation because light-driven electron flow ultimately powers it. The overall linear pathway can be summarized as:

$\ce{2H2O + 2NADP+ -> O2 + 2NADPH + 2H+}$

with about 2.6 ATP made per 2 NADPH, from the proton gradient formed along the way.

Counting protons shows a small shortfall. Passing four electrons through the chain moves about 12 protons into the lumen. That is enough for roughly 2.6 ATP, but the Calvin cycle needs 3 ATP for every 2 NADPH, so linear flow alone slightly under-supplies ATP.

Chloroplasts make up the difference with cyclic electron flow. In this route, electrons from ferredoxin return to plastoquinone or the b6f complex instead of reducing $\ce{NADP+}$. They cycle through the pump again, which adds more protons to the gradient. Cyclic flow makes ATP without making NADPH or oxygen, so the cell can adjust the ATP-to-NADPH ratio to its needs.

• The light reactions do not fix carbon. They make the ATP and NADPH that the Calvin cycle uses to do so.

• Oxygen comes from water, at PSII, not from carbon dioxide.

• Photosystems are numbered by discovery, not by order. Electrons meet PSII first, then PSI.

• Photophosphorylation resembles oxidative phosphorylation in mitochondria in using a proton gradient and a rotary ATP synthase. The difference is that the energy source is light, not oxidized food molecules.

In summary, light is converted into chemical energy in two linked ways. Excited electrons are captured and stored in the reducing power of NADPH, and the energy released as they move along the chain is stored in a proton gradient that ATP synthase turns into ATP.