The light-dependent reactions capture sunlight and use it to produce two energy-rich molecules: ATP and NADPH. As a byproduct, they split water and release the oxygen we breathe. These reactions take place in the thylakoid membrane, a system of flattened, interconnected sacs inside the chloroplast. Everything that happens here sets the stage for the second half of photosynthesis, where carbon dioxide is converted into sugar.
How Light Energy Gets Captured
The process starts when pigment molecules, primarily chlorophyll, absorb photons of light. Chlorophyll absorbs most efficiently in two bands: blue light (400 to 500 nm) and red light (650 to 680 nm). Green wavelengths are mostly reflected, which is why plants look green. Accessory pigments like carotenoids broaden the range of usable light slightly, funneling that energy toward chlorophyll at the center of each photosystem.
The absorbed light energy excites electrons in the chlorophyll to a higher energy state. These energized electrons are the currency that drives everything downstream. Two large protein complexes embedded in the thylakoid membrane, called photosystem II and photosystem I, each absorb light and boost electrons in sequence. The arrangement is sometimes called the “Z-scheme” because, when diagrammed by energy level, the electrons zigzag from one photosystem to the next.
Water Splitting and Oxygen Release
Photosystem II is where the light-dependent reactions truly begin. When its chlorophyll absorbs a photon, it loses a high-energy electron to the next carrier in the chain. That lost electron needs to be replaced, and the replacement comes from water. A cluster of manganese, calcium, and oxygen atoms called the oxygen-evolving complex strips electrons from water molecules in a four-step cycle. After four electrons have been removed, the overall reaction looks like this: two water molecules yield one molecule of oxygen gas, four protons (hydrogen ions), and four electrons.
This is the origin of all the oxygen released by photosynthesis. The protons freed from water are dumped into the interior of the thylakoid, which becomes important later for ATP production. The electrons, meanwhile, enter the electron transport chain.
The Electron Transport Chain
After leaving photosystem II, the energized electrons don’t jump straight to photosystem I. They pass through a series of carrier molecules embedded in the thylakoid membrane, losing a little energy at each step. The key players are a small mobile molecule called plastoquinone and a large protein complex called the cytochrome b6f complex.
Plastoquinone picks up electrons from photosystem II, along with protons from the fluid outside the thylakoid (the stroma), and shuttles them to the cytochrome b6f complex. This complex sits at the heart of the chain, connecting the two photosystems. As electrons move through it, the complex actively pumps additional protons from the stroma into the thylakoid interior. Each step adds to a growing concentration difference of protons across the membrane.
From the cytochrome b6f complex, electrons are handed off by a small copper-containing protein called plastocyanin, which ferries them to photosystem I. At this point, the electrons have lost much of the energy they gained in photosystem II, but they’re about to get a second energy boost.
Photosystem I and NADPH Production
Photosystem I absorbs another photon and re-energizes the electrons. Instead of pumping protons like photosystem II’s side of the chain, photosystem I directs its high-energy electrons to a small iron-sulfur protein called ferredoxin. Ferredoxin then passes the electrons to an enzyme that combines them with a proton and the carrier molecule NADP+, producing NADPH.
NADPH is essentially a packet of stored electrons and hydrogen. It will carry that reducing power into the Calvin cycle, where it helps convert carbon dioxide into sugar. Linear electron flow through both photosystems produces ATP and NADPH at a fixed ratio of roughly 2.6 ATP for every 2 NADPH.
How ATP Is Made
All that proton pumping has a purpose. As electrons move through the chain, protons accumulate inside the thylakoid, creating a steep concentration gradient across the membrane. The inside becomes more acidic (more protons) relative to the stroma outside. This stored energy is called the proton motive force.
Protons can only escape through one route: a large enzyme called ATP synthase, which spans the thylakoid membrane. As protons flow back out through ATP synthase, the enzyme physically rotates, and that mechanical energy drives the attachment of a phosphate group onto ADP, forming ATP. This process is called chemiosmosis, and it’s remarkably similar to how mitochondria produce ATP during cellular respiration. The difference is the energy source: in chloroplasts, light powers the proton gradient rather than food molecules.
Cyclic Electron Flow
Under certain conditions, plants run a modified version of the light reactions called cyclic electron flow. In this pathway, electrons from photosystem I cycle back through the cytochrome b6f complex instead of being used to make NADPH. This still pumps protons and generates ATP, but it produces no NADPH and releases no oxygen.
Why would a plant do this? The Calvin cycle requires slightly more ATP than NADPH. Linear electron flow alone doesn’t always produce enough ATP to keep up. Cyclic electron flow tops off the ATP supply without generating excess NADPH. It also appears to be especially important when plants first transition from dark to light. Research in algae has shown that when cyclic flow is blocked, carbon fixation stalls during the initial moments of illumination because the pools of phosphorylated sugar intermediates in the Calvin cycle haven’t been replenished yet. Once linear flow ramps up fully, cyclic flow plays a smaller, supplementary role.
What the Light Reactions Produce Overall
Putting it all together, the light-dependent reactions take in water and light energy and produce three things:
- ATP, the cell’s main energy currency, generated by chemiosmosis
- NADPH, a carrier of high-energy electrons
- Oxygen, released as a byproduct of water splitting
ATP and NADPH then move into the stroma, where the Calvin cycle uses them to fix carbon dioxide into three-carbon sugars. Without the light reactions continuously regenerating these two molecules, the Calvin cycle would grind to a halt within seconds. Oxygen diffuses out of the chloroplast, out of the leaf, and into the atmosphere. Every breath you take depends on photosystem II splitting water somewhere on Earth.