Light-dependent reactions take place in the thylakoid membrane, a specialized internal membrane system inside chloroplasts. This is where sunlight is captured, water is split, and the energy carriers ATP and NADPH are produced for the next stage of photosynthesis. Every step of the light-dependent reactions, from absorbing photons to generating ATP, is physically tied to this membrane and the spaces it creates.
The Thylakoid Membrane Inside the Chloroplast
Chloroplasts have three distinct membranes. The outer two form a double-walled envelope that separates the organelle from the rest of the cell. Inside that envelope sits the thylakoid membrane, a third membrane folded into a network of flattened, disc-like sacs called thylakoids. The fluid-filled space surrounding the thylakoids is called the stroma, while the narrow space inside each thylakoid sac is the thylakoid lumen.
All of the electron-transport chains, light-capturing pigment systems, and ATP-producing machinery of the light reactions are embedded in or associated with this thylakoid membrane. The green pigment chlorophyll sits here, which is why chloroplasts appear green. When a photon of light hits chlorophyll, it energizes an electron that then moves through a chain of protein complexes in the membrane, much like electrons move through an energy-producing chain in mitochondria.
Grana Stacks and Stroma Lamellae
The thylakoid membrane is not one uniform sheet. It forms two distinct regions: grana stacks and stroma lamellae. Grana stacks look like columns of coins under an electron microscope, with multiple thylakoid discs pressed tightly together. Stroma lamellae are the unstacked portions that extend outward and connect grana stacks to one another, spiraling around them in right-handed helices.
These two regions house different parts of the light reactions. Photosystem II, the complex that captures light energy and splits water, is concentrated in the tightly stacked grana. Photosystem I and ATP synthase are mostly found in the unstacked stroma lamellae and the exposed end membranes of grana. ATP synthase is excluded from the interior of grana stacks for a simple physical reason: the gap between stacked thylakoids is too narrow to fit its bulky head, which extends about 16 nanometers above the membrane surface.
Despite this separation, the entire thylakoid network is connected. The inner spaces of grana and stroma lamellae are joined through narrow slit openings called frets at the edges of each granum. This means the whole thylakoid system encapsulates a single continuous water-filled lumen, allowing molecules to travel between the two regions.
How Electrons Move Between Regions
Because Photosystem II sits mainly in grana stacks and Photosystem I sits in stroma lamellae, electrons need a way to travel between them. Small, mobile electron carriers handle this job. One carrier called plastoquinone moves through the membrane itself, while another called plastocyanin diffuses through the thylakoid lumen. Plastocyanin is the long-range carrier, shuttling electrons from grana stacks to the distant unstacked regions where Photosystem I waits.
This arrangement means the thylakoid lumen is not just a passive space. It actively swells and shrinks in response to light, and the ease with which plastocyanin can diffuse through it directly regulates how fast the light reactions run.
Where Water Splitting Happens
The splitting of water, one of the most important steps in the light reactions, takes place on the lumen side of the thylakoid membrane. A cluster of proteins and metal ions known as the oxygen-evolving complex is bound to Photosystem II, facing into the thylakoid lumen. This complex pulls electrons from water molecules, releasing oxygen gas as a byproduct and dumping protons (hydrogen ions) directly into the lumen.
This is significant because those protons contribute to a concentration gradient across the thylakoid membrane, which is the driving force behind ATP production.
The Proton Gradient Across the Membrane
As electrons move through the transport chain in the thylakoid membrane, protons are pumped from the stroma into the thylakoid lumen. Combined with the protons released from water splitting, this creates a steep difference in proton concentration across the membrane. The lumen becomes acidic, dropping to a pH of roughly 5.7 to 6.5, while the stroma becomes slightly alkaline, rising to about pH 7.8 to 8.0. In the dark, both compartments sit near pH 7.
This gradient is a form of stored energy. Protons naturally want to flow back out of the lumen into the stroma, and ATP synthase provides the channel for that flow. As protons pass through ATP synthase, the enzyme spins like a molecular turbine and assembles ATP. The catalytic head of ATP synthase protrudes into the stroma, so the newly made ATP is released right where it is needed for the Calvin cycle, the next stage of photosynthesis that converts carbon dioxide into sugar.
Light Reactions in Organisms Without Chloroplasts
Plants and algae run their light reactions in chloroplasts, but photosynthetic bacteria called cyanobacteria do not have these organelles. Cyanobacteria still have thylakoid membranes, though. These membranes exist as a distinct intracellular membrane system within the bacterial cytoplasm and serve as the sole site of photosynthetic electron transport. In cyanobacteria, the same thylakoid membranes also handle much of the cell’s respiration, performing double duty that chloroplasts in plant cells do not.
This shared reliance on thylakoid membranes across both plants and cyanobacteria reflects the evolutionary origin of chloroplasts. Chloroplasts are descended from ancient cyanobacteria that were engulfed by early eukaryotic cells, which is why the basic architecture of the light reactions, a proton gradient across a thylakoid membrane driving ATP synthesis, is essentially the same in both groups.