Light-harvesting reactions take place in the thylakoid membranes inside chloroplasts. More specifically, they occur within protein complexes embedded in these membranes, where pigment molecules absorb sunlight and funnel the energy toward reaction centers that convert it into chemical energy. In plants and algae, chloroplasts contain these thylakoid membranes as an internal system of flattened, interconnected sacs. In cyanobacteria, which lack chloroplasts entirely, the thylakoid membranes float freely inside the cell but serve the same function.
Thylakoid Membranes: The Precise Location
Chloroplasts have a double outer membrane, but the real action happens deeper inside, on a third membrane system: the thylakoids. These are stacked into coin-like columns called grana (singular: granum), connected by unstacked sheets called stroma lamellae. Each granal layer consists of four strata: two lipid bilayers with proteins embedded in them, a narrow interior space called the lumen, and a thin stromal gap between layers.
The light-harvesting machinery isn’t spread evenly across these membranes. Photosystem II clusters mainly in the stacked grana regions, while Photosystem I concentrates in the unstacked stroma lamellae. This separation matters because the two photosystems do different jobs and hand off energy in sequence. The enzyme that produces ATP also sits in the stroma lamellae alongside Photosystem I, physically separated from the Photosystem II clusters in the grana stacks.
How Pigments Capture Light
The actual light-catching molecules are pigments, primarily chlorophyll a and chlorophyll b, along with carotenoids like beta-carotene and lutein. These pigments are not floating loose. They are held in precise arrangements within protein structures called light-harvesting complexes (LHCs), which act as antennas surrounding the two photosystems.
Chlorophyll a and b have absorption peaks that differ by about 20 nanometers, which lets them collectively capture a broader slice of the light spectrum. Both absorb strongly in the blue (around 430 to 460 nm) and red (around 640 to 680 nm) ranges, while absorbing only a few percent of green light (500 to 600 nm), which is why leaves look green. Carotenoids fill in part of the gap, absorbing high-energy blue and blue-green photons in the 400 to 520 nm range. Beta-carotene sits almost exclusively in the core of Photosystems I and II, while lutein and other carotenoids are found in the surrounding antenna complexes.
Some photosynthetic organisms push these boundaries further. Certain cyanobacteria and algae use chlorophyll d and f, which have red-shifted absorption peaks that let them harvest infrared light between 700 and 750 nm, wavelengths that chlorophyll a cannot use.
How Energy Moves to the Reaction Center
When a pigment molecule absorbs a photon, it doesn’t perform the chemistry itself. Instead, the energy hops from pigment to pigment through a process called fluorescence resonance energy transfer, first described by physicist Theodor Förster in the late 1940s. The transfer rate depends steeply on distance: it falls off with the sixth power of the gap between pigment molecules, meaning even small increases in separation dramatically slow the process. This is why the precise protein scaffolding that holds pigments at exact distances and orientations is so critical.
The energy migrates stochastically, generally moving from higher-energy pigments to lower-energy ones, like water flowing downhill. This funneling continues over distances of hundreds of angstroms until the excitation reaches a reaction center, where it triggers a charge separation event. At that point, light energy becomes electrical energy in the form of separated electrons and positive charges across the membrane.
The Two Photosystems and What They Produce
Photosystem II and Photosystem I work in series, connected by a chain of electron carriers. Photosystem II uses absorbed light energy to split water molecules into molecular oxygen, protons, and electrons. This is where all the oxygen from photosynthesis originates. The electrons then pass through a series of carriers, including a complex that pumps protons across the thylakoid membrane, building up a concentration gradient that drives ATP production.
Photosystem I picks up those electrons and, using energy from a second photon absorption, boosts them to an even higher energy level. Rather than pumping more protons, it uses these high-energy electrons to produce NADPH, a molecule that carries chemical reducing power. Together, ATP and NADPH are the two key outputs of the light-harvesting reactions, and both are consumed by the Calvin cycle in the surrounding stroma fluid to build sugars from carbon dioxide.
The yield from this process: for each pair of electrons that passes through both photosystems (called noncyclic electron flow), the cell generates between 1 and 1.5 molecules of ATP, plus NADPH. A separate pathway called cyclic electron flow loops electrons back through Photosystem I without producing NADPH, yielding a smaller 0.5 to 1 ATP per electron pair. Plants use this cyclic route to fine-tune the ratio of ATP to NADPH based on metabolic demand.
How the Antenna Complexes Are Organized
Each photosystem sits at the center of a larger assembly called a supercomplex. Photosystem II supercomplexes are dimeric, meaning two copies of the core sit side by side, surrounded by two to four copies of trimeric antenna complexes (LHCII). These trimers are the most abundant membrane protein on Earth, and each one holds multiple chlorophyll and carotenoid molecules in precise positions. Three specific antenna proteins mediate how each trimer docks onto the Photosystem II core, and recent structural studies have shown that additional proteins help link neighboring supercomplexes into even larger megacomplexes within the membrane plane.
Photosystem I takes a different form. Its core is a single unit (monomeric) with four different antenna proteins attached, plus binding sites for additional antenna complexes. This structural difference, along with the membrane’s lipid composition, is a key reason why the two photosystems segregate into different regions of the thylakoid membrane rather than mixing together.
Light Harvesting Without Chloroplasts
Cyanobacteria are the evolutionary ancestors of chloroplasts, and they carry out light-harvesting reactions on thylakoid membranes that run through their cytoplasm without being enclosed in a separate organelle. The core machinery is the same: photosystems embedded in lipid bilayer membranes, surrounded by antenna systems that capture and funnel light. The main difference is the antenna type. Many cyanobacteria use large, water-soluble antenna structures called phycobilisomes instead of the membrane-embedded LHC proteins found in plants. These phycobilisomes sit on the outer surface of the thylakoid membrane and absorb wavelengths that chlorophyll misses, particularly in the green, yellow, and orange range.
Plants also adapt their thylakoid architecture to changing light conditions. When illumination shifts, the membrane reorganizes its composition and shape at the nanoscale, adjusting the ratio and arrangement of antenna complexes to optimize energy capture under dim light or protect against damage under intense light.