A thylakoid is a membrane-bound compartment in which the light-dependent reactions of oxygen-producing photosynthesis occur. Thylakoids form an internal membrane system within the chloroplasts of plants and algae, and occur in most cyanobacteria. Their membranes contain the molecular machinery that captures light, transfers electrons, and generates chemical energy. A thylakoid comprises both its enclosing membrane and its aqueous interior, called the lumen; it is not simply another name for a chloroplast or a stack of photosynthetic membranes. (pmc.ncbi.nlm.nih.gov)
Structure and organization
In plant chloroplasts, thylakoids are embedded in the stroma, the aqueous compartment enclosed by the chloroplast envelope. The thylakoid membrane separates the lumen from this surrounding space. Many thylakoid regions are flattened and closely appressed into cylindrical stacks called grana, singular granum. Unstacked regions, known as stroma lamellae, connect the stacks. Grana are therefore domains of an interconnected membrane network rather than collections of completely independent sacs. (pmc.ncbi.nlm.nih.gov)
The network has a complex three-dimensional geometry. Electron tomography has revealed helical connections between grana and stroma lamellae, with membrane surfaces joining stacked and unstacked regions. These connections provide pathways for movement within both the membrane and its enclosed aqueous compartment. Consequently, the familiar textbook image of separate piles of discs linked by simple tubes is a useful schematic rather than a complete representation of thylakoid architecture. (pmc.ncbi.nlm.nih.gov)
Thylakoid membranes contain a high concentration of proteins within a specialized lipid matrix. Their major lipid classes include the galactolipids monogalactosyldiacylglycerol and digalactosyldiacylglycerol, together with sulfoquinovosyldiacylglycerol and phosphatidylglycerol. Lipids contribute to membrane organization and influence the behavior of embedded photosynthetic complexes; experiments with reconstituted membranes show that lipid composition can alter light-harvesting activity. (pmc.ncbi.nlm.nih.gov)
Photosynthetic machinery
The membrane contains chlorophyll and carotenoid pigments associated with light-harvesting proteins. These antenna complexes absorb light and transfer excitation energy toward reaction centers. The principal photochemical complexes are photosystem II and photosystem I, which operate in sequence during linear electron flow. Other essential components include the cytochrome b₆f complex, mobile electron carriers, and ATP synthase. (pmc.ncbi.nlm.nih.gov)
These complexes are not distributed uniformly throughout plant thylakoids. Photosystem II and its major antenna complex, LHCII, are concentrated in appressed grana membranes. Photosystem I and ATP synthase are enriched in non-appressed regions, including stroma lamellae and exposed portions of grana. This spatial differentiation links membrane architecture to the organization of photosynthetic reactions while retaining communication between the different domains. (pmc.ncbi.nlm.nih.gov)
Electron transport and ATP production
Linear electron flow transfers electrons from water to NADP⁺ through two successive light-driven reaction centers. At photosystem II, water oxidation supplies replacement electrons and releases oxygen and protons. Electrons pass through an electron transport chain that includes plastoquinone, cytochrome b₆f, and plastocyanin before reaching photosystem I. A second photochemical step permits transfer through ferredoxin toward NADP⁺ reduction, producing NADPH. These transfers comprise a coordinated sequence of oxidation–reduction reactions. (pmc.ncbi.nlm.nih.gov)
Electron transport is coupled to proton accumulation in the lumen. Water oxidation releases protons on the lumenal side, while plastoquinone-mediated transport and cytochrome b₆f activity contribute additional proton transfer from the stroma. The resulting electrochemical gradient includes a difference in proton concentration and an electrical component; together these constitute the proton-motive force. The membrane thus separates compartments in which different proton conditions can be maintained. (pmc.ncbi.nlm.nih.gov)
Protons return from the lumen to the stroma through ATP synthase. This flow drives synthesis of adenosine triphosphate (ATP) from adenosine diphosphate and inorganic phosphate. The coupling mechanism is chemiosmosis, and light-driven ATP formation is called photophosphorylation. ATP synthase’s catalytic portion faces the stroma, where the newly produced ATP becomes available. (pmc.ncbi.nlm.nih.gov)
ATP and NADPH support stromal reactions, including the Calvin–Benson cycle responsible for carbon fixation. Cyclic electron flow around photosystem I provides an additional route for building the proton gradient without net NADPH production. It helps adjust ATP production relative to reducing-power supply as metabolic requirements change. (pmc.ncbi.nlm.nih.gov)
Variation among organisms
Plant-style grana are not universal. Diatoms, for example, possess loosely stacked thylakoid membranes whose organization differs from that of land plants, although photosynthetic complexes remain spatially differentiated. In cyanobacteria, thylakoids occur within the cell rather than inside chloroplasts and may form peripheral layers or extensive internal networks. Tomographic studies have identified membrane bridges and perforations that permit intracellular traffic. (nature.com)
Cyanobacterial thylakoids can accommodate components of both photosynthesis and respiration. Some cyanobacteria, notably Gloeobacter, lack thylakoids and locate their photosynthetic machinery in the cytoplasmic membrane. Thylakoids are therefore characteristic of most cyanobacteria, not a defining structure present in every member of the group. (pmc.ncbi.nlm.nih.gov)
Remodeling and investigation
Thylakoid organization is dynamic. Changes in antenna associations redistribute excitation between photosystems, while protective mechanisms dissipate excess excitation energy. Repair of damaged photosystem II involves disassembly and replacement of affected subunits, particularly D1. Experiments with spinach thylakoids show that unstacking can facilitate access to damaged proteins and their degradation during light stress. (nature.com)
An electron microscope reveals membrane ultrastructure, and electron tomography reconstructs connections in three dimensions. Live-cell chlorophyll-fluorescence imaging complements these structural methods by following changes over time. Observations in moss chloroplasts demonstrate that individual thylakoid regions can remain flexible even when the overall network retains a broadly stable organization. (pmc.ncbi.nlm.nih.gov)