Thylakoid membranes are the internal membrane system inside chloroplasts where the light-dependent reactions of photosynthesis take place. They convert sunlight into the chemical energy that drives nearly all life on Earth, producing the ATP and NADPH that fuel sugar synthesis while releasing oxygen as a byproduct.1PubMed Central. Structure, biogenesis, and evolution of thylakoid membranes What makes them remarkable is not just what they do but how they are organized: a highly ordered, lipid-rich architecture that separates distinct photosynthetic tasks into different physical zones, all while dynamically reshuffling its own components in response to shifting light and temperature.
How Thylakoid Membranes Are Arranged Inside the Chloroplast
A chloroplast viewed under an electron microscope reveals two visually distinct membrane domains. The first is a set of coin-like discs stacked on top of each other, called grana. A single granum can contain anywhere from a few to several dozen of these stacked discs. The second domain consists of unstacked sheets called stroma lamellae, which thread through the surrounding fluid (the stroma) and connect adjacent grana stacks to one another.
These two domains are not just visually different; they house different molecular machinery. The grana stacks are densely packed with photosystem II and its associated light-harvesting antenna complexes, while the stroma lamellae are enriched in photosystem I and ATP synthase.2PubMed Central. Granal thylakoid structure and function: explaining an enduring mystery of higher plants This physical separation creates a division of labor: different zones of the same continuous membrane system handle different stages of light energy capture and conversion.3PubMed. The structural and functional domains of plant thylakoid membranes
Electron tomography has shown that the connection between grana and stroma lamellae is more intricate than older textbook illustrations suggest. The stroma lamellae wrap around grana stacks in the form of multiple right-handed helices, tilted at roughly 20 to 25 degrees, and connect through narrow tubular junctions.4Plant Physiology. Three-Dimensional Architecture of Grana and Stroma Thylakoids of Higher Plants as Determined by Electron Tomography This helical winding ensures that the internal water-filled space, the lumen, forms one continuous compartment even though the membrane itself is partitioned into functionally different zones. That continuity matters because the proton gradient used to make ATP depends on protons accumulating inside the lumen regardless of where in the membrane they were pumped in.
A Membrane Unlike Most Others
The lipid makeup of thylakoid membranes is unusual compared to the membranes found in animal cells or even elsewhere in the plant cell. Most biological membranes are dominated by phospholipids, but thylakoid membranes are built primarily from galactolipids, specifically two types abbreviated MGDG and DGDG. These uncharged, sugar-bearing lipids form the bulk of the lipid bilayer and provide the matrix in which photosynthetic protein complexes are embedded.5PubMed Central. Role of membrane glycerolipids in photosynthesis, thylakoid biogenesis and chloroplast development
This galactolipid dominance is not just a biochemical curiosity. The ratio of MGDG to DGDG plays a direct role in shaping chloroplast structure. DGDG molecules can form hydrogen bonds between the sugar-bearing heads of lipids on adjacent membrane layers, helping to hold grana stacks together. Charged lipids in the membrane push layers apart, and DGDG interactions counterbalance that repulsion, allowing the regularly stacked architecture to persist.6PubMed. Contribution of galactoglycerolipids to the 3-dimensional architecture of thylakoids Recent work has demonstrated that changing the galactolipid ratio alone is enough to reshape the entire chloroplast, underscoring how tightly form and function are linked through membrane lipids.7PubMed. Shaping chloroplasts via galactolipids
These galactolipids also respond to environmental stress. Under phosphate deprivation, temperature extremes, or drought, galactolipids undergo dynamic remodeling, shifting their composition to keep the membrane functional under conditions that might otherwise disrupt photosynthesis.
Capturing Light and Moving Electrons
The core job of the thylakoid membrane is to absorb photons and use that energy to move electrons through a chain of protein complexes, ultimately generating ATP and NADPH. The process starts at photosystem II, concentrated in the grana stacks, where light energy splits water molecules. This water-splitting reaction releases oxygen, donates electrons into the transport chain, and deposits protons into the thylakoid lumen. Measurements of the photosystem II content across different membrane zones confirm that the grana core holds the highest concentration and the most active reaction centers.8PubMed. EPR characterization of photosystem II from different domains of the thylakoid membrane
Electrons from photosystem II are then shuttled to the cytochrome b6f complex, which sits at a crossroads in the chain. This complex oxidizes plastoquinol, a small mobile carrier that ferries electrons from photosystem II, and uses that energy to pump additional protons across the membrane into the lumen.9PubMed. The Cytochrome b6f Complex: Biophysical Aspects of Its Functioning in Chloroplasts The complex operates as a dimer, enclosing a central chamber where plastoquinone and its partially reduced forms couple proton movement with electron transfer.10Trends in Plant Science. Thylakoid Membrane: Function, Structure, and Photosynthesis
From the cytochrome b6f complex, electrons move via plastocyanin to photosystem I, which is concentrated in the stroma lamellae. Photosystem I absorbs a second photon and uses that energy boost to reduce ferredoxin, which in turn donates electrons to make NADPH. The whole chain, from water to NADPH, is called linear electron flow, and it is the primary route for generating the two energy currencies (ATP and NADPH) that the Calvin cycle needs to build sugars.
The Proton Gradient and ATP Production
The electron transport chain does not make ATP directly. Instead, it builds up a steep difference in proton concentration across the thylakoid membrane: high inside the lumen, low in the stroma. This proton gradient is the energy source that drives ATP synthase, the molecular turbine embedded in stroma lamellae and at the flat end-caps of grana stacks.11PubMed Central. Proton gradient across the chloroplast thylakoid membrane governs the redox regulatory function of ATP synthase Protons flow back out through ATP synthase down their concentration gradient, and the mechanical rotation this drives stitches ADP and phosphate together into ATP.12PubMed Central. Proton Gradients and Proton-Dependent Transport Processes in the Chloroplast
Structural studies in spinach and pea chloroplasts have found that ATP synthase covers about 20 percent of the surface area at grana end membranes and stroma lamellae, with the vast majority existing as individual molecules rather than clusters.13PubMed Central. Arrangement of Photosystem II and ATP Synthase in Chloroplast Membranes of Spinach and Pea ATP synthase is excluded from the tightly stacked interior of grana, which makes sense given that its bulky stroma-facing headpiece would not fit in the narrow gap between stacked discs.
Beyond linear electron flow, plants also run cyclic electron flow, in which electrons loop back from photosystem I through the cytochrome b6f complex without passing through photosystem II. Cyclic flow does not produce NADPH or release oxygen, but it pumps additional protons into the lumen, boosting the gradient and therefore ATP production. This is increasingly recognized as essential for plant growth, especially under stress, because it fine-tunes the ATP-to-NADPH ratio and helps activate protective mechanisms.14PubMed Central. Regulation of cyclic and linear electron flow in higher plants
The Thylakoid Lumen Is More Than a Proton Reservoir
The water-filled interior of the thylakoid is easy to think of as just a proton holding tank, but the lumen hosts its own suite of proteins and chemical reactions. It is where oxygen evolution takes place, where plastocyanin shuttles electrons between complexes, and where key photoprotective processes are triggered. Lumenal proteins also play roles in regulating the assembly and turnover of photosynthetic complexes, including the repair of damaged photosystem II components.15PubMed Central. Understanding the roles of the thylakoid lumen in photosynthesis regulation The pH inside the lumen is not just a power source for ATP synthase. It also serves as a signal: when the lumen becomes very acidic, it triggers protective responses in the membrane above, a feedback loop discussed in the next section.
Balancing Energy Input When Light Changes
Sunlight is not constant. A cloud passing overhead can cut light intensity in seconds, and the spectrum shifts as leaves shade one another in a canopy. Thylakoid membranes cope with these fluctuations through several dynamic adjustments.
One of the fastest is a process called state transitions. When photosystem II absorbs more light than photosystem I can keep up with, a kinase enzyme phosphorylates the light-harvesting antenna complexes attached to photosystem II. This weakens their grip on photosystem II and increases their affinity for photosystem I, effectively redirecting some of the captured light energy to the complex that needs it most.16PubMed. Role of chloroplast protein kinase Stt7 in LHCII phosphorylation and state transition in Chlamydomonas Modeling work suggests that this redistribution does not require a large-scale migration of antenna proteins from grana to stroma lamellae; instead, photosystem I more efficiently recruits antenna complexes that already reside nearby in the unstacked regions.17PubMed Central. Modeling the Role of LHCII-LHCII, PSII-LHCII, and PSI-LHCII Interactions in State Transitions
When light is too intense rather than imbalanced, plants activate nonphotochemical quenching (NPQ), a set of processes that safely dump excess absorbed energy as heat before it can damage the photosynthetic machinery. A protein called PsbS senses the drop in lumenal pH that comes with intense light and triggers a conformational switch in a small number of antenna complexes. A pigment called zeaxanthin then extends the reach of this quenching effect across a broader domain of the antenna system.18The Plant Cell. Nonphotochemical quenching in plants: Mechanisms and mysteries The interplay between the pH signal, PsbS, and zeaxanthin gives plants a rapid, reversible safety valve against photodamage.
When Damage Happens Anyway
Despite these protective mechanisms, photosystem II inevitably sustains damage during normal operation. The same water-splitting chemistry that makes oxygenic photosynthesis possible generates reactive oxygen species that attack the D1 protein at the core of photosystem II. Plants have evolved an elaborate repair cycle to deal with this. Damaged photosystem II complexes are phosphorylated, which triggers their separation from the tightly packed grana. The damaged complex then migrates laterally to the unstacked stroma lamellae, where it is partially disassembled. The degraded D1 protein is removed and replaced with a freshly synthesized copy, after which the reassembled complex migrates back to the grana for reactivation.19PubMed Central. Revisiting the photosystem II repair cycle
The degradation of damaged D1 protein is carried out cooperatively by at least two families of proteases. FtsH, positioned on the stromal side, handles the bulk of steady-state D1 degradation. Under severe photoinhibitory conditions, lumenal Deg proteases contribute by cleaving D1 into fragments that FtsH and other proteases can process more rapidly.20PubMed Central. Cooperative D1 Degradation in the Photosystem II Repair Mediated by Chloroplastic Proteases in Arabidopsis This two-pronged system ensures that repair keeps pace with damage across a wide range of light intensities.
The repair cycle has an interesting side benefit: it protects photosystem I. When the repair of photosystem II is blocked experimentally (for instance, by inhibiting protein synthesis), the loss of functional photosystem II centers actually reduces electron flow to photosystem I, preventing the kind of over-reduction that causes irreversible photodamage there. In effect, controlled sacrifice of photosystem II activity acts as a brake that shields the harder-to-replace photosystem I from permanent harm.21Biochimica et Biophysica Acta (BBA) – Bioenergetics. Photosystem II photoinhibition-repair cycle protects Photosystem I from irreversible damage
How Thylakoid Membranes Are Assembled
Building and maintaining thylakoid membranes requires coordination between two genomes. About half of the proteins in the thylakoid membrane are encoded by nuclear DNA, made in the cytoplasm, and imported into the chloroplast through specialized pore complexes in the chloroplast envelope. Once inside, these proteins are routed to the thylakoid membrane through several distinct insertion pathways. The other half are encoded by the chloroplast’s own genome and are synthesized right in the stroma, where they are inserted into the membrane as they are being made.22PubMed Central. Protein Targeting Into the Thylakoid Membrane Through Different Pathways
Lipid supply is another logistical challenge. The galactolipids that dominate thylakoid membranes are synthesized at the chloroplast’s inner envelope membrane, not at the thylakoid itself. How these lipids travel from the envelope to the thylakoid has long been a puzzle. Recent proteomic work has identified a membrane fraction intermediate in density between the inner envelope and the thylakoid that is enriched in lipid metabolic enzymes and transport proteins, suggesting the existence of dedicated contact sites where lipid handoff occurs.23Nature Communications. Lipid metabolism and contact site homologs are present at the chloroplast envelope-thylakoid interface This is still an active area of research, but the picture emerging is one of specialized molecular bridges that shuttle lipids to where they are needed.
Adjusting to Temperature
Because thylakoid function depends so heavily on membrane fluidity, temperature is a constant challenge. Membranes that work well at 20°C can become dangerously fluid at 45°C, disrupting the precise arrangement of protein complexes. Plants acclimate by adjusting the fatty acid composition of their thylakoid lipids: higher growth temperatures lead to more saturated fatty acids, which stiffen the membrane and lower its fluidity. Studies on oleander plants shifted between cool and hot growth conditions showed that the thermal stability of energy transfer to photosystem II shifted by about 10 degrees Celsius, tracking changes in lipid fluidity and fatty acid composition.24Biochimica et Biophysica Acta (BBA) – Biomembranes. Correlations between the thermal stability of chloroplast (thylakoid) membranes and the composition and fluidity of their polar lipids upon acclimation of the higher plant, Nerium oleander, to growth temperature The relationship between fatty acid makeup and fluidity is not simple or linear, but across species, there appears to be a limiting maximum fluidity beyond which membrane structure breaks down, and plants adjust their lipid chemistry to stay below that threshold.25PubMed Central. Response of isolated thylakoid membranes with altered fluidity to short term heat stress
Thylakoid Membranes in Other Organisms
The grana-and-stroma-lamellae layout described above is specific to higher plants and some green algae. Cyanobacteria, the free-living bacteria that gave rise to chloroplasts through an ancient endosymbiotic event, also possess thylakoid membranes, but theirs typically form concentric or parallel sheets without grana stacking.26PubMed. Biogenesis and origin of thylakoid membranes Molecular dynamics simulations comparing cyanobacterial and plant thylakoid membranes find that the cyanobacterial version is considerably less fluid, even when the higher growth temperatures of thermophilic cyanobacteria are accounted for. The plant membrane also has a stronger tendency to undergo phase transitions, which may relate to its ability to rapidly restructure under changing conditions.27PubMed. Characterization of thylakoid lipid membranes from cyanobacteria and higher plants by molecular dynamics simulations
Diatoms, a major group of single-celled algae responsible for a large share of oceanic photosynthesis, have their own thylakoid arrangement. Their membranes typically run in sets of three loosely stacked layers rather than forming the tall grana columns seen in plants. Structural analysis of diatom photosystem II has revealed a supercomplex with an unusual configuration, featuring a dimeric core flanked by trimeric and monomeric antenna subunits in a layout distinct from the plant version.28Oxford University Press (Plant Physiology). Revealing the architecture of the photosynthetic apparatus in the diatom Thalassiosira pseudonana These differences reflect over a billion years of independent evolution since diatom chloroplasts were acquired through a secondary endosymbiosis event, and they serve as a reminder that there is no single “correct” thylakoid blueprint. Evolution has arrived at multiple workable designs.
Thylakoid Membranes in Biotech and Artificial Cells
The efficiency of thylakoid membranes at converting light into chemical energy has attracted interest from researchers working on bio-inspired solar technologies. One recent approach uses extracted thylakoid membranes immobilized on a graphene-cellulose nanofiber matrix as a photoanode in a biophotovoltaic cell. This system achieved a peak power density of about 56 milliwatts per square meter and a stable current density of 100 milliamps per square meter, and it doubles as a sensor for detecting waterborne herbicides that disrupt photosynthetic electron transport.29Journal of Photochemistry and Photobiology A: Chemistry. Environmentally benign biophotovoltaic cell using thylakoid membrane and graphene–cellulose nanocomposite for solar energy harvesting and photoelectrochemical monitoring of waterborne herbicides
On the more fundamental science end, researchers have built artificial cells containing real thylakoid membranes. By encapsulating thylakoids inside giant lipid vesicles and creating compartmentalized internal structures that mimic chloroplasts, these so-called TA-cells can carry out light-driven electron transfer when illuminated.30ACS Synthetic Biology. Thylakoid Containing Artificial Cells for the Investigation of Light-Driven Electron Transfer during Photosynthesis Others are working on fully synthetic versions that use purified photosynthetic proteins reconstituted into artificial membranes to power internal metabolic reactions.31PubMed Central. Light energy transduction in liposome-based artificial cells These are still proof-of-concept experiments, far from practical energy harvesting. But they demonstrate that the modular, self-contained design of thylakoid-based photosynthesis can be extracted from living cells and made to work in entirely synthetic contexts, which opens doors for future applications in green chemistry and bioenergy.