Grana are stacks of flattened, disc-shaped membrane compartments called thylakoids, found inside the chloroplasts of land plants and some green algae. They serve as the primary platform for the light-dependent reactions of photosynthesis, concentrating the molecular machinery that captures sunlight and splits water. A single chloroplast can contain dozens of these stacks, and their layered architecture is not just a structural quirk. It directly shapes how efficiently a plant harvests light, balances energy flow between its two photosystems, and repairs photosynthetic damage.
What Grana Actually Look Like
Under a light microscope, grana were first spotted centuries ago as tiny green granules inside chloroplasts. The word “granum” literally means “grain” in Latin. When transmission electron microscopes arrived in the mid-twentieth century, researchers discovered that each of those granules is actually a stack of tightly pressed membrane discs, interconnected by unstacked membrane sheets called stroma lamellae (or stroma thylakoids).1PubMed Central. A brief history of how microscopic studies led to the elucidation of the 3D architecture and macromolecular organization of higher plant thylakoids Think of a granum as a short cylinder of stacked coins, where each “coin” is a thylakoid disc. The stroma lamellae connecting adjacent grana spiral around each stack in a helical pattern, with a tilt angle of roughly 20 to 25 degrees, joining the grana through narrow slit-like openings that range from about 15 by 30 nanometers to 15 by 435 nanometers.2Plant Physiology. Three-Dimensional Architecture of Grana and Stroma Thylakoids of Higher Plants as Determined by Electron Tomography
The diameter and height of grana stacks are not fixed. In the model plant Arabidopsis, grana diameters measured under different light conditions range from about 0.36 micrometers under light that preferentially excites Photosystem II to about 0.58 micrometers under high-intensity white light. The number of grana per chloroplast also shifts, from roughly 47 in the dark to around 69 under Photosystem II light.3PubMed Central. Dynamic Thylakoid Stacking Is Regulated by LHCII Phosphorylation but Not Its Interaction with PSI These numbers make clear that grana are not static scaffolding. They reshape themselves constantly in response to light.
Why the Stacking Matters for Photosynthesis
Plants run two photosystems in series. Photosystem II (PSII) absorbs light to split water and feed electrons into a transport chain, while Photosystem I (PSI) absorbs light at a slightly different wavelength to push those electrons onward to make the energy carrier NADPH. For the system to work efficiently, the two photosystems need to receive fairly balanced amounts of excitation energy. If too much energy spills from PSII directly into PSI, PSII becomes starved and the whole chain backs up.
Grana stacking solves this by physically separating the two photosystems. PSII and its associated light-harvesting antenna complexes are concentrated in the tightly stacked grana membranes, while PSI and a minor fraction of PSII sit in the unstacked stroma lamellae.4PubMed Central. Localization of different photosystems in separate regions of chloroplast membranes Recent work confirms that this physical distance prevents unwanted energy spillover from PSII to PSI, and that the degree of spillover tracks with the depletion of light-harvesting complexes rather than with grana shape alone.5PubMed Central. Specific light-harvesting complexes mediate grana stacking and prevent energy spillover between photosystems in plant chloroplasts In other words, stacking creates the segregation, but the antenna proteins are the active enforcers of balanced energy flow.
Beyond separating the photosystems, the stacked architecture also affects how protons accumulate inside the thylakoid lumen. The tight geometry of grana restricts how freely water and ions can move through the lumen, which means the pH inside grana lumens can differ from the pH in the lumens of stroma lamellae. That pH gradient is what drives ATP synthase to produce ATP, so the three-dimensional shape of grana directly influences the energetics of photosynthesis.6PubMed Central. Granal thylakoid structure and function: explaining an enduring mystery of higher plants
What Holds the Stack Together
Thylakoid membranes are made largely of specialized lipids found almost nowhere else in biology. Two of these, abbreviated DGDG and MGDG, play opposite but complementary roles in maintaining grana architecture. DGDG, a lipid with two sugar head groups, forms hydrogen bonds between the polar surfaces of adjacent membrane layers, essentially gluing them face-to-face. Those bonds counterbalance the electrical repulsion from negatively charged lipids in the same membrane, allowing the stack to persist even when the space between layers is fully hydrated.7PubMed. Contribution of galactoglycerolipids to the 3-dimensional architecture of thylakoids MGDG, meanwhile, has a cone-like molecular shape that naturally favors curved surfaces. It concentrates at the highly curved rim of each granum disc, stabilizing the tight bends where the flat stacked region transitions into the rounded margin.8PubMed. The Nonbilayer Lipid MGDG and the Major Light-Harvesting Complex (LHCII) Promote Membrane Stacking in Supported Lipid Bilayers
Proteins matter too. The major light-harvesting antenna complex of PSII (known as LHCII) is one of the most abundant membrane proteins on Earth, and it actively promotes membrane stacking through interactions between its portions that stick out into the space between layers. A family of small proteins called CURT1 takes care of a different structural job: inducing the sharp curvature at grana margins, where each flat disc bends back on itself. Plants with more CURT1 protein have more layers per granum, while plants lacking CURT1 produce fewer, flatter stacks.9PubMed Central. Arabidopsis CURVATURE THYLAKOID1 Proteins Modify Thylakoid Architecture by Inducing Membrane Curvature CURT1 proteins also participate in the earliest stages of thylakoid development, shaping the geometry of precursor structures called prolamellar bodies before grana even exist.10PubMed Central. Curvature thylakoid 1 proteins modulate prolamellar body morphology and promote organized thylakoid biogenesis in Arabidopsis thaliana
How Grana Reshape Themselves in Real Time
One of the more surprising discoveries about grana is how quickly and reversibly they change size. When light conditions shift, a plant needs to rebalance the workload between its two photosystems. It does this partly through “state transitions,” which involve shuffling antenna complexes between PSII and PSI. The molecular switch behind this is the phosphorylation of LHCII by a kinase called STN7 and its reversal by a phosphatase called TAP38. When LHCII gets phosphorylated, grana shrink in diameter; when it gets dephosphorylated, they expand. In mutants missing the kinase, grana stay large regardless of light changes, while mutants missing the phosphatase keep their grana small.3PubMed Central. Dynamic Thylakoid Stacking Is Regulated by LHCII Phosphorylation but Not Its Interaction with PSI
The protein Lhcb2, a specific member of the LHCII family, turns out to be a major driver of this dynamism. When researchers engineered chloroplasts to contain only Lhcb2, oversized grana largely disassembled upon illumination. Triggering Lhcb2 phosphorylation significantly reduced both grana diameter and height.11The Plant Cell. Specific light-harvesting complexes mediate grana stacking and prevent energy spillover between photosystems in plant chloroplasts Interestingly, computational simulations suggest that state transitions do not require large-scale migration of antenna complexes from grana to stroma lamellae. Instead, PSI in the stroma lamellae simply recruits antenna proteins that are already nearby when those proteins become phosphorylated, and PSII in grana does the same with dephosphorylated antennae that are already in the stack.12Biophysical Journal. Molecular Model of the Thylakoid Membrane Reveals the Different Contributions of Lateral and Stacking Interactions to Photosynthetic State Transitions The system is more like shuffling cards within two separate hands than moving cards from one hand to the other.
The Repair Shuttle
Bright light is a double-edged sword for plants. It provides the energy for photosynthesis, but it also damages PSII, particularly a core protein called D1 that sits at the heart of the water-splitting reaction. This damage is constant and unavoidable even under normal outdoor light. Plants survive it by running a continuous repair cycle, and grana architecture is central to how it works.
When a PSII complex gets damaged, its core subunits become phosphorylated. The damaged complex then breaks apart from its supercomplex partners and migrates laterally out of the crowded grana stack, traveling through the margins into the stroma lamellae, where the repair machinery has room to operate. Once the broken D1 protein is removed and replaced with a fresh copy, the reassembled PSII complex migrates back into the grana, reattaches to its antenna partners, and resumes work.13PubMed Central. Revisiting the photosystem II repair cycle Plants that cannot phosphorylate their PSII core proteins struggle to disassemble damaged supercomplexes at high light, which blocks this migration and slows repair.14PubMed. Core protein phosphorylation facilitates the repair of photodamaged photosystem II at high light
The narrow slit-like junctions between grana and stroma lamellae are not just architectural details. They function as controlled bottlenecks, ensuring that only disassembled, phosphorylated PSII cores can squeeze through for repair, while intact supercomplexes stay put in the grana where they belong.
How Grana Form When a Seedling First Sees Light
Seeds germinate underground in complete darkness, and their chloroplast precursors (called etioplasts) do not have grana. Instead, they contain a lattice-like structure called a prolamellar body, an intricate network of branching membrane tubules. When a seedling breaks through the soil and encounters light, the transformation is remarkably fast. Within about an hour of illumination, the first double-layered thylakoid sheets appear near the prolamellar body. By two hours, the crystalline lattice collapses from its outer surface inward, and the freed tubules merge into flat sheets that fold over each other to create the earliest grana stacks. By eight hours, the prolamellar bodies are completely gone and the chloroplast has taken on the characteristic grana-and-stroma-lamellae architecture of a mature organelle.15PubMed Central. Electron tomography of prolamellar bodies and their transformation into grana thylakoids in cryofixed Arabidopsis cotyledons
Plants That Do Without Grana
Not every chloroplast in every plant has grana. The most well-studied exception comes from C4 plants like maize and sugarcane. These species divide the labor of photosynthesis between two cell types. Mesophyll cells contain normal chloroplasts with well-developed grana stacks, but bundle sheath cells, which surround the leaf veins, contain chloroplasts that are partly or entirely agranal. These agranal chloroplasts lack functional PSII and cannot perform the water-splitting reaction; they are specialized instead for PSI-driven cyclic electron flow and the Calvin cycle.16PubMed Central. Deficient Photosystem II in Agranal Bundle Sheath Chloroplasts of C(4) Plants The absence of grana in these cells is not a defect. It is a deliberate adaptation that prevents oxygen production at the site where COâ‚‚ is being fixed, which would otherwise fuel wasteful photorespiration.
Looking beyond flowering plants, algae generally lack true grana. Their thylakoid membranes are arranged in loose stacks or parallel sheets, but without the tight appression and distinct grana-stroma-lamella organization seen in land plants. A vesicle trafficking system that helps build and maintain the complex thylakoid architecture of grana has been found exclusively in embryophytes, the lineage that includes mosses, ferns, and seed plants. This lineage evolved alongside the move from aquatic to terrestrial habitats, suggesting that the challenges of life on land selected for more elaborate internal membrane organization.17PubMed Central. A brief history of thylakoid biogenesis One influential hypothesis proposes that land plants evolved grana as a way to photosynthesize efficiently in shade, since terrestrial environments involve far more variable and often dimmer light conditions than aquatic ones.18PubMed. Function and evolution of grana
Shade, Sun, and Stress
The evolutionary connection between grana and shade tolerance plays out visibly in modern plants. Shade-adapted species generally have more thylakoid layers per granum, increasing the total membrane area available for light capture. Sun-adapted species tend toward fewer layers per stack but more grana overall. This pattern is well established for flowering plants, but at least one exception is instructive: the lycophyte Selaginella martensii, a shade-adapted fern relative with unusually giant chloroplasts, increases granum diameter rather than layer count when adapting to low light, essentially breaking the standard shade paradigm.19PubMed. Thylakoid membrane appression in the giant chloroplast of Selaginella martensii Spring: A lycophyte challenges grana paradigms in shade-adapted species Different lineages have found different structural solutions to the same problem.
When conditions become truly hostile, grana are among the first casualties. Under combined high-temperature and drought stress, the orderly stacking of thylakoids can disintegrate entirely. In one study of the cycad Cycas panzhihuaensis subjected to heat and drought together, the maximum efficiency of PSII dropped by about 50%, thylakoid structure broke apart, and lipid-rich osmophilic particles accumulated in the stroma, a sign that membranes were being broken down for salvage.20PubMed Central. Transcriptome-based investigation of the response and repair mechanisms in the photosynthetic system of Cycas panzhihuaensis under dual high-temperature and drought stress The destruction of grana under stress is not just correlated with photosynthetic decline; it is mechanistically connected, because the organized stacking is what keeps PSII complexes packed at the density needed for efficient light harvesting.
Grana as Inspiration for Artificial Photosynthesis
The stacked architecture of grana has caught the attention of engineers working on artificial photosynthesis, particularly systems that split water into hydrogen and oxygen using sunlight. The natural design is appealing because it packs a huge amount of active surface area into a tiny volume, something solar energy devices also need to do. One research team built multilayered systems containing real PSII and ATP synthase enzymes assembled layer by layer on artificial supports, mimicking the grana stacking pattern. Under illumination, the PSII layers split water and generated a proton gradient that drove ATP production, and the yield scaled with the number of PSII layers, just as more thylakoid layers in a natural granum increase capacity.21PubMed. Supramolecular Assembly of Photosystem II and Adenosine Triphosphate Synthase in Artificially Designed Honeycomb Multilayers for Photophosphorylation
Fully synthetic approaches are also underway. A recent artificial photosynthetic cell used a molecular catalyst inspired by thylakoid chemistry to split water into hydrogen and oxygen at a low driving voltage of 1.1 volts under near-neutral conditions, mimicking the mild environment inside a real chloroplast.22PubMed. Artificial Photosynthetic Cell with Molecular Biomimetic Thylakoid Other concept designs propose building multilayer stacks from cheap, abundant nanomaterials that replicate the tandem light-absorbing architecture of grana without using any biological components at all.23Int J Nanomater Nanotechnol Nanomed. Tandem Architectures for Artificial Thylakoid Membranes These projects are still in early stages, but they underscore a point about grana that goes beyond botany: the stacked disc is a genuinely efficient geometry for packing light-driven chemistry into small spaces, and it took evolution roughly 450 million years of land-plant history to refine it.