Any cell that contains the right light-harvesting machinery can perform photosynthesis, but in plants the job falls mainly to mesophyll cells, the densely packed interior cells of a leaf. These cells are loaded with chloroplasts, the organelles where sunlight is converted into chemical energy. Plants are far from the only photosynthesizers, though. Cyanobacteria, single-celled algae, diatoms, dinoflagellates, and even some animal cells that harbor photosynthetic partners all carry out the same basic conversion of light into fuel.
Mesophyll Cells and the Interior of a Leaf
When most people picture photosynthesis, they picture a green leaf. The green comes from chlorophyll packed inside mesophyll cells, which make up most of the tissue between a leaf’s upper and lower surfaces. A typical leaf has two layers of mesophyll. The palisade layer sits just below the upper skin of the leaf and is made of tall, tightly arranged cells optimized for catching direct sunlight. Below that, the spongy mesophyll is more loosely organized, with air spaces that allow carbon dioxide to circulate.
Both layers are photosynthetically active, but the palisade cells do the heaviest lifting because they sit where light intensity is greatest. Each mesophyll cell can contain dozens to over a hundred chloroplasts, and each chloroplast runs its own photosynthetic assembly line. Carbon dioxide enters the leaf through tiny pores called stomata, which are flanked by guard cells that open and close the pore in response to changing carbon dioxide levels and water availability.1PubMed Central. CO2 Sensing and CO2 Regulation of Stomatal Conductance: Advances and Open Questions Guard cells themselves contain chloroplasts and can photosynthesize at a low level, but their primary job is regulating gas exchange, not producing sugar.
Inside the Chloroplast
A cell does not perform photosynthesis with its whole body. The work happens inside chloroplasts, which are bounded by a double membrane and contain an elaborate internal system of flattened sacs called thylakoid membranes. These thylakoids house the protein complexes that capture light energy and use it to split water molecules, releasing oxygen and generating the energy carriers that ultimately drive sugar production. In vascular plants, thylakoids stack into structures called grana, connected by unstacked regions called stroma thylakoids.2PubMed Central. Structure, biogenesis, and evolution of thylakoid membranes
A cell that lacks chloroplasts cannot photosynthesize on its own. Root cells, for example, never see light and contain no chloroplasts. What makes a mesophyll cell a photosynthetic cell is not some unique gene that other plant cells lack; rather, it is the fact that chloroplasts have fully developed there. In the growing tip of a shoot, young cells start with small, undifferentiated organelles called proplastids. As the tissue matures and is exposed to light, proplastids develop into chloroplasts through a regulated process that builds up the thylakoid membranes and loads them with chlorophyll.3PubMed Central. Establishment of Photosynthesis through Chloroplast Development Is Controlled by Two Distinct Regulatory Phases This differentiation begins in the shoot tip and continues as leaves expand.4PubMed. Initial proplastid-to-chloroplast differentiation in the developing vegetative shoot apical meristem of Arabidopsis
Cyanobacteria Do It Without a Nucleus
Plants were not the first photosynthesizers. Cyanobacteria, single-celled organisms without a nucleus, have been converting sunlight into energy for roughly two to three billion years. They are the organisms that originally oxygenated Earth’s atmosphere. Unlike plant cells, cyanobacteria have no chloroplasts. Instead, their thylakoid membranes run through the interior of the cell itself, performing both photosynthesis and cellular respiration on the same membrane system.5PubMed Central. Distribution and dynamics of electron transport complexes in cyanobacterial thylakoid membranes
This matters because the chloroplasts inside every plant and algal cell are descended from ancient cyanobacteria. Over a billion years ago, a non-photosynthetic eukaryotic cell engulfed a cyanobacterium. Instead of digesting it, the host cell kept it, and the cyanobacterium eventually evolved into the chloroplast. The thylakoid membranes found in chloroplasts today likely originated in those photosynthetic bacteria.6PubMed. Biogenesis and origin of thylakoid membranes How many times this kind of event occurred, and how chloroplasts subsequently spread between different lineages of eukaryotes, remains an active debate among researchers.7PubMed. Endosymbiosis and Eukaryotic Cell Evolution
Cyanobacteria are not the only photosynthetic bacteria, either. Purple and green sulfur bacteria carry out a form of photosynthesis that does not produce oxygen. Instead of splitting water, these bacteria oxidize hydrogen sulfide or other sulfur compounds to fuel their energy production.8PubMed Central. Molecular Physiology of Anaerobic Phototrophic Purple and Green Sulfur Bacteria This kind of photosynthesis predates the oxygen-producing version and thrives in environments like hot springs and deep-sea vents where oxygen is scarce.
Algae and Diatoms
When people think of photosynthetic cells, they usually think of land plants. But in the ocean, single-celled eukaryotic algae are the dominant photosynthesizers. Marine phytoplankton, and diatoms in particular, account for close to half of all primary production on Earth.9PubMed Central. Divergence of photosynthetic strategies amongst marine diatoms Each diatom is a single cell encased in an intricate silica shell, with chloroplasts that drive photosynthesis just as they do in a leaf cell.
The chloroplasts of diatoms and many other algal groups have an unusual structure, though. Rather than two surrounding membranes like those in a plant chloroplast, they are wrapped in three or even four membranes.10PubMed Central. More membranes, more proteins: complex protein import mechanisms into secondary plastids This extra wrapping is a relic of “secondary endosymbiosis,” where an already photosynthetic alga was engulfed by another eukaryotic cell. Diatoms and dinoflagellates, for instance, acquired their chloroplasts by swallowing a red alga ancestor.11PubMed. Protein import pathways in ‘complex’ chloroplasts derived from secondary endosymbiosis involving a red algal ancestor The extra membranes are essentially the leftover “wrappers” from that ancient engulfment event.
Recent work suggests the extra membranes around these chloroplasts are not just passive packaging. In diatoms, dinoflagellates, and coccolithophores, proton pumps on the membranes surrounding the chloroplast appear to concentrate carbon dioxide in the space just outside the chloroplast, boosting photosynthetic efficiency. Blocking these pumps reduced oxygen production by roughly 16 to 40 percent depending on the species.12Current Biology. V-type H+-ATPase is localized around diatom chloroplasts and contributes to photosynthesis and ocean primary production So the machinery of an algal photosynthetic cell is not identical to a plant’s, even though the core chemistry is the same.
C4 and CAM Plants Use Two Cell Types
Not all plant photosynthesis happens the same way, and some plants have evolved strategies that divide the labor between different cell types. In C4 plants like corn and sugarcane, mesophyll cells initially capture carbon dioxide using a specialized enzyme and then shuttle it into deeper bundle-sheath cells, where a second enzyme fixes it into sugar. Confining the sugar-making step to bundle-sheath cells creates a high concentration of carbon dioxide around the enzyme that needs it most, which reduces wasteful side reactions that would otherwise occur in hot, dry conditions.13Journal of Experimental Botany. Deconstructing Kranz anatomy to understand C4 evolution
In this arrangement, both cell types are photosynthetic, but they do different parts of the job. Mesophyll cells handle the initial carbon capture, and bundle-sheath cells finish the process. This division of labor depends on a distinctive leaf anatomy where bundle-sheath cells form a ring around each vein, with mesophyll cells arranged around them.
CAM plants, like cacti and many succulents, take a different approach. Rather than splitting the work between two cell types in space, CAM plants split it in time within the same mesophyll cell. At night, when opening stomata will not cause excessive water loss, these cells capture carbon dioxide and store it as malic acid in their vacuoles. During the day, stomata close tightly to conserve water, and the stored acid is broken down to release carbon dioxide for the standard photosynthetic cycle.14PubMed. Crassulacean acid metabolism photosynthesis: working the night shift The cell itself is the same mesophyll cell you would find in any leaf, but its biochemistry runs on a day-night schedule that ordinary plant cells do not follow.
Photosynthesis Outside the Leaf
Leaves are the organs most visibly dedicated to photosynthesis, but they are not the only plant tissues that contain chloroplasts. Green stems, unripe fruit, flower parts, and even bark often contain chlorophyll and carry out a limited form of photosynthesis. These tissues are not built for the job the way leaves are. They primarily serve other functions like structural support, reproduction, or protection. But the chlorophyll-containing cells within them typically recycle carbon dioxide released by the tissue’s own respiration, recapturing some of that carbon rather than letting it escape.15Flora – Morphology, Distribution, Functional Ecology of Plants. Non-foliar photosynthesis – a strategy of additional carbon acquisition
Think of a green pepper or an unripe tomato. The green color comes from chloroplasts in the fruit’s outer cells. As the fruit ripens, those chloroplasts convert into chromoplasts that produce red or yellow pigments, and the photosynthetic activity fades. The same cells that were once photosynthetic become color-display cells. This flexibility is a reminder that being a “photosynthetic cell” is not always a permanent identity. Cells can gain or lose the ability depending on developmental stage and environmental signals.
How Cells Adapt Their Photosynthetic Hardware
The internal structure of a photosynthetic cell is not fixed. Cells adjust their chloroplast architecture depending on how much light they receive. Plants grown in low light develop chloroplasts with notably large grana stacks, sometimes containing as many as 160 thylakoids layered on top of each other.16PubMed Central. Acclimation of leaves to low light produces large grana: the origin of the predominant attractive force at work These larger stacks increase the surface area available for capturing scarce photons. In bright light, the grana tend to be smaller, with fewer layers per stack.
This plasticity means two mesophyll cells from the same species can look quite different inside if one grew in deep shade and the other in full sun. Shade-adapted cells invest more in light-harvesting antenna complexes, while sun-adapted cells invest more in the enzymes that fix carbon dioxide. The cell’s identity as a photosynthetic unit stays the same, but its internal toolkit shifts to match its circumstances.
Animals That Borrow Photosynthesis
Some animals have found ways to harness photosynthesis without evolving their own chloroplasts. The most familiar example is coral. Coral polyps host single-celled dinoflagellate algae inside specialized gastrodermal cells, creating a partnership where the algae photosynthesize and share sugar with the coral, while the coral provides shelter and nutrients.17PubMed Central. Photosynthesis and other factors affecting the establishment and maintenance of cnidarian–dinoflagellate symbiosis When this relationship breaks down, usually due to heat stress, the coral expels its algae and bleaches. Without its photosynthetic partners, the coral can starve.18The ISME Journal. Cell type-specific immune regulation under symbiosis in a facultatively symbiotic coral
An even stranger case involves sea slugs in the genus Elysia. These animals eat algae and retain the chloroplasts from their food, incorporating them into their own body cells. The stolen chloroplasts, called kleptoplasts, continue to photosynthesize inside the slug’s tissues. Slugs that fed on the alga Codium fragile and retained functional chloroplasts nearly doubled their growth efficiency in bright light compared to dim light, while slugs with poorly functioning stolen chloroplasts showed no such benefit.19PLOS ONE. Acquired Phototrophy through Retention of Functional Chloroplasts Increases Growth Efficiency of the Sea Slug Elysia viridis The slugs are not truly photosynthetic organisms in the way a plant is, because they cannot make new chloroplasts. They have to keep eating algae to replenish their supply. But while the stolen chloroplasts last, the slugs get a genuine energy boost from sunlight.
Engineering Photosynthesis Into New Cell Types
If ancient cells could acquire photosynthesis by swallowing a cyanobacterium, could scientists recreate that process in the lab? Several research groups have tried. One team engineered cyanobacteria to live inside budding yeast cells, creating chimeric organisms that could grow under photosynthetic conditions. The engineered yeast-cyanobacteria hybrids propagated for 15 to 20 generations, with the cyanobacteria performing energy-producing functions inside their yeast hosts.20Nature Communications. Engineering artificial photosynthetic life-forms through endosymbiosis
A more ambitious experiment pushed the concept into mammalian cells. Researchers identified the cyanobacterium Synechocystis as a good candidate for living inside animal cells and found that macrophages (immune cells that naturally engulf other cells) could internalize the cyanobacteria. Under illumination, the internalized cyanobacteria rescued energy deficiencies in their host cells. Using a membrane-coating delivery method, the team then introduced cyanobacteria into human embryonic kidney cells, which proved to be well-suited hosts for sustained endosymbiosis.21PubMed. Toward Photosynthetic Mammalian Cells through Artificial Endosymbiosis
These experiments are proofs of concept, not practical technologies yet. The cyanobacteria do not replicate in sync with their hosts the way a true chloroplast does, and the systems break down after a limited number of cell divisions. But they demonstrate that the boundary between “photosynthetic cell” and “non-photosynthetic cell” is not as rigid as it seems. Given the right partner and the right conditions, even a human kidney cell can host a photosynthetic engine.
The Forgotten Photosynthesizers
Discussions of photosynthetic cells tend to focus on green plants and maybe cyanobacteria, skipping over some less familiar lineages. Glaucophytes are a small group of freshwater algae whose chloroplasts still retain a thin layer of bacterial cell wall material between the two surrounding membranes, a direct holdover from the original cyanobacterial ancestor. Their blue-green color comes from a combination of chlorophyll a with accessory pigments called phycocyanins, the same pigments found in cyanobacteria.2PubMed Central. Structure, biogenesis, and evolution of thylakoid membranes Glaucophytes, red algae, and green plants (including land plants) are thought to descend from the same original endosymbiosis event, but their chloroplasts ended up with different pigment profiles and different internal architectures.
Red algae, for example, use pigments called phycobilins to harvest wavelengths of light that penetrate deep water, which is why some red algal species thrive at ocean depths where green plants could not survive. The photosynthetic cells in these organisms work the same way at the chemical level, splitting water and fixing carbon, but the light-harvesting antennas are tuned to different parts of the spectrum. This diversity of pigments across photosynthetic lineages is one reason aquatic ecosystems can support photosynthesis at such a range of depths and conditions.
So the question “what type of cell performs photosynthesis” does not have a single neat answer. The list includes mesophyll cells, bundle-sheath cells, guard cells (modestly), algal cells with two, three, or four chloroplast membranes, cyanobacteria with no chloroplasts at all, anoxygenic bacteria that do not even produce oxygen, coral gastrodermal cells hosting dinoflagellate tenants, and sea slug cells running on stolen chloroplasts. What unites them is a membrane system loaded with pigments and protein complexes that capture light and use it to build chemical energy. The cell types that can host that system turn out to be far more varied than a leaf might suggest.