Photosynthesis happens primarily inside chloroplasts, the small green organelles packed into the cells of plant leaves, but that familiar answer barely scratches the surface. The process also takes place in stems, roots, fruits, and seeds of plants; inside cyanobacteria floating in oceans and lakes; within algae wrapped in layers of stolen membranes; and even, remarkably, inside certain animal cells. Some organisms photosynthesize in complete darkness at the bottom of the ocean, using faint infrared glow from hydrothermal vents instead of sunlight. The geography of photosynthesis stretches from a single membrane inside a bacterium to the canopy of a rainforest and well beyond.
Inside the Chloroplast
In a typical green plant cell, photosynthesis is concentrated in chloroplasts. Each chloroplast contains an elaborate internal membrane system called the thylakoid network. Under a light microscope, the thylakoid stacks show up as dark-green dots known as grana. These grana house the molecular machinery for the light-dependent reactions, where water is split and light energy is converted into chemical energy.1Trends in Plant Science. Where Does Photosynthesis Occur in Plants and Beyond? The fluid surrounding these stacks, called the stroma, is where carbon dioxide gets fixed into sugar. That carbon-fixing chemistry is sensitive to conditions in the stroma; it works best at a slightly alkaline pH around 8.1, and essentially stalls below about 7.3.2Biochimica et Biophysica Acta (BBA) – Bioenergetics. The role of pH in the regulation of carbon fixation in the chloroplast stroma The light reactions themselves create this alkaline environment in the stroma by pumping protons across the thylakoid membrane, so the two halves of photosynthesis are tightly coupled inside this one organelle.
Which Cells in a Leaf Do the Work
Within a leaf, not every cell is equally photosynthetic. In most plants, the mesophyll cells, the soft tissue between the upper and lower surfaces of the leaf, are loaded with chloroplasts and do the bulk of the work. But the arrangement varies between plant species, and in some cases that arrangement is itself a key part of the photosynthetic strategy.
Many grasses, crops like maize and sugarcane, and various desert-adapted species use a two-step carbon-fixation process that relies on a specific leaf architecture called Kranz anatomy. In these plants, a ring of tightly packed bundle sheath cells surrounds each vein, and mesophyll cells surround the bundle sheath. Carbon dioxide is initially captured in the mesophyll, then shuttled into the bundle sheath for the final fixation step, which concentrates COâ‚‚ and reduces wasteful side reactions.3PubMed Central. Screening of Mutants Related to the C4 Photosynthetic Kranz Structure in Foxtail Millet Interestingly, the exact shape and layout of the Kranz structure varies considerably even among related species. A comparative study of eudicot plants found at least four distinct Kranz anatomical types, and the shape of the anatomy did not predict which biochemical variant of the carbon-fixation pathway the plant used.4PubMed. Diversity of Kranz anatomy and biochemistry in C4 eudicots Plants have converged on this general strategy many times independently, producing a surprising diversity of leaf-level solutions to the same problem.
Beyond the Leaf
Leaves get nearly all the credit, but many other plant organs are photosynthetically active. Stems, roots, flowers, fruits, and even seeds can contain chloroplasts and fix carbon to varying degrees.5PubMed Central. Fruit Photosynthesis: More to Know about Where, How and Why The reasons these organs photosynthesize differ from organ to organ, and the contribution to the plant’s total carbon budget ranges from trivial to essential.
Woody stems, for example, contain green tissue beneath the bark that recaptures some of the COâ‚‚ released by the stem’s own respiration. In Scots pine, bark photosynthesis accounted for up to about 13% of the stem’s net COâ‚‚ flux.6PubMed Central. Partitioning seasonal stem carbon dioxide efflux into stem respiration, bark photosynthesis, and transport-related flux in Scots pine That number sounds modest, but over the lifetime of a large tree, it adds up. Avocado trees also re-assimilate COâ‚‚ through their green stems.7PubMed Central. Does green stem photosynthesis affect plant drought tolerance and recovery in avocado? Desert trees like the palo verde (Parkinsonia florida) take the concept further: their stems are a primary photosynthetic surface, and stem photosynthesis plays a measurable role in the tree’s ability to survive drought.8PubMed Central. The benefits of woody plant stem photosynthesis extend to hydraulic function and drought survival in Parkinsonia florida
Fruit photosynthesis serves a different purpose. Green, unripe fruits are photosynthetically active, and the energy they produce appears to support local biosynthetic work: building fatty acids, generating oxygen to prevent oxygen starvation in the fruit’s dense interior, and providing carbon skeletons for the sugars, flavonoids, and lipids that accumulate as the fruit develops.5PubMed Central. Fruit Photosynthesis: More to Know about Where, How and Why
Roots That Photosynthesize
The most surprising site of photosynthesis in plants is probably underground, or at least out of sight. Several groups of epiphytic orchids have green, photosynthetically active roots. In most orchids, the leaves remain the main photosynthetic organ, and root photosynthesis primarily recycles respiratory COâ‚‚ back into useful carbon. But the oxygen produced by root photosynthesis may be just as important as the carbon fixed. The thick, fleshy roots of epiphytic orchids can become oxygen-starved in the dark, and light exposure triggers enough photosynthetic oxygen production to relieve that internal hypoxia.9PubMed. Root photosynthesis prevents hypoxia in the epiphytic orchid Phalaenopsis
Some orchids have dispensed with leaves entirely. The genus Taeniophyllum, for instance, consists of leafless epiphytes whose aerial roots are the sole photosynthetic organs. These roots use a specialized form of photosynthesis called crassulacean acid metabolism, the same water-conserving strategy used by cacti, opening their pores at night to take in COâ‚‚ and fixing it into sugars during the day.10PubMed. Aerial roots of the leafless epiphytic orchid Taeniophyllum are specialized for performing crassulacean acid metabolism photosynthesis A plant that photosynthesizes entirely through its roots is a long way from the textbook picture of a green leaf soaking up sunlight.
Cyanobacteria and the Evolutionary Origin of Chloroplasts
Chloroplasts did not originate inside plants. They descend from free-living cyanobacteria that were engulfed by an ancient single-celled organism in an event known as primary endosymbiosis. Over vast stretches of time, the engulfed cyanobacterium lost its independence and became the chloroplast. This origin is shared by all green plants, red algae, and a small group called glaucophytes.11PubMed Central. The endosymbiotic origin, diversification and fate of plastids
Modern cyanobacteria still photosynthesize without chloroplasts. Instead, their thylakoid membranes run directly through the cell interior, forming complicated branched networks. In some species, these networks have a rudimentary helical structure that loosely parallels the stacked grana of plant chloroplasts, but without the neat compartmentalization.12PubMed Central. Insights into the complex 3-D architecture of thylakoid membranes in unicellular cyanobacterium Cyanothece sp. ATCC 51142 Cyanobacteria are enormously abundant in oceans and freshwater, and they collectively account for a substantial fraction of global photosynthesis.
Algae and Second-Hand Chloroplasts
The story gets more convoluted in algae. Green plants and red algae got their chloroplasts from that single ancient endosymbiosis event. But many other photosynthetic organisms, including diatoms, brown algae, dinoflagellates, and euglenoids, got their chloroplasts secondhand: one of their ancestors engulfed an alga that already had chloroplasts. This process, called secondary endosymbiosis, left these organisms with plastids wrapped in three or even four membranes, rather than the two membranes of a standard plant chloroplast.13PubMed Central. More membranes, more proteins: complex protein import mechanisms into secondary plastids Those extra membranes are relics of the successive engulfing events. In some lineages, the plastids descend from red algae rather than green algae, adding yet another layer of evolutionary complexity.14PubMed Central. A New Model and Dating for the Evolution of Complex Plastids of Red Alga Origin
Because secondary endosymbiosis shuffled genes between the engulfed alga’s nucleus and the host’s nucleus, these organisms now face a logistical challenge: proteins made in the host cell’s cytoplasm need to be imported back across three or four membranes to reach the plastid interior.15PubMed. Translocation of proteins across the multiple membranes of complex plastids The elaborate targeting machinery that evolved to solve this problem is one of the more remarkable feats of cellular engineering in biology.
Animals That Borrow Photosynthesis
No animal has evolved its own chloroplasts, but several have found ways to borrow them. The most celebrated example involves sacoglossan sea slugs, particularly Elysia chlorotica, found along the eastern coast of the United States. These slugs eat algae but retain the functional chloroplasts, which continue photosynthesizing inside the slug’s digestive cells. Elysia chlorotica can survive for months without food, apparently sustained in part by these stolen chloroplasts.16PubMed Central. A draft genome assembly of the solar-powered sea slug Elysia chlorotica Among animals, only a handful of sacoglossan species can maintain photosynthetically active chloroplasts for weeks to months.17PLoS Biology. Kleptoplasty: Getting away with stolen chloroplasts
Recent work has clarified how the slugs manage this trick. The stolen chloroplasts are sequestered inside arrested phagosomes, dubbed “kleptosomes,” which protect the organelles from the normal digestive process. Under well-fed conditions the plastids are maintained, but starvation triggers their degradation, suggesting the slugs actively digest them for nutrition when other food runs out.18Current Biology. A new study shows that Sacoglossan sea slugs sequester stolen plastids in arrested phagosomes called ‘kleptosomes’
A less flashy but equally fascinating example occurs in spotted salamanders. Eggs of Ambystoma maculatum are typically green because symbiotic algae colonize the jelly envelope surrounding each egg.19Canadian Journal of Zoology. Symbiosis between salamander eggs and green algae: microelectrode measurements inside eggs demonstrate effect of photosynthesis on oxygen concentration But the relationship goes deeper than surface colonization. Imaging and genetic analysis have shown that algal cells actually invade embryonic salamander tissues and individual cells during development, making this the only known vertebrate with intracellular photosynthetic symbionts.20PubMed Central. Intracellular invasion of green algae in a salamander host
Lichens and the Fungal Partnership
Lichens look like a single organism but are actually a partnership, most commonly between a fungus and a photosynthetic partner (called a photobiont) that can be a green alga, a cyanobacterium, or both. The fungus provides structure and mineral absorption; the photobiont provides sugars through photosynthesis. Some cyanobacterial lichens simultaneously house green algae in their photosynthetic layer, and experiments have confirmed that both partners contribute to the lichen’s photosynthesis.21PubMed Central. Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis
The fungus does not passively receive sugars. In many lichen species, the fungal hyphae form specialized contact structures called haustoria that penetrate the cell walls of the photobiont, creating intimate physical connections for nutrient transfer.22New Phytologist. ULTRASTRUCTURAL STUDIES IN LICHENS Lichens colonize bare rock, tree bark, and exposed soil surfaces worldwide, making them important photosynthesizers in habitats where vascular plants cannot grow.
Organelle Theft in Microscopic Predators
The practice of stealing photosynthetic machinery is not limited to sea slugs. Single-celled organisms do it too. The marine ciliate Mesodinium rubrum feeds on cryptophyte algae and retains not just their chloroplasts but also a remnant of the prey’s nucleus, called a kleptokaryon. Multi-omics analysis of an Antarctic strain showed that the ciliate depends on these stolen organelles not only for photosynthesis but also for building fatty acids, amino acids, and other macromolecules. In brighter light, transcription from the stolen nucleus increased, implying a degree of functional regulation despite the absence of a stable, permanent symbiosis.23Current Biology. Dynamic Chloroplast Acquisition and Metabolic Integration in an Organelle-Stealing Ciliate
The chain can get even longer. The dinoflagellate Dinophysis caudata feeds on Mesodinium rubrum, stealing the chloroplasts that the ciliate already stole from a cryptophyte. These third-hand plastids are structurally remodeled and retained as the compound chloroplasts characteristic of Dinophysis species.24PubMed. DINOPHYSIS CAUDATA (DINOPHYCEAE) SEQUESTERS AND RETAINS PLASTIDS FROM THE MIXOTROPHIC CILIATE PREY MESODINIUM RUBRUM Photosynthesis, in other words, can pass through multiple unrelated organisms like a hand-me-down.
Bacteria That Photosynthesize Without Oxygen
All the photosynthesis discussed so far produces oxygen as a byproduct, but that is not the only flavor. Anoxygenic photosynthesis, which predates the oxygen-producing kind by hundreds of millions of years, is still practiced by several groups of bacteria. Green sulfur bacteria, for instance, use hydrogen sulfide instead of water as an electron donor, producing elemental sulfur rather than oxygen. Their photosynthetic pigments are housed not in thylakoid membranes but in unique structures called chlorosomes, vesicles enclosed by a single lipid layer that serve as extremely efficient light-harvesting antennas.25PubMed Central. Anoxygenic photosynthesis with emphasis on green sulfur bacteria and a perspective for hydrogen sulfide detoxification of anoxic environments These bacteria thrive in sulfur-rich, oxygen-poor environments like hot springs and sediment layers, places where oxygen-producing photosynthesizers cannot compete.
Photosynthesis in the Dark
Perhaps the most striking departure from the textbook picture is photosynthesis that operates without any sunlight at all. A green sulfur bacterium was isolated from a deep-sea hydrothermal vent, where the only available light comes from the faint glow of geothermal radiation emitted by superheated water. This dim infrared and near-infrared light includes wavelengths that the bacterium’s photosynthetic pigments can absorb.26PubMed Central. An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent Subsequent research at deep-sea vents on the Southwest Indian Ridge confirmed that infrared light from vent chimneys promotes bacterial growth and shifts population composition, supporting the idea that geothermal light can sustain photosynthetic life independently of the sun.27The Innovation Geoscience. Illuminating a bacterial adaptation mechanism: Infrared-driven cell division in deep-sea hydrothermal vent environments
Above water, some cyanobacteria have expanded their usable light spectrum in a different way. Most photosynthetic organisms rely on chlorophyll a, which absorbs visible red and blue light. But certain cyanobacteria produce chlorophylls d and f, pigments that absorb far-red light well beyond the wavelengths that standard chlorophyll can use.28PubMed. Far-red light acclimation in diverse oxygenic photosynthetic organisms The gene responsible for chlorophyll f synthesis has been identified: it encodes a highly divergent version of a photosystem II protein, and heterologous expression of that gene successfully enabled chlorophyll f production in a cyanobacterium that does not naturally make it.29PubMed. Light-dependent chlorophyll f synthase is a highly divergent paralog of PsbA of photosystem II Researchers have speculated that introducing chlorophyll f biosynthesis into crop plants could expand the portion of the solar spectrum they can harvest. The biophysics of how a small number of chlorophyll f molecules transfer far-red energy “uphill” to the standard reaction center pigments is still being worked out.30PubMed. Key Chlorophyll a Molecules in the Uphill Energy Transfer from Chlorophyll f to P700 in Far-Red Light-Adapted Photosystem I
When Plants Lose Photosynthesis
If photosynthesis is so central to plant life, can a plant give it up? Yes. Parasitic plants that steal nutrients from a host’s roots or stems face little evolutionary pressure to maintain their own photosynthetic machinery, and over millions of years, some lineages have lost it entirely. The beechdrops (Epifagus virginiana), a parasite of American beech trees, has a plastid genome that has lost every gene related to photosynthesis and chlororespiration.31PubMed. Function and evolution of a minimal plastid genome from a nonphotosynthetic parasitic plant The plastid genome itself is dramatically shrunk but still present, retained for non-photosynthetic functions.
Rafflesia, the genus famous for producing the world’s largest flowers, may have gone even further. Research suggests Rafflesia lagascae has possibly lost its plastid genome altogether, an extreme rarity in flowering plants. Despite this, the cells still contain plastid-like compartments with a uniform interior but none of the layered membrane systems found in functional chloroplasts, hinting that these remnant organelles serve metabolic roles unrelated to photosynthesis.32PubMed Central. Possible Loss of the Chloroplast Genome in the Parasitic Flowering Plant Rafflesia lagascae (Rafflesiaceae) The broader pattern across parasitic plant lineages is a gradual accumulation of mutations and deletions in photosynthesis genes once the selective pressure to maintain them disappears.33PubMed. From chloroplasts to “cryptic” plastids: evolution of plastid genomes in parasitic plants
Semi-Artificial Photosynthesis
Understanding where and how photosynthesis works has inspired efforts to build it from scratch, or at least halfway. Semi-artificial photosynthesis combines biological components, often enzymes or whole photosynthetic proteins, with synthetic materials like organic semiconductors. The goal is to harness the efficiency of natural light-harvesting and catalytic machinery in engineered systems that can produce hydrogen fuel or fix carbon dioxide into useful chemicals. A recent proof-of-concept system using organic semiconductors paired with enzymes demonstrated that biohybrid platforms can match or exceed the performance of fully synthetic alternatives for certain reactions.34Joule. Sustainable semi-artificial photosynthesis using organic semiconductors and enzymes The field has grown substantially over the past decade, driven by the integration of diverse biological systems with synthetic materials.35PubMed. Toward Next-Generation Semiartificial Photosynthesis: Multidisciplinary Engineering of Biohybrid Systems These systems are still in early stages, but the underlying logic is straightforward: nature spent billions of years optimizing the molecular machinery of photosynthesis, and rather than designing replacements from scratch, researchers are learning to plug those biological parts into artificial frameworks.