A process by which living organisms use light to convert carbon dioxide (CO2) and water (H2O) into organic compounds, such as sugars (CH2O)n
Photosynthesis has two important inputs: carbon dioxide and water. Both of these are used differently to create sugars (CH2O)n
CO2 + H2O = CH2O + O2
The process of converting carbon dioxide (CO2) in the atmosphere into organic carbon, like sugar, is called carbon fixation. The plant will use the carbon and oxygen from gaseous carbon dioxide to incorporate them into sugars.
(Aerobic) photosynthesis involves splitting water molecules into hydrogen and oxygen atoms (called hydrolysis) in the presence of sunlight.
The process keeps the hydrogen but discards the water's oxygen atoms, which are released and become the O2 that we breathe and use in respiration
Some prokaryotic organisms, such as the green sulfur bacteria, conduct anaerobic photosynthesis by splitting hydrogen sulfide [H2S](instead of water) to make carbohydrates. Instead of oxygen, they release sulfur.
CO2 + H2S = CH2O + S2
In plants, photosynthesis takes place within an organelle called the chloroplast, where (simply put) the hydrogen from water is combined with the carbon dioxide from the air, to make sugars. It's more complicated than that...
Organisms that "fix carbon" in the presence of light are called autotrophs (e.g., plants, cyanobacteria, many algae)
Organisms that survive by consuming the carbon created by autotrophs are called heterotrophs (e.g., animals, fungi)
Chemosynthesis is the fixing of carbon in the absence of sunlight
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the world's most abundant enzyme and is the first major player in carbon fixation.
This enzyme is used by autotrophs, from bacteria to plants.
The Good, the Bad, and the Ugly: RuBisCO has tricky chemistry such that it can react with either carbon dioxide or oxygen.
The Good: Binding with CO2 leads to carbon fixation and the creation of sugars (glucose).
The Bad: RuBisCO can also bind with oxygen if levels are sufficiently high within plant tissues.
The Ugly: Binding with O2 leads to photorespiration, which greatly reduces the photosynthetic capacity of many plants
Therefore, many autotrophs have mechanisms to increase the concentration of CO2 around the enzyme, to prevent O2 binding and photorespiration
Some groups of plants have evolved different metabolic pathways, called C4 and CAM carbon fixation (see below), to increase photosynthetic efficiency. This is particularly found in drought-adapted plants that close their stomata to prevent moisture loss, thus reducing the amount of carbon dioxide inside the plant that RuBisCO can fix.
Algae have a structure called a pyrenoid that concentrates carbon dioxide around the chloroplast. Carbon dioxide is frequently limiting in water bodies and slow to repopulate.
This could be thought of as the "normal" version of photosynthesis, or more specifically, with plants in which water is not limiting in the environment.
Called "C3" because during photosynthesis, carbon dioxide that enters the leaf through the stomata is first incorporated into a 3-carbon molecule (3-phosphoglyceric acid or 3-PGA).
The plants use RuBisCO to combine RuBP with CO2 to create this 3-carbon molecule.
Photosynthesis takes place throughout the leaf, and stomata are open during the day. The leaves have a plentiful supply of carbon dioxide since these pores are open.
As a result, a vast portion of water that enters through the roots exits through the stomata during evapotranspiration.
This form of metabolism is found in plants that live in environments where water is more abundant (e.g., wet temperate & wet tropical zones)
Above: Cross-section of a typical C3 dicot leaf
C3 photosynthesis is more efficient than C4 and CAM plants under cool and moist conditions, and under normal light because it requires less machinery (fewer enzymes and no specialized anatomy).
In hot and dry environments, C3 plants close their stomata, and RuBisCO adds O2 (not CO2) to RuBP, causing photorespiration, producing a product that cannot be used in photosynthesis.
Many plants in temperate zones have C3 metabolism (e.g., eudicot trees, wheat, rice, barley, etc.)
This metabolism is found in plants that live in water-limited environments (e.g., grasslands, rocky outcrops, savannas, some deserts)
Called "C4" because carbon dioxide is first incorporated into a 4-carbon compound called oxaloacetate (or OAA).
Plants create and use an enzyme called PEP Carboxylase to take up CO2
This enzyme allows CO2 to be taken into the plant very quickly, convert to a 4-carbon molecule, and ultimately deliver the CO2 directly to RuBisCO (in bundle sheath cells --> see below) for carbon fixation
Stomata are open during the day, but close intermittently, to prevent water loss
Although it appears that some plants can effectively control water loss from their leaves while stomata remain open, using special "water-gating" proteins called aquaporins
Carbon fixation takes place in specialized cells in the leaf
OAA is first converted to malic acid, and then delivered to bundle sheath cells, which surround the veins of C4 plant leaves (See monocot leaf image).
The malic acid is converted back to CO2, which then enters the C3 pathway.
This special anatomy with bundle sheath cells around the veins in their leaves is called Kranz Anatomy (Kranz means "wreath")
Above: Cross-section of a C4 monocot leaf, displaying bundle sheath cells (called Kranz anatomy)
Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RuBisCO, not allowing it to grab oxygen and undergo photorespiration.
Has better water-use efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.
In wet and moist conditions, C4 is more energetically expensive due to additional enzymes and anatomy.
C4 plants include several thousand species in at least 19 plant families.
e.g., corn, millet, sugar cane, and many summer annuals.
Above: Drawing of the inside of a C4 monocot leaf with lavender-colored bundle sheath cells
Many algae (and the bryophyte hornworts) have a structure called a pyrenoid, which is associated with the chloroplast(s) in the cell
In aquatic environments, carbon dioxide is sometimes limited, and can be quickly depleted, and is slow to be replenished in water.
The pyrenoid functions to concentrate carbon dioxide around RuBisCO and prevent photorespiration.
1/3 of the world's photosynthesis may occur through algae using pyrenoids
This metabolism is found in plants that live in dry and arid environments (e.g., deserts)
Called "Crassulacean Acid Metabolism" after the plant family in which it was first found (Crassulaceae - stonecrop family),
CO2 is stored in the form of malic acid before use in photosynthesis.
Stomata open at night (when evaporation rates are usually lower), but they are closed during the day.
Plants absorb CO2 at night and use C4 metabolism (above)
PEP Carboxylase combines CO2 with a 3-carbon molecule to make malic acid that is stored in the vacuole during the night.
During the day, the malic acid leaves the vacuole and enters the chloroplast, where it is broken down and the CO2 is released to RuBisCO for carbon fixation
Above: Visual diagram of CAM metabolism
Better water-use efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
Plants may "CAM-idle"
When conditions are extremely arid, CAM plants can just leave their stomata closed night and day.
Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis
This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever
CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells)
In wet and moist conditions, where carbon dioxide is plentiful, CAM is more energetically expensive due to synthesis of additional enzymes
e.g. CAM plants include many stem and leaf succulents such as a cactus and agave, respectively.
Some epiphytes such as orchids and bromeliads have CAM due to limited water availability in canopy
There are aquatic CAM plants, such as Sagittaria (arrowhead), Littorella, Vallisneria (eelgrass), Crassula aquatica (pigmyweed) and Isoetes (quillwort), which are limited by CO2 availability in water (Keely 1998)
Isoetes, an ancient lineage of spore-bearing plants, may represent some of the earliest plants to evolve CAM photosynthesis.
Some species of Isoetes have been shown to be able to switch between C3 and CAM photosynthesis
Above: Jade plant (Crassula), a plant with CAM metabolism
Below: Aquatic CAM plant Sagittaria latifolia
Global study identifies upswing in photosynthesis driven by land, offset by oceans (Phys.org 1Aug2025)
└Zhang et al. (2025) Contrasting biological production trends over land and ocean
Scientists trace photosynthesis-enabling protein to earliest land plants (Phys.org 31Jul2025)
└Kanaris et al. (2025) Shikimate Kinase-Like 1 Participates in an Ancient and Conserved Role Contributing to Chloroplast Biogenesis in Land Plants
Newly found plant pathway helps detoxify cells under high light stress (Phys.org 18Jul2025)
└Jiang et al. (2025) A cytosolic glyoxylate shunt complements the canonical photorespiratory pathway in Arabidopsis
How plants manage light: New insights into nature's oxygen-making machinery (Phys.org 8Jul2025)
└Leonardo et al. (2025) Bidirectional Energy Flow in the Photosystem II Supercomplex
Mutagenesis technique boosts the efficiency of rubisco, a key enzyme in photosynthesis (Phys.org 7Jul2025)
└McDonald et al. (2025) In vivo directed evolution of an ultrafast Rubisco from a semianaerobic environment imparts oxygen resistance
Fig Trees Turn Atmospheric Carbon Into Stone (ScienceBlog 6Jul2025)
Study finds world's most prolific CO₂-fixing enzyme is slowly getting better (Phys.org 7Mar2024)
└Bouvier et al. (2024) Rubisco is evolving for improved catalytic efficiency and CO2 assimilation in plants
Why is photosynthetic light-harvesting is so efficient? (MIT 3Jul2023)
└Wang et al. (2023) Elucidating interprotein energy transfer dynamics within the antenna network from purple bacteria
An 'artificial photosynthesis' system ten times more efficient than existing systems (Phys.org 11Nov2022)
└Lan et al. (2022) Biomimetic active sites on monolayered metal–organic frameworks for artificial photosynthesis
Cyanobacteria reveal a blueprint for photosynthesis (Phys.org 1Sep2022)
└Dominguez-Martin et al. (2022) Structures of a phycobilisome in light-harvesting and photoprotected states
Plants Have Been Keeping a Secret From Us About How Thirsty They Actually Are (13Aug2022ScienceAlert)
└Wong et al. (2022) Humidity gradients in the air spaces of leaves
Artificial photosynthesis can produce food without sunshine (23Jun2022, UC Riverside)
└Hann et al. (2022) A hybrid inorganic–biological artificial photosynthesis system for energy-efficient food production
‘Reverse Photosynthesis’ Process Discovered (Science Alert 5Apr2016)
C4 photosynthesis boosts growth by altering size and structure of leaves and roots (Univ of Sheffield 18Apr2016)
New “Bionic” Leaf Is Roughly 10 Times More Efficient Than Natural Photosynthesis (SciAm 1Aug2016)