Photosynthesis
What is photosynthesis?
A process by which living organisms use light to convert carbon dioxide (CO2) and water (H2O) into organic compounds, such as sugars (CH2O).
The process of converting carbon dioxide (CO2) in the atmosphere into organic carbon, like sugar, is called carbon fixation
Photosynthesis involves the splitting of water molecules into hydrogen and oxygen atoms (called hydrolysis) in the presence of sunlight.
The process discards the oxygen, which becomes the O2 that we breathe and use in respiration
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
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; instead using other energy sources; this process is found in archaea and some bacteria
Autotrophs use a very important enzyme called RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) in the first major step of carbon fixation.
RuBisCo may be the most abundant enzyme in the world,
This enzyme has tricky chemistry for autotrophs such that it can react with either carbon dioxide or oxygen.
Binding with CO2 leads to carbon fixation and the creation of sugars (glucose)
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
During drought, plants close their stomata, reducing the amount of carbon dioxide inside the plant that RuBisCo can fix.
Some groups of plants have evolved different metabolic pathways, called C4 and CAM photosynthesis (see below), to increase the carbon fixation efficiency when living in water-restricted environments
C3 Metabolism
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.
Therefore, a vast portion of water that enters through the roots, exits through the stomata during evapotranspiration.
This usually requires that C3 plants live in environments in which water is more abundant (e.g. wet temperate & tropical zones)
Above: Cross-section of a typical C3 dicot leaf
Adaptive Value of C3 Photosynthesis
C3 photosynthesis is more efficient than C4 and CAM plants under cool and moist conditions, and under normal light because requires less machinery (fewer enzymes and no specialized anatomy).
Disadvantages of C3 Photosynthesis
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.
Examples
Most plants have C3 metabolism (e.g. eudicot trees, wheat, rice, barley, etc.)
C4 Metabolism
Called "C4" because carbon dioxide is first incorporated into a 4-carbon compound called oxaloacetate (or OAA).
Plant create and use an enzyme called PEP Carboxylase to uptake 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 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.
Above: Cross-section of a C4 monocot leaf, displaying bundle sheath cells (called Kranz anatomy)
Adaptive Value of C4 photosynthesis
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.
Disadvantages of C4 photosythesis
In wet and moist conditions, C4 is more energetically expensive due to additional enzymes and anatomy.
Examples
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 C4 monocot leaf with Kranz anatomy
C4 Metabolism in Non-Angiosperms
Many algae (and 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
Crassulacean Acid Metabolism (CAM) Photosynthesis
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
Adaptive Value of CAM photosynthesis
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)
Disadvantages of CAM Photosynthesis
In wet and moist conditions, where carbon dioxide is plentiful, CAM is more energetically expensive due to synthesis of additional enzymes
Examples
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
Additional Resources
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)