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Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in carbohydrate molecules, such as sugars and starches, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek phōs (φῶς), "light", and synthesis (σύνθεσις), "putting together".[1][2][3] Most plants, algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.[4]
Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centers that contain green chlorophyll (and other colored) pigments/chromophores. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.
In plants, algae and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP).[5] Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose. In other bacteria, different mechanisms such as the reverse Krebs cycle are used to achieve the same end.
The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.[6] Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth,[7] which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts,[8][9][10] which is about eight times the current power consumption of human civilization.[11] Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams, or billion metric tons), of carbon into biomass per year.[12][13] That plants receive some energy from light – in addition to air, soil, and water – was first discovered in 1779 by Jan Ingenhousz.
Photosynthesis is vital for climate processes, as it captures carbon dioxide from the air and then binds carbon in plants and further in soils and harvested products. Cereals alone are estimated to bind 3,825 Tg (teragrams) or 3.825 Pg (petagrams) of carbon dioxide every year, i.e. 3.825 billion metric tons.[14]
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can be later released to fuel the organism's activities through cellular respiration. Some of this chemical energy is stored in carbohydrate molecules, such as sugars and starches, which are synthesized from carbon dioxide and water. Hence the name photosynthesis, derived from the Greek words phōs (φῶς), meaning "light," and synthesis (σύνθεσις), meaning "putting together."
Most plants, algae, and cyanobacteria perform photosynthesis, and these organisms are called photoautotrophs. Photosynthesis plays a crucial role in producing and maintaining the oxygen content of the Earth's atmosphere and supplies most of the energy necessary for life on Earth.
Although photosynthesis varies among different species, the process always begins with the absorption of light energy by proteins called reaction centers. These reaction centers contain green chlorophyll and other pigments/chromophores. In plants, these proteins are found inside organelles called chloroplasts, primarily in leaf cells. In bacteria, they are embedded in the plasma membrane. During the light-dependent reactions, energy from light is used to separate electrons from suitable substances, like water, resulting in the production of oxygen gas. The hydrogen released during the splitting of water is used to create two compounds that store energy, namely reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), which serves as the "energy currency" of cells.
In plants, algae, and cyanobacteria, sugars are synthesized through a series of light-independent reactions known as the Calvin cycle. In this cycle, atmospheric carbon dioxide is incorporated into existing organic carbon compounds. Using the ATP and NADPH produced during the light-dependent reactions, these compounds are then reduced and transformed into carbohydrates, such as glucose. Other bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same goal.
Photosynthesis first evolved early in the history of life, and the first photosynthetic organisms likely used reducing agents like hydrogen or hydrogen sulfide as sources of electrons instead of water. Cyanobacteria emerged later and the excess oxygen they produced directly contributed to the oxygenation of the Earth, making the evolution of complex life forms possible. Currently, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about eight times the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams, or billion metric tons) of carbon into biomass per year.
Photosynthesis is crucial for climate processes as it captures carbon dioxide from the air and stores carbon in plants, soils, and harvested products. Cereals alone are estimated to bind 3,825 Tg (teragrams) or 3.825 Pg (petagrams) of carbon dioxide every year, which amounts to 3.825 billion metric tons.