Photosynthesis is the process in which Light energy is used to reduce Carbon dioxide (CO2) to sugars and other carbohydrates and in which molecular oxygen (O2) is evolved. It is central to all life, in that photosynthesis ultimately produces all sources of food. Past photosynthetic activity also produced the present oxygen-rich atmosphere, and the oil, gas and hydrocarbon-rich sediments.
The basic photosynthetic process takes place in all green higher plant and algal cells, within organelles termed chloroplasts, which have the necessary pigments (chlorophylls) to absorb and utilize light energy. A similar process also occurs in the cyanobacteria and in photosynthetic bacteria. The term photosynthesis is also used generally to indicate the uptake of CO2 by plants, which involves the diffusion of CO2 from the atmosphere into the leaves through the stomatal pores, and into the photosynthetic cells. Because of this diffusion, photosynthesis is intimately linked with transpiration — the loss of water by evaporation from cells in the leaf, and diffusion of the water vapour out through the pores into the air.
Photosynthesis is actually the coupling of two processes.
1. Light capture, electron transfer and photophosphorylation on the thylakoid membranes within the chloroplasts to generate reduced assimilatory power in the form of NADPH and ATP.
2. Use of these reduced products to drive the ‘dark reactions’ that make carbohydrates in the stroma of the chloroplasts.
For the ‘light reactions’ the pigments are organized into two photosystems, termed PSI and PSII, and a light-harvesting complex (LHCP). Light absorption by chlorophyll molecules raises their excitation state, and electrons are passed to a reaction centre in the photosystem. In PSII, oxygen is evolved and ATP produced, while in PSI, NADPH is produced.
All green plants use the NADPH and ATP from the light reactions for the carbon fixation in the dark reactions, in the photosynthetic carbon reduction (PCR) or Calvin cycle. The key PCR enzyme is ribulose bisphosphate carboxylase-oxygenase (Rubisco), which catalyses the uptake of CO2 through incorporation with a 5-carbon compound, ribulose bisphosphate, into a 3-carbon compound, phosphoglyceric acid. The enzyme can also oxygenate the ribulose bisphosphate, producing phosphoglycollate and releasing CO2 in the process known as photorespiration. At typical temperatures and at the CO2 and O2 concentrations in most plant cells this oxygenase activity loses about 20–50% of the carbon fixed by carboxylation. This is the situation for the major part of the world's vegetation, which is of the C3 photosynthetic type.
Some species of plants, the C4 group, have evolved a secondary carbon fixation pathway, using the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), which has no oxygenase activity, and a higher affinity for CO2. In these species, CO2 is first combined into a 4-carbon compound, in the mesophyll leaf cells, and then passed to the cells around the veins in the leaf (the bundle sheath cells) where the CO2 is released at high concentrations for recarboxylation in the usual PCR pathway. These high CO2 concentrations inside the bundle sheath suppress photorespiration.
These C4 species (which are of diverse genera) are typically found in warmer and drier environments and, while not important in terms of the overall proportion of the world's vegetation, are locally and agriculturally very important (e.g. maize, sorghum, millet, sugar-cane and many subtropical and tropical forage grasses). As their photosynthetic rates are typically higher than those of the C3 types, they also lose less water per molecule of CO2 taken up, and therefore have a higher water use efficiency.
Another locally important group of plants with a different photosynthetic physiology are the CAM plants, such as many cacti and succulents, typical of arid zones. The problem in these environments is that CO2 uptake involves the risk of dehydration. These species therefore harvest light energy during the day, while maintaining the stomata shut, but providing CO2 for fixation in the PCR cycle from the decarboxylation of stored malate. At night they open the stomata, and accumulate CO2 into malate; they have therefore separated the CO2 fixation part of photosynthesis from the CO2 uptake part, and thereby conserve water.
Photosynthesis is somewhat inefficient in energy terms. Firstly, only about 47% of the solar spectrum — the region between 400 and 700 nm (termed photosynthetically active radiation) — can be absorbed by the photosynthetic pigments. Secondly, between 10 and 30% of the light reaching the leaf is reflected. Thirdly, although the theoretical minimum quantum yield (the number of absorbed quanta required to fix one molecule of CO2) is 8, reflection, absorption by non-photosynthetic pigments, fluorescence and decay of excited pigment molecules increase the value to about 12. In addition, substantial amounts of CO2 are lost due to respiration and photorespiration, so that the real quantum requirement at low light levels is about 15–20 quanta. At higher light levels this requirement increases, because other factors in addition to light start to influence the photosynthetic rate. The overall energy efficiency of even highly productive agricultural crops is only around 2%, in terms of the amount of energy in the crop as a fraction of the solar energy incident. The rest of the solar energy incident on the crop is dissipated in heating the crop and the air in contact, and in evaporation. See also c3/c4 plants; ecological efficiency; Ecological energetics; primary productivity. [J.I.L.M.]
Hall, D.O. & Rao, K. (1994) Photosynthesis, 5th edn. Cambridge University Press, Cambridge.
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