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RuBisCO is important biologically because it catalyzes the primary chemical reaction by which inorganic carbon enters the biosphere.While many autotrophic bacteria and archaea fix carbon via the reductive acetyl CoA pathway, the 3-hydroxypropionate cycle, or the reverse Krebs cycle, these pathways are relatively small contributors to global carbon fixation compared to that catalyzed by RuBisCO.
C 2 photosynthesis (also called glycine shuttle and photorespiratory CO 2 pump) is a CCM that works by making use of – as opposed to avoiding – photorespiration. It performs carbon refixation by delaying the breakdown of photorespired glycine, so that the molecule is shuttled from the mesophyll into the bundle sheath .
This pathway allows C4 photosynthesis to efficiently shuttle CO 2 to the RuBisCO enzyme and maintain high concentrations of CO 2 within bundle sheath cells. These cells are part of the characteristic kranz leaf anatomy, which spatially separates photosynthetic cell-types in a concentric arrangement to accumulate CO 2 near RuBisCO. [21]
C 4 photosynthesis reduces photorespiration by concentrating CO 2 around RuBisCO. To enable RuBisCO to work in a cellular environment where there is a lot of carbon dioxide and very little oxygen, C 4 leaves generally contain two partially isolated compartments called mesophyll cells and bundle-sheath cells.
The CO 2 is stored as four-carbon malic acid in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO 2, which is then used during photosynthesis. The pre-collected CO 2 is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency.
Under these conditions, CO 2 will decrease and oxygen gas, produced by the light reactions of photosynthesis, will increase, causing an increase of photorespiration by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and decrease in carbon fixation.
[2] [3] Pyrenoids are associated with the operation of a carbon-concentrating mechanism (CCM). Their main function is to act as centres of carbon dioxide (CO 2) fixation, by generating and maintaining a CO 2-rich environment around the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).
2-Phosphoglycolate (chemical formula C 2 H 2 O 6 P 3-; also known as phosphoglycolate, 2-PG, or PG) is a natural metabolic product of the oxygenase reaction mediated by the enzyme ribulose 1,5-bisphosphate carboxylase (RuBisCo). Photorespiration serves as a salvage pathway that converts 2-PG into non-toxic metabolites. Contrary to the Calvin ...