explain calvins cycle,c3 and c4 pathway
The Calvin Cycle : The Three Carbon Pathway
The C3 type of carbon reactions occurs in the strome of the chloroplast. It is also called Calvin cycle after its discoverer, Melvin Calvin.
a) In the first step CO2 is accepted by a 5- carbon molecule, ribulose -1,5 biphosphate (RuBP) and 2 molecules of 3-carbon compound that is 3- phosphoglycerate (PGA) are formed. This reaction is catalysed by an enzyme called Rubisco-Ribulose biphosphate carboxylase oxygenase. Formation of PGA is called carboxylation.
b) The 3 - carbon compound formed is the first stable product of this pathway and hence the name C3 pathway.
c) After carboxylation, reduction of PGA occurs by utilising ATP and NADPH2 formed during photochemical reactions. Reduction results in the formation of glyceraldehyde-3-phosphate.
d) These 3 - carbon molecules, also called triose phosphates are diverted from the Calvin cycle and act as precursors for the synthesis of sucrose and starch.
The Calvin cycle proceeds in three stages: (1) carboxylation, during which CO2 combines with ribulose-1,5-bisphosphate (2) reduction, during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH; and (3) regeneration during which the CO2 acceptor ribulose 1,5-bisphosphate is formed again so that the cycle continues
For the cycle to continue on its own regeneration of the initial 5- carbon acceptor molecule i.e., RuBP takes place, from glyceraldehyde 3- phosphate using an ATP molecule
The Calvin Cycle : The four Carbon Pathway
C4 plants like maize, sugarcane; pearl millet, etc. have evolved a wonderful mechanism to avoid photorespiration, which is considered a wasteful process.
In these plants three or 2 types of photosynthetic cells namely mesophyll cells and bundle sheath cells. The bundle sheath cells are arranged in a wreath like manner, thus calling them as kranz anatomy (kranz : wreath).
One more speciality of C4 plants is that they contain dimorphic chloroplasts, that is chloroplasts in bundle sheath cells are agranal. The presence of two types of cells allows the occurrence of light reactions and carbon reactions separately.
In C4 plants, light reactions occur in the mesophyll cells, whereas CO2 assimilation occurs in the bundle sheath cells. This type of separation does not allow O2 released in mesophyll cells to escape in to the bundle sheath cells. This prevents the oxygenation of RuBP, which is present in the bundle sheath cells.
Transverse section of maize leaf showing the arrangement of mesophyll and bundle-sheath cells. The C4 pathway takes place in the mesophyll cells, and the C3 pathway (Calvin cycle) operates in the bundle-sheath cells. Both types of cells contain chloroplasts.
To further avoid photorespiration, C4 plants have evolved a CO2 concentrating mechanism called the C4 pathway. The main objective of C4 pathway is to build up a high concentration of CO2 in the vicinity of Rubisco, thus favouring carboxytition and suppressing photorespiration.
Many plants like maize and sugar cane, are far more efficient at taking up CO2 than C3 plants. The CO2 acceptor in C4 plants is phosphoenolpyruvate (PEP). PEP reacts with CO2 to form oxaloacetic acid which is reduced by NADPH to form malic acid. The malic acid then reacts with RUBP to form pyruvic acid and PGA. The pyruvic acid is then phosphorylated by ATP to regenerate PEP while PGA is converted to triose phosphate as far as C3 plants. These reactions are called the Hatch - Slack pathway.
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