C4 Cycle...

There are many photosynthetic cycles but The most important of these photosynthetic adaptations are C4 photosynthetic carbon cycle.(C4 cycle, named because initial CO2 fixation is four carbon acid) 

The C4 cycle was discovered by Hatch and slack in Australia in 1966, so it's sometimes called      The Hatch and Slack Pathway.

The C4 cycle concentrates Co2 at the site of carboxylation, keeping the Co2 conc^ high enough for RuBisCo to bind carbon dioxide rather than oxygen. In this way, C4 cycle has evolved to minimize photorespiration. Plants that have a C4 cycle are called C4 plants. C4 plants include both monocots and dicots, Ex:- the C4 cycle is particularly prominent in Gramineae ( corn, sugarcane), Chenopodiaceae(Atriplex), Cyperaceae( sedges) family. In C4 plants, there are two distinct types of photosynthetic cells:-

1- Bundle sheath cells

2- mesophyll cells

In bundle sheath cells and mesophyll cells, the basic C4 cycle completes in four stages:-

i)- Fixation of Co2 by the carboxylation of PEP(phosphoenolpyruvate) in the mesophyll cells to form a c4 acid ( malate and/or aspartate ).

ii)-Transport of the C4 acids (malate and aspartate) to the bundle sheath cells.

iii)-Decarboxylation of the C4 acids within the bundle sheath cells and generation of Co2, which enters in the C3 cycle.( there are three different types of C4 cycles based on the enzyme which is employed to decarboxylate C4 acids in the bundle sheath.these are NADP+-malic enzyme type, NAD+-malic enzyme type, and PEP carboxykinase type)

iv)- Transport of the C3 acid(pyruvate or alanine) that is formed by the decarboxylation step back to the mesophyll cell.

  

Mechanism of C4 cycle

In the first step, PEP is carboxylated to the C4 acid such as malate and this is the first product of photosynthesis, In contrast to 3-phosphoglycerate in C3 plants. C4 acids are formed in the mesophyll cells and are then transferred, by diffusion, to the bundle sheath, where they are decarboxylated.This generates a high conc^ of Co2 within the bundle sheath.RuBisCo and the majority of the enzymes of the Calvin cycle are confined to the bundle sheath. The co2 conc^ in the bundle sheath is sufficient to suppress the oxygenase activity of RuBisCo.The co2 released is fixed by RuBisCo and converted to Triose phosphate by the Calvin cycle. A C3 compound such as pyruvate returns to the mesophyll cells from the bundle sheath. In mesophyll cells, pyruvate is converted into PEP. This step is ATP dependent. 

 C4 cycle is, therefore, an ATP-driven Co2 enrichment cycle. The cost of concentrating co2 within the bundle sheath cell is 2 ATP per CO2. Thus total energy requirement for fixing CO2 by the combined C4 and Calvin cycle in five ATP( 2 ATP for co2 enrichment and 3 ATP for co2 fixation)

and plus two NADPH per co2 fixed.

In this way, C4 photosynthesis minimizes photorespiration and enhances carbohydrate production.