C4 carbon fixation
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- For other meanings of C4, see C4 (disambiguation)
C4 carbon fixation is a common metabolic pathway found in land plants (C4 plants). They are competitively superior to plants possessing the more common C3 pathway under the conditions of drought, high temperatures and nitrogen limitation. The C4 plants possess a characteristic leaf anatomy. Their vascular bundles are surrounded by two rings of cells. The inner ring, called Bundle Sheath Cells, contain starch-rich chloroplasts lacking grana which differ from those in mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic.This peculiar anatomy is called Kranz Anatomy.(Kranz-Crown/Halo)The C4 cycle allows for a spacial separation of carbon fixation from respiration, thus allowing C4 plants to increase concentration of CO2 within their leaves. This increases the amount of photosynthesis and decreases the chances of photorespiration, a harmful process in which organic material and energy is lost from the plant due to high concentrations of oxygen. This was proved by two Australian scientists Hatch and Slack in 1966. Therefore, it is also called Hatch-Slack pathway. It is called "C4" because the product, oxaloacetate, contains four carbon atoms. It occurs in the mesophyll of the leaf, specifically in the mesophyll cells and the bundle sheath cells. The chemical equation is:
- PEP carboxylase + PEP + CO2 → oxaloacetate
The product is usually converted to malate, a simple organic compound that gives up its CO2 to the Calvin cycle after being shipped off to bundle sheath cells surrounding a nearby vein. After losing the CO2, it becomes pyruvate, and can be phosphorylated into PEP at the cost of a phosphorus group and one ATP. It can then be reused in the above equation. Since every CO2 molecule has to be fixed twice, the C4 pathway is more energy consuming than the C3 pathway. The C3 pathway requires 18 ATP for the synthesis of one molecule of glucose while the C4 pathway requires 30 ATP. But since otherwise tropical plants lose more than half of photosynthetic carbon in photo-respiration, the C4 pathway is an adaptive mechanism for minimizing the loss.
Plants which use C4 metabolism include sugarcane, maize (corn), sorghum, Eleusine, Amaranthus, Euphorbia, and Cannabis.