C4 carbon fixation

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Overview of C4 carbon fixation

C4 carbon fixation is one of three methods, along with C3 and CAM photosynthesis, used by land plants to "fix" carbon dioxide (binding the gaseous molecules to dissolved compounds inside the plant) for sugar production through photosynthesis. Along with CAM photosynthesis, C4 fixation is an improvement over the simpler and more ancient C3 carbon fixation strategy used by most plants. Both methods overcome the tendency of rubisco (the first enzyme in the Calvin cycle) to photorespire, or waste energy by using oxygen to break down carbon compounds to CO₂. C4 plants separate rubisco from atmospheric oxygen, fixing carbon in the mesophyll cells and using oxaloacetate and malate to ferry the fixed carbon to rubisco and the rest of the Calvin cycle enzymes isolated in the bundle-sheath cells. The intermediate compounds both contain four carbon atoms, hence the name C4.

The Reaction

The C4 pathway was discovered by M. D. Hatch and C. R. Slack, two Australian researchers, in 1966, so it is sometimes called the Hatch-Slack pathway.

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 photorespiration, the C₄ pathway is an adaptive mechanism for minimizing the loss.

C4 Leaf Anatomy

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). It occurs in the mesophyll of the leaf, specifically in the mesophyll cells and the bundle-sheath cells.

The Evolution and Advantages of the C4 Pathway

C4 plants have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures and nitrogen or carbon dioxide limitation. C4 carbon fixation has evolved on several occasions in different groups of plants, so is an example of convergent evolution. Plants which use C4 metabolism include sugarcane, maize, sorghum, Eleusine, Amaranthus, and Switchgrass (Panicum virgatum). C4 plants arose during the Cenozoic Era and did not become common until the Miocene Period. Today they represent about 5% of Earth's plant biomass.

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