An alternative, very efficient metabolic pathway used by plants living in areas with low levels of carbon dioxide. They use this pathway to convert carbon dioxide into a form usable during photosynthesis. Plants in environments with normal carbon dioxide concentrations use the Calvin cycle instead of this pathway, which is also called the C4 cycle.

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The C4 pathway was first discovered in the 1960's by scientists in Russia and the USA. However, it was the Australian Marshall Hatch and Englishman Roger Slack that dedicated their research to understanding the process and identifying the enzymes involved. The Hatch-Slack pathway is essentially the same process as that described in photosynthesis, except that a four carbon compound is the first stable product. C4 plants also have different leaf anatomy, enzymes and location of choroplasts.

The Hatch-Slack pathway is an adaptation by plants to conditions of high temperatures and low humidity. Carbon dioxide is concentrated in the bundle sheath cells, and so only small amounts of carbon dioxide are required by gas exchange through the stomata. The stomata can therefore be fairly closed, reducing water loss. High carbon dioxide concentration also reduces the rate of photorespiration, so C4 plants have a higher maximum rate of photosynthesis than those plants using the C3 pathway.

The leaf anatomy in C4 plants feature bundle sheaths surrounding the vascular bundles, and choroplasts located in the surrounding mesophyll cells. The cell walls are relatively thicker to inhibit the diffusion of carbon dioxide and maintain a high concentration.

C4 plants occur mostly in hot climates and can reach rates of photosynthesis that C3 plants can only dream about.

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