C4 Carbon Fixation

C4 carbon fixation is a variant of photosynthesis in which plants first capture carbon dioxide in a four-carbon molecule, then release it near the enzyme Rubisco. The system concentrates CO₂ and greatly reduces photorespiration, making C4 plants efficient in heat, strong light and dry conditions.

In the outer mesophyll cells of the leaf, an enzyme called PEP carboxylase, which does not react with oxygen, joins CO₂ to phosphoenolpyruvate to make oxaloacetate, which becomes malate. The malate moves into tightly packed bundle sheath cells that surround the veins, in a ring-like pattern called Kranz anatomy, and there it releases CO₂ at a high concentration for the Calvin cycle. The pump costs extra ATP, so it pays off mainly where photorespiration would be serious.

Only about 3 percent of plant species use C4, but they account for roughly a fifth or more of the land's primary production. Maize, sugarcane, sorghum and many tropical grasses are C4 plants. The pathway has evolved independently more than 60 times, and it spread widely when falling CO₂ levels about 30 million years ago favored grasses. It was worked out in the 1960s by Marshall Hatch and Roger Slack, building on earlier work by Hugo Kortschak.

C4 plants use water more efficiently because they can keep their stomata partly closed, and they grow quickly in warm climates. Researchers are trying to engineer the pathway into rice to raise yields, a project in synthetic biology and biotechnology. Compared with C3 plants, C4 species respond less to extra CO₂ in the air.

• C3 Carbon Fixation — the more common pathway
• Photorespiration — the loss it avoids
• Rubisco — the enzyme it supplies with CO₂