Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
The C4 pathway provides the necessary CO2 concentration

Why might it be advantageous for plants to initially fix СО2 in one Cell type, only to release and re-fix it immediately in another, especially given that this complex pathway requires a substantial energy investment? Fundamental research into the biochemistry and Histology of СО2 fixation in tropical plants has helped reveal the adaptive Significance of the С4 cycle. Tropical plants must minimize excessive Water loss through Transpiration. They achieve this by closing their Stomata, which act as a kind of respiratory tract for the leaves. However, stomatal closure also restricts the diffusion of atmospheric СО2 into the bundle-sheath Cells. As a result, the intracellular СО2 concentration in the bundle sheath remains relatively low, preventing ribulose bisphosphate carboxylase from operating at maximum velocity. In contrast, phosphoenolpyruvate carboxylase, located in the mesophyll cells, exhibits a much higher affinity for СО2, allowing carbon fixation to proceed much more efficiently there. The reaction catalyzed by phosphoenolpyruvate carboxylase ensures the fixation and accumulation of СО2 in the form of malate. Subsequent decarboxylation of malate within the bundle-sheath cells elevates the local СО2 concentration to a level sufficient to drive The activity of ribulose bisphosphate carboxylase close to its maximum.

An intriguing paradox arises: although fixing a single molecule of СО2 via the Hatch-Slack pathway requires C4 plants to expend five high-energy phosphate groups compared to only three in C3 plants, tropical C4 plants nevertheless grow faster than temperate C3 species and produce greater biomass per unit of leaf area. (Unfortunately for gardeners, crabgrass and many other resilient weeds are of tropical origin and utilize the C4 pathway, meaning they are exceptionally efficient at converting light energy into biomass).

Let us explore the underlying mechanisms of this paradox.



Last update: 06/08/2026

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