BIOLOGY Volume 1 - A Guide to General Biology - 2004

7. AUTOTROPHIC NUTRITION

7.9. C₄ Photosynthesis

7.9.3. Refixation of Carbon Dioxide in Bundle Sheath Cells

In the METABOLISM/14.html">Chloroplasts of bundle sheath Cells, In addition to Pyruvate, carbon dioxide and reduced NADP are produced (see the Description of the malate shunt above). The Carbon dioxide is then refixed by RuBP carboxylase, which participates in the Reactions of the C₃ pathway, while the reduced NADP is used to reduce PGA to sugar (section 7.6.3).

Since each CO₂ molecule must be fixed twice, C₄ Photosynthesis requires approximately twice as much energy as C₃ photosynthesis. At first glance, The transport of CO₂ and hydrogen in the C₄ pathway seems pointless. However, fixation via PEP carboxylase is so efficient that it leads to a high accumulation of carbon dioxide within the bundle sheath cells. This means that RuBP carboxylase operates under conditions of carbon dioxide excess, unlike in C₃ plants where the CO₂ concentration is atmospheric. This can be explained as follows: firstly, like any enzyme, this enzyme performs better when substrate is abundant; secondly, oxygen is competitively excluded from the enzyme by carbon dioxide.

Consequently, the primary advantage of C₄ photosynthesis is enhanced efficiency of carbon dioxide fixation. It can be viewed as a Supplement rather than an alternative to the C₃ pathway. Thus, C₄ plants are photosynthetically more efficient, since The rate of carbon dioxide fixation is usually the limiting factor. Although utilizing the C₄ pathway requires C₄ plants to consume more energy, the energy factor is generally not limiting in photosynthesis; C₄ plants grow in regions with high light intensity, and furthermore, the chloroplasts in these plants are modified to maximize the utilization of available energy (see below).