C4 carbon fixation

C4 Carbon Fixation

Diagram illustrating the C4 carbon fixation pathway.

C4 carbon fixation is a biochemical process that plants use to efficiently capture and fix carbon dioxide (CO2) from the atmosphere. It is an adaptation mechanism found in certain plants, particularly in hot and arid environments, where high temperatures and low CO2 concentrations pose challenges to photosynthesis. This process allows plants to minimize water loss and maximize carbon assimilation, making them more efficient in these challenging conditions.

Overview

C4 carbon fixation involves a specialized pathway that operates alongside the traditional photosynthetic pathway known as C3 carbon fixation. In C3 plants, CO2 is directly fixed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) in the mesophyll cells of the leaves. However, in C4 plants, CO2 is initially fixed into a four-carbon compound called oxaloacetate in the mesophyll cells, which are then transported to bundle sheath cells where the actual carbon fixation occurs.

The key enzyme responsible for the initial fixation of CO2 in C4 plants is phosphoenolpyruvate carboxylase (PEP carboxylase). This enzyme has a higher affinity for CO2 than RuBisCO, allowing it to effectively capture CO2 even at low concentrations. The oxaloacetate produced by PEP carboxylase is then converted into malate or aspartate, which are transported to the bundle sheath cells.

In the bundle sheath cells, malate or aspartate is decarboxylated, releasing CO2 that can be fixed by RuBisCO. This spatial separation of initial CO2 fixation and subsequent carbon assimilation in different cell types minimizes the oxygenation reaction of RuBisCO, reducing photorespiration and increasing the efficiency of carbon fixation.

Significance

C4 carbon fixation confers several advantages to plants in hot and arid environments. By concentrating CO2 in the bundle sheath cells, C4 plants can maintain higher CO2 concentrations around RuBisCO, which reduces the oxygenation reaction and minimizes photorespiration. This leads to increased photosynthetic efficiency and improved water-use efficiency.

Furthermore, the spatial separation of the C4 pathway from the oxygen-sensitive RuBisCO allows C4 plants to keep their stomata partially closed during the day, reducing water loss through transpiration. This adaptation helps them conserve water in environments where water availability is limited.

C4 plants are typically found in grasses, such as maize, sugarcane, and sorghum, as well as in certain dicotyledonous plants like Amaranthus and Chenopodium. These plants have evolved specialized leaf anatomy, with distinct mesophyll and bundle sheath cells, to facilitate the C4 carbon fixation pathway.

Examples of C4 Plants

• Maize (Zea mays)
• Sugarcane (Saccharum officinarum)
• Sorghum (Sorghum bicolor)
• Amaranthus (Amaranthus spp.)
• Chenopodium (Chenopodium album)