C3 and C4 pathways

It was once thought that all green plants fix CO2 through Calvin cycle only. Now, we know that certain plants fix CO2 in a different photosynthetic mechanism called C4 pathway.

In this chapter, we will know more about this. Hatch and Slack observed that 4C compounds such as oxaloaceticacid, malate and aspartate were the first formed compounds, when the leaves of sugarcane were exposed to 14CO2 for one second. So, sugarcane is an example for C4 plant. When the leaves of rice plant are exposed to 14CO2 3C compound called phosphoglyceric acid is formed. So, rice plant is an example for C3 plant.

In C3 plants, photosynthesis occurs only in mesophyll cells. We already learnt that photosynthesis has two types of reactions – light reactions and dark reactions (Calvin cycle). In light reactions ATP and NADPH2 are produced and oxygen is released as a byproduct. CO2 is reduced to carbohydrates by dark reactions.

In C3 plants both light reactions and dark reactions occur in mesophyll cells, whereas in C4 plants, the mechanism of photosynthesis requires two types of photosynthetic cells – mesophyll cells and bundle sheath cells. The C4 plants contain dimorphic chloroplasts

i.e. chloroplasts in mesophyll cells are granal (with grana) whereas in bundle sheath chloroplasts are agranal (without grana). The presence of two types of cells leads to segregation of photosynthetic work i.e. light reactions and dark reactions separately.

Hatch-Slack pathway involves two carboxylation reactions.

One takes place in chloroplasts of mesophyll cells and another in chloroplasts of bundle sheath cells.
1. The first step involves the carboxylation of phosphoenol pyruvic acid in the chloroplasts of mesophyll cells to form a 4C compound, oxaloacetic acid. This reaction is catalysed by the enzyme phosphoenol pyruvate carboxylase

2. Oxaloacetic acid is converted into aspartic acid by the enzyme transaminase or it may be reduced to malic acid by NADP+ specific malate dehydrogenase.

3. Malic acid or aspartic acid formed in chloroplast of mesophyll cells is transferred to the chloroplasts of bundle sheath where it is decarboxylated to form CO2 and pyruvic acid in the presence of NADP+specific malic enzyme.

4. Now, second carboxylation occurs in chloroplasts of bundle sheath cells. Ribulose bisphosphate accepts CO2 produced in step (3) in the presence of RuBP carboxylase and yields 3-phosphoglyceric acid. Some of the 3-phosphoglyceric acid molecules are utilised to produce sucrose and starch, while remaining PGA molecules are used for the regeneration of RuBP.

5. The pyruvic acid produced in step (3) is transferred to the chloroplasts of mesophyll cells where it is phosphorylated to regenerate phosphoenolpyruvic acid . This reaction is catalysed by pyruvate kinase in the presence of Mg 2+.

The AMP is phosphorylated by ATP in the presence of adenylate kinase to form 2 molecules of ADP.
C4 plants are photosynthetically more efficient than C3 plants, because the net requirement of ATP and NADPH2 for the fixation of one molecule of CO2 is considerably lower in C4 plants than in C3 plants.