C3 and C4 Pathways of Photosynthesis

C3 And C4 Cycle

High crop yield plants are essential for meeting the demands of the ever-growing population and, therefore, need to be ensured that photosynthesis occurs in an optimal way for such plants. As most of the crop plants are C3 in nature (where the first carbon intermediate compounds formed by such plants have three carbon atoms), they tend to bond with the photosynthetic enzyme Rubisco, instead of Carbon dioxide, and thus, wasting energy. These plants then start closing up their stomata to lower water loss, especially in arid regions or hot climates. This is where the evolved C4 pathway comes in as it keeps a high concentration of Carbon dioxide gas in the process, preventing the Rubisco - Oxygen bonding.  

However, before going into details about these pathways, here's a detailed version of photosynthesis - the crux to the C3  and C4 pathways.

Photosynthesis is popularly perceived as the formation of energy with sunlight, atmospheric carbon dioxide, and water. This energy then flows throughout the plant, ensuring its nutrition and health. The chemical equation for the process is: 

6CO2+ 6H2O ---> C6H12O6 + 6H2O

Generally, The Photosynthesis Process Occurs in Two Main Phases

• Photosynthetic: In this phase, the plants utilize the light energy to form energy in ATP and NADPH, which serve as the intermediate for the biosynthetic step.

• Biosynthetic: In this phase, the CO2and H2O can combine to yield carbohydrates, and therefore popularly called Carbon Fixation. Different plants obey distinct methods for carbon fixation, and these are known as the C3  and C4 pathways. 

C3 Pathway Plants

The C3  pathway plants follow the Calvin cycle in the dark reaction of photosynthesis. Here, the photosynthetic efficiency is lesser because of excess photorespiration. The first step causes the fixation of carbon dioxide by rubisco and, therefore, needs correction. As many as 85% of crop plants like rice, wheat, and trees follow the C3  pathways. These plants also yield a 3-carbon atom acid known as the Phosphoglyceric Acid as its initial product, and the overall process occurs in the following stages:

• Carboxylation - Here, the PGA gets generated with the help of the Rubisco enzyme carboxylase. 

• Reduction - The PGA gets reduced to Adenosine Tri Phosphate, and Nicotinamide Adenine Dinucleotide Phosphate phosphorylate in this stage.

• Regeneration - Once it yields glucose, the cycle goes on a loop with the restoration of the RuBP enzyme.

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C4 Pathway Plants

These plants follow an additional step to the usual dark reaction followed by the C4 plants. Here, the first compound that gets formed for these plants has four carbon atoms (also known as the OAA) in it, and therefore, has high photosynthetic abilities. Here is a C4 and pathway circle diagram:

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Found in only 5% of the plants worldwide, the C4 pathway is an ability that comes with tropical desert plants. As soon as the formation of the 3-Carbon ‘ferry’ molecule phosphoenolpyruvate is witnessed, the enzyme works with a PEP carboxylase to form the Oxaloacetic Acid (OAA).

In the next stage, the OAA is further converted into a 4-Carbon compound called Malic acid. The OAA gets reduced into Carbon dioxide and a 3-Carbon molecule that helps in the regeneration of the PEP.

C4 Pathway of Photosynthesis

The C4 plants work on enlarged physiological functions that are a direct connection with the CO2 concentration of these plants, therefore influencing the plant's metabolism. As the cells in the C4 species are enlarged, there is always a close contact between the mesophyll and bundle sheath cells and are interconnected via plasmodesmata. For the direct connection between the bundle sheath and the mesophyll cells, the 5% plants that follow the C4 pathway, have their distinct anatomy of leaves where they have a high vein density, making the ratio of mesophyll and bundle sheath tissues as 1:1. Also termed as the Kranz anatomy, these plants use two-cells mode for the C4 photosynthesis. 

Compared to the C3 photosynthesis process, the C4 process takes one or two additional molecules of ATP per fixed particle, without requiring any other reduction equivalents. Such an increase leads to the ATP and NADPH ratio enhancement for the C4 plants, which positively affects ATP production. 

Difference Between C3 and C4 Plants