Introduction:
C3 and C4 plants are two types of photosynthetic pathways found in different plant species. C3 plants use the C3 pathway, while C4 plants use the C4 pathway. The primary difference between these two pathways is the way carbon dioxide is initially fixed during photosynthesis. This difference has consequences for the energy requirements of the two pathways.

Explanation:
1. C3 Pathway:
In C3 plants, carbon dioxide is initially fixed by the enzyme Rubisco into a three-carbon compound called 3-phosphoglycerate (3PGA) in the Calvin cycle. This process is called C3 because the first stable compound formed during carbon fixation has three carbon atoms. The Calvin cycle then proceeds to produce hexose sugars, such as glucose, by further enzymatic reactions.

During carbon fixation, C3 plants require 3 molecules of ATP to produce 1 molecule of 3PGA. In the subsequent steps of the Calvin cycle, an additional 9 molecules of ATP are required to produce 1 molecule of hexose sugar. Therefore, in total, C3 plants require 12 molecules of ATP for the net production of 1 molecule of hexose sugar.

2. C4 Pathway:
In C4 plants, carbon dioxide is initially fixed into a four-carbon compound called oxaloacetate by the enzyme phosphoenolpyruvate carboxylase (PEPCase). This process occurs in the mesophyll cells of the plant. The oxaloacetate is then converted into malate or aspartate, which are transported into bundle sheath cells.

In the bundle sheath cells, the four-carbon compounds release carbon dioxide, which is then fixed by Rubisco into 3PGA in the Calvin cycle, similar to C3 plants. However, since the initial carbon fixation occurs in the mesophyll cells, C4 plants can efficiently concentrate carbon dioxide around Rubisco, minimizing photorespiration and increasing photosynthetic efficiency.

Due to the extra steps involved in the C4 pathway, C4 plants require additional ATP to carry out the process. However, the net production of ATP in C4 plants is sufficient to meet the energy demands. Therefore, no additional molecules of ATP are needed for the net production of one molecule of hexose sugar in C4 plants.

Conclusion:
In summary, C3 plants require a total of 12 molecules of ATP for the net production of one molecule of hexose sugar, while C4 plants do not require any additional molecules of ATP beyond what is produced during photosynthesis. This difference in energy requirements is one of the advantages of C4 plants in terms of their photosynthetic efficiency and ability to thrive in environments with high light and temperature.

It is 12