Question

$C_4$- plants differ from $C_3$-plants in respect to

• A. Number of $C{O}_2$ molecules used
• B. Substrate, which accept the $C{O}_2$ molecules
• C. The final product
• D. Number of ATP formed

Answer 1

The correct option is B Substrate, which accept the $C{O}_2$ molecules

In $C_3$-plants, carbon dioxide combines with RuBP to form phosphoglyceric acid, on the other hand in $C_4$-plants, carbon dioxide is first picked up by Phosphoenol Pyruvate (PEP) to form oxaloacetic acid.
In C4 plants, the oxaloacetic acid formed is converted in to malic acid or aspartic acid and transported to the bundle sheath cells. Inside the bundle sheath cells, carbon dioxide from malic acid or aspartic acid is extracted and used in the C3 cycle.
Hence,the number of carbon dioxide molecules used, the final product and the amount of ATP formed remains the same in both C3 and C4 plants.
However, the net ATP generated, which is the difference between the the total ATP formed and the ATP consumed during the process, is different for the two type of plants. This is because, in C4 plants, 2 ATP per cycle is consumed to transport malate or aspartate in to the bundle sheath cells and generate phosphoenol pyruvate.