Question

In \[{C_4}\] pathway initial carbon dioxide fixation occurs in the chloroplast of
A. Guard cells
B. Mesophyll
C. Bundle sheath
D. All of the above

Answer 1

Hint:-During photosynthesis in plants two types of reactions occur in their chloroplasts namely- light reaction and dark reaction. The light reaction is responsible for trapping the light energy and ultimately producing the ATP and NADPH. These products of the light reaction are further utilized during the dark reaction to drive the process leading to the synthesis of food (more accurately sugars). This is called a biosynthetic phase of photosynthesis.

Complete solution:Dark reaction operates in different photosynthetic organisms through three different ways- \[{C_3}\]pathway, \[{C_4}\] pathway, and CAM pathway.\[{C_4}\] pathway is a cyclic process that occurs in most of the monocots plants. These plants have two types of cells called mesophyll cells and bundle sheath cells. Both these cells have their respective chloroplasts.

In the mesophyll cells, a three-carbon molecule called phosphoenolpyruvate (PEP) is present. It acts as the primary carbon dioxide acceptor in this pathway. The enzyme responsible for this fixation is PEP carboxylase or PEPase. This results in the formation of a four-carbon acid called Oxaloacetic acid (OAA).
Malic acid or aspartic acid is then formed by OAA in the mesophyll cells, which are transported to bundle sheath cells. In bundle sheath cells these acids are broken down to release carbon dioxide and a three-carbon molecule called pyruvic acid.

Pyruvic acid is then transported back to the mesophyll cell where it is converted to PEP again, thus completing the cycle. The carbon dioxide released in the bundle sheath cells then enters the Calvin cycle, a pathway common to all plants. Therefore, the first carbon dioxide fixation in these plants occurs in the chloroplasts of mesophyll cells. Thus, the correct answer is option B.

Note:- The evolution of the \[{C_4}\]- photosynthetic system is probably one of the strategies for maximizing the availability of carbon dioxide while minimizing water loss. These plants are twice as efficient as \[{C_3}\] plants in terms of fixing carbon (making sugars). \[{C_4}\] plants lose only half as much water as a \[{C_3}\] plant for the same amount of carbon dioxide fixed.