Question

In Maize, mesophyll cells perform photosynthesis cycle
(a) ${ C }_{ 4 }$
(b) ${ C }_{ 3 }$
(c) ${ C }_{ 2 }$
(d) ${ C }_{ 1 }$

Answer 1

In plants like maize and sugarcane which are adapted to dry tropical regions have a cycle in which double fixation of carbon dioxide occurs. This cycle was discovered by Hatch and Slack- Two Australian botanists.

Complete Step by Step Answer:
In Maize, the mesophyll cell performs the photosynthetic cycle by using ${ C }_{ 4 }$ Pathway. Most of the plants that are adapted to the dry tropical regions have the ${ C }_{ 4 }$ Pathway, for example, sugarcane, maize, Sorghum, Amaranthus, etc.
In these plants, a double fixation of carbon dioxide takes place. The initial product of this pathway is a four- carbon compound - Oxaloacetic acid and hence the name.
- This pathway is also named as the Hatch and Slack pathway after the scientists who discovered this pathway.
- The ${ C }_{ 4 }$ pathway requires the presence of two types of cells- mesophyll cells and the bundle sheath cells. The ${ C }_{ 4 }$ pathway is a cyclic process.
- The primary Carbon dioxide acceptor is the three- carbon molecule phosphoenolpyruvate (PEP) and it is present in mesophyll cells. The enzyme that catalyzes this ${ C }O_{ 2 }$ fixation is PEP carboxylase or PEPcase.
The four- carbon oxaloacetic acid is formed in the mesophyll cells which is then converted to other four- carbon compounds like malic acid or aspartic acid in the mesophyll cells itself and then transported to the bundle sheath cells where this ${ C }_{ 4 }$ is broken down into ${ C }O_{ 2 }$ and three- carbon molecule. The ${ C }O_{ 2 }$ is released in the bundle sheath cells where it enters the ${ C }_{ 3 }$ pathway.
- The three- carbon molecule is transported back to the mesophyll cells where it is converted into PEP by enzyme PEP synthetase, thus completing the cycle.
So, the correct answer is, “(a) ${ C }_{ 4 }$”.

Note:
- ${ C }_{ 4 }$ plants are special as they have special leaf anatomy, they can tolerate higher temperatures, they show a response to high intensities of light, they lack a wasteful process called photorespiration, thus they show greater productivity and higher yield than ${ C }_{ 3 }$ plants.
- Bundle sheath cells may form several layers around the vascular bundle. They are characterized by having several chloroplasts, grana are absent, thick walls impervious to gaseous exchange, and no intercellular spaces. This special anatomy of leaves of ${ C }_{ 4 }$ plants is called “Kranz Anatomy”.
- Kranz means wreath and is a reflection of the arrangement of the cell.
- The mesophyll cells of ${ C }_{ 4 }$ plants lack RuBisCO.
- The bundle sheath cells contain RuBisCO but lack PEPcase.
- The basic pathway leads to the formation of sugar. The Calvin pathway is common in both ${ C }_{ 4 }$ and ${ C }_{ 3 }$ plants.