Question

Name the first stable product of ${ C }_{ 4 }$ cycle.

Answer 1

The first stable product ${ C }_{ 4 }$ cycle is a four-carbon compound and is characteristic to the naming of the cycle. This cycle is reported in members of families inhabiting tropical or subtropical areas.

Complete answer:
${ C }_{ 4 }$ cycle is an alternate process of carbon fixation which has been evolved to minimize photorespiration and increase the efficiency of carbon fixation from whatever level of carbon dioxide is available in the immediate environment. This is named so due to the four-carbon acid ‘oxaloacetate’ being the first stable product. Examples of ${ C }_{ 4 }$ plants are Maize, sugarcane, Amaranthus etc.
${ C }_{ 4 }$ cycle is also known as Hatch and Slack pathway, named in honour of Marshall Davidson Hatch and Charles Roger Slack - the discoverers of this cycle. In the ${ C }_{ 4 }$ cycle, compartmentalisation occurs in the form of Kranz anatomy. ${ C }_{ 3 }$ cycle occurs in bundle sheath cells.
In kranz anatomy, the mesophyll cells are concentrically arranged around bundle sheath cells. Bundle sheath cells, in turn, surround vascular bundles in a wreath-like manner in one to several layers. Both mesophyll and bundle sheath cells are connected via plasmodesmata or cytoplasmic bridges.
This cycle occurs through three basic steps which are as follows:
Carboxylation: ${ CO }_{ 2 }$ is accepted by phosphoenolpyruvate and fixed into a four-carbon acid i.e oxaloacetate. This takes place in mesophyll cells. It is converted to malic acid
Diffusion: Transport of the fixed malic acid from mesophyll to bundle sheath cells.
Decarboxylation: ${ CO }_{ 2 }$ and pyruvate is released from malic acid by its decarboxylation. The ${ CO }_{ 2 }$ enters the C3 cycle. It is accepted by RuBisCo and the first step of ${ C }_{ 3 }$ cycle, carboxylation, begins.
Regeneration: Phosphoenolpyruvate is regenerated from the 3-Carbon pyruvate. It is again transported to the mesophyll cells to accept and fix another ${ CO }_{ 2 }$ entering the cell.

Note: The ${ C }_{ 4 }$ cycle concentrates ${ CO }_{ 2 }$ in bundle sheath cells which is the site of carboxylation for ${ C }_{ 3 }$ cycle by removing ${ CO }_{ 2 }$ from malic acid. This is done to keep the concentration high enough in bundle sheath for RuBisCo to bind ${ CO }_{ 2 }$ rather than oxygen via ${ C }_{ 3 }$ cycle. In this way, this cycle has evolved to minimize photorespiration.