Question

Explain the ${C_4}$ Cycle of photosynthesis.

Answer 1

Photosynthesis is nothing but the utilisation of atmospheric carbon dioxide in the manufacture of glucose and oxygen by the plants. There are various cycles involved in photosynthesis. The plants fix the carbon molecules taken from the atmosphere in the form of carbon dioxide. There are three known ways of carbon fixation. ${C_4}$ cycle is one of them.

Complete answer:
Until the 1950s majority of scientists were convinced of the fact that all plants use the same pathway for the process of respiration. This pathway was referred to as the Calvin cycle or the photosynthetic carbon reduction cycle. But by the time, it is discovered that some plants exhibit another mechanism for carbon fixation which is termed as the ${C_4}$ cycle. This is described by Marshall Hatch and Charles Slack.
The process of the ${C_4}$ cycle is as follows-
The first and the major step in the NADP-ME type ${C_4}$ pathway is the conversion of pyruvate to phosphoenolpyruvate (PEP) with the help of an enzyme Pyruvate phosphate dikinase (PPDK). So, it is called ${C_4}$ because the first fixation is of a 4 carbon compound.
This reaction involves the usage of the inorganic phosphate and ATP plus pyruvate, producing PEP, AMP, and inorganic pyrophosphate (PPi).
The next step follows with the fixation of $C{O_2}$ into oxaloacetate by the PEP carboxylase enzyme (PEPC).
Both of these steps occur inside the mesophyll cells of plants.
Reactions involved are as follows-
$Pyruvate + Pigment + ATP \to PEP + AMP + PPi$
$PEP + C{O_2} \to oxaloacetate$
PEPC shows lower Km for HCO−3 — and, thus, higher affinity — than RuBisCO.
Furthermore, ${O_2}$ is a poor substrate for this enzyme. Thus, at relatively lower concentrations of Carbon dioxide, most $C{O_2}$ will be fixed by this pathway.
The final product is usually converted to malate (M), a simple organic compound, which is transported to the bundle sheath cells. Here, it is decarboxylated by the NADP-malic enzyme (NADP-ME) to form $C{O_2}$ and pyruvate.
The Carbon dioxide now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell.

Note: Difference between ${C_3}$ and ${C_4}$ pathways

${C_3}$ pathway	${C_4}$ pathway
The process of photosynthesis occurs in mesophyll cells.	The process of photosynthesis occurs in mesophyll and bundle sheath cells.
The carbon dioxide acceptor molecule is RUBP	The carbon dioxide acceptor molecule is Phosphoenolpyruvate.
It operates at a temperature ranging from 20-250 C.	It operates at a temperature ranging from 30-450 C.
Kranz anatomy is not present.	Kranz anatomy is present.
Eg- Paddy, wheat, potato.	Eg- Sugarcane, Maize, etc.