Photosynthesis in Higher Plants - C3 and C4 pathways

The biosynthetic phase of photosynthesis (Dark Reaction) occurs in the stroma of chloroplasts and does not directly depend on light but relies on the products of the light reaction, $ATP$ and $NADPH$.

In the $C_3$ pathway (Calvin Cycle), the primary acceptor of $CO_2$ is a 5-carbon sugar called $RuBP$ (Ribulose-1,5-bisphosphate), and the first stable product is a 3-carbon compound, $3-PGA$ (3-phosphoglyceric acid).

The Calvin cycle consists of three stages: Carboxylation (fixation of $CO_2$ into $3-PGA$), Reduction (formation of glucose using $ATP$ and $NADPH$), and Regeneration (renewal of the $CO_2$ acceptor $RuBP$).

In the $C_4$ pathway (Hatch-Slack pathway), plants exhibit 'Kranz anatomy'. The primary $CO_2$ acceptor is $PEP$ (Phosphoenolpyruvate) located in the mesophyll cells, and the first stable product is $OAA$ (Oxaloacetic acid), a 4-carbon compound.

Photorespiration is a wasteful process occurring in $C_3$ plants when $RuBisCO$ binds with $O_2$ instead of $CO_2$ at high temperatures and high $O_2$ concentration, leading to the formation of phosphoglycolate and no synthesis of sugar or $ATP$.

$C_4$ plants are more efficient than $C_3$ plants in high temperatures and low $CO_2$ concentrations because they have a mechanism to increase the concentration of $CO_2$ at the enzyme site, thus minimizing photorespiration.