Plant Physiology - Photosynthesis in Higher Plants (Light and Dark Reactions, C3 and C4 pathways)

Photosynthesis is a physico-chemical process by which green plants use light energy to synthesize organic compounds. The overall reaction is represented as: $6CO_2 + 12H_2O \xrightarrow{\text{Light/Chlorophyll}} C_6H_{12}O_6 + 6H_2O + 6O_2$.

The process occurs in the chloroplast: the Light Reaction takes place in the thylakoids/grana, and the Dark Reaction (Biosynthetic phase) occurs in the stroma.

Light Reaction involves: Light absorption, water splitting ($Photolysis$), oxygen release, and the formation of high-energy chemical intermediates, $ATP$ and $NADPH$.

Photophosphorylation is the synthesis of $ATP$ from $ADP$ and inorganic phosphate ($P_i$) in the presence of light. It can be Non-cyclic (Z-scheme involving $PS \ I$ and $PS \ II$) or Cyclic (involving only $PS \ I$).

The Calvin Cycle ($C_3$ pathway) involves three stages: Carboxylation (primary $CO_2$ acceptor is $RuBP$, catalyzed by $RuBisCO$), Reduction (using $ATP$ and $NADPH$), and Regeneration of $RuBP$.

The $C_4$ pathway (Hatch-Slack pathway) is an adaptation to dry tropical regions. It involves 'Kranz anatomy' and uses $PEP$ (Phosphoenolpyruvate) as the primary $CO_2$ acceptor in mesophyll cells, forming $OAA$ (Oxaloacetic acid).

Photorespiration is a wasteful process where $RuBisCO$ binds with $O_2$ instead of $CO_2$ under high light intensity and temperature, leading to a loss of fixed carbon. It is absent in $C_4$ plants.

Blackman's Law of Limiting Factors states that if a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value.