Plant Physiology - Respiration in Plants (Glycolysis, Krebs Cycle, ETS, Amphibolic Pathway)

Cellular Respiration is the mechanism of breaking down food materials within the cell to release energy, and the trapping of this energy for synthesis of $ATP$.

Glycolysis, also known as the EMP pathway, occurs in the cytoplasm. One molecule of glucose ($C_6H_{12}O_6$) is broken down into two molecules of pyruvic acid ($CH_3COCOOH$) with a net gain of $2 ATP$ and $2 NADH + H^+$.

The Link Reaction involves the oxidative decarboxylation of pyruvate into Acetyl $CoA$ in the mitochondrial matrix, mediated by the enzyme pyruvate dehydrogenase, producing $CO_2$ and $NADH + H^+$.

The Krebs Cycle (TCA Cycle) occurs in the mitochondrial matrix. It starts with the condensation of Acetyl $CoA$ ($2C$) with Oxaloacetic acid ($4C$) to form Citric acid ($6C$). For every turn, $3 NADH$, $1 FADH_2$, and $1 GTP/ATP$ are produced.

The Electron Transport System (ETS) is located on the inner mitochondrial membrane (cristae). It involves the transfer of electrons through complexes I to IV, eventually reducing $O_2$ to $H_2O$.

Oxidative Phosphorylation is the process of $ATP$ synthesis driven by the proton gradient across the inner mitochondrial membrane, catalyzed by Complex V ($ATP$ synthase/$F_0-F_1$ particles).

Respiration is considered an Amphibolic Pathway because it involves both catabolism (breakdown of substrates) and anabolism (intermediates like $\alpha$-ketoglutaric acid are used for synthesis of amino acids).

Respiratory Quotient ($RQ$) depends on the type of respiratory substrate used. $RQ = 1$ for carbohydrates, $RQ < 1$ for fats/proteins, and $RQ > 1$ for organic acids.