Respiration in Plants: Exam-Eve Review

Cellular respiration is the stepwise, controlled oxidation of glucose ( $C_{6}H_{12}O_{6}$ + $6O_{2}$ → $6CO_{2}$ + $6H_{2}O$ + 38 ATP) occurring through four stages: glycolysis, oxidative decarboxylation, TCA cycle, and ETS with oxidative phosphorylation.
Glycolysis occurs in the cytoplasm, requires no oxygen, and is universal — converting one glucose (6C) into two pyruvate (3C) with a net yield of 2 ATP and 2 NADH through 10 enzymatic steps regulated by phosphofructokinase (PFK).
Under anaerobic conditions, pyruvate undergoes fermentation in the cytoplasm — either alcoholic fermentation (yeast → ethanol + $CO_{2}$ ) or lactic acid fermentation (muscles/Lactobacillus → lactate) — yielding only 2 ATP and regenerating N $AD^{+}$ for continued glycolysis.
In aerobic respiration, pyruvate enters the mitochondrial matrix where the pyruvate dehydrogenase complex (requiring TPP, N $AD^{+}$ , CoA) converts it to acetyl CoA, releasing 1 $CO_{2}$ and 1 NADH per pyruvate.
The TCA cycle (Krebs cycle) operates in the mitochondrial matrix through 8 steps; per glucose (two turns), it yields 6 NADH, 2 $FADH_{2}$ , 2 GTP, and 4 $CO_{2}$ , making it an amphibolic pathway serving both energy production and biosynthesis.
The Electron Transport System (ETS) on the inner mitochondrial membrane transfers electrons from NADH (at Complex I, yielding 3 ATP) and $FADH_{2}$ (at Complex II, yielding 2 ATP) through ubiquinone and cytochrome c to molecular oxygen at Complex IV, forming water.
As electrons flow through ETS Complexes I, III, and IV, protons are pumped into the intermembrane space, creating an electrochemical gradient that drives $F_{0}$ - $F_{1}$ ATP synthase via chemiosmosis — the mechanism proposed by Peter Mitchell (Nobel 1978).
The Respiratory Quotient (RQ = $CO_{2}$ evolved / $O_{2}$ consumed) identifies the respiratory substrate: carbohydrates (RQ = 1.0), fats (≈ 0.7), proteins (≈ 0.8), organic acids (>1, e.g., malic acid ≈ 1.33), and CAM plants at night (RQ → ∞).
The total ATP from one glucose in aerobic respiration is 38 ATP (2 glycolysis + 6 link reaction NADH + 24 TCA + ETS contributions), reduced to 36 ATP only when the glycerol-3-phosphate shuttle converts cytoplasmic NADH to mitochondrial $FADH_{2}$ .
Key NEET trap: glycolysis occurs in the cytoplasm (not mitochondria), $FADH_{2}$ yields 2 ATP (not 3), the TCA cycle is amphibolic (not purely catabolic), and fermentation is anaerobic yielding only 2 ATP per glucose.