Respiration in Plants

Cellular respiration is the controlled oxidation of organic molecules to release energy, summarised as: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). Unlike combustion, this energy release occurs in a stepwise manner through four stages: glycolysis, oxidative decarboxylation, the TCA cycle, and the electron transport system with oxidative phosphorylation.

Glycolysis (Embden-Meyerhof pathway) occurs in the cytoplasm and is common to both aerobic and anaerobic respiration. It does not require oxygen. Through 10 enzymatic steps, one glucose molecule (6C) is converted into two molecules of pyruvate (3C). Three key regulatory enzymes govern the pathway: hexokinase (step 1, phosphorylates glucose), phosphofructokinase (step 3, rate-limiting enzyme), and pyruvate kinase (step 10, generates ATP by substrate-level phosphorylation). The net yield is 2 ATP (4 produced minus 2 invested) and 2 NADH.

Under anaerobic conditions, pyruvate undergoes fermentation. Alcoholic fermentation (in yeast — Saccharomyces) converts pyruvate to acetaldehyde and then to ethanol + CO₂ via pyruvate decarboxylase and alcohol dehydrogenase. Lactic acid fermentation (in skeletal muscles and Lactobacillus) converts pyruvate directly to lactate via lactate dehydrogenase. Both pathways yield only 2 ATP net — representing incomplete oxidation of glucose.

In aerobic respiration, pyruvate enters the mitochondrial matrix where oxidative decarboxylation converts it to acetyl CoA by the pyruvate dehydrogenase complex (requiring TPP, NAD⁺, and CoA as cofactors), releasing 1 CO₂ and generating 1 NADH per pyruvate (2 NADH per glucose).

The TCA cycle (Krebs cycle/Citric acid cycle) operates in the mitochondrial matrix. Acetyl CoA (2C) condenses with oxaloacetate (OAA, 4C) to form citrate (6C), which undergoes eight sequential reactions: citrate → isocitrate → $\alpha$-ketoglutarate → succinyl CoA → succinate → fumarate → malate → OAA. Each turn yields 3 NADH, 1 FADH₂, and 1 GTP (equivalent to 1 ATP). Since two acetyl CoA molecules enter per glucose, the total TCA yield per glucose is 6 NADH, 2 FADH₂, and 2 GTP.

The Electron Transport System (ETS) is located on the inner mitochondrial membrane. NADH donates electrons at Complex I (NADH dehydrogenase), while FADH₂ enters at Complex II (succinate dehydrogenase). Electrons pass through Complex III (cytochrome bc1) and Complex IV (cytochrome c oxidase), with ubiquinone (CoQ) and cytochrome c serving as mobile carriers. Molecular oxygen is the final electron acceptor, forming water. As electrons flow through Complexes I, III, and IV, protons are pumped from the matrix to the intermembrane space, generating an electrochemical gradient. This gradient drives ATP synthase (F₀-F₁ particle) via chemiosmosis (Peter Mitchell hypothesis): each NADH yields approximately 3 ATP (via Complex I), while each FADH₂ yields approximately 2 ATP (entering at Complex II, bypassing one proton pump).

The total ATP balance sheet: Glycolysis (2 ATP + 2 NADH = 6 ATP) + Oxidative decarboxylation (2 NADH = 6 ATP) + TCA cycle (2 GTP + 6 NADH = 18 ATP + 2 FADH₂ = 4 ATP) = 38 ATP per glucose. In some cells, cytoplasmic NADH uses the glycerol-3-phosphate shuttle (yielding FADH₂ instead of NADH), reducing the total to 36 ATP.

The TCA cycle is an amphibolic pathway — it serves both catabolic (energy production) and anabolic (biosynthetic) functions. OAA can be transaminated to aspartate, $\alpha$-ketoglutarate to glutamate, and acetyl CoA to fatty acids.

The Respiratory Quotient (RQ) is the ratio of CO₂ evolved to O₂ consumed. For carbohydrates RQ = 1.0 (equal amounts of CO₂ produced and O₂ consumed), for fats RQ is approximately 0.7 (more O₂ required for oxidation of long hydrocarbon chains), for proteins approximately 0.8, and for organic acids greater than 1 (e.g., malic acid approximately 1.33, as these substrates are already partially oxidised).

The key testable concept is the ATP yield balance sheet, the location of each respiratory stage, and the distinction between aerobic and anaerobic pathways.