Respiration in Plants Class 11 Notes

Overview of Cellular Respiration

Cellular respiration is the process by which cells break down organic molecules (mainly glucose) to release energy in the form of ATP. It can be aerobic (with O₂) or anaerobic (without O₂, i.e. fermentation). In eukaryotes, glycolysis occurs in the cytoplasm, while the Krebs cycle and ETS occur in the mitochondria.

• Glycolysis (cytoplasm): glucose → 2 pyruvate + 2 ATP + 2 NADH.
• Link reaction (mitochondrial matrix): pyruvate → acetyl CoA + CO₂ + NADH.
• Krebs cycle / TCA cycle (mitochondrial matrix): acetyl CoA is fully oxidised to CO₂; produces NADH, FADH₂ and ATP.
• Electron Transport System and Oxidative Phosphorylation (inner mitochondrial membrane): NADH and FADH₂ donate electrons to generate a proton gradient that drives ATP synthase — the bulk of ATP production.

Glycolysis (EMP Pathway)

Glycolysis (Embden–Meyerhof–Parnas pathway) is the universal first step of glucose breakdown, occurring in the cytoplasm of all living cells. It is anaerobic — it does not require oxygen. A 6-carbon glucose is split into two 3-carbon pyruvate molecules through a 10-step pathway.

Energy investment and payoff

• Energy investment phase: 2 ATP are consumed to phosphorylate glucose (glucose → fructose-1,6-bisphosphate), making it reactive.
• Energy payoff phase: the 6-carbon sugar is split into two 3-carbon molecules, each of which is oxidised and converted to pyruvate. In the process 4 ATP (by substrate-level phosphorylation) and 2 NADH are produced.
• Net gain per glucose: 2 ATP + 2 NADH + 2 pyruvate.

Fermentation

When oxygen is unavailable (anaerobic conditions), pyruvate from glycolysis is further metabolised by fermentation to regenerate NAD⁺ from NADH, allowing glycolysis to continue producing a small amount of ATP.

• Alcoholic fermentation: pyruvate → acetaldehyde + CO₂ → ethanol (C₂H₅OH), catalysed by pyruvate decarboxylase and alcohol dehydrogenase. Occurs in yeast and some bacteria.
• Lactic acid fermentation: pyruvate is directly reduced to lactate (CH₃CH(OH)COOH) by lactate dehydrogenase, regenerating NAD⁺. Occurs in animal muscle cells during intense exercise and in certain bacteria (Lactobacillus).
• Fermentation yields only 2 ATP per glucose (from glycolysis) — far less efficient than aerobic respiration.

Link Reaction and Krebs Cycle (TCA Cycle)

Krebs cycle / TCA cycle

The Krebs cycle (tricarboxylic acid cycle) takes place in the mitochondrial matrix. Acetyl CoA (2C) combines with oxaloacetate (OAA, 4C) to form citrate (6C). Through eight enzymatic steps citrate is oxidised and decarboxylated, regenerating OAA. Each turn produces 3 NADH, 1 FADH₂ and 1 GTP (equivalent to 1 ATP). Since one glucose yields 2 acetyl CoA, two turns of the cycle occur per glucose.

• Key intermediates: citrate (6C) → isocitrate → α-ketoglutarate (5C) → succinyl CoA → succinate → fumarate → malate → OAA (4C).
• Two decarboxylation steps release 2 CO₂ per turn (one at isocitrate dehydrogenase, one at α-ketoglutarate dehydrogenase).
• Net per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 GTP + 4 CO₂.

Electron Transport System and Oxidative Phosphorylation

The ETS is a series of protein complexes (Complex I–IV) embedded in the inner mitochondrial membrane. NADH donates electrons at Complex I; FADH₂ donates at Complex II. Electrons pass through ubiquinone (CoQ), the cytochrome bc₁ complex, cytochrome c, and finally to Complex IV (cytochrome c oxidase), where O₂ is the terminal electron acceptor, forming H₂O.

• As electrons flow through Complexes I, III and IV, protons (H⁺) are pumped from the matrix into the intermembrane space, creating an electrochemical proton gradient (proton motive force).
• Protons flow back into the matrix through ATP synthase (Complex V), driving the phosphorylation of ADP to ATP — this is chemiosmosis (Peter Mitchell's hypothesis).
• Oxygen is essential as the final electron acceptor: O₂ + 4H⁺ + 4e⁻ → 2H₂O. Without O₂ the ETS stalls, the proton gradient collapses, and oxidative phosphorylation ceases.

Respiratory Balance Sheet

The respiratory balance sheet tracks ATP production through each phase of aerobic respiration per molecule of glucose. It is theoretical and assumes ideal conditions; actual yields may be slightly lower due to membrane leakage and energy costs of metabolite transport.

• Glycolysis: 2 ATP + 2 NADH → ~5 ATP (via shuttle to mitochondria).
• Link reaction: 2 NADH → ~5 ATP.
• Krebs cycle (2 turns): 2 GTP (2 ATP) + 6 NADH (~15 ATP) + 2 FADH₂ (~3 ATP).
• Total: ~36–38 ATP per glucose. The range depends on whether the glycerol-phosphate shuttle (~36 ATP) or the malate-aspartate shuttle (~38 ATP) is used to transfer cytoplasmic NADH into mitochondria.

Respiratory Quotient (RQ)

• Carbohydrates (glucose): RQ = 1 (6CO₂/6O₂ = 1).
• Fats: RQ ≈ 0.7 (more O₂ is required per carbon for oxidation of fats).
• Proteins: RQ ≈ 0.9.
• Anaerobic respiration in yeast: RQ is infinite (CO₂ released but no O₂ consumed).
• Organic acids (e.g. oxalic acid): RQ > 1 (more CO₂ released than O₂ consumed).

Solved Examples

Example: Calculate the total number of ATP molecules produced from complete aerobic oxidation of one molecule of glucose, assuming the malate-aspartate shuttle.

Solution: Glycolysis yields 2 ATP (net) + 2 NADH. Using the malate-aspartate shuttle, each NADH yields ~2.5 ATP → 2 × 2.5 = 5 ATP. Link reaction yields 2 NADH → 5 ATP. Krebs cycle (2 turns): 2 GTP = 2 ATP, 6 NADH → 15 ATP, 2 FADH₂ → 3 ATP. Total = 5 + 5 + 2 + 15 + 3 = 30 ATP from oxidative phosphorylation + 2 ATP from glycolysis + 2 GTP = 34? Recalculating: 2 (glycolysis ATP) + 5 (glycolysis NADH) + 5 (link NADH) + 2 (Krebs GTP) + 15 (Krebs NADH) + 3 (Krebs FADH₂) = 32 ATP. Different textbooks give 36–38 depending on the shuttle and rounding; the key is that glycolysis NADH and Krebs FADH₂ contribute differently.

Example: A fat molecule with the formula C₅₇H₁₁₀O₆ is completely oxidised. Why is its RQ lower than that of glucose?

Solution: Fats have a higher proportion of carbon and hydrogen relative to oxygen compared to carbohydrates. More O₂ molecules are needed to fully oxidise the extra C–H bonds to CO₂ and H₂O. For a typical fat, RQ ≈ 0.7 because the volume of CO₂ released is only about 70% of the O₂ consumed. Glucose (C₆H₁₂O₆) already contains sufficient oxygen for its own oxidation, giving RQ = 1.