Respiration in PlantsNEET Botany · Class 11 · NCERT Chapter 14

Respiration in Plants is a Medium to High Weightage chapter with 3 to 5 questions in most NEET exams. Questions focus on glycolysis intermediates and net ATP, the link reaction, Krebs cycle products per turn, the electron transport chain (Complex I, II, III, IV and ATP synthase), ATP yield per glucose (NCERT 38 ATP), fermentation pathways (alcoholic and lactic acid), respiratory quotient (RQ) values, and the amphibolic nature of the respiratory pathway. Master each phase quantitatively for a strong NEET score.

Glycolysis takes place in the cytoplasm. It does not need oxygen and breaks one glucose (6C) into two pyruvate (3C). Net products per glucose: 2 ATP and 2 NADH. The Krebs cycle (TCA cycle) takes place in the mitochondrial matrix. It needs oxygen indirectly (because the ETC must run to keep the cycle going). It oxidises acetyl CoA (2C) to 2 CO2. Per turn it produces 3 NADH, 1 FADH2, and 1 GTP (or ATP). Per glucose, 2 turns of the Krebs cycle produce 6 NADH, 2 FADH2, 2 ATP, and 4 CO2.

According to NCERT (theoretical maximum): 38 ATP per glucose. The breakdown is: glycolysis gives 2 ATP (net) + 2 NADH; the link reaction gives 2 NADH (1 per pyruvate, 2 pyruvates per glucose); the Krebs cycle gives 2 ATP + 6 NADH + 2 FADH2. Total reduced coenzymes: 10 NADH + 2 FADH2. Each NADH yields 3 ATP and each FADH2 yields 2 ATP via the ETC and oxidative phosphorylation, giving 30 + 4 = 34 ATP from oxidative phosphorylation. Total: 2 (glycolysis) + 2 (Krebs GTP) + 34 (ETC) = 38 ATP. The actual yield in modern biochemistry is around 30-32 ATP because of proton leak and the cost of the malate-aspartate shuttle, but NEET expects the NCERT value of 38.

Aerobic respiration uses oxygen as the final electron acceptor. It completely oxidises glucose to CO2 and H2O, takes place in the cytoplasm + mitochondria, and yields 38 ATP per glucose (NCERT). It involves glycolysis + link reaction + Krebs cycle + ETC. Fermentation does not use oxygen. It is incomplete oxidation. It takes place entirely in the cytoplasm. The only ATP produced is the 2 ATP from glycolysis. The pyruvate is converted to ethanol + CO2 (alcoholic fermentation in yeast like Saccharomyces) or to lactate (lactic acid fermentation in muscles under O2 stress and in Lactobacillus). The point of fermentation is to regenerate NAD+ so that glycolysis can continue.

The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration: RQ = volume of CO2 evolved / volume of O2 consumed. Standard NEET values: carbohydrates (such as glucose) RQ = 1.0; fats (such as tripalmitin) RQ ≈ 0.7; proteins RQ ≈ 0.9; organic acids (such as malic acid) RQ > 1.0 (typically 1.33 for malic acid); anaerobic respiration RQ = infinity (CO2 is produced but no O2 is consumed). The RQ depends on the substrate being respired. Pure water-loving fatty substrates have lower RQ because they need more oxygen to oxidise.

A pathway is amphibolic when it serves both catabolic (breakdown) and anabolic (synthesis) roles. The respiratory pathway is amphibolic because its intermediates are not just energy currency; they are also building blocks for biosynthesis. For example: acetyl CoA is the precursor for fatty acids, sterols, and ketone bodies; alpha-ketoglutarate gives rise to glutamate and other amino acids; oxaloacetate gives rise to aspartate and other amino acids; pyruvate gives rise to alanine. So the same pathway that breaks down glucose to release energy can also be tapped at any intermediate to build up other biomolecules. Respiration is therefore catabolic and anabolic, hence amphibolic.

Glycolysis occurs in the cytoplasm (cytosol). Link reaction (pyruvate dehydrogenase complex) occurs in the mitochondrial matrix. Krebs cycle (TCA cycle) occurs in the mitochondrial matrix. Electron transport chain and oxidative phosphorylation (ATP synthase) occur on the inner mitochondrial membrane (cristae). Glycolysis is the only phase that does not require mitochondria, which is why anaerobic organisms (those without mitochondria) can still do glycolysis but not the later phases. The inner mitochondrial membrane is folded into cristae to provide a large surface area for the electron transport chain and ATP synthase.