Chapter 12 of Class 11 Biology is Respiration in Plants. It covers cellular respiration — glycolysis, fermentation, the Krebs cycle, and the electron transport chain — and the energy yield from glucose breakdown. These Class 11 Biology Chapter 12 solutions are also useful as quick revision notes before exams.
Glycolysis
Occurs in the cytoplasm, common to both aerobic and anaerobic respiration. One glucose molecule (6C) is broken down into two pyruvate molecules (3C), yielding a net gain of 2 ATP and 2 NADH.
Fermentation (Anaerobic Respiration)
Occurs in absence of oxygen. Alcoholic fermentation (yeast): pyruvate → ethanol + CO₂. Lactic acid fermentation (some bacteria, muscle cells): pyruvate → lactic acid. Both regenerate NAD⁺ but yield very little energy (net 2 ATP from glycolysis only), and are incomplete oxidation.
Aerobic Respiration: Krebs Cycle
Pyruvate is first converted to Acetyl CoA (in the mitochondrial matrix), which enters the Krebs cycle (citric acid cycle), producing CO₂, NADH, FADH₂, and a small amount of ATP (via substrate-level phosphorylation).
Electron Transport System (ETS) and Oxidative Phosphorylation
Occurs on the inner mitochondrial membrane. NADH and FADH₂ donate electrons to the ETS; as electrons pass through complexes, protons are pumped, creating a gradient used by ATP synthase to produce the bulk of ATP (chemiosmosis). Oxygen is the final electron acceptor, forming water.
Energy Yield (Aerobic Respiration)
Complete aerobic oxidation of one glucose molecule yields a theoretical maximum of about 36-38 ATP, vastly more than the 2 ATP from fermentation, because oxygen allows complete oxidation of glucose to CO₂+H₂O.
Respiratory Quotient (RQ)
RQ = volume of CO₂ evolved / volume of O₂ consumed. RQ=1 for carbohydrates, <1 for fats/proteins, >1 for organic acids.
Class 11 Biology Chapter 12 – Notes and Extra Questions
Along with these NCERT Solutions, students can also use the Class 11 Biology Chapter 12 Extra Questions and Class 11 Biology Chapter 12 Revision Notes for quick revision and extra practice.
