(a) Respiration releases energy stepwise in living cells and stores part of it as ATP; combustion releases energy suddenly as heat/light. (b) Glycolysis converts glucose to pyruvate and gives a small ATP/NADH yield; Krebs' cycle oxidises acetyl CoA to CO2 and produces NADH, FADH2 and ATP/GTP. (c) Aerobic respiration completely oxidises glucose to CO2 and H2O; fermentation incompletely oxidises glucose to ethanol or lactic acid.
Respiration is a controlled enzymatic oxidation in cells, while combustion is uncontrolled burning. Glycolysis occurs in cytoplasm and partially oxidises glucose; Krebs' cycle occurs in mitochondrial matrix and completes oxidation of acetyl CoA. Aerobic respiration uses oxygen and yields much more ATP; fermentation occurs without oxygen and yields little ATP.
Carbohydrates, fats and proteins can serve as respiratory substrates, but carbohydrates, especially glucose, are most commonly used in cellular respiration.
Respiratory substrates are organic substances oxidised during respiration to release energy. Glucose is the most common respiratory substrate.
Glycolysis occurs in the cytoplasm. Pyruvate is transported into mitochondria and converted to acetyl CoA in the mitochondrial matrix. Krebs' cycle occurs in the matrix. Electron transport and oxidative phosphorylation occur on the inner mitochondrial membrane.
The main steps are glycolysis, pyruvate oxidation, Krebs' cycle and electron transport/oxidative phosphorylation.
Electrons from NADH enter Complex I and those from FADH2 enter through Complex II. They pass through ubiquinone, Complex III, cytochrome c and Complex IV to oxygen, which is reduced to water. The released energy pumps protons to the intermembrane space; proton flow back through ATP synthase drives ATP synthesis.
ETS, or electron transport system, transfers electrons from NADH and FADH2 through carriers on the inner mitochondrial membrane to oxygen, producing ATP by oxidative phosphorylation.
(a) Aerobic respiration requires O2 and completely oxidises substrate to CO2 and H2O with high ATP yield; anaerobic respiration occurs without O2 and yields less energy. (b) Glycolysis is the pathway converting glucose to pyruvate in cytoplasm; fermentation is anaerobic conversion of pyruvate to ethanol/lactic acid with regeneration of NAD+. (c) Glycolysis occurs in cytoplasm and produces pyruvate; the citric acid cycle occurs in mitochondrial matrix and oxidises acetyl CoA to CO2.
The pairs differ in oxygen requirement, completeness of oxidation, location, products and ATP yield.
NCERT notes that theoretical ATP yield assumes sequential functioning of glycolysis, Krebs' cycle and ETS; NADH from glycolysis enters mitochondria and is oxidised; intermediates are not used for synthesis of other compounds; and each NADH/FADH2 gives a fixed ATP yield. In living cells, these assumptions are not always fully true.
ATP-yield calculations assume complete oxidation of one glucose molecule, orderly pathway operation, fixed ATP equivalents for NADH/FADH2 and no diversion of intermediates.
Respiration breaks down glucose, fats and proteins to release energy, so it is catabolic. At the same time, intermediates such as acetyl CoA and Krebs' cycle acids are used to synthesise fatty acids, amino acids and other compounds, so the pathway is also anabolic.
The respiratory pathway is amphibolic because it functions in both breakdown and synthesis.
RQ = CO2 evolved / O2 consumed. Fats are more reduced than carbohydrates and require more oxygen for oxidation, so the ratio is below one.
Respiratory quotient, RQ, is the ratio of volume of CO2 evolved to volume of O2 consumed in respiration. For fats, RQ is less than 1, typically about 0.7.
As electrons pass through ETS to oxygen, energy is used to pump protons across the inner mitochondrial membrane. The proton gradient drives ATP synthase to phosphorylate ADP to ATP. Because phosphorylation is coupled with oxidation, it is called oxidative phosphorylation.
Oxidative phosphorylation is ATP synthesis linked to electron transport and oxidation of NADH/FADH2.
Respiration oxidises substrates through many enzyme-controlled reactions. Energy is released in small packets and conserved in ATP and reduced coenzymes. This prevents wasteful heat release and allows energy to be used for cellular work.
Step-wise release lets cells capture energy efficiently as ATP instead of losing it all as heat.