9700 · 12.2
Common exam mistakes on 9700 Respiration. Learn what loses marks, then practise the topic with Examiner’s Ink.
Examiners often look for precision in terminology. Don't confuse 'reduced NAD' with 'NAD⁺' or simply 'NAD'. Be clear about the specific locations of each stage (cytoplasm, mitochondrial matrix, inner mitochondrial membrane) and the exact number of ATP, reduced NAD, and reduced FAD molecules produced at each step. Also, practice explaining the role of oxygen as the final electron acceptor in the ETC - it's a common point of confusion!
Aerobic respiration requires oxygen, occurs in the cytoplasm and mitochondria, produces a high yield of ATP (around 30-32 per glucose), and results in carbon dioxide and water as end products. Anaerobic respiration occurs without oxygen, is confined to the cytoplasm, yields only 2 ATP per glucose (from glycolysis), and produces lactate (in animals) or ethanol and carbon dioxide (in yeast).
Chemiosmosis is the process where ATP is generated using energy from a proton (H⁺) gradient. In respiration, the electron transport chain pumps protons from the mitochondrial matrix into the intermembrane space. This creates a high concentration of protons, which then flow back into the matrix through the enzyme ATP synthase. This flow of protons provides the energy to drive the synthesis of ATP from ADP and inorganic phosphate.
Oxygen is crucial because it acts as the final electron acceptor at the end of the electron transport chain. It removes electrons and combines with protons to form water. Without oxygen, electrons would accumulate, halting the ETC. This would stop the pumping of protons and prevent the formation of the proton gradient needed for chemiosmosis, thereby stopping the majority of ATP synthesis.
Lipids have a higher proportion of carbon-hydrogen bonds and a lower proportion of oxygen compared to carbohydrates. This means that for the same mass, lipids provide more hydrogen atoms to the electron transport chain via reduced NAD and FAD. This leads to a larger proton gradient and ultimately the production of more ATP molecules per gram, giving lipids a higher energy density.