Summary
Highlights
Cellular respiration is a complex process with three main stages: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis occurs in the cytosol, the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the cristae of the inner mitochondrial membrane. While remembering specific numbers of NADH, FADH2, CO2, and O2 isn't usually required for VCE Biology, the number of ATP produced at each stage is crucial.
Glycolysis begins with glucose (C6H12O6), which is broken down into two pyruvate molecules. This process releases energy, producing two ATP molecules (from two ADP and two inorganic phosphates) and two NADH molecules (from two NAD+ and two hydrogens). ATP acts as the cell's energy currency, while NADH is an 'energy gift card' that can only be spent in specific cellular processes, like the electron transport chain. An initial hydrogen imbalance is noted, with two unaccounted for hydrogens.
The two pyruvate molecules from glycolysis enter the Krebs cycle, where they are further broken down into carbon dioxide. This releases more energy, generating two additional ATP molecules and eight NADH molecules. The video notes a temporary balance for hydrogens from pyruvate to NADH, but an additional four hydrogens are needed to produce two FADH2 molecules from two FAD. This creates a net deficit of two hydrogens and six oxygen atoms that are temporarily 'borrowed' from the cell.
The NADH and FADH2 molecules, acting as energy gift cards, move to the electron transport chain on the cristae. Here, their hydrogens are released, and the electrons are passed along a protein chain, generating 26 to 28 ATP molecules. Oxygen acts as the final electron acceptor, combining with hydrogens to form water. Six oxygen molecules and twelve hydrogens are consumed to produce six water molecules. Initially, there appear to be two leftover hydrogens and six leftover oxygens after water formation.
The 'leftover' hydrogens and oxygens from the electron transport chain precisely account for the 'borrowed' atoms from the Krebs cycle and glycolysis. The six leftover oxygen atoms pay back the six oxygens borrowed for the Krebs cycle, and the two leftover hydrogens pay back the two hydrogens borrowed earlier in the process. Thus, all atoms are accounted for at the end of cellular respiration, achieving a perfect balance.