This complex process has three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. First, glycolysis occurs in the cytoplasm, which is the watery fluid inside the cell. This stage breaks one glucose molecule into two smaller molecules of pyruvate.
Glycolysis does not need oxygen and only makes a small amount of ATP. Next, the pyruvate molecules enter the mitochondria, which are the powerhouses of the cell. Here, the citric acid cycle breaks down the pyruvate completely.
This cycle releases carbon dioxide gas as waste and transfers energy to carrier molecules. These carriers are nicotinamide adenine dinucleotide, or NADH, and flavin adenine dinucleotide, or FADH2. In the final stage, these carriers deliver high-energy electrons to the electron transport chain.
The chain is a series of proteins embedded in the inner mitochondrial membrane. As electrons flow, they pump protons across the membrane to build a strong gradient. This gradient acts like a dam to power a protein machine called ATP synthase.
The machine spins to make a large amount of ATP. A common student mistake is thinking that plants only do photosynthesis and not respiration. In reality, plants must also perform cellular respiration to use their stored sugars.
Oxygen is the final electron acceptor at the end of the transport chain. It joins with protons to form water molecules.