Glycolysis is the energy-harvesting process of all cells, according to a lecture given by Kim Largen, Ph.D., of George Mason University.
Glycolysis, which means "splitting of sugar," is the breakdown of glucose into pyruvic acid and its complete oxidation into carbon dioxide and water.
The process occurs in the cytoplasm outside mitochondria in an animal cell, according to Kimball's Biology Pages. The mitochondria are membrane-enclosed organelles, with the main function of converting the energy of food molecules into ATP.
The outer membrane of the mitochondria contains channels, formed by proteins, to allow the entry and flow of various molecules and ions. The matrix, within the mitochondria, contains a mix of soluble enzymes to catalyze the respiration of pyruvic acid and other small, organic molecules.
Simplistically, within the matrix, a two-carbon fragment of acetate is created, bound to coenzyme A to form acetyl-CoA. The fragment is donated to a molecule of oxaloacetic acid, creating a molecule of citric acid necessary for the Krebs Cycle, the second level of cellular respiration.
In the Krebs Cycle, glucose is broken completely to produce carbon dioxide. The process, also called the tricarboxylic acid cycle or citric acid cycle, produces molecules of ATP, and other energy-rich molecules. The cycle also contributes electrons to the electron transport chain, used the third level of cellular respiration.
The Krebs Cycle occurs in the mitochondria, where the citric acid undergoes a cycle of enzymatic steps. The result is a new molecule of oxaloacetic acid. This connects to new acetate fragments, once again formed by Glycolysis, the first level of cellular respiration.
So, the cycle repeats. Meanwhile, it's forming electrons for the electron transport chain, the third level of cellular respiration.
The electron transport chain uses the downhill flow of electrons to transport electrons from their carriers to oxygen. Three complexes of integral membrane proteins and two freely-diffusible molecules shuttle the electrons from complex to complex.
The steps to transfer electrons into oxygen molecules form water molecules, with the aid of protons. The energy of the transfer pumps protons from the matrix to the intermembrane space, according to Kimball's Biology Pages. The electrons necessary to reduce oxygen to water pass through this respiratory chain.
The gradient of the protons across the inner membrane, formed by the process of active transport, form a battery. The protons flow down the gradient, to re-enter the matrix, through another complex of integral proteins, the ATP synthase complex.
Ultimately, "as in chloroplasts, the energy released as these protons flow down their gradient is harnessed to the synthesis of ATP," in the chemiosmosis process.