Cellular respiration is the set of metabolic reactions used by cells to harvest energy from food. The catabolism of glucose under aerobic conditions occurs in three sequential metabolic pathways: glycolysis, pyruvate oxidation, and the citric acid cycle. The reduced coenzymes produced from these metabolic pathways are then oxidized by the respiratory chain, and ATP is made. By these pathways, glucose has been completely oxidized and the cell has gained many molecules of ATP—a versatile energy carrier that fuels most kinds of cellular work.
In this tutorial we will examine the operation of the electron transport chain and the production of ATP. In cellular respiration, it is the action of the electron transport chain that produces the bulk of the ATP for the cell.
During the early phases of cellular respiration, glucose is completely broken down. CO2 is liberated into the atmosphere, and the hydrogen atoms from glucose are donated to the energy carriers NAD+ and FAD to form NADH + H+ and FADH2. In order for cellular respiration to continue to operate on additional glucose molecules, these energy carriers must be recycled.
The work of the respiratory chain is, in part, to recycle these carriers. The carriers donate their extra hydrogen atoms to the respiratory chain and thereby convert back into NAD+ and FAD. In the accompanying animation, we focused on NADH, which donates a hydrogen atom to the first complex in the chain. FADH2 (not shown in the animation) donates to a different complex.
The other work of the respiratory chain is to transform the chemical energy of the hydrogen atoms (specifically, their electrons) into potential energy. In a series of redox reactions, electrons jump from one complex to another and, in the process, release energy. The chain uses the released energy to pump protons across the membrane, from a region of low concentration inside the mitochondrion to a region of high concentration within the intermembrane space. This concentration gradient represents potential energy.
The cell taps the potential energy of the gradient when protons flow back across the membrane through a pore in the ATP synthase complex. As the protons flow, they release energy, which the complex uses to convert ADP and inorganic phosphate to ATP. The production of ATP from energy derived from the flow of electrons through the respiratory chain is referred to as oxidative phosphorylation. Chemiosmosis is another term for ATP synthesis, referring to the use of a proton gradient to fuel the production of ATP.
Textbook Reference: Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy