Summary
Highlights
Living cells require energy for various activities like active transport, anabolic reactions, and movement. Respiration releases energy from organic molecules, converting chemical potential energy into a usable form called ATP (adenosine triphosphate). Organisms are classified as autotrophs (synthesize their own carbon compounds) or heterotrophs (obtain carbon compounds from food).
ATP is the universal energy currency in all organisms, functioning like money for various cellular processes. It is a small, soluble, and rapidly hydrolyzable molecule, stable at cellular pH, and can be recycled. ATP is composed of ribose, an adenine base, and three phosphate groups. Hydrolysis of ATP releases energy, with removal of each phosphate group yielding different amounts of free energy.
Cells continuously synthesize ATP as they cannot store large amounts. There are two primary methods of ATP synthesis: substrate-linked phosphorylation, where a phosphate group is directly transferred to ADP, and chemiosmosis, which uses energy from proton movement down a concentration gradient to synthesize ATP via ATP synthase.
Coenzymes NAD and FAD are crucial in respiration, accepting hydrogen ions and becoming reduced. They transport these hydrogen ions to the electron transport chain, where protons are pumped across the inner mitochondrial membrane, creating a proton gradient. The movement of protons back into the matrix through ATP synthase generates ATP. These coenzymes undergo reversible reactions and are recycled.
Glucose is the primary respiratory substrate, but cells can also use other carbohydrates, proteins, and lipids. Lipids provide the most energy per gram due to their high hydrogen concentration. The Respiratory Quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration, indicating which substrate is being metabolized. Molecules with higher hydrogen content result in a greater proton gradient and thus more ATP formation.