All cells must obtain energy, synthesize their own internal structure, control much of their own activity, and guard their boundaries. Cellular metabolism refers to the collective chemical processes that occur within living cells to accomplish these activities.
Especially important for living organisms is chemical energy, stored in the chemical bonds of molecules. Much of the work done by living organisms involves conversion of potential energy to kinetic energy. Free energy is the energy in a system available for doing work.
Catalysts are chemical substances that accelerate reaction rates without affecting the products of the reaction and without being altered or destroyed by the reaction. Enzymes are catalysts of the living world. The special catalytic talent of an enzyme is its power to reduce the amount of activation energy required for a reaction. Many enzymes are pure proteins. Other enzymes require participation of small nonprotein groups called cofactors. An enzyme functions by associating in a highly specific way with its substrate, the molecule whose reaction it catalyzes. The binding of enzyme to substrate forms an enzyme-substrate complex (ES complex). One of the most distinctive attributes of enzymes is their high specificity. Specificity is a consequence of the exact molecular fit required between enzyme and substrate, so that both are specific to each other. Furthermore, an enzyme catalyzes only one reaction. No side reactions or by-products result. Enzyme-catalyzed reactions are reversible, but for various reasons reactions catalyzed by enzymes tend to go predominantly in one direction.
The quantity of some enzymes is regulated by certain molecules that switch enzyme synthesis on or off. Enzyme activity may be altered by the presence or absence of metabolites that cause conformational changes in enzymes and thus improve or diminish their effectiveness as catalysts.
Endergonic reactions are those that do not proceed spontaneously because their products require an input of free energy. However, an endergonic reaction may be driven by coupling the energy-requiring reaction with an energy-producing reaction. ATP is one of the most common intermediates in coupled reactions, and because it can drive such energetically unfavorable reactions, it is of central importance in metabolic processes. ATP is an energy-coupling agent and not a fuel. ATP is formed as it is needed, primarily by oxidative processes in mitochondria. Metabolism is mostly self-regulating.
All cells obtain their chemical energy requirements from oxidation-reduction (“redox“) reactions, which involves a transfer of electrons from an electron donor (the reducing agent) to an electron acceptor (the oxidizing agent). The electron donor becomes oxidized and the electron acceptor becomes reduced. For every oxidation there must be a corresponding reduction. Aerobic metabolism is vastly more efficient than anaerobic metabolism. Aerobic metabolism (usually called cellular respiration) is defined as the oxidation of fuel molecules to produce energy with molecular oxygen as the final electron acceptor. Oxidation of fuel molecules describes the removal of electrons and not the direct combination of molecular oxygen with fuel molecules. Molecular oxygen is involved only at the very end of the pathway. There are three stages in the complete oxidation of fuel molecules to carbon dioxide and water: digestion, glycolysis, and the final common pathway, which includes the Krebs cycle. The final step of the third stage is the electron transfer chain, where most of the ATP in living organisms is produced. Oxidation of one glucose molecule may yield a total of 38 ATP molecules (36 net). Pyruvic acid is the final electron acceptor in anaerobic metabolism.
Fats are more concentrated fuels than carbohydrates. Because fats are almost pure hydrocarbons, they contain more hydrogen per carbon atom than sugars do, and it is the energized electrons of hydrogen that generate high-energy bonds when they are carried through the mitochondrial electron-transport chain. Fat stores are derived principally from surplus fats and carbohydrates in the diet. Stored fats are the greatest reserve fuel in the body. Excess proteins serve as fuel as do carbohydrates and fats. Amino acid degradation yields two main products, carbon skeletons and ammonia. Aquatic animals get rid of ammonia by diffusion into the surrounding medium. Terrestrial animals must detoxify it by converting it; the two principal compounds formed are urea and uric acid.
—May 2021
—June 2023
Lance Jones 