metabolism
Summary
Metabolism is the sum of all enzyme-catalyzed reactions in a cell or organism. Catabolism breaks molecules down and captures energy as ATP and reduced carriers; anabolism builds molecules and uses that energy. Pathways are linked by ATP and are regulated, often by feedback inhibition.
Detail
Meaning: Metabolism is the network of enzyme-catalyzed reactions by which a cell obtains energy and builds what it needs. Catabolism breaks down molecules such as glucose, usually by oxidation, and stores part of the energy in ATP, NADH and FADH₂. Anabolism builds proteins, glycogen and fatty acids using ATP and reducing power. Energy coupling links them: an exergonic reaction (ΔG < 0), such as ATP hydrolysis, drives an endergonic one. Cells control flux through key enzymes, often by feedback inhibition, and insulin and glucagon shift the body between fed and fasting states.
Equation: ΔG = ΔH − TΔS, where ΔG is free-energy change (J/mol), ΔH enthalpy change (J/mol), T temperature (K) and ΔS entropy change (J/(mol·K)). Coupled steps proceed if their summed ΔG is negative.
On the MCAT: A passage shows a pathway and asks what happens when an enzyme is inhibited, oxygen is withheld or a hormone changes. Predict which intermediates accumulate and whether ATP output rises or falls.
Common confusion: Metabolism is not only breakdown; it includes catabolism and anabolism. Catabolic pathways release energy overall, but some steps spend ATP first: phosphorylating glucose is endergonic by itself and becomes favorable when coupled to ATP hydrolysis (the first step of glycolysis).
Sources
- Campbell Biology
- Lehninger Principles of Biochemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.