Introduction
Metabolism is the network of chemical reactions that sustain living organisms, extracting usable energy from nutrients and using it to build the biomolecules described in Chapter 8 — this chapter applies both thermodynamics (Chapter 3) and enzyme-catalysed kinetics (Chapter 4, Chapter 5) to a biological setting.
1. Catabolism and anabolism
Catabolism is the breakdown of larger, complex molecules into smaller ones, generally releasing energy (an exergonic, energy-releasing process, connecting to Chapter 3’s spontaneity criteria). Anabolism is the reverse — building larger, complex molecules from smaller ones, generally requiring an energy input. Together, catabolic and anabolic pathways form a cell’s overall metabolism, constantly balancing energy release and energy consumption.
2. Energy carriers: ATP and redox coenzymes
ATP (adenosine triphosphate) is the cell’s principal short-term energy currency: hydrolysing its terminal phosphate bond releases a substantial, usable amount of free energy, which cells couple to otherwise unfavourable (positive ΔG) reactions to drive them forward overall. Redox coenzymes (such as NAD⁺/NADH) shuttle electrons (and thus reducing power, tying directly back to the oxidation/reduction concepts of Chapter 2) between metabolic pathways — catabolic pathways typically generate NADH by oxidising nutrients, and this NADH’s electrons are ultimately used to generate large amounts of ATP.
3. Major carbohydrate metabolic pathways (introductory level)
Glycolysis breaks down glucose into pyruvate in the cell cytoplasm, producing a small net amount of ATP directly and some NADH, without requiring oxygen. The citric acid cycle (Krebs cycle) further oxidises the products of glycolysis, releasing carbon dioxide and generating substantially more NADH (and related carriers), which then feed into the electron transport chain, where their stored reducing power drives the production of most of a cell’s ATP, consuming oxygen in the process — the reason aerobic metabolism extracts far more energy from a given amount of glucose than fermentation (anaerobic breakdown) can.
4. Connections with lipid and amino-acid metabolism
Fatty acids (from lipids, Chapter 8) are broken down via beta-oxidation into two-carbon units that feed into the same citric acid cycle used by carbohydrate metabolism, explaining why fats yield considerably more energy per gram than carbohydrates (their carbon is more reduced, so more electrons are available to generate NADH). Amino acids (from protein breakdown) can likewise be converted, after removal of their nitrogen-containing amine group, into intermediates that feed into the same central catabolic pathways — illustrating that carbohydrate, lipid, and amino-acid metabolism are not separate systems but converge on shared central pathways.
5. Enzyme control and pathway regulation
Metabolic pathways are controlled primarily through enzyme regulation rather than by simply having reactants always fully react: key regulatory enzymes (often catalysing an early, rate-determining, and effectively irreversible step of a pathway) are activated or inhibited by molecules signalling the cell’s current energy status (such as high ATP levels inhibiting an early glycolytic enzyme) — a feedback mechanism ensuring metabolic output matches the cell’s actual needs rather than proceeding at a fixed, wasteful rate.
Key Takeaways
- Catabolism releases energy by breaking down molecules; anabolism consumes energy to build them, together forming the cell’s overall metabolic balance.
- ATP is the short-term energy currency, and redox coenzymes like NADH shuttle electrons between pathways, ultimately feeding the electron transport chain that produces most cellular ATP.
- Glycolysis, the citric acid cycle, and the electron transport chain form the central pathway of carbohydrate catabolism, with lipid and amino-acid metabolism converging into the same central pathways.
- Metabolic pathways are regulated by controlling key enzymes’ activity in response to the cell’s energy status, not simply left to run at a constant rate.