Metabolism Basics — energy, matter and nutritional types
Metabolism is all of the chemical reactions inside a cell — stepwise, interconnected metabolic pathways. Exergonic reactions release energy; endergonic reactions need it. Anabolism builds complex molecules (endergonic, spends ATP) while catabolism breaks them down (exergonic, harvests energy). Organisms are also classified by where their carbon and energy come from — explore all three layers.
Energy Carriers — electrons and the currency of the cell
Most of the energy stored in atoms and used to fuel cell functions is carried by high-energy electrons — transferred in small, usable packages. Redox reactions move those electrons between molecules, and a small class of molecules shuttles them: NAD⁺/NADH (the most common mobile carrier, used in catabolism), NADP⁺/NADPH (anabolism & photosynthesis) and FAD/FADH₂. The captured energy is banked in ATP — the cell’s energy currency.
Enzyme Lab — biological catalysts at work
Proteins called enzymes are the catalysts of the cell: they are not used up and are reusable, and they work by lowering the activation energy of a reaction. Their specificity comes from a jigsaw-like match with their substrates at the active site — and they are exquisitely sensitive to temperature, pH and substrate concentration. Many also need cofactors or coenzymes to function at all.
Enzyme Control — inhibitors and feedback
Enzyme activity can be promoted or reduced. A competitive inhibitor resembles the substrate and blocks the active site; a noncompetitive (allosteric) inhibitor binds elsewhere and changes the enzyme’s shape — while an allosteric activator does the opposite. And in feedback inhibition, a pathway’s own end product regulates its further production.
Glycolysis & Other Fuels — the central route of catabolism
Glycolysis is the most common route for glucose catabolism — and every living organism runs some form of it, suggesting an ancient, universal process. It needs no oxygen, happens in the cytoplasm, and turns one glucose into two pyruvate. Microbes also feed lipids and proteins into the same central routes.
Krebs & Respiration — oxidative phosphorylation
Before the cycle, pyruvate loses CO₂ and rides on coenzyme A into the Krebs cycle. The Krebs (citric acid / TCA) cycle strips the remaining electrons onto carriers — and then the electron transport system converts them into the bulk of the cell’s ATP: oxidative phosphorylation, aerobic or anaerobic.
Fermentation — life without an electron transport system
When respiration isn’t possible, the cell must still reoxidize NADH back to NAD⁺ or glycolysis grinds to a halt. Fermentation uses an organic final electron acceptor (commonly pyruvate), involves no ETS, and produces no additional ATP — just the 2 ATP of glycolysis. Yet it feeds the world: yogurt, cheese, pickles, bread, wine, biofuels — and some of microbiology’s best diagnostic tests.
Photosynthesis — light into chemical energy
Photosynthesis runs in two stages: light-dependent reactions capture solar energy into ATP and NADPH, and light-independent reactions use that energy to fix CO₂ into sugar (they still depend on the light reactions because their products are short-lived). Explore the machinery, the pigments — and the two great strategies: oxygenic and anoxygenic.
Biogeochemical Cycles & Bioremediation — microbial ecology at work
The six elements of organic molecules — C, H, N, O, P, S — take many chemical forms and recycle endlessly between organisms and their environment in biogeochemical cycles, where microorganisms play a role at every step, interconverting oxidized and reduced versions of molecules. And where pollution strikes, we enlist those same metabolic superpowers: bioremediation.
Chapter Quiz — Microbial Metabolism
Twenty-six questions covering the whole chapter: energy & nutrition, carriers and ATP, enzymes and their control, glycolysis, the Krebs cycle, electron transport, fermentation, photosynthesis, and the great biogeochemical cycles.