Chapter 11: Catabolism – How Microbes Release & Conserve Energy

g., chemoorganoheterotroph or photolithoautotroph). Chemoorganotrophs typically break down complex organic molecules through one of three central processes: aerobic respiration, which uses oxygen as the final electron acceptor; anaerobic respiration, which employs alternative exogenous acceptors like nitrate or sulfate; or fermentation, which uses an internal, endogenous acceptor and generates ATP solely via substrate-level phosphorylation without involving an electron transport chain (ETC). Glucose catabolism often proceeds via the Embden-Meyerhof, Entner-Doudoroff, or pentose phosphate pathways, funneling intermediates into the Tricarboxylic Acid (TCA) cycle where substrates are fully oxidized to carbon dioxide. In respiration, the vast majority of ATP is produced through oxidative phosphorylation, where the flow of electrons through an ETC creates a Proton Motive Force (PMF) across the cell membrane, which drives ATP synthase. Microbes also catabolize lipids through β-oxidation and proteins following deamination, converting them into central metabolic intermediates. Conversely, chemolithotrophs gain energy by oxidizing inorganic compounds (like reduced sulfur or nitrogen), utilizing an ETC and PMF. Because many inorganic electron donors possess a more positive standard reduction potential than NAD(P)+, these microbes often must expend energy via reverse electron flow to synthesize the necessary reducing power for biosynthesis. Finally, phototrophs convert light energy into chemical energy through photophosphorylation using pigments like chlorophylls or bacteriochlorophylls, which drive electron flow to create a PMF. This includes oxygenic photosynthesis (two photosystems, uses water, produces O2, makes ATP and NADPH) and anoxygenic phototrophy (one photosystem, uses non-water donors). Some organisms bypass the ETC entirely using rhodopsin pigments as light-driven proton pumps, generating PMF without substrate oxidation.