5 Microbial Metabolism
Learn how microbes capture and transfer energy, obtain carbon and electrons, and use metabolic pathways to build the materials needed for growth.
Metabolism, enzymes, and energy transfer
Microbial metabolism is the network of reactions that sustains cell growth and activity. Two complementary processes organize much of this network: breaks molecules down and captures energy, while uses energy and small precursors to build cellular components. Catabolic reactions can therefore supply both the energy and the building blocks needed for growth.
Most cellular reactions are accelerated by enzymes, which bind particular reactants called substrates and lower the activation energy required for a reaction. Enzymes are not consumed by the reactions they catalyze. Their activity can depend on temperature, pH, substrate availability, and regulatory molecules. Cells can also regulate enzyme production or activity; for example, occurs when a pathway’s end product inhibits an enzyme earlier in the pathway.
Cells transfer energy and electrons between reactions. acts as an immediate energy carrier, coupling energy-releasing reactions to cellular work. Electron carriers such as NAD⁺/NADH and FAD/FADH₂ commonly transfer electrons from , while NADP⁺/NADPH often provides reducing power for . In redox reactions, electron movement can release energy that cells capture in or an ion gradient.
Microbes can be described by where they obtain energy, electrons, and carbon. Phototrophs obtain energy from light; chemotrophs obtain it from chemical reactions. Organotrophs use organic electron donors, while lithotrophs use inorganic donors. Autotrophs build cellular carbon from , whereas heterotrophs obtain carbon from organic compounds. These categories describe different metabolic features and can be combined: a photoautotroph uses light for energy and as its carbon source.
Takeaway: Metabolism connects the breakdown of nutrients, the transfer of energy and electrons, and the regulation of reactions.
and respiration
makes without an electron transport chain. In substrate-level phosphorylation, an enzyme transfers a phosphate group directly to ADP. also regenerates NAD⁺ so glycolysis can continue. An organic molecule derived from the original nutrient accepts electrons, rather than an external terminal electron acceptor such as oxygen. generally produces less per sugar than respiration.
products depend on the organism and pathway. Lactic acid bacteria can produce lactate, which contributes to the acidity of yogurt. Yeasts commonly produce ethanol and carbon dioxide, products used in brewing and bread-making.
uses an electron transport system (ETS) to transfer electrons from an energy source to a terminal electron acceptor. Electron flow helps establish an electrochemical gradient across a membrane. As ions flow back through synthase, the enzyme uses the gradient to produce by oxidative phosphorylation. can also be made directly by substrate-level phosphorylation, including in steps of glycolysis. In bacteria, respiratory electron transport systems are typically located in the cell membrane.
In aerobic respiration, oxygen is the terminal electron acceptor. In anaerobic respiration, another external acceptor is used; examples include nitrate and sulfate. Both forms of respiration use an ETS, distinguishing them from , which does not. The available pathways depend on the microbe’s genes and environmental conditions.
Takeaway: The key distinction is how is generated and whether electrons pass through an ETS to an external terminal acceptor.
and carbon fixation
converts light energy into chemical energy. Pigments absorb light and transfer energy to reaction centers, where electron flow begins. In photosynthetic membranes, this flow can create an ion gradient that drives production by photophosphorylation. Light reactions also provide reducing power, often as NADPH, for carbon fixation and other cellular reactions.
Photosynthetic microbes differ in their electron donors and whether they release oxygen. Oxygenic , carried out by cyanobacteria as well as plants and algae, uses water as an electron donor and releases oxygen. Anoxygenic , used by some bacteria, uses other donors such as hydrogen sulfide and does not release oxygen.
In photoautotrophs, and reducing power generated using light can support carbon fixation: the incorporation of into organic molecules that provide carbon for growth.
Takeaway: Light can supply the energy for production and reducing power; the electron donor used determines whether releases oxygen.
and metabolic integration
assembles small molecules into the materials a cell needs, including amino acids and proteins, nucleotides and nucleic acids, and lipids and carbohydrates. These pathways require energy, often supplied by , and building blocks called precursors. Reducing power, often supplied by NADPH, supports reactions that add electrons to precursor molecules.
Catabolic pathways support in two connected ways: they provide and reducing power, and they supply intermediates that can be diverted into the production of cellular components. For example, intermediates from carbohydrate metabolism can serve as starting materials for other molecules. Carbon-fixing microbes can also build organic compounds from , using energy from light or chemical reactions.
Because metabolism is interconnected, a change in nutrient availability can affect both energy production and the materials available for growth. The balance between breaking down nutrients and building cellular components helps determine how a microbe grows and functions.
Takeaway: depends on energy, reducing power, and precursors, many of which are supplied by connected catabolic pathways.