Summary
Highlights
The video introduces microbial metabolism as a complex but fascinating topic involving biochemical reactions within microbial cells. Unlike introductory biology, this session specifically focuses on metabolic properties and processes in microorganisms. Metabolism includes both catabolic (breaking down) and anabolic (building up) processes.
Metabolism is guided by eight basic statements: cells acquire nutrients, require energy (from light or nutrient catabolism), store energy as ATP, catabolize nutrients into precursor metabolites, use ATP and enzymes in anabolic reactions, form macromolecules (proteins, carbohydrates, nucleic acids, lipids), grow by assembling macromolecules, and reproduce after doubling in size. These principles apply similarly to prokaryotic and eukaryotic cells.
Catabolism involves breaking larger molecules into smaller products, releasing energy (exergonic reactions). Anabolism involves synthesizing larger molecules from smaller building blocks, requiring energy input (endergonic reactions). This process uses the ATP and precursors generated during catabolism to build cellular structures, support growth, and enable energy storage.
Metabolic reactions are primarily driven by oxidation-reduction (redox) reactions. Oxidation is the loss of electrons and hydrogens (OIL - Oxidation Is Loss), while reduction is the gain of electrons and hydrogens (RIG - Reduction Is Gain). These reactions always occur simultaneously, transferring electrons from a donor to a receiver. Key electron carriers are NAD+, NADP+, and FAD.
ATP (adenosine triphosphate) stores energy in high-energy phosphate bonds. The process of adding a phosphate group to ADP (adenosine diphosphate) to form ATP is called phosphorylation. There are three main types: substrate-level phosphorylation, oxidative phosphorylation (more complex, tied to cellular respiration), and photophosphorylation (using light energy).
Enzymes are biological catalysts that speed up reactions by lowering the activation energy. Most enzymes are proteins, though some RNA molecules (ribozymes) can also act as enzymes. Enzymes can be complete (holoenzymes) or require cofactors (inorganic ions) or coenzymes (organic molecules) to become active. The specificity of an enzyme is determined by its active site, which binds to a specific substrate in an induced-fit model.
An example from glycolysis demonstrates enzymatic activity: fructose 1,6-bisphosphate is broken down by aldolase into smaller products. The enzyme itself remains unchanged and can facilitate further reactions. Proper fitting of the substrate into the enzyme's active site forms an enzyme-substrate complex, leading to product formation.
Enzyme activity is affected by several factors: the shape and fit of the active site, temperature and pH (which can cause denaturation and loss of function if not optimal), enzyme and substrate concentrations (activity increases with substrate until saturation point), and the presence of inhibitors. Inhibitors can be competitive (mimicking the substrate and binding to the active site) or non-competitive (binding to an allosteric site and changing the active site's shape).
Feedback inhibition is a regulatory mechanism where the end product of a metabolic pathway acts as a non-competitive inhibitor by binding to an allosteric site on an enzyme earlier in the pathway, stopping further product synthesis when sufficient product is available, thus conserving energy.
Carbohydrate catabolism is a universal and well-understood process where organisms oxidize carbohydrates (primarily glucose) for energy. Glucose can be catabolized through two main processes: cellular respiration (more complex, involving glycolysis, the Krebs cycle, and electron transport chain, yielding more ATP) and fermentation (simpler with fewer ATP, primarily anaerobic). Glycolysis is the first step common to both.
Glycolysis occurs in the cytoplasm and breaks down a six-carbon glucose molecule into two three-carbon pyruvate molecules. This process yields a net gain of 2 ATP (via substrate-level phosphorylation), 2 NADH (electron carriers), and 2 pyruvate molecules. NADH carries electrons to the electron transport chain in respiration or participates in fermentation pathways.