Interaction and Interdependence: IB Biology's Big-Picture Theme

Metabolism is the sum of all cellular reactions, split into catabolism (breaking bonds, releasing energy, like respiration) and anabolism (forming bonds, consuming energy, like protein synthesis). Every step is enzyme-catalysed: the active site binds substrate via induced fit, lowering activation energy so reactions proceed far faster. Rate rises with temperature up to an optimum, then collapses as heat denatures the enzyme's tertiary structure; pH shifts disrupt the same bonds holding the active site's shape.

The Michaelis-Menten equation links reaction velocity to substrate concentration through $K_m$ (substrate concentration at half-maximal rate) and $V_{max}$ (the saturated rate). The single most-tested numerical fact: $v=V_{max}/2$ when $[S]=K_m$. Competitive inhibitors raise $K_m$ but leave $V_{max}$ unchanged, since more substrate can still outcompete the inhibitor; non-competitive inhibitors lower $V_{max}$ while $K_m$ stays the same, because they block the active site regardless of substrate concentration.

First-line defences like skin and mucous membranes stop most pathogens before infection; if breached, innate responses such as phagocytosis and inflammation act fast but non-specifically. Specific immunity is slower but targeted: B-lymphocytes recognise antigens and differentiate into plasma cells that secrete antibodies, and memory cells that enable a faster, stronger secondary response. Antibiotics disrupt bacterial cell walls or ribosomes, structures viruses simply don't have, which is why they can't treat colds, flu or HIV.

A population grows in a new environment along a sigmoid curve: a slow lag phase, a rapid exponential phase, then a plateau at the carrying capacity (K) — the maximum population a habitat's resources can sustain. Density-dependent factors like food, disease and predation intensify as numbers rise, slowing growth as it approaches K. Growth rate is the gradient of the curve, not its height, so it peaks mid-exponential phase and falls to near zero at K even though population size (N) is greatest there.

Cell respiration releases chemical energy from organic molecules to make ATP. Aerobic respiration proceeds through glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane, where oxygen is the final electron acceptor and most ATP is produced. When oxygen is unavailable, anaerobic pathways regenerate NAD+ so glycolysis can continue — lactic acid fermentation in animal muscle, or alcoholic fermentation (ethanol + CO2) in yeast and plants — but yield far less ATP per glucose, making anaerobic respiration a stopgap rather than an efficient alternative.