Interaction and Interdependence
This domain examines interactions from molecular reactions to ecosystems. The exam tests enzyme action and metabolic pathways, energy transformations, signalling and defence, population interactions, and the transfer of energy and matter through communities.
Core rules and concepts
- Enzymes are globular protein catalysts that lower activation energy. Their active sites are specific. Rate is affected by substrate concentration, temperature, pH and inhibitors. A competitive inhibitor competes for the active site and can be overcome by raising substrate concentration; a noncompetitive inhibitor binds elsewhere and lowers maximum rate.
- Cell respiration transfers chemical energy to ATP, while photosynthesis captures light energy. These processes link organisms within ecosystems.
- Populations interact through competition, predation, parasitism and mutualism. Population growth may be limited by carrying capacity and density-dependent or density-independent factors.
- Energy flows one way through trophic levels, whereas nutrients cycle. Energy is lost through respiration, heat, excretion and uneaten material, so biomass pyramids usually narrow upward.
- Chemical and neural signalling coordinate responses, and defence systems distinguish self from non-self.
Worked example
An enzyme is assayed with and without an inhibitor. At low substrate concentration the inhibited rate is lower. As substrate concentration rises, the inhibited rate approaches the uninhibited maximum rate. This pattern indicates competitive inhibition: extra substrate outcompetes the inhibitor for active sites. A noncompetitive inhibitor would reduce the maximum rate even at high substrate concentration because it does not compete for the active site.
Traps that cost marks
Do not claim a fixed “10% rule” for every trophic transfer; efficiency varies. Do not say energy cycles. Distinguish gross from net productivity, and do not confuse competitive with noncompetitive inhibition without citing the substrate-concentration evidence.