Cellular Energetics
What you must understand
Cells couple exergonic (energy-releasing) reactions to endergonic (energy-requiring) ones via ATP. Enzymes lower activation energy without changing the overall free-energy change; they are not consumed and do not alter equilibrium. Enzyme activity depends on temperature, pH, substrate concentration, and inhibitors. Competitive inhibitors bind the active site; noncompetitive/allosteric inhibitors bind elsewhere and change shape.
Photosynthesis converts light energy to chemical energy. Light reactions (thylakoid membranes) split water, release O2, and produce ATP and NADPH. The Calvin cycle (stroma) fixes CO2 using ATP and NADPH. Know that O2 comes from water, not CO2.
Cellular respiration oxidizes glucose to make ATP. Glycolysis (cytoplasm) yields net 2 ATP and 2 NADH. The Krebs cycle (mitochondrial matrix) and oxidative phosphorylation (inner membrane, electron transport chain + chemiosmosis) produce most ATP. Fermentation regenerates NAD+ without oxygen.
Traps
- The ETC produces ATP via a proton gradient (chemiosmosis), not by directly phosphorylating.
- Photosynthesis and respiration both use ETCs but in different organelles and directions.
- A competitive inhibitor can be overcome by more substrate; a noncompetitive one cannot.
Rule of thumb
Trace electrons and protons: in respiration electrons flow to O2; in photosynthesis light excites electrons and water provides replacements.
Worked example
Adding more substrate restores reaction rate in the presence of a competitive inhibitor but not a noncompetitive one-because the latter changes enzyme shape regardless of substrate.