Expansion
Unfavourable reactions are driven by coupling them to ATP hydrolysis
Expansion
- Negative Gibbs free energy change: exergonic, spontaneous, releases energy
- Positive: endergonic, requires energy input
- Zero: at equilibrium
Two points that are commonly confused:
- Free energy predicts whether a reaction can proceed, not how fast. Rate is determined by the activation energy, which is what enzymes lower. Enzymes do not change the equilibrium position, only the speed of reaching it
- The standard free energy change uses defined conditions; the actual value depends on the real concentrations in the cell, which is why reactions that look unfavourable on paper proceed readily when products are continuously removed
Coupling is the cell’s solution to endergonic reactions: linking them to ATP hydrolysis, which releases about 30 kJ/mol. The classic example is hexokinase, coupling the unfavourable phosphorylation of glucose to ATP hydrolysis.
ATP is not a store but a currency: total body content is only about 250 g, yet the turnover approaches body weight each day. This is why any interruption of ATP production, as in cyanide poisoning or ischaemia, causes failure within minutes.