Enzymes and Cellular Energy
Enzymes are biological catalysts, almost always proteins, that speed up chemical reactions without being consumed. They work by lowering a reaction's activation energy, the energy barrier reactants must overcome.
How enzymes work
Each enzyme has an active site shaped to fit a specific substrate (the induced-fit model). When the substrate binds, the enzyme stabilizes the transition state, lowering activation energy and speeding the reaction. Enzymes are sensitive to temperature and pH; extremes can denature them, destroying the active site's shape.
ATP, the energy currency
Cells store usable energy in ATP (adenosine triphosphate). Breaking the bond to its third phosphate releases energy:
ATP -> ADP + Pi + energy.
This energy powers active transport, muscle contraction, and synthesis reactions. Respiration recharges ADP back into ATP.
Worked example
If you raise temperature from 20 C to 37 C, an enzyme's reaction rate usually increases because molecules collide more often. But beyond an optimum (around 40 C for many human enzymes), the rate drops sharply as the protein denatures. So the rate-vs-temperature graph rises to a peak, then falls.
Key takeaways
- •Enzymes lower activation energy and are reusable catalysts.
- •The active site is substrate-specific; denaturation destroys function.
- •ATP releases energy when it loses a phosphate, becoming ADP.
