Explain the effects of temperature, substrate concentration and pH on…

The idea

Enzymes are proteins that catalyse reactions by binding substrate at their active site. Temperature raises the frequency of successful collisions between enzyme and substrate, so rate rises up to an optimum; beyond it, bonds holding the active site break, the enzyme denatures and the substrate no longer fits, so rate falls sharply. Substrate concentration increases rate while active sites are available, but once all sites are occupied the rate plateaus because enzyme concentration becomes limiting. pH alters the charges and shape of the active site; each enzyme has an optimum pH, and moving away from it distorts the active site and reduces or stops activity. Explain each effect by linking molecular shape and collision frequency to the observed rate.

What earns the marks

• Temperature: increasing temperature increases kinetic energy and the frequency of successful enzyme–substrate collisions, so rate rises towards an optimum.
• Above the optimum temperature, bonds in the enzyme break, the active site changes shape (denaturation) and substrate can no longer bind, so rate falls.
• Substrate concentration: rate increases as more substrate molecules collide with active sites, then plateaus when all active sites are occupied and enzyme concentration limits the rate.
• pH: each enzyme has an optimum pH; changes in pH alter the shape and charge of the active site, reducing binding and activity away from the optimum.
• Denaturation is a change in the shape of the active site, not the destruction of the substrate, and is generally irreversible for temperature and extreme pH.

Common mistakes and how to put them right

• Saying the enzyme is 'killed' by high temperature.

  Put it right: Enzymes are not alive, so the correct idea is that the enzyme is denatured and its active site loses shape.

• Claiming rate keeps rising with substrate concentration indefinitely.

  Put it right by explaining the plateau once all active sites are saturated.

• Confusing the effect of pH with temperature.

  Put it right by stating that pH changes the charges and shape of the active site rather than the kinetic energy of particles.

Exam tips

• Always name the factor and then explain the mechanism through active-site shape or collision frequency, not just 'it gets faster'.
• For temperature, describe both the rising section and the fall after the optimum, using the word denature.
• For substrate concentration, mention the plateau and link it to enzyme concentration becoming limiting.