Homeostasis maintains optimal conditions for enzyme action and all cell…

The idea

Enzymes are proteins that catalyse reactions in cells, and each enzyme has an optimum temperature and pH at which its rate of action is highest. Homeostasis keeps these conditions close to the optimum so that metabolic reactions, including respiration and digestion, continue at a suitable rate. If temperature rises too far, enzyme molecules denature as the shape of the active site changes, so substrate molecules no longer fit and the rate falls. If pH moves away from the optimum, bonds in the enzyme are affected and activity decreases. Homeostasis also maintains conditions such as blood glucose concentration and water content so that cells can function.

For example: Sweating cools the body and insulin lowers blood glucose, protecting enzyme-controlled reactions.

What earns the marks

• Enzymes have an optimum temperature and pH at which their rate of action is highest.
• Homeostasis keeps internal conditions close to these optima so metabolic reactions continue.
• Above the optimum temperature, enzymes denature because the active site changes shape and substrate molecules no longer fit.
• Changes in pH away from the optimum reduce enzyme activity.
• Homeostasis also regulates conditions such as blood glucose concentration and water content for cell function.
• Examples include sweating to cool the body and insulin to lower blood glucose concentration.

Common mistakes and how to put them right

• Saying enzymes are killed by high temperature.

  Put it right: Enzymes are proteins and are denatured, meaning the active site changes shape.

• Stating that any temperature above 0 °C denatures enzymes.

  Put it right: Denaturation occurs above the optimum temperature, and the optimum differs between enzymes.

• Claiming homeostasis keeps conditions exactly constant.

  Put it right: It maintains conditions within narrow limits around the optimum.

Exam tips

• State the optimum temperature and pH as conditions that maximise enzyme activity, then explain what happens when they are not maintained.
• Use the lock-and-key idea to explain why a changed active site stops the substrate fitting.
• Give a named homeostatic response, such as sweating or insulin release, and link it to protecting enzyme action.