OxfordAQA International A Level Biology 3.1.3 Biochemical Reactions in Cells are Controlled by Enzymes Exam Questions

About OxfordAQA International A Level Biology 3.1.3 Biochemical Reactions in Cells are Controlled by Enzymes

This Unit 1 topic explains how enzymes speed up the reactions in cells. You need to describe enzymes as proteins whose active site has a specific tertiary structure, explain the lock and key and induced fit models, and explain how temperature, pH, substrate concentration, enzyme concentration and inhibitors affect the rate of reaction. Some effects are explained by collisions and some by changes to the active site, and examiners reward answers that use the right explanation. It links directly to protein structure, digestion and respiration, and to Required practical 1 on the effect of temperature, pH or substrate concentration on an enzyme-controlled reaction.

What you need to know

• Enzyme action Enzymes are proteins that act as biological catalysts. The substrate binds to the active site to form an enzyme-substrate complex, which lowers the activation energy of the reaction.
• Models of enzyme action In the lock and key model the substrate is complementary to a rigid active site. The induced fit model says the active site changes shape slightly as the substrate binds, putting strain on bonds in the substrate. Induced fit is the currently accepted model.
• Specificity Each enzyme catalyses one reaction because the tertiary structure of its active site is complementary to only one substrate.
• Temperature and pH Raising temperature increases kinetic energy and the frequency of successful collisions, until high temperatures break hydrogen and ionic bonds and the enzyme denatures. pH away from the optimum alters charges on R groups, changing the active site so fewer enzyme-substrate complexes form.
• Concentration Increasing substrate concentration increases rate until all active sites are occupied, when enzyme concentration becomes limiting. Increasing enzyme concentration increases rate while substrate is in excess.
• Inhibitors Competitive inhibitors have a similar shape to the substrate and bind to the active site; adding more substrate reduces their effect. Non-competitive inhibitors bind elsewhere on the enzyme and change the shape of the active site, so extra substrate does not overcome them.

Common mistakes in 3.1.3 Biochemical Reactions in Cells are Controlled by Enzymes

• Saying the enzyme is killed at high temperature. Enzymes are not alive; they are denatured when bonds holding the tertiary structure break.
• Saying the substrate and active site have the same shape. They are complementary in shape.
• Explaining the effect of low temperature using denaturation. At low temperatures molecules have less kinetic energy so there are fewer collisions and fewer enzyme-substrate complexes form.
• Stating that a non-competitive inhibitor competes with the substrate for the active site. It binds at a site other than the active site and changes the active site shape.

Try three 3.1.3 Biochemical Reactions in Cells are Controlled by Enzymes exam-style questions