Enzymes and metabolism practice questions.

In aerobic respiration, the majority of ATP is produced during

• glycolysis
• the link reaction (pyruvate oxidation)
• the Krebs cycle
• oxidative phosphorylation

Answer: oxidative phosphorylation

Oxidative phosphorylation on the inner mitochondrial membrane yields the bulk of ATP via the electron transport chain and chemiosmosis.

In a plant cell, photosynthesis takes place in the

• mitochondrion
• chloroplast
• ribosome
• nucleus

Answer: chloroplast

Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis.

Which of the following is NOT normally a limiting factor for the rate of photosynthesis?

• light intensity
• carbon dioxide concentration
• temperature
• oxygen concentration

Answer: oxygen concentration

Light, CO₂ and temperature can each limit the rate; oxygen is a product and does not normally limit photosynthesis.

A reversible reaction is allowed to reach equilibrium in a test tube. An enzyme for that reaction is then added. What is the effect on the reaction?

• Equilibrium is reached faster, its position unchanged
• Equilibrium shifts towards the products of the reaction
• Equilibrium shifts back towards the reactants again
• The reaction stops because the enzyme is consumed

Answer: Equilibrium is reached faster, its position unchanged

An enzyme lowers the activation energy of both the forward and the reverse reaction equally, so it changes only the rate, never the equilibrium position. A common error is to assume catalysts create more product.

One enzyme catalyses the breakdown of only one type of molecule. What accounts for this specificity?

• The overall number of amino acids in the protein
• The shape and chemical properties of its active site
• The speed at which the molecules move in solution
• The temperature at which the reaction is carried out

Answer: The shape and chemical properties of its active site

Only a substrate whose shape and charge distribution are complementary to the active site can bind and form an enzyme-substrate complex; size of the whole protein is irrelevant.

The rate of an enzyme-catalysed reaction falls sharply when the temperature is raised from 40 °C to 60 °C. What is the reason for this fall?

• The enzyme molecules are used up more quickly than before
• The substrate molecules move too fast to be bound
• The bonds holding the tertiary structure are disrupted
• The peptide bonds of the enzyme are hydrolysed by heat

Answer: The bonds holding the tertiary structure are disrupted

Heat breaks the hydrogen and ionic bonds that hold the tertiary structure, so the active site loses its complementary shape; the strong peptide bonds of the primary structure remain intact.