All Free Biology MCQs with Answers

591. Enzymes that catalyse the splitting of bonds with the addition of water belong to the class called

• A. hydrolases
• B. ligases
• C. transferases
• D. isomerases

Explanation: Hydrolases, such as amylase, lipase and peptidase, break bonds by inserting water. Transferases move groups between molecules, ligases join molecules using ATP, and isomerases rearrange a molecule without changing its formula.

592. Which statement about enzymes as biological catalysts is correct?

• A. They are consumed during the reaction
• B. They allow energetically impossible reactions to occur
• C. They increase the energy content of the products
• D. They are regenerated unchanged at the end of the reaction

Explanation: An enzyme leaves the reaction exactly as it entered, which is why tiny amounts can process huge quantities of substrate. It only speeds a reaction that could happen anyway by lowering the activation energy; it never changes the energy of substrates or products.

593. Storing food in a refrigerator slows its decay mainly because low temperature

• A. permanently denatures the enzymes of microbes
• B. greatly reduces the rate of enzyme controlled reactions
• C. kills all bacteria and fungi at once
• D. changes the pH of the food

Explanation: Cold reduces the kinetic energy of molecules so enzyme catalysed spoilage reactions slow dramatically, but the enzymes are not destroyed and work again on warming. This reversibility is exactly why freezing preserves food without sterilising it.

594. The specific region of an enzyme to which the substrate binds and where catalysis occurs is called the

• A. allosteric site
• B. active site
• C. cofactor site
• D. regulatory site

Explanation: The active site is a cleft formed by the folding of the chain, whose shape and chemistry fit the substrate. Allosteric sites are separate locations where regulators bind, so a question about the substrate's own binding place points only to the active site.

595. Koshland's induced fit model improved on the lock and key hypothesis by proposing that

• A. the active site is completely rigid
• B. the substrate changes its shape before binding
• C. enzymes have no binding site at all
• D. the active site moulds itself around the substrate as it binds

Explanation: Koshland argued in 1966 that binding itself induces a conformational change that tightens the fit and stresses bonds in the substrate, whereas the older lock and key model pictured the active site as a fixed shape. Both models agree that the fit is specific.

596. In the lock and key analogy for enzyme action, the lock represents the

• A. enzyme
• B. substrate
• C. product
• D. coenzyme

Explanation: Emil Fischer's analogy pictures the enzyme's rigid active site as the lock and the substrate as the one key that fits it, explaining specificity. The products simply leave the site once the reaction is complete.

597. Which sequence correctly represents enzyme catalysis?

• A. E + P → EP → ES → E + S
• B. ES → E + S → E + P
• C. E + S → ES → EP → E + P
• D. EP → ES → E + S

Explanation: Enzyme and substrate first form the enzyme-substrate complex, which converts to an enzyme-product complex before releasing product and regenerating the free enzyme. Any sequence that starts from product runs the pathway backwards.

598. An enzyme changes the rate of a reaction without altering its

• A. activation energy
• B. speed of completion
• C. overall energy difference between substrates and products
• D. reaction pathway

Explanation: Catalysts lower the activation energy and speed the reaction, but the energy of substrates and products is fixed by chemistry, so the enzyme cannot change it. This is why an enzyme never makes an unfavourable reaction favourable.

599. Enzymes are needed inside cells in only minute quantities because they

• A. are very large molecules
• B. are made of DNA
• C. work only at very high temperatures
• D. are not consumed and can be reused repeatedly

Explanation: Since the enzyme emerges unchanged from each reaction, one molecule can process thousands of substrate molecules per second. Cells therefore regulate metabolism by controlling enzyme activity rather than by stockpiling enzyme.

600. The activation energy of a reaction could in principle be supplied by heat, but cells use enzymes instead because heating would

• A. slow every reaction down
• B. damage cellular molecules and speed all reactions indiscriminately
• C. destroy the substrates
• D. prevent products from forming

Explanation: Raising temperature speeds every reaction and denatures proteins, so it cannot give the selective control life needs. An enzyme lowers the barrier for one specific reaction at body temperature, which is precisely why metabolism can run fast at 37 degrees.