All Free Biology MCQs with Answers

601. When the substrate is present in excess, doubling the concentration of enzyme will

• A. approximately double the rate of the reaction
• B. halve the rate of the reaction
• C. leave the rate unchanged
• D. denature the substrate

Explanation: With abundant substrate every added enzyme molecule finds work at once, so rate is directly proportional to enzyme concentration. The rate only becomes independent of enzyme amount when substrate runs short and active sites compete for it.

602. Trypsin, working in the small intestine, has an optimum pH of about

• A. 8
• B. 2
• C. 5
• D. 11

Explanation: The alkaline environment of the duodenum, fed by bicarbonate from the pancreas, suits trypsin at pH 8. Pepsin in the acidic stomach works best near pH 2, so each digestive enzyme matches the pH of the compartment where it acts.

603. In an experiment on enzyme activity a buffer solution is used in order to

• A. increase the substrate concentration
• B. supply the necessary coenzyme
• C. keep the pH constant throughout
• D. raise the temperature slowly

Explanation: Because pH changes can alter ionisation of the active site and denature the enzyme, a fair test holds pH steady with a buffer. Without one, the acid or alkali produced by the reaction itself would shift the result.

604. A very high fever above 41 degrees Celsius is medically dangerous mainly because

• A. it makes the blood too alkaline
• B. vital enzymes begin to denature and lose function
• C. it stops all diffusion in the body
• D. it destroys red blood cells instantly

Explanation: Proteins start to unfold irreversibly a few degrees above body temperature, so sustained high fever disrupts the enzyme systems of the brain and other organs. This is the same denaturation that cooks an egg, happening inside the body.

605. Below the optimum, raising the temperature of an enzyme controlled reaction from 15 to 25 degrees Celsius will roughly

• A. halve the rate
• B. stop the reaction
• C. have no effect on the rate
• D. double the rate

Explanation: Each 10 degree rise roughly doubles the rate because molecules collide more often and with more energy, until heat damage overtakes this gain near the optimum. Above the optimum the curve falls steeply as the enzyme denatures.

606. Which description best fits the graph of reaction rate against increasing substrate concentration at a fixed enzyme concentration?

• A. A straight line rising forever
• B. A curve that rises and then levels off as active sites fill
• C. A curve that falls from the start
• D. A horizontal line from the beginning

Explanation: At low substrate, rate climbs steeply with each added molecule, but once every active site are occupied the enzyme is saturated and the curve plateaus. A straight line would imply no saturation, which contradicts the finite number of active sites.

607. Which treatment permanently destroys the activity of an enzyme?

• A. Strong heating well above its optimum
• B. Cooling the solution to 0 degrees Celsius
• C. Diluting the enzyme solution
• D. Adding more substrate

Explanation: Strong heat breaks the weak bonds holding the tertiary structure, so the active site is permanently distorted. Cold only slows the molecules down and activity returns on warming, which is why refrigeration preserves without sterilising.

608. Sulfa drugs control bacterial infections because they

• A. denature every bacterial protein at once
• B. dissolve the bacterial membrane
• C. raise the temperature inside the bacterium
• D. resemble PABA and competitively block the bacterial enzyme that uses it

Explanation: The drug mimics para-aminobenzoic acid, a substrate bacteria need to make folic acid, so it occupies the enzyme's active site and starves the pathway. Humans obtain folate from food, which is why the drug harms bacteria selectively.

609. Many modern medicines are designed as competitive inhibitors. To work this way, the drug molecule must

• A. bind permanently to the enzyme by covalent bonds
• B. denature the whole protein
• C. mimic the shape of the enzyme's natural substrate
• D. block the synthesis of the enzyme

Explanation: A competitive inhibitor succeeds only if it fits the active site, so medicinal chemists build molecules shaped like the substrate. Permanent covalent binding would make the drug an irreversible inhibitor instead, a different mechanism.

610. An irreversible inhibitor differs from a reversible one in that it

• A. can be removed simply by dilution
• B. binds only at the allosteric site
• C. attaches by weak hydrogen bonds
• D. forms permanent covalent bonds with the enzyme

Explanation: Irreversible poisons chemically modify amino acids at or near the active site, so the enzyme is permanently disabled and the cell must make new molecules. Reversible inhibitors bind weakly and fall off, so their effect depends on concentration.