Enzymes — Practice · Grade 10 Science

Chapter: Investigating the Chemistry of Life

What this practice covers

By the end of Enzymes you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: enzyme, catalyst, substrate, active site, activation energy, denature, ribozyme.

Problem 1

An enzyme is best described as a
  1. protein that speeds up a chemical reaction without being used up
  2. sugar that stores energy for the cell
  3. type of lipid that builds the cell membrane
  4. molecule that stores genetic information
Answer and explanation

protein that speeds up a chemical reaction without being used up

Enzymes are protein catalysts — they speed up specific reactions and are not consumed, so they can be reused.

Problem 2

The specific molecule that an enzyme acts on is called its
  1. active site
  2. substrate
  3. product
  4. catalyst
Answer and explanation

substrate

The substrate is the molecule that binds the active site and gets changed into product.

Problem 3

Why does an enzyme stop working at very high temperatures?
  1. The substrate becomes too cold to move
  2. High heat gives the enzyme an extra active site
  3. The heat denatures the enzyme, changing the shape of its active site so the substrate no longer fits
  4. The enzyme is used up in the reaction
Answer and explanation

The heat denatures the enzyme, changing the shape of its active site so the substrate no longer fits

Too much heat unravels the protein's folding (denaturing), distorting the active site so the substrate can't bind.

Problem 4

Without an enzyme, a reaction makes 5 molecules of product per second. With its enzyme, the reaction runs 200 times faster. How many molecules of product form per second with the enzyme?
Answer and explanation

1000 molecules per second

5 products/second × 200 = 1000 products per second.

Problem 5

Pepsin, a stomach enzyme, works best at about pH 2, while most enzymes in your cells work best near pH 7. Use what you know about enzyme shape to explain why an enzyme has an optimum pH.
Answer and explanation

An enzyme's function depends on its precise folded shape, which is held by chemical attractions sensitive to the surrounding H⁺ concentration (pH). At its optimum pH the active site keeps the right shape to fit the substrate; at the wrong pH those attractions are disrupted, the active site distorts, and the enzyme works poorly or denatures. Pepsin evolved to fold correctly in stomach acid, so its optimum is pH 2.

Problem 6

Rate increases with substrate concentration then levels off while enzyme amount is fixed. Give a plausible explanation. Explain the reasoning in your own words.
Answer and explanation

Enzyme saturation can explain the plateau; the design must rule out other limiting conditions.

At low substrate, more substrate increases encounters with active sites.

At high substrate, available sites can become occupied much of the time.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

An inhibitor resembles the substrate and occupies the active site. What mechanism fits? Show the reasoning, not just the conclusion.
Hints

A falling rate does not establish the exact molecular mechanism without additional tests.

Answer and explanation

Competitive inhibition is consistent with competing for the substrate-binding site.

Problem 8

Does an enzyme supply all the energy required by an unfavorable reaction?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. It lowers activation barriers; it does not change the overall free-energy requirement or create an energy source.

Problem 9

An optimum is a result under specified conditionsAn optimum is a result under specified conditionsInitial reaction rate (model units/min)04812102030405060Temperature (°C)The supplied optimum is near 40 °C under these test conditions.
Original invented initial-rate versus temperature example, not a measured universal enzyme optimum or storage recommendation.
Interpret the supplied visual. Would this curve justify storing all enzymes at 40 °C indefinitely?
Answer and explanation

No. It describes an initial-rate example under selected conditions; long-term stability and other enzyme identities require different evidence.

A reaction rate measures change per time; an initial rate uses a defined early interval before large substrate depletion or product buildup. Warming can increase productive collisions, but higher temperature can disrupt an enzyme’s fold and stability. The balance creates a condition-dependent optimum. pH, enzyme identity, exposure duration and substrate conditions matter too. Low rate in the cold is not automatically denaturation; activity may return on warming. A steep loss after heating needs suitable recovery and structural evidence before its cause is assigned.

No. It describes an initial-rate example under selected conditions; long-term stability and other enzyme identities require different evidence.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

An enzyme has the same reactant and product energy levels with and without a catalyst, but a lower pathway peak with it. Explain rate, equilibrium and energy implications.
Answer and explanation

The lower barrier can increase the approach rate in both directions. It does not by itself change the equilibrium or net free-energy change, and an unfavorable isolated process still needs suitable coupling or conditions.

The lower barrier can increase the approach rate in both directions. It does not by itself change the equilibrium or net free-energy change, and an unfavorable isolated process still needs suitable coupling or conditions.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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