AP Biology: Enzyme Catalysis — Concentration and Saturation Practice Questions
Test yourself on AP Biology: Enzyme Catalysis — Concentration and Saturation with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
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1. A purified enzyme follows a simple substrate-saturation model. At fixed enzyme concentration, its initial rate rises strongly at low substrate concentration but changes little after much more substrate is added. Which explanation best fits this pattern?
Explanation. A near-plateau on a rate-versus-substrate plot is consistent with substrate saturation of the available enzyme. A substantial positive rate still occurs; the observation does not imply enzyme consumption or production of additional enzyme.
2. At saturating substrate concentration, a reaction forms product at 6 micromoles per minute. The concentration of the same active enzyme is doubled in the same reaction volume. With unchanged temperature and pH, no other limitations, and negligible substrate depletion during measurement, what initial rate is expected?
Explanation. Twice as much active enzyme provides twice as much catalytic capacity under the stated matched conditions. The predicted initial rate is 2 times 6, or 12 micromoles per minute. This prediction concerns the early rate, not the final amount of product.
3. An idealized enzyme has initial rate v = Vmax times S divided by (K plus S), where S is substrate concentration and K is a positive constant. If Vmax is 18 rate units and S equals K, what is v?
Explanation. Substituting S = K makes the fraction K divided by (K plus K), which is one-half. Thus v is one-half of 18, or 9 rate units. The given equation supplies all the information needed; no numerical value of K is required.
4. An enzyme obeys v = Vmax times S divided by (K plus S). Its Vmax is 30 rate units and K is 2 millimolar. Raising S from 2 to 8 millimolar changes the predicted initial rate from 15 units to what value?
Explanation. At S = 8, the stated equation gives 30 times 8 divided by (2 plus 8), or 24. Quadrupling substrate therefore does not quadruple the rate in this example. The predicted value remains below the stated limiting rate of 30.
5. To isolate the effect of substrate concentration on an enzyme's initial rate, which design is most appropriate?
Explanation. Only the first design makes substrate concentration the intended changing factor while maintaining a comparable rate measurement. The other designs change additional variables or compare different reaction stages, leaving the substrate effect unresolved.
6. Two identical closed reactions start with equal amounts of substrate. One contains twice the active enzyme. Assume that each substrate molecule is eventually converted to one product molecule, both reactions reach complete conversion, and the enzyme does not supply substrate. How do the final product amounts compare?
Explanation. Under the stated complete-conversion assumption, each reaction can produce one product molecule per starting substrate molecule. Equal starting substrate therefore gives equal final product. More enzyme can change how quickly conversion occurs without increasing the available substrate.
7. At substrate concentrations of 1, 2, 4, and 8 millimolar, independently measured initial rates are 4, 7, 9, and 9.5 units. Conditions and enzyme concentration are matched. Which statement best describes these observations?
Explanation. The successive rate increases are 3, 2, and 0.5 units. They become smaller even though substrate doubles each time. This describes the measurements; it does not establish an exact maximum rate beyond the measured range.
8. In an enzyme time-course experiment, product amounts are 0, 3, 6, and 7 nanomoles at 0, 10, 20, and 30 seconds. The first 20 seconds form the validated early linear interval. What rate should be estimated from that interval?
Explanation. Use the product change over the stated linear interval: (6 minus 0) divided by (20 minus 0) equals 0.30 nanomoles per second. The later interval has a smaller slope and is excluded from this particular initial-rate estimate.
9. A substrate-series experiment appears to reach a plateau, but the product detector may stop responding proportionally at high signal. Which additional check most directly tests this measurement limitation?
Explanation. Known product standards test whether detector response tracks product over the relevant range. A flattened calibration response could mimic a rate plateau. Checking that response addresses the proposed measurement problem directly; the other choices do not.
10. Two enzyme preparations are assayed in equal volumes with the same saturating substrate concentration and matched conditions. Preparation A contains 2 micrograms of enzyme protein and has a rate of 8 units; preparation B contains 5 micrograms and has a rate of 20 units. How do their rates per microgram compare?
Explanation. Normalize each measured rate by its own protein mass: 8 divided by 2 and 20 divided by 5 both equal 4. B's larger total rate is accompanied by proportionally more protein. These observations alone do not establish equal purity or molecular identity.
11. A student prepares 4.0 milliliters of reaction mixture using 1.0 milliliter of 12 millimolar substrate stock and 3.0 milliliters of substrate-free reagents. Assuming additive volumes and no reaction before measurement begins, what is the initial substrate concentration?
Explanation. The stock is diluted to four times its original volume. The concentration is 12 times 1.0 divided by 4.0, or 3 millimolar. Substrate-free reagents increase the volume but add no substrate, so the stock concentration is not the final concentration.
12. A product-versus-time curve from one closed enzyme reaction becomes flatter after several minutes. Which conclusion is justified by that trace alone?
Explanation. A flatter time trace means product accumulates more slowly. It does not by itself distinguish substrate depletion, product effects, enzyme instability, or a measurement limitation. A substrate-saturation study instead compares initial rates from separate reactions at different starting substrate concentrations.
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FAQ
Does adding more substrate always double an enzyme's rate?
No. At fixed enzyme concentration, the rate may rise less strongly as substrate increases and the enzyme approaches its catalytic capacity. Any proportional prediction needs a stated model and appropriate conditions.
Why distinguish a time-course plateau from substrate saturation?
A time course follows one reaction as it progresses. A substrate-saturation experiment compares initial rates at different starting substrate concentrations. A flat region in one type of plot does not automatically explain a flat region in the other.
Are these numbers measured experimental results?
No. The numerical examples are invented for independent study practice. Equations and assumptions are supplied where needed; the questions do not report measurements from a particular enzyme or reproduce a real exam.