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AP Biology — Unit 1 — pH, Buffers & Biological Molecules — Drill 3

Drill 3 ·

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About This Drill

AP Biology — Unit 1 — pH, Buffers & Biological Molecules — Drill 3 is a practice drill. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.

Practice analyzing pH, acid-base chemistry, and buffer systems in biological contexts with this AP Biology drill. You will interpret pH scale relationships, evaluate how buffers maintain homeostasis, and explain how pH changes affect enzyme function and protein structure.

Passage

The pH scale measures the concentration of hydrogen ions (H+) in a solution. It is a logarithmic scale ranging from 0 to 14, where pH 7 is neutral. Solutions with pH below 7 are acidic (higher H+ concentration) and solutions above 7 are basic (lower H+ concentration). Because the scale is logarithmic, each unit represents a 10-fold difference in H+ concentration. Living systems must maintain stable pH because even small deviations can denature proteins, disrupt enzyme active sites, and alter membrane function. Biological buffer systems resist pH change by absorbing excess H+ (acting as a base) or releasing H+ (acting as an acid) when needed. Bicarbonate Buffer System in Human Blood The primary buffer in human blood is the bicarbonate system: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- Normal blood pH is maintained between 7.35 and 7.45. When blood becomes too acidic (pH drops), the reaction shifts left, consuming H+. When blood becomes too basic, carbonic acid dissociates to release more H+. pH and Enzyme Function
EnzymeOptimal pHLocation
Pepsin2.0Stomach
Salivary amylase6.8–7.0Mouth
Trypsin7.5–8.5Small intestine
Urease7.0Soil bacteria
Each enzyme has an optimal pH at which its active site geometry and charge distribution best fit the substrate. Deviations from optimal pH alter the ionization state of amino acid R groups, changing the shape of the active site.

Questions in This Drill

  1. A solution has a pH of 5. A second solution has a pH of 3. How does the H+ concentration in the pH 3 solution compare to the pH 5 solution?
  2. Based on the table, which enzyme would be most active in the stomach immediately after a meal, and why?
  3. During intense exercise, CO2 production increases dramatically. Using the bicarbonate buffer equilibrium shown in the passage, predict the effect on blood pH if the respiratory system does not increase ventilation rate to compensate.
  4. A researcher adds a small amount of strong acid to a buffered solution (pH 7.4) and to an unbuffered solution (pH 7.4). The buffered solution's pH drops from 7.4 to 7.3, while the unbuffered solution's pH drops from 7.4 to 4.9. Which of the following best explains the difference?
  5. A student is studying an enzyme that normally functions optimally at pH 7.4. When the solution is acidified to pH 5, enzyme activity drops to nearly zero. Which of the following provides the best molecular explanation for this loss of activity?