Question

rate of pyruvic acid formation (moles/liter/min) a c b d 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 time (min) ue culture of vertebrate muscle was provided with a constant excess supply of glucose under robic conditions starting at time zero and the amounts of pyruvic acid and atp produced were ured. the solid line in the graph above represents the pyruvic acid produced in moles per liter inute. atp levels were also found to be highest at points a and c, lowest at b and d. a second re was set up under the same conditions, except that substance x was added, and the results dicated by the dotted line. h of the following best accounts for the shape of the solid line between a and d? after ten minutes, the cellular enzymes became ineffective atp acted as an allosteric inhibitor on one or more of the enzymes. the measurements of pyruvic acid were unreliable. the cells required more glucose than was being provided.

Explanation:

• Option A: If cellular enzymes became ineffective after ten minutes, the rate of pyruvic acid formation would likely decrease more drastically and not show the oscillating pattern seen between A and D.
• Option B: In allosteric regulation, a molecule (in this case, ATP) binds to an enzyme at a site other than the active site, causing a conformational change. When ATP levels are high (at points A and C), it can act as an allosteric inhibitor of enzymes involved in glycolysis (the pathway that produces pyruvic acid). As ATP is used (and its levels drop, at points B and D), the inhibition is relieved, and pyruvic acid production (and ATP production, since glycolysis produces ATP) can increase again. This explains the oscillating pattern of pyruvic acid (and ATP) levels.
• Option C: There is no indication in the problem that the measurements of pyruvic acid were unreliable. The problem provides a consistent pattern (solid - line graph) and a comparison (dotted - line graph with substance X added), suggesting that the measurements are at least internally consistent.
• Option D: The problem states that there is a “constant excess supply of glucose”. So, the cells are not glucose - limited.

Answer:

B. ATP acted as an allosteric inhibitor on one or more of the enzymes.