A tissue culture of vertebrate muscle was provided with a constant excess supply of glucose under anaerobic conditions starting at time zero and the amounts of pyruvic acid and ATP produced were measured. The solid line in the graph above represents the pyruvic acid produced in moles per liter per minute. ATP levels were also found to be highest at points A and C, lowest at B and D. A second culture was set up under the same conditions, except that substance X was added, and the results are indicated by the dotted line.

The rate of pyruvic acid formation fluctuates because

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A tissue culture of vertebrate muscle was provided with a constant excess supply of glucose under anaerobic conditions starting at time zero and the amounts of pyruvic acid and ATP produced were measured. The solid line in the graph above represents the pyruvic acid produced in moles per liter per minute. ATP levels were also found to be highest at points A and C, lowest at B and D. A second culture was set up under the same conditions, except that substance X was added, and the results are indicated by the dotted line.

The rate of pyruvic acid formation fluctuates because

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Consider the scenario: a tissue culture is supplied with excess glucose under anaerobic conditions, and measurements show that pyruvic acid production fluctuates, with ATP levels peaking at A and C and dropping at B and D. The same setup with substance X added is also tracked.
Option 1: 'coenzymes have begun to function' — While coenzymes are essential cofactors for many enzymes, their mere presence or activity does not by itself explain oscillations in the rate of pyruvate production over time. Coenzyme involvement would more likely affect the rate uniformly or in a more predictable manner, not produce the observed fluctuations tied to ATP levels across time points.
Option 2: 'all enzymes have been used up' — Enzymes are proteins that catalyze reactions; they are not consumed in chemical reactions in the way substrates are. Therefore, the idea that all enzymes have been used up is biologically incorrect and cannot account for alternating rates of pyruvate formation.
Option 3: 'all glucose has reacted' — If all glucose were exhausted, the rate would drop to zero and remain low, not exhibit the described alternating high and low points in pyruvate production correlating with ATP levels. The system under continuous glucose supply would not show sustained oscillations solely from substrate depletion.
Option 4: 'the reaction is affected by negative feedback' — This matches known metabolic regulation: high ATP (an energy-rich state) inhibits glycolytic enzymes (e.g., phosphofructokinase) via feedback mechanisms, causing the rate of pyruvate production to slow when energy charge is high and to increase when ATP is low. The observed high ATP at certain points (A and C) and lower ATP at others (B and D) fit a scenario where the pathway is throttled in response to energy status, producing the fluctuating rates seen in the graph.
Option 5: 'the reaction is accelerated by positive feedback' — Positive feedback in metabolic pathways is less common for central catabolic processes like glycolysis, especially under steady substrate supply; it would drive runaway activation rather than produce the rhythmic fluctuations aligned with ATP levels. This makes it a poor explanation for the observed pattern.
In summary, the fluctuating rate of pyruvic acid formation aligns with a regulatory mechanism where the pathway is modulated by the cell’s energy state, i.e., negative feedback. The other options describe scenarios that either contradict basic enzyme biology, rely on substrate exhaustion, or invoke an unlikely positive feedback loop for this context.

APBIO141-031 with labs S11 - CollegeBoard* AP Exam Prep: Multiple Choice

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