Bioenergetics of working muscle — which listed compound is NOT a direct source of usable energy for active muscle cells?

Difficulty: Easy

Correct Answer: Lactic acid (lactate)

Explanation:


Introduction / Context:
During intense exercise, muscle fibers rely on immediate and short-term energy systems: ATP, phosphocreatine, anaerobic glycolysis, and aerobic metabolism of glucose and fatty acids. Clarifying which molecules actually supply usable energy in the moment helps interpret fatigue and metabolite measurements.



Given Data / Assumptions:

  • Active muscle requires rapid ATP turnover.
  • Phosphocreatine buffers ATP via creatine kinase.
  • Glucose and fatty acids fuel ATP production via glycolysis and oxidative phosphorylation.
  • Lactate accumulates during high glycolytic flux but is not oxidized significantly by the same fiber during peak anaerobic output.


Concept / Approach:

ATP is the immediate energy currency. Creatine phosphate rapidly regenerates ATP. Glucose and fatty acids are substrates for ATP generation. Lactate is primarily an end-product of anaerobic glycolysis under oxygen-limited conditions and is exported; it can be metabolized later (e.g., in liver via the Cori cycle or in oxidative fibers/heart) but is not the primary contemporaneous energy donor at the moment of its formation in active anaerobic fibers.



Step-by-Step Solution:

Identify immediate ATP sources: ATP and phosphocreatine.Identify metabolic fuels: glucose and fatty acids.Recognize lactate as a by-product/temporary sink for reducing equivalents.Therefore lactate is not a direct energy source in active contracting fibers.


Verification / Alternative check:

Physiology texts describe lactate shuttling and subsequent utilization by other tissues or later time points, not as the principal contemporaneous fuel within glycolytic bursts.


Why Other Options Are Wrong:

ATP powers myosin ATPase; phosphocreatine buffers ATP; glucose and fatty acids provide ATP via metabolic pathways.


Common Pitfalls:

Equating “can be metabolized somewhere later” with “is a present energy source.”


Final Answer:

Lactic acid (lactate)

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