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:
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:
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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