During a slow jog, a runner's muscles get enough oxygen delivered to release energy from glucose. When the same runner sprints flat out, their muscles cannot get oxygen fast enough, yet the muscles keep working for a while before starting to ache and tire. What is happening in the muscles during the sprint?
- The muscles briefly stop respiring altogether and rely only on the oxygen already stored in the blood from before the sprint began
- The muscles switch to burning stored fat directly, a process that also happens to require no oxygen at all
- The muscles switch to anaerobic respiration, breaking down glucose without oxygen to release energy more quickly, but producing lactic acid as a waste product that builds up and causes the aching, tired feeling
- The muscles increase their rate of aerobic respiration only, extracting extra oxygen that was previously unused in the surrounding tissue
Why C? And why not the others?
Correct answer: C. The muscles switch to anaerobic respiration, breaking down glucose without oxygen to release energy more quickly, but producing lactic acid as a waste product that builds up and causes the aching, tired feeling
When the demand for energy outpaces the oxygen the blood can deliver, muscle cells switch to anaerobic respiration, breaking down glucose without oxygen to release energy faster; the trade-off is that this process produces lactic acid as a waste product, which builds up in the muscles and causes the aching, tired feeling the runner notices. The option claiming the muscles stop respiring is wrong because muscle cells must keep respiring to keep producing energy; stopping respiration entirely would mean the muscles could no longer function at all. The option about burning stored fat is wrong because breaking down fat for energy actually requires oxygen, so it is not a pathway the body can rely on when oxygen is in short supply during a sprint. The option about increasing aerobic respiration alone is wrong because the whole problem during a sprint is that oxygen delivery cannot keep up with demand, so relying only on the oxygen-using pathway cannot explain how the muscles keep working.
Source: BBC Bitesize GCSE Biology / PE: Aerobic and anaerobic respiration