When you exercise, your muscles begin to undergo anaerobic respiration. But does aerobic respiration still go on (i.e. anaerobic just tops it up), or does it stop (i.e. anaerobic takes over)?
It's almost axiomatic. Think about it, if anaerobic respiration occurred, that suggests a lack of oxygen and so aerobic respiration cannot occur. A cell cannot aerobically and anaerobically respire at the same time can it.
When you exercise, your muscles begin to undergo anaerobic respiration. But does aerobic respiration still go on (i.e. anaerobic just tops it up), or does it stop (i.e. anaerobic takes over)?
I'll have to check in with the contrary view as a sports scientist. Yes aerobic respiration continues during anaerobic activity - perhaps with the exception of bursts of activity lasting less than a few seconds and rely on the phosphagen system. It's just that aerobic respiration is incredibly rate-limited and can't make a significant contribution over the time course of anaerobic activity, but it is still going on.
Anaerobic respiration kicks in when aerobic respiration can't meet the metabolic demand of exercise. All 3 energy systems are used in different proportions for all kinds of activities.
They both do happen at the same time in a single cell yes.
A cell will never have 0 oxygen. Under normal circumstances blood will still be flowing and oxygen will still be delivered. It will just not be enough and have to be supplemented. Remember how much more efficient aerobic is - it would make little sense for a cell desperate for energy to stop using it.