system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a fly have so much energy to use it's wings for as long as it does?
gpt
Super adapted muscles/metabolism for flight. The flight muscles in insects can be up to 1/5th of their body mass. But it's really all about efficiency. Your cells can use aerobic or anaerobic metabolism to produce ATP from glucose (and sometimes lipids and other stuff). During aerobic metabolism - when oxygen is available to the cell - breaking down one glucose molecule yields something like 30 ATP. When cells can't get enough oxygen, they instead use other molecules in its place, but the process is way less efficient - you get 3 molecules of ATP per glucose. In mammals, oxygen is carried through our blood by proteins called hemoglobins. The eli5 version of this is that it means if your muscles are working really hard, eg you're sprinting, they will use more oxygen than can be released from the hemoglobin and your cells will have to use anaerobic respiration for energy. Flying insects, on the other hand, don't have hemoglobin at all. Oxygen travels through their bodies as a gas, and they have a network of tubes designed to provide it directly to their muscle cells, so their muscle cells have virtually infinite oxygen and never use anaerobic metabolism for energy. They also have by far the largest aerobic scope in the animal kingdom. That means they can generate energy by breaking down glucose around 75 times faster when working hard then when resting, and they are able to switch between these rates nearly instantaneously by very tight regulation of the enzymes involved. By comparison, we can at best increase respiration rates by about 20 times, and it takes a long time for our cells to slow down respiration so a lot of "fuel" is wasted.