system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If mitochondria are small even through microscopes, how do we know how they work?? How do we "know" how body cells function??
gpt
Much of what we know about cells comes from a) direct observation under electron microscopy and b) knock-out mutations/disease in people and animal subjects. Some structures we can see under electron microscopes, and we have a saying in physiology of “form follows function.” Basically, the way something looks can inform what it does. So we look at mitochondria and see the double membranes, and then we make hypotheses. What’s the double membrane for? Is it for energy production? (It is—it’s for the electron transport chain)

Then we look at what happens if there are no mitochondria at all. What happens to the cells? If they don’t have mitochondria, they don’t seem to make any energy, and they die. Some people have mitochondrial disorders, too, and what happens in their bodies can inform us of what the purpose of the structure is. Finally, animal experiments where genes are artificially silenced or enhanced can prove or disprove our hypotheses.

Biochemistry can be frustrating because we don’t normally see exactly what is going on. We have to use surrogates sometimes to make a theory about what is happening, and—when it comes to physiology—sometimes when something goes wrong, we can try to find what is missing and determine the function from there.