It feels really weird that we have proteins that transport something small (like oxygen or iron, below 100 daltons heavy) that can transport something like four of the things in question at a time and weigh _tens of kilodaltons_. And they're still more efficient than direct dissolution.
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Like, why does transferrin exist? Why isn't all iron transport done with ferritin? (There's probably some extremely obvious reason, but I'm not a biochemist sadly.)
@whitequark would your headmate perhaps be willing and able to give a pointer to a reasonable point to start reading about this?