instruction:
Given a scientific question, generate a correct answer to it.
question:
What do we call the diffusion of water across a membrane because of a difference in concentration?
answer:
osmosis

question:
The pelvic girdle, consisting of a hip bone, serves to attach a lower limb to this?
answer:
axial skeleton

question:
Ion pumps, the biological machines responsible for the selective transport of metal ions, are complex assemblies of what?
answer:
proteins