Answer:
A. Protein pumps
Explanation:
Molecules usually move down their concentration gradient - in other words, they move from high to low concentration.
Here, in the problem, we are moving molecules against their concentration gradient - from low to high. So, we require an input of energy.
Look at each of the answer choices:
A) Protein pumps
Protein pumps do require an input of ATP (energy), in order to push molecules up their concentration gradient. This is the answer.
B) Diffusion
Diffusion is the transport of molecules down their gradient, from high to low concentration, so this is wrong.
C) Osmosis
Osmosis is essentially diffusion of water molecules, so it's again from high to low concentration. This is wrong.
D) Passive transport
Like the name suggests, this transport is "passive", or it doesn't require energy. Because it doesn't require energy, we know it moves molecules down their concentration gradients. This is wrong.
The answer is thus A.
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