Final answer:
Moving molecules through specific protein channels down the concentration gradient is called facilitated diffusion, which is a passive transport mechanism and does not require energy.
Step-by-step explanation:
Moving molecules through protein channels that allow only specific types of molecules or ions per channel, both from high to low and low to high concentrations, describes facilitated diffusion and active transport respectively. However, since the question specifies that the direction can be either high to low or low to high without mentioning the use of energy, this narrows down the answer. For facilitated diffusion, molecules move through protein channels along the concentration gradient, which means from a place of higher concentration to a lower one, and this process does not require energy. Moving molecules against their concentration gradient, which requires energy, is known as active transport.
In summary, the transport mechanism described in the question for moving specific molecules through protein channels down their concentration gradient is known as facilitated diffusion (Option A), while the movement against the gradient would involve active transport (Option B), making Option A the correct choice.