Answer:
facilitated diffusion
Explanation:
Substances tend to move in and out of living cells via the selectively permeable membrane (plasma membrane), which helps controls what goes in and out of the cell. Movement of these substances can either be passive or active depending on whether energy is needed for the transport to take place.
Passive transport of substances is that which does not require energy input before it occurs. According to the question, FACILITATED DIFFUSION suits this category. Facilitated diffusion is the movement of substances (ions) that cannot freely diffuse into a cell due to their polarity, via carrier molecules (protein channels and carrier proteins). They help diffuse molecules through the plasma membrane of the cell. This form of transport is passive i.e. does not expend energy, because the substances move down their concentration gradient. The helper proteins are just channels/carriers to get the substances in or out of the cell.
A good example of facilitated diffusion is the movement of water molecules across the cell membrane with the help of carrier proteins called AQUAPORINS.
