Question

Facilitated diffusion
(a) Needs a carrier protein
(b) Is an active process
(c) Occurs against the concentration gradient
(d) Needs ATP

Answer 1

Hint: Facilitated diffusion is a process which helps compounds that lack sufficient solubility to move rapidly across the membrane. The transport of oxygen in the blood and muscles is an example of facilitated diffusion. In blood, haemoglobin helps in the transportation of oxygen whereas, in the muscles, myoglobin helps in oxygen transport.

Complete answer:
Facilitated diffusion is a type of passive transport of a single molecule (uniport) or co-transport of two different molecules in the same (symport) or opposite (antiport) direction. This diffusion occurs via ion channels and or carrier proteins, structures that exhibit specificity for the transported molecules. Not only is this process faster than simple diffusion, but is also responsible for providing a pathway for ions and large polar molecules to transverse membranes that would otherwise be impermeable to them. This process uses ion channel proteins that form small aqueous pores across the membrane through which small water-soluble molecules and ions such as chlorine, pass down a concentration gradient. Aquaporins are channels which help in the rapid transport of water across the cell membrane without permitting an accompanying flow of protons to pass through the channels.
Because the cell membrane allows the passage of small, charged and polar molecules through the channel proteins due to difference in the concentration of the molecules inside and outside of the cell membrane, this process takes place on its own due to difference in concentration. Hence there is no requirement of external energy in the form of ATP. Substances having a hydrophilic moiety find it difficult to pass through the membrane. Their movement has to be facilitated. So, facilitated diffusion needs a carrier protein.
So the correct answer is ‘(a) Needs a carrier protein’.

Note: There are two different types of proteins involved in facilitated diffusion. They are channel proteins and carrier proteins. Channel proteins are embedded in the biological membrane and create a channel for transport whereas carrier proteins embed themselves in the molecule they’re transporting.