A protein helps transport a neutral substance from a region of higher concentration to a region of lower concentration. The transport is slow and therefore saturable. This is

(a) passive transport

(b) active transport

(c) facilitated diffusion

(d) simple diffusion

(e) impossible

The correct answer is:

facilitated diffusion (c)

Explanation:

Facilitated diffusion is an assisted transport mechanism where molecules move from areas of higher concentration to areas of lower concentration across a membrane with the help of transport proteins. Molecules that are too big or polar, and cannot cross the hydrophobic lipid bi-layer membrane into the cells are aided (facilitated) through, with the help of transporter proteins, example is permease. Example of molecules that are transported this way is glucose, sodium ions, and chloride ions.

Unlike active transport, this mode of transport does not utilize energy directly from ATP to help transportation, but instead, relies on the interaction between the molecule and the transport protein, where the molecule binds to the transport protein, causing it to change its shape and permit the inward movement of the molecules into the cell. Also, since the movement is down the concentration gradient (higher to lower concentration), energy is not required.

Facilitated diffusion is similar to simple diffusion, but unlike simple diffusion which does not require any specialized media/channels for transportation, it does, and it is saturable with respect to the concentration difference of the substance unlike simple diffusion that has a linear saturation.

Active transport on the other hand, although also requires  transport channel, also use energy directly to enhance transportation, and it is sometimes against the transportation gradient.

protist eukaryotes that are not plants, animals, or fungi ("misfits")