Question

In which situation would passive transport use a transport protein for entry into a cell?

a) Movement of water through aquaporins
b) Facilitated diffusion of ions
c) Simple diffusion of small nonpolar molecules
d) Osmosis of solvents

Answer 1

Passive transport uses a transport protein for entry into a cell when molecules cannot pass through the membrane on their own or when they are polar. Facilitated diffusion of ions is an example of passive transport that requires transport proteins.

Step-by-step explanation:

Passive transport of a molecule or ion depends on its ability to pass through the membrane, as well as the existence of a concentration gradient that allows the molecules to diffuse from an area of higher concentration to an area of lower concentration. Some molecules, like gases, lipids, and water itself (which also utilizes water channels in the membrane called aquaporins), slip fairly easily through the cell membrane; others, including polar molecules like glucose, amino acids, and ions do not. Some of these molecules enter and leave cells using facilitated transport, whereby the molecules move down a concentration gradient through specific protein channels in the membrane. This process does not require energy. For example, glucose is transferred into cells by glucose transporters that use facilitated transport (Figure 26.8).