Final answer:
The transfer of molecules in and out of a cell depends on the ability of the molecules to pass through the cell membrane and the presence of a concentration gradient. Some molecules can pass through easily, while others require facilitated transport using protein channels. The process of molecule transfer does not require energy.
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).