Lesson 7.2: Cell Transport and Homeostasis

Lesson Objectives

  • Identify two ways that molecules and ions cross the plasma membrane.
  • Distinguish between diffusion and osmosis.
  • Identify the role of ion channels in facilitated diffusion.
  • Compare passive and active transport.
  • Identify the connection between vesicles and active transport.
  • Compare endocytosis and exocytosis.
  • Outline the process of cell communication.


Probably the most important feature of a cell’s phospholipid membranes is that they are selectively permeable. A membrane that is selectively permeable has control over what molecules or ions can enter or leave the cell, as shown in Figure below. The permeability of a membrane is dependent on the organization and characteristics of the membrane lipids and proteins. In this way, cell membranes help maintain a state of homeostasis within cells (and tissues, organs, and organ systems) so that an organism can stay alive and healthy.

A selectively permeable membrane allows certain molecules through, but not others.

CK-12 Biology I book:
  • CK-12 Foundation, Barabara Akre, Jean Brainard, Niamh Gray-Wilson, Douglas Wilson
Last modified: Sunday, October 3, 2010, 8:38 PM