In this module:

Bilayers and Membranes

Because phospholipids have both polar and hydrophobic parts, when they are in water they will spontaneously arrange themselves into ordered structures. In the cell, the most important of these is a bilayer. As the diagram below shows, water is excluded from the center of the bilayer where the fatty acids are, while the polar headgroups have access to the water. Phospholipid bilayers form the basis for cell membranes and all the other organelle boundaries found in cells. Membranes found in cells also contain many other components, such as free fatty acids, proteins, and other lipids.

What sort of molecules, atoms, or ions do you think might be able to diffuse across a phospholipid bilayer? Enter your answer in the space, then click on the Check button to check your answer.

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Since the center of the bilayer is nonpolar, only uncharged molecules, like O₂, H₂O, CO and CO₂, can cross the bilayer. Charged molecules like K⁺, Na⁺, and Cl^- are completely blocked by bilayers. Because of the large number of atoms that would have to be rearranged to allow passage, large molecules like fatty acids, complex carbohydrates, proteins, and nucleic acids are also blocked by bilayers.