Concept preview

What you will be able to do

1. 01Describe the phospholipid bilayer and explain why it self-assembles in water.
2. 02Distinguish integral (including transmembrane) from peripheral proteins and state the roles of channel, carrier, receptor, enzyme and recognition proteins.
3. 03Explain how cholesterol regulates membrane fluidity.
4. 04Explain how glycoproteins and glycolipids allow cell recognition.
5. 05Relate membrane structure to selective permeability.

Every one of your cells is wrapped in a boundary only a few nanometres thick — the membrane is only about $5\text{ nm}$ across. This boundary is the plasma membrane (also called the cell surface membrane), and it is far more than a passive bag. It decides what may enter and leave the cell, it carries the sensors that let the cell listen to its surroundings, and it holds the identity tags that mark the cell as belonging to a particular tissue and a particular organism. To understand how a cell lives, you must first understand the extraordinary sheet of molecules that forms its skin.