Let's explore the structure and function of cell membranes so we understand why bacteria like Staph aureus can make us so sick.

The most abundant molecules in the membrane are lipids, biomolecules that are insoluble in water. Fatty acids, phospholipids, and steroids like cholesterol are all lipids. The most abundant lipids in membranes are phospholipids. Their hydrophilic heads are more stable in contact with water, while their hydrophobic tails are more stable when they pack together to keep water away. Since both the extracellular fluid and the cytoplasm are aqueous, the hydrophobic tails can best avoid contact with water if the phospholipid molecules form a double layer, or bilayer. This means the hydrophilic heads end up on the exterior of the bilayer, facing the aqueous environment.

At normal human body temperature, about 37 degrees Celsius, the hydrocarbon tails of membrane phospholipids are pretty mobile. But when cholesterol, another important membrane lipid, is inserted between the phospholipids, it alters the mobility of these molecules in the membrane.

Proteins are also part of the cell membrane. Like phospholipids, some proteins also have hydrophilic and hydrophobic regions. There are two classes of membrane proteins: integral and peripheral. Integral membrane proteins lie in the membrane with their hydrophobic region surrounded by the nonpolar chains of the membrane phospholipids. The hydrophilic regions extend out of the membrane. Integral proteins that span the membrane are called transmembrane proteins. Peripheral membrane proteins aren't embedded in the membrane like integral membrane proteins. Instead, they're held in place by interactions with the inner membrane surface or with integral membrane proteins.

Some membrane proteins and lipids have sugars attached to them. The branchlike sugars of these glycoproteins and glycolipids are only present on the outside of the cell. Because the inner and outer lipid layers are different, cell membranes are asymmetric.

Together the lipids and proteins make up the fluid mosaic model of cell membrane structure. The "mosaic" part of the name comes from the many different molecules in a membrane, just like a tile mosaic contains many different colors. "Fluid" means that molecules move around rather than remain stationary. Membranes are fluid because the hydrophobic tails of the phospholipids are not tightly bound to each other. The lipids and many membrane proteins are free to move laterally in the plane of the membrane.

Let's see how well you understand the fluid mosaic model. Select the image that shows the correct orientation of an integral membrane protein by clicking on it. Click Submit to check your answer. Click Jump Ahead to skip this step.

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Sorry, that's not right. This is a peripheral membrane protein. Integral membrane proteins have at least part of their structure in contact with the hydrophobic lipid side chains.

Sorry, that's incorrect. The interior of the membrane is hydrophobic, so the hydrophilic parts of the protein won't be found there.

Not exactly. You're correct that the protein is inserted in the membrane, but it won't be very stable when the hydrophobic amino acids are exposed to the aqueous extracellular fluid.

This is how integral membrane proteins are oriented. A protein with hydrophilic and hydrophobic regions will span the membrane. The hydrophilic regions are in the extracellular fluid and the cytosol, and the hydrophobic region is in contact with the membrane lipids.

To see how the fluid mosaic model explains the relationship between the structure and function of a membrane, let's return to Staph. aureus. The toxic protein secreted by Staph. aureus is called alpha-toxin. So how do alpha-toxin proteins damage membranes? Several copies of this integral membrane protein can end up in each cell membrane. Membrane fluidity enables proteins to aggregate, or come together, in groups. A cluster of 7 Staph. aureus alpha-toxin molecules forms a channel or hole in the membrane. What happens to the cells after this type of damage? Let's move on to membrane function and see.

The most basic function of cell membranes is to provide a barrier between cells or organelles and their environment. If cells were built like fortresses, they'd be protected from their environment, but would die from isolation. Instead, membranes are built to control the movement of substances in and out of cells. Membrane proteins are the doors and windows of the cell membrane. They work in a variety of ways to allow the passage of specific molecules across membranes.

What happens when cells lose control of traffic across the membrane? We can get an idea by looking at cells that have been damaged by Staph. aureus alpha-toxin. Small molecules and ions enter and exit the cells through the pores formed by the toxin. This alters the normal concentrations of these substances inside the cells, and the cells die. It's clear that the integrity of the membrane is crucial to the cell's vitality.

In the next section, we'll focus on several ways that cells control substance movement, then later we'll explore some additional functions of membranes.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education