1Phospholipids: two ends, two behaviours

A phospholipid has two regions.

• The phosphate head is polar, and often charged. It is hydrophilic: it attracts water.
• The two fatty acid tails are long nonpolar hydrocarbon chains. They are hydrophobic: water does not attract them, and they cluster away from it.

Both sides of a plasma membrane are watery: the cytosol inside and the extracellular fluid outside. So phospholipids arrange themselves into two layers. The heads of the outer layer face the external fluid; the heads of the inner layer face the cytosol. The tails of both layers point inward and face each other, forming a nonpolar core with no water in it. No energy is needed for this: it is the arrangement in which water molecules can make the most hydrogen bonds with each other and with the heads.

That nonpolar core is what makes the membrane a barrier, which 2.4 develops.

2Membrane proteins: positioned by their side groups

Proteins set into the membrane are made of amino acids whose side groups can be:

• hydrophilic: polar or charged side groups;
• hydrophobic: nonpolar side groups;
• and most membrane proteins have both, in different regions.

The rule for where each region goes is the same as for phospholipids.

• Hydrophobic regions form the protein surface that touches the fatty acid tails in the interior of the membrane. Nonpolar meets nonpolar.
• Hydrophilic regions are either exposed to the cytosol (or the external fluid), or tucked inside the protein, for example lining a water-filled pore through which ions can pass.

So a protein that crosses the membrane once typically has a stretch of nonpolar amino acids, long enough to span the core, with stretches of polar and charged amino acids on each side of it. A protein made entirely of hydrophilic regions cannot sit in the core; it can only attach to the surface.

The structure decides the job. A channel protein, for instance, has a hydrophobic outer surface that holds it in the bilayer, and a hydrophilic tunnel through its middle that lets particular ions cross (2.6).

3The rest of the mosaic

Besides phospholipids and proteins, the membrane contains:

• Steroids, such as cholesterol in the membranes of vertebrate animals. Cholesterol sits among the phospholipid tails and helps keep the membrane's fluidity within a working range as temperature changes.
• Glycoproteins: proteins with carbohydrate chains attached.
• Glycolipids: lipids with carbohydrate chains attached.

The carbohydrate chains of glycoproteins and glycolipids face the outside of the cell. They act as identification tags, so cells can recognise each other, which matters in development and in the immune system (Unit 4).

4The fluid mosaic model

The fluid mosaic model describes the membrane as a framework of phospholipids with proteins, steroids, glycoproteins and glycolipids embedded in it.

• Mosaic, because the membrane is a patchwork of many different molecules.
• Fluid, because those molecules are not locked in place. Phospholipids move sideways within their layer constantly, and many proteins drift through the bilayer like boats on a pond.

Figure 1 is the model. Find each part: phospholipids with heads out and tails in; integral proteins crossing the bilayer, including a channel; a peripheral protein on the inner surface; cholesterol among the tails; carbohydrate chains on the glycoprotein and the glycolipid, on the outside.

Hydrophilic parts face water; hydrophobic parts face each other. Every position in the membrane follows that rule.

5Evidence that the membrane is fluid

A model is only as good as its evidence. One classic kind of experiment fuses two cells whose surface proteins carry different fluorescent labels, then watches what happens. An invented version: a mouse cell with proteins labelled green is fused with a human cell with proteins labelled red, and the fused cells are kept at 37 °C or 5 °C.

At first the two colours sit on opposite halves of the fused cell. At 37 °C they mix within 40 minutes, so the proteins must move sideways through the membrane. At 5 °C they barely mix, because the membrane is far less fluid in the cold. The data support the "fluid" part of the model.