The cell membrane

The boundary of every cell

Every cell, from a bacterium to a nerve cell, is wrapped in a thin cell membrane, or plasma membrane. It is far too thin to see with a light microscope. Yet it decides what enters the cell and what leaves.

Think of the gate of a busy school. It lets students in, keeps strangers out, and opens for deliveries at fixed times. The cell membrane does the same work for the cell.

What the membrane is made of

The first detailed study of the membrane came from the human red blood cell, or erythrocyte. These cells have no nucleus and no organelles. So when they are burst, the membrane can be collected almost pure.

These studies showed that the membrane is mainly made of lipids arranged in two layers, a bilayer.

• Each lipid has a polar head, which likes water, and hydrophobic tails, which avoid it.
• The heads face outwards, towards the watery fluid on either side. The tails point inwards, to the middle of the membrane.
• The tails are therefore shielded from water.
• Most of the lipid is phosphoglyceride. Cholesterol is also present.

Later studies found proteins and carbohydrates in the membrane too. The ratio of protein to lipid differs a great deal from one kind of cell to another. In the human erythrocyte, the membrane is about 52 per cent protein and 40 per cent lipid.

The proteins are of two kinds.

• Integral proteins are buried in the membrane, partly or right through it.
• Peripheral proteins lie on the surface of the membrane only.

The carbohydrates are short chains joined to proteins or lipids on the outer surface.

The fluid mosaic model

In 1972, Singer and Nicolson proposed the model that is widely accepted today: the fluid mosaic model.

• The lipid layer is quasi-fluid: it behaves partly like a liquid.
• So the proteins can move sideways within the bilayer. This is called lateral movement.
• The proteins are dotted through the lipid like the tiles of a mosaic, hence the name.

The fluid nature of the membrane matters. It is needed for cell growth, for forming junctions between cells, for secretion, for endocytosis (taking in material) and for cell division.

Moving things across the membrane

The membrane is selectively permeable: some molecules cross it, others do not.

Passive transport needs no energy from the cell.

• In simple diffusion, neutral solutes move from a region of higher concentration to one of lower concentration.
• Water also moves by diffusion. The movement of water across a membrane is called osmosis. Water moves towards the side where the solutes are more concentrated.
• Polar molecules cannot easily cross the non-polar lipid layer. They need a carrier protein to pass.

Active transport carries ions or molecules against their concentration gradient, from lower to higher concentration. This needs energy, supplied by ATP.

The best-known example is the Na+/K+ pump. It moves sodium ions out of the cell and potassium ions into it. Each turn of the pump uses one ATP. It carries three sodium ions out and two potassium ions in.

You should find that $20$ thousand pumps turning $50$ times a second for $5$ seconds use $5$ million ATP. They carry $15$ million sodium ions out and $10$ million potassium ions in. More ions go out than come in, so the inside of the cell becomes negative.

Try it in Python

The program stores the make-up of the red blood cell membrane and works out the rest, and the protein for each gram of lipid.

You should see $8$ per cent left over, and $1.3$ g of protein for each gram of lipid. For myelin the ratio falls to about $0.23$ g. That membrane is mostly lipid, which suits its job as an insulator.

Practise with new numbers

Common mistakes

• Saying the tails face the water. The polar heads face the water on both sides; the hydrophobic tails face each other.
• Thinking the proteins are fixed in place. In the fluid mosaic model they can drift sideways in the lipid.
• Calling osmosis a kind of active transport. Osmosis is the passive movement of water; it uses no ATP.
• Mixing up the pump's numbers. It moves three sodium ions out and two potassium ions in for each ATP.

Remember these

• The membrane is a lipid bilayer, mostly phosphoglycerides, with cholesterol, proteins and carbohydrates.
• Integral proteins are buried in it; peripheral proteins sit on its surface.
• Fluid mosaic model: Singer and Nicolson, 1972; a quasi-fluid lipid with proteins that move sideways.
• Passive transport needs no energy; active transport moves substances against the gradient and uses ATP.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why were red blood cells chosen for the first studies of the cell membrane?

2. In the bilayer, which part of each lipid faces the watery fluid?

3. What is the difference between an integral and a peripheral protein?

4. Give two processes of the cell that depend on the fluidity of the membrane.

5. Is osmosis active or passive? Explain.