The cell membrane and osmosis

Cells in a tissue work as a team, and even a single-celled organism lives by trading with its surroundings: food and oxygen in, waste out, signals both ways. All of that trading happens at one place, the boundary of the cell.

So before looking inside a cell, start at its edge.

The cell membrane, also called the plasma membrane, is the thin covering around every cell. It keeps the contents together and protected, and it marks out each cell as a separate unit with its own identity.

Its key property is that it is selectively permeable. Think of it as a gate with a guard: some substances are let through, others are turned back. This is how the lining of the alveoli in your lungs can take in oxygen and give out carbon dioxide.

Cut a potato into two pieces of about the same size and weigh both. Drop one into a beaker of plain water (A) and the other into a 20 per cent salt or sugar solution (B). Wait about an hour, then weigh them again.

Run it below. Drag the clock, or press play, and watch the water.

The piece in plain water swells and gets heavier. The piece in salt solution shrinks and gets lighter. The cell membranes of the potato let water pass both ways, but not the salt or sugar.

Water keeps moving from where it is plentiful and the solute is scarce (a dilute solution) to where water is scarcer and the solute is crowded (a concentrated solution), until both sides are equally concentrated. Movement of water across a selectively permeable membrane like this is called osmosis.

You have met diffusion before: particles spreading from a higher concentration to a lower one, like ink in water or a smell in a room. Diffusion needs no membrane. Osmosis is a special case: the diffusion of water through a selectively permeable membrane. This is also how water from the soil gets into the root cells of a plant.

What happens to a single cell depends on how the solute concentration outside it compares with the concentration inside it. Pick a solution and watch the water arrows.

Isotonic: the outside has the same solute concentration as the inside. Water goes in and out equally, so the cell stays the same.

Hypotonic: the outside has a lower solute concentration than the inside. More water enters than leaves, and the cell swells.

Hypertonic: the outside has a higher solute concentration than the inside. More water leaves than enters, and the cell shrinks.

The membrane is astonishingly thin, about 7 to 10 nanometres. It is built from lipids (fats) and proteins, and the picture scientists use for it is the fluid mosaic model.

The lipids form a bilayer, two sheets of special fat molecules. Their water attracting heads face outwards, towards the watery inside and outside of the cell, and their water repelling tails point inwards, away from water.

It is fluid because the molecules are not locked in place: they slide sideways, flip and spin within the membrane. It is a mosaic because the lipids and proteins are fitted together like tiles in a mosaic. Many of the proteins act as gatekeepers that help particular substances across.

Every living cell talks to its neighbours through its membrane. But plant, fungal and bacterial cells have something extra outside the membrane. To see why it matters, compare two slides: a thin peel of an onion leaf or a Rhoeo (cradle lily) leaf stained with safranin, and cells gently scraped from the inside of your cheek stained with methylene blue.

The onion or Rhoeo cells look like neat boxes in rows; the cheek cells are flat and scattered with no fixed pattern. Add a 20 per cent sugar solution and wait half an hour: the plant cells keep their outline while the living contents inside shrink away from it, but the cheek cells shrink as a whole.

The plant cells kept their shape because of a rigid layer outside the membrane, the cell wall. That is the next topic.