The cell membrane and osmosis

Every cell is wrapped in a thin skin called the cell membrane (or plasma membrane). It protects the cell and keeps it separate from its neighbours. It is selectively permeable: like a guard at a gate, it lets some things in and out and stops others.

The potato experiment. Put potato pieces in plain water, and others in a strong (20 per cent) salt or sugar solution.

• In plain water, the potato swells and gets heavier.
• In salt or sugar water, it shrinks and gets lighter.

The membrane lets water through but not salt or sugar. So only the water moves. You can try the same at home with raisins: overnight in water they plump up; in thick sugar syrup they don't.

Diffusion first. Light an incense stick in the prayer corner and soon the whole room smells of it. Particles spread from where there are many of them to where there are few, with or without any membrane.

Osmosis is a special kind of diffusion: water moving through a selectively permeable membrane, from the dilute side (more water) to the concentrated side (more salt or sugar). Simply put: water moves towards the saltier or sweeter side. Roots take in water from the soil by osmosis.

What happens to a cell depends on the liquid around it:

A memory trick: hypo sounds like hippo, and the cell swells up like one.

"What is the membrane made of?" It is only about 7 to 10 nanometres thick, made of lipids (fats) and proteins. Scientists describe it with the fluid-mosaic model. The lipids form two layers (a bilayer): their water-loving heads face outwards and their water-hating tails face inwards. The molecules keep sliding sideways, flipping and turning, so the membrane is fluid; and proteins sit in it like tiles in a pattern, so it is a mosaic. These proteins act as gatekeepers, helping substances across.