The cell wall

Cells of plants, fungi and bacteria have an extra layer outside the membrane: the cell wall.

"Why do plants need it?" A plant cannot walk away from wind and rain. It needs a strong frame. The rigid wall keeps leaves and flowers firm and helps the whole plant stand upright.

Don't mix up the wall and the membrane:

• The wall is rigid and permeable: water and some dissolved minerals pass through it freely.
• The membrane, just inside it, is selectively permeable: it chooses what passes.

Working together, they help roots take in water and nutrients.

Compare an onion cell and a cheek cell. Onion (or Rhoeo) cells look like neat boxes in rows. Cheek cells are irregular in shape. Now put both in a 20 per cent sugar solution:

• In the onion cell, the inside shrinks and the membrane pulls away from the wall, but the wall keeps its box shape.
• The cheek cell has no wall, so the whole cell shrinks a lot.

The wall also stops a plant cell bursting when it takes in a lot of water.

Having no rigid wall is useful for animals: their cells can change shape easily, which helps movement.

The plant cell wall is made mainly of cellulose, a carbohydrate built from many glucose units joined in a chain. When we eat vegetables, cellulose is the roughage that helps our digestion.