Animal and plant cells: Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

The idea

Plant and algal cells are eukaryotic cells that, unlike animal cells, possess a cell wall. This wall is made of cellulose, a carbohydrate polymer that forms strong fibres. It lies outside the cell-surface membrane and provides structural support, helping the cell keep its shape and resist bursting when water enters by osmosis. For example, a root hair cell in a plant has a cellulose wall that prevents the cell from bursting in dilute soil water, while still allowing water and mineral ions to pass to the membrane. The wall is fully permeable, so it does not control what enters or leaves; that is the role of the cell-surface membrane. Algal cells also have cell walls, although the wall material can vary between algal species. Recognising the presence of a cell wall helps distinguish plant and algal cells from animal cells in microscopy and in cell models.

What earns the marks

• Plant and algal cells have a cell wall outside the cell-surface membrane.
• The cell wall is made of cellulose.
• Cellulose forms strong fibres that give the wall strength.
• The wall strengthens the cell and helps maintain its shape.
• The wall prevents the cell from bursting when it takes in water by osmosis.
• The wall is fully permeable and does not control entry or exit of substances.
• Animal cells do not have a cellulose cell wall, so this is a distinguishing feature.

Common mistakes and how to put them right

• Confusing the cell wall with the cell-surface membrane.

  Put it right: The wall is a cellulose layer outside the membrane, whereas the membrane is a partially permeable layer that controls exchange.

• Stating that the cell wall is made of protein or chitin.

  Put it right: In plant cells it is cellulose; chitin is found in fungal cell walls.

• Claiming that the cell wall controls what enters or leaves the cell.

  Put it right: It is fully permeable, so it offers little barrier to water or small solutes; the cell-surface membrane regulates transport.

Exam tips

• When labelling a plant cell diagram, draw the cell wall as a thick outer layer and label it clearly as cellulose; place the cell-surface membrane just inside it.
• Use the phrase 'made of cellulose' rather than just 'cell wall' when explaining strength, because the material is part of the specification.
• Link the wall to a named plant cell example, such as a root hair cell or palisade cell, to show how the structure relates to function.