The cell wall

A coat outside the membrane

A tender stem bends in the wind and springs back. A dry coconut shell is hard enough to crack a tooth. Both owe their strength to the cell wall.

The cell wall is a non-living, rigid structure. It forms an outer covering outside the plasma membrane of fungi and plants. Animal cells have no cell wall.

What the wall does

The cell wall has four main jobs.

• It gives the cell its shape.
• It protects the cell from mechanical damage and from infection.
• It helps in cell-to-cell interaction.
• It acts as a barrier to large molecules that the cell does not want.

The wall is not a sealed box, though. Water and small molecules pass through it freely. The plasma membrane inside it still decides what enters the cell.

What the wall is made of

The make-up of the wall differs between groups.

• Algae: a wall of cellulose, galactans and mannans, with minerals such as calcium carbonate.
• Other plants: a wall of cellulose, hemicellulose, pectins and proteins.
• Fungi: the wall is made largely of chitin, the same tough material found in the shells of insects.

Cellulose is common to the walls of algae and land plants. Its long fibres give the wall its great strength.

Primary and secondary walls

The wall of a young plant cell is the primary wall. It is thin and stretchy, so it can grow as the cell grows. As the cell matures, its power to grow slowly fades.

In many cells, a secondary wall then forms. It is laid down on the inner side of the primary wall, towards the membrane. It is thick and strong, as in the cells of wood.

Holding cells together, and joining them

Between the walls of two neighbouring cells lies the middle lamella. It is a layer made mainly of calcium pectate. It glues neighbouring cells together.

The walls and the middle lamella are not unbroken. They are crossed by fine strands of cytoplasm called plasmodesmata. These connect the cytoplasm of neighbouring cells, so materials and signals can pass from cell to cell.

You should find that a cell $30$ µm across, touching neighbours on $4$ faces, shares $3600$ µm² of wall with them. At one channel in each square micrometre, it has $3.6$ thousand plasmodesmata. These numbers are a model. Real densities vary from one tissue to another.

Try it in Python

The program keeps the parts of two walls in sets, and asks which parts they share.

You should see cellulose in both walls. The fungus shares nothing with the land plant here, so Python prints an empty set, set().

Practise with new numbers

Common mistakes

• Giving animal cells a wall. Only plants, fungi, algae and most bacteria have one; animal cells have only the plasma membrane.
• Calling the wall living. The cell wall is non-living, made by the cell outside its membrane.
• Putting the secondary wall outside the primary wall. It forms on the inner side, towards the membrane.
• Saying the middle lamella is cellulose. It is mainly calcium pectate.

Remember these

• Wall jobs: shape, protection from damage and infection, cell-to-cell interaction, a barrier to unwanted large molecules.
• Algae: cellulose, galactans, mannans, calcium carbonate. Land plants: cellulose, hemicellulose, pectins, proteins.
• Primary wall: young and able to grow. Secondary wall: inside it, towards the membrane.
• Middle lamella: calcium pectate, gluing cells. Plasmodesmata: strands joining the cytoplasm of neighbours.

Check yourself

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

1. Name the two groups of living things in which the cell wall lies outside the plasma membrane, as described in this lesson.

2. Which three substances are found in the walls of algae besides cellulose?

3. Why can a young plant cell grow bigger, while an old woody cell cannot?

4. What would happen to a piece of plant tissue if its middle lamella were dissolved?

5. How can a sugar made in one leaf cell reach the next cell, though both have walls?