A sea of tiles

The idea

In the fluid-mosaic model, the 7–10 nm thick membrane is a lipid bilayer (water-attracting heads outward, water-repelling tails inward) with embedded gatekeeper proteins; its molecules move sideways, flip and rotate (fluid) and are arranged like mosaic tiles.

Oil and water

Oil and water do not mix. Fat molecules have a part that likes water and a part that avoids it.

In a membrane between two watery places, where would the water-avoiding parts go?

• Inside, away from the water on both sides
• Outside, facing the water
• It makes no difference

Tap the parts

The molecules keep moving. Tap a head, a tail and a protein.

Fluid and mosaic

The membrane is a lipid bilayer: two layers of fat molecules with water-attracting heads outwards and water-repelling tails inwards, with proteins embedded in it. Its molecules can move sideways, flip and rotate, so it is fluid. They are arranged like tiles in a mosaic. It is only 7 to 10 nm thick.

1 nanometre (nm) is 0.000001 mm. The membrane is incredibly thin.

• Part | Role
• Heads | attract water; face outwards
• Tails | repel water; face inwards
• Proteins | gatekeepers for passing substances

Notes

The membrane is a fluid-mosaic: a lipid bilayer (heads out, tails in) with embedded protein gatekeepers. Its molecules move sideways, flip and rotate. It is only 7–10 nm thick.

Check yourself

In the lipid bilayer, the water-attracting heads point…

Why is the membrane called fluid?

Why is it called a mosaic?

What do the proteins in the membrane do?

The cell membrane is 7 to 10 nm thick. What is the greatest thickness, in nm?

Answer: 10 nm

The range is 7 to 10 nm, so the thickest is 10 nm.

• outwards, towards the water — correct. Yes.
• inwards, towards each other. The tails point inwards.
• sideways only. Heads face the watery places on either side.
• Its molecules can move sideways, flip and rotate — correct. Yes.
• It is full of water only. Fluid describes the movement of its molecules.
• It is made of one big rigid sheet. It is not rigid.
• The molecules are arranged like tiles in a mosaic — correct. Yes.
• It is made of glass. A mosaic is a pattern of many small pieces.
• It has only one kind of molecule. It has lipids and proteins.
• Act like gatekeepers, helping substances pass through — correct. Yes.
• Make the nucleus. They are part of the membrane.
• Store water. Their job is to help substances cross.