Fluid mosaic membranes
The cell membrane
- Every cell is wrapped in a cell surface membrane.
- We describe it with the fluid mosaic model.
- It is partially permeable — it lets some things through and blocks others.
The phospholipid bilayer
- The membrane is built from phospholipids: each has a hydrophilic head and two hydrophobic tails.
- With water on both sides, they line up in two layers — a bilayer — heads facing the water, tails hidden inside.
- This forms by itself because of those water-loving and water-fearing parts.
Practice
In the phospholipid bilayer, the molecules arrange so that:
Heads are attracted to the water on both sides; the tails hide in the middle, away from water.
Fluid and mosaic
- Fluid: the phospholipids slide past each other, so the membrane bends and moves.
- Mosaic: many proteins are dotted through it, like tiles in a picture.
| Part | Role |
|---|---|
| channel proteins | water-filled pores for ions |
| carrier proteins | carry specific molecules across |
| cholesterol | controls fluidity and adds strength |
| glycolipids / glycoproteins | cell recognition; some are antigens |
Practice
Why is the model called "fluid mosaic"?
"Fluid" = the phospholipids move; "mosaic" = proteins scattered through the bilayer like tiles.
Practice
Cholesterol in the membrane mainly:
Cholesterol sits between the phospholipid tails, controlling fluidity and stabilising the membrane.
Practice
Match each membrane component to its role.
Channel = ion pore; carrier = specific transport; glycoprotein = recognition (some act as antigens).
Cell signalling
- Cells talk to each other by cell signalling:
- a cell secretes a signal chemical — a ligand (e.g. a hormone).
- it travels (often in blood) to a target cell.
- it binds a receptor whose shape matches that ligand only — triggering a response inside the cell.
Practice
In cell signalling, a ligand causes a response because it:
The ligand binds a specific receptor (matching shape) on the target cell, triggering a response inside.
You've got it
Key idea
- fluid mosaic model: a fluid phospholipid bilayer (heads out, tails in) with proteins as a mosaic
- channel/carrier proteins transport; cholesterol controls fluidity; glycoproteins = recognition/antigens
- the membrane is partially permeable
- cell signalling: ligand → target cell → matching receptor → response