Lesson 1 of 5 / Membranes and transport
A fluid bilayer with many jobs
Why is the membrane more than a phospholipid sheet?
In this lesson: Explain the fluid mosaic model and the roles of its constituent biomolecules.
About 6 min
The key ideaA mobile phospholipid bilayer forms the barrier; proteins, carbohydrate-bearing molecules and cholesterol give it selective and regulatory functions.
Explore the idea
Locate the membrane components
Heads face water on both sides. Tails occupy the hydrophobic interior, restricting ions and many polar molecules.
Original simplified bilayer model. One highlighted crossing represents a mechanism, not a measured rate. Passive molecules move both ways; the gradient determines net movement.
Explanation
Phospholipid heads face the watery cytosol and extracellular fluid, while hydrophobic tails face each other. This creates a hydrophobic interior that restricts ions and many polar substances. The bilayer is fluid because many lipids and proteins can move laterally; it is not a rigid wall.
Membrane proteins form a mosaic within or on the bilayer. Channel and carrier proteins provide selective transport routes. Other proteins act as receptors, enzymes or anchors. A receptor binds a particular signal; a transport protein moves a substance, so the roles should not be treated as identical.
Glycoproteins and glycolipids bear carbohydrate chains on the non-cytosolic surface. Their exposed patterns contribute to cell recognition, adhesion and signalling. The carbohydrate chains are attached to proteins or lipids, not free threads floating through the bilayer.
Cholesterol fits between phospholipids in animal membranes. It helps regulate fluidity and permeability: it restrains excessive lipid movement at higher temperatures and reduces tight packing at lower temperatures. It does not simply make every membrane either completely rigid or more fluid under every condition.
Step by step
- 1
Start with orientation
Hydrophilic heads face water and hydrophobic tails form the interior.
- 2
Add specific components
Match channels, carriers, receptors and recognition molecules to their roles.
- 3
Explain fluidity
Lateral movement and cholesterol-mediated regulation make the membrane dynamic.
Worked example
An ion meets a bilayer
Why does adding a suitable channel protein greatly increase ion movement through an otherwise intact phospholipid bilayer?
One way to explain it
The ion is charged and crosses the hydrophobic interior poorly. A suitable channel provides a hydrophilic route across the membrane, allowing selective movement down its electrochemical gradient.
Why this answer works
- Use the ion's charge.
- Explain the barrier before explaining the protein's benefit.
Is this true? "The carbohydrate chains face the cytosol on both sides of the cell surface membrane."
In the cell surface membrane, the carbohydrate chains of glycoproteins and glycolipids project from the extracellular surface.