Lesson 1 of 5 / Membranes and transport
A fluid bilayer with many jobs
Why is the membrane more than a phospholipid sheet?
I can explain the fluid mosaic model and the roles of its constituent biomolecules.
I can explain the fluid mosaic model and constituent biomolecule roles.Syllabus
Syllabus 8876, Core 1(h). Explain the fluid mosaic model and constituent biomolecule roles.
About 6 min
Make a guess. You are not marked.
On the cell surface membrane, which side do the carbohydrate chains of glycoproteins face?
Show the answer
The outside, away from the cytoplasm.
They project from the extracellular surface, where they act in cell recognition and adhesion.
The key idea
A 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.
Mark schemes name two kinds of membrane protein. Integral proteins are embedded in the bilayer, and many span it from one side to the other; their hydrophobic regions sit in the hydrophobic core and their hydrophilic regions face the water on each side. Channel and carrier proteins are integral. Peripheral proteins are attached loosely to the surface, to the phospholipid heads or to integral proteins, and do not enter the hydrophobic core. Some receptors and enzymes are peripheral.
The hydrophobic core is the region of fatty-acid tails in the middle of the bilayer. Small non-polar molecules such as oxygen and carbon dioxide dissolve in it and diffuse straight through. Ions and large polar molecules cannot, so they need proteins. Because the phospholipids are always moving, brief (transient) gaps open between them, which lets a small amount of water cross the bilayer.
The fatty-acid tails also set the fluidity. Unsaturated tails have kinks at their carbon-carbon double bonds, so the phospholipids cannot pack tightly and the membrane is more fluid. Saturated tails are straight and pack closely, so the membrane is less fluid. Cholesterol then buffers fluidity across temperatures, as described above.
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.
Watch out for this
A student says: "The carbohydrate chains face the cytosol on both sides of the cell surface membrane." What is wrong with this?
Show the answer
In the cell surface membrane, the carbohydrate chains of glycoproteins and glycolipids project from the extracellular surface.
Check your understanding
Which component provides a selective hydrophilic passage for an ion?
Show the answer
A suitable membrane channel protein.
Its hydrophilic passage allows an otherwise restricted substance to cross.
Quick recall
Why is the membrane more than a phospholipid sheet?