Topic 2.4 Notes – Membrane Permeability

Selective permeability and the hydrophobic barrier

Every cell needs to keep its internal environment different from the outside. The plasma membrane makes that possible by separating the cytoplasm from the external environment.

Selective permeability means:

• Some molecules cross easily.
• Others cross slowly or not at all.
• Some require specific proteins.

The reason comes down to the membrane’s hydrophobic interior.

Inside the membrane, the fatty acid tails of phospholipids form a nonpolar core. Nonpolar substances can dissolve in this region. Polar and charged substances cannot.

So the membrane does not actively “block” most molecules. Its structure naturally creates a barrier.

Big connection:
Structure → hydrophobic core → selective permeability → homeostasis

On quizzes, you’ll often be asked to explain permeability in terms of structure, not just state what can cross.

What the plasma membrane is made of

Phospholipid bilayer

Phospholipids are amphipathic. That means they have two different regions:

• Hydrophilic (polar) phosphate head → interacts with water
• Hydrophobic (nonpolar) fatty acid tails → avoid water

In water, they form a bilayer with heads facing the extracellular and intracellular fluids and tails tucked toward each other in the center.

Phospholipid bilayer structure

That inward-facing tail region is the hydrophobic barrier that determines permeability.

Membrane proteins

Proteins are embedded throughout the bilayer. Their placement depends on their amino acids:

• Hydrophobic regions sit in the membrane interior.
• Hydrophilic regions face water.

Important types for permeability:

• Channel proteins
  • Form hydrophilic tunnels
  • Allow specific ions or polar molecules to move across
  • Passive transport
• Transport proteins (carriers/pumps)
  • Bind substances and change shape to move them
  • Some use ATP (active transport, covered more in 2.5)

Other membrane proteins you should recognize:

• Receptor proteins → receive signals
• Adhesion proteins → attach cells together
• Cell surface markers (glycoproteins) → cell ID

For this topic, channel and transport proteins matter most because they allow hydrophilic substances to bypass the hydrophobic core.

Fluid mosaic model

The membrane is described as a fluid mosaic:

• Fluid → phospholipids and many proteins move laterally.
• Mosaic → mixture of lipids and proteins embedded together.

This flexibility allows proteins to function and the membrane to self-repair.

What can and cannot cross the membrane

Permeability depends on size, polarity, and charge.

Freely cross

Small, nonpolar molecules dissolve in the hydrophobic interior:

• O₂
• CO₂
• N₂

They cross by simple diffusion.

Cross slowly in small amounts

Small, polar, uncharged molecules:

• H₂O
• NH₃

They can slip through the bilayer, but slowly. In cells, water mainly uses aquaporin channel proteins for efficient movement.

Cannot cross without proteins

• Ions (Na⁺, Cl⁻, Ca²⁺)
• Large polar molecules (glucose)

The nonpolar hydrocarbon tails repel these substances. They require channel or transport proteins.

Here’s a clean comparison:

A common AP move is giving you a new molecule and asking you to predict movement based on polarity and charge. Always think about the hydrophobic core.

The role of the cell wall

Some organisms have a cell wall outside the plasma membrane.

Found in:

• Plants → cellulose
• Fungi → chitin
• Bacteria → peptidoglycan
• Archaea → other polymers

Animal cells do not have cell walls.

Functions:

• Structural support → maintains shape
• Protection from osmotic lysis → prevents bursting in hypotonic environments
• Additional permeability barrier → filters some substances before they reach the membrane

In plant cells, you can see the thick cell wall forming the rigid outer boundary, with the plasma membrane just inside it.

Labeled plant cell showing the cell wall outside the plasma membrane

In a hypotonic environment, water enters the cell. The plasma membrane allows water movement, but the rigid cell wall prevents overexpansion and bursting.

So the membrane controls molecular movement. The wall provides structural protection and an extra layer of filtering.