Topic 2.3 Notes – Plasma Membrane

1. What the Plasma Membrane Is

Every cell has a plasma membrane that separates the inside of the cell from the outside environment. Its main job is maintaining homeostasis, which means keeping internal conditions stable even when the outside changes.

The membrane is selectively permeable. Some substances cross easily, others cannot cross at all without help.

It also:

• Regulates transport in and out
• Allows cells to communicate
• Helps cells recognize each other
• Provides structural support

The key idea you’ll see over and over is this:

Structure determines function.

How the membrane is built explains everything it can do.

2. Components of the Plasma Membrane

a. Phospholipids and the Bilayer

Phospholipids form the structural foundation.

Each phospholipid is amphipathic, meaning it has two different regions:

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

In water, they arrange themselves into a bilayer:

Phospholipid bilayer structure

Heads face the watery cytosol and extracellular fluid, as labeled in the diagram. Tails face inward toward each other.

This creates a hydrophobic interior barrier.

What that means functionally:

• Small nonpolar molecules (O₂, CO₂) pass easily
• Small uncharged polar molecules (like water, slowly) may pass
• Ions and large polar molecules cannot cross without help

That hydrophobic core is the physical basis of selective permeability.

b. Membrane Proteins

Proteins are embedded within or attached to the bilayer. Their structure matches the membrane’s structure.

Integral (Transmembrane) Proteins

These span the entire membrane.

• Hydrophobic amino acids face outward toward the fatty acid tails
• Hydrophilic amino acids face the cytosol, extracellular fluid, or line channels

They function in:

• Transport (channels, carriers, pumps)
• Receptors for signaling molecules
• Cell adhesion

Peripheral Proteins

• Loosely attached to the membrane surface
• Often connected to integral proteins or phospholipid heads
• Involved in signaling or structural support

A common exam move is giving you a mutated protein and asking why it no longer embeds properly. If hydrophobic regions are altered, it may not interact correctly with the lipid tails.

c. Steroids and Cholesterol

In animal cells, cholesterol sits between phospholipids.

It acts as a fluidity buffer:

• High temperature → reduces excessive movement
• Low temperature → prevents tight packing

It stabilizes the membrane so it doesn’t become too rigid or too fluid.

d. Glycoproteins and Glycolipids

These are carbohydrates attached to proteins or lipids on the extracellular surface.

Together they form the glycocalyx.

Functions:

• Cell recognition (like blood type markers)
• Cell-to-cell adhesion
• Immune identification
• Protection

On exams, if immune cells recognize “self” vs “non-self,” think membrane carbohydrates.

3. The Fluid Mosaic Model

Singer and Nicolson proposed this model in 1972.

It describes the membrane as:

Fluid

• Phospholipids move laterally
• Many proteins also drift within the membrane

This movement allows:

• Self-repair
• Redistribution of proteins
• Vesicle fusion

Mosaic

• Patchwork of lipids, proteins, steroids, carbohydrates
• Proteins “float” in a sea of lipids

The diagram below shows the phospholipid bilayer with embedded integral and peripheral proteins, cholesterol within the hydrophobic interior, and carbohydrate chains attached to proteins and lipids on the outer surface.

Fluid mosaic model of the plasma membrane

The membrane is dynamic, not rigid. That dynamic quality is essential for transport and signaling.

4. Membrane Fluidity

Fluidity depends on three main factors:

Temperature

• Higher temperature → more fluid
• Lower temperature → less fluid

Fatty Acid Composition

• Unsaturated tails (double bonds, kinks) → prevent tight packing → increase fluidity
• Saturated tails (straight) → pack tightly → decrease fluidity

Cholesterol

Buffers extremes in fluidity.

If a question asks how a cell adapts to cold, increasing unsaturated fatty acids is a classic answer.

5. How the Plasma Membrane Maintains Homeostasis

Selective Permeability

Substances that cannot diffuse require membrane proteins.

Transport and Communication

Membrane proteins allow:

• Channel proteins → create hydrophilic pores
• Carrier proteins → change shape to move molecules
• Pumps → use ATP to move substances against gradients
• Receptor proteins → bind signaling molecules
• Cell adhesion molecules → connect cells

Membrane Dynamics

Because it’s fluid, the membrane can:

• Form vesicles in endocytosis
• Fuse with vesicles in exocytosis
• Merge membranes during cell processes

Without fluidity, none of that would work.