4.1 Fluid Mosaic Membranes

Cambridge AS Level Biology (9700)

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe the fluid mosaic model of membrane structure, including how hydrophobic and hydrophilic interactions cause the phospholipid bilayer to form and how proteins are arranged within it.
  2. Describe the arrangement of cholesterol, glycolipids, and glycoproteins in cell surface membranes.
  3. Describe the roles of phospholipids, cholesterol, glycolipids, proteins, and glycoproteins in cell surface membranes — including their effects on stability, fluidity, permeability, transport, cell signalling, and cell recognition.
  4. Outline the main stages of cell signalling: secretion of ligands, transport of ligands to target cells, and binding of ligands to cell surface receptors.

Main Content

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Section 1: The Fluid Mosaic Model

What is the cell surface membrane?

Every cell in your body is surrounded by a very thin layer called the cell surface membrane. This membrane is only about 7 nm thick (7 nanometres — that is 7 billionths of a metre!), which means it is invisible to the naked eye and can only be seen under an electron microscope at very high magnification.

The cell surface membrane does two critical jobs:

  • It separates the inside of the cell from the outside environment.
  • It controls what enters and leaves the cell — it is selectively permeable, meaning it only lets certain substances through.

The Fluid Mosaic Model — What Does It Mean?

In 1972, scientists proposed a model to explain how the cell membrane is structured. It is called the fluid mosaic model. The name tells you two important things:

  • Fluid — the molecules that make up the membrane are not fixed rigidly in place. They can move sideways and drift around, a bit like people moving slowly in a crowd.
  • Mosaic — when you look at the membrane from above, the many different proteins scattered throughout it create a patterned appearance — like a mosaic tile artwork.

Section 2: The Phospholipid Bilayer

What is a Phospholipid?

The membrane is mainly made of molecules called phospholipids. Each phospholipid molecule has two distinct parts:

  • A phosphate head — this part is hydrophilic, which means it loves water (hydro = water, philic = loving). It is polar, meaning it has a slight electric charge, and it mixes well with water.
  • Two fatty acid tails — these are hydrophobic, which means they fear water (phobic = fearing). They are non-polar (no electric charge) and do not mix with water. They are attracted to other non-polar molecules instead.

Think of the phospholipid like a lollipop — a round head on two wavy sticks.

How Does the Bilayer Form?

When phospholipids are placed in water, they automatically arrange themselves to solve a problem: the tails hate water, but the heads love it. So the phospholipids organise themselves so that:

  • The hydrophilic heads face outward — pointing towards the watery environment both inside and outside the cell.
  • The hydrophobic tails face inward — pointing towards each other, away from the water.

This creates a double layer of phospholipids, called a phospholipid bilayer. This arrangement is entirely driven by the hydrophobic and hydrophilic interactions of the two regions — no energy is needed. The bilayer forms spontaneously (on its own) because this arrangement is the most stable.

💡 Think of it like this: Imagine two rows of people standing back-to-back. The people represent phospholipids. Their heads (hydrophilic) face outward, and their backs (hydrophobic tails) are pressed together on the inside. This is the bilayer.

Arrangement of Proteins in the Membrane

Proteins are found embedded within and around the phospholipid bilayer. There are two types:

  • Intrinsic proteins (also called integral proteins) — these are embedded within the bilayer. Most span the entire membrane from one side to the other; these are called transmembrane proteins. Their position in the membrane is determined by their own hydrophilic and hydrophobic regions — the hydrophobic parts sit in the fatty tail region, and the hydrophilic parts are exposed to the watery environment.
  • Extrinsic proteins (also called peripheral proteins) — these sit on the inner or outer surface of the membrane only. They do not go all the way through.

The proteins are scattered throughout the bilayer like icebergs floating in the sea — some move freely, while others may be held in a fixed position.

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