Fluid mosaic membranes

1. Overview

The cell surface membrane surrounds every cell and controls what enters and leaves it. Its structure is described by the fluid mosaic model.

Two words explain the whole idea:

• Fluid – the phospholipids form a bilayer that can flow sideways, so the membrane is flexible and can self-seal.
• Mosaic – many different molecules (proteins, cholesterol, glycolipids and glycoproteins) are scattered through the bilayer like tiles in a mosaic.

In this topic you will cover:

• how the phospholipid bilayer forms from hydrophobic and hydrophilic interactions;
• the position and role of each membrane component;
• the main stages of cell signalling.

Key Definitions

• Cell surface membrane: the partially permeable membrane that surrounds a cell and controls what enters and leaves it.
• Fluid mosaic model: the accepted model of membrane structure, in which a fluid phospholipid bilayer has proteins and other molecules embedded in it in a constantly changing mosaic pattern.
• Phospholipid: a molecule with a hydrophilic phosphate-containing head and two hydrophobic fatty acid tails, forming the bilayer of membranes.
• Hydrophilic: attracted to water; the phosphate heads face the watery surroundings on each side of the membrane.
• Hydrophobic: repelled by water; the fatty acid tails face inwards, away from water, forming the core of the bilayer.
• Intrinsic (integral) protein: a protein embedded within the bilayer, often spanning it, including channel and carrier proteins.
• Extrinsic (peripheral) protein: a protein attached to the inner or outer surface of the membrane and not embedded in the bilayer.
• Cholesterol: a lipid positioned between the phospholipids that regulates the fluidity and stability of the membrane.
• Glycolipid: a lipid with a carbohydrate chain attached, found on the outer membrane surface and acting in cell recognition.
• Glycoprotein: a protein with a carbohydrate chain attached, found on the outer surface and acting as a receptor or antigen.
• Cell signalling: communication between cells in which a chemical signal (ligand) released by one cell binds to a receptor on a target cell and triggers a specific response.

Content

The phospholipid bilayer

Each phospholipid has a hydrophilic (water-loving) head containing a phosphate group and two hydrophobic (water-hating) fatty acid tails. With water on both sides of the membrane, these two properties drive the phospholipids to arrange themselves automatically into a bilayer:

• the hydrophilic phosphate heads point outwards, towards the watery solutions inside and outside the cell;
• the hydrophobic tails point inwards, towards each other and away from the water, forming the core of the membrane.

These hydrophobic and hydrophilic interactions hold the bilayer together — no chemical bonds are needed between the phospholipids. Because the phospholipids are not bonded to one another, they can move sideways within their own layer, which is why the membrane is fluid.

The hydrophobic core also makes the membrane partially permeable:

• small, non-polar molecules (such as oxygen and carbon dioxide) can dissolve through it;
• large or charged (polar) molecules and ions cannot cross freely.

The arrangement of proteins

The membrane is a mosaic because proteins of different shapes and sizes are scattered throughout the bilayer. There are two arrangements:

Protein type	Position	Examples / role
Intrinsic (integral)	Embedded in the bilayer; many span it from one side to the other	Channel and carrier proteins used in transport
Extrinsic (peripheral)	On the inner or outer surface only, not in the hydrophobic core	Support the membrane, or act as enzymes or receptors

Note that not all membrane proteins go all the way through the membrane — only the transmembrane intrinsic proteins do.

The pattern is called a mosaic because the components are scattered rather than in a regular order, and fluid because they are not fixed in place: the proteins (and other components) can move sideways through the fluid bilayer. When asked to describe the fluid mosaic model, state both ideas — the bilayer arrangement (heads outwards, tails inwards) and that the components drift laterally within it.

Cholesterol, glycolipids and glycoproteins

Three further components complete the membrane:

• Cholesterol molecules fit between the phospholipids in the bilayer. Cholesterol is a lipid, not a phospholipid.
• Glycolipids are lipids with a short carbohydrate chain attached directly to them. The carbohydrate projects from the outer surface.
• Glycoproteins are proteins with a carbohydrate chain attached. The carbohydrate also projects from the outer surface.

The carbohydrate chains of glycolipids and glycoproteins are found only on the outer face of the cell surface membrane, where they are exposed to the cell's surroundings — exactly the position needed for recognition and signalling.

Roles of the membrane components

Each component contributes to one or more functions of the membrane:

• Phospholipids form the basic bilayer. Their fluidity lets the membrane flex and self-seal, and their hydrophobic core makes the membrane partially permeable, blocking large and charged molecules.
• Cholesterol controls fluidity and stability (it acts as a fluidity buffer):
  • At high temperatures – it restrains the moving phospholipids, stopping the membrane becoming too fluid and leaky.
  • At low temperatures – it keeps the phospholipids apart, stopping the membrane becoming too rigid.
  • By filling the spaces between phospholipids it also reduces leakage of water and ions, helping to maintain permeability.
  • Separately from fluidity, cholesterol gives the membrane mechanical stability — it makes the bilayer stronger and less likely to break or tear.
• Channel proteins form water-filled pores that let specific ions and small polar molecules pass through; carrier proteins change shape to move specific molecules across. Both are central to transport.
• Glycoproteins act as cell surface receptors in cell signalling and as cell surface antigens in cell recognition (see 11.1.2), letting cells identify one another and tell "self" from foreign cells.
• Glycolipids also act in cell recognition, helping cells attach to one another and forming part of the markers that identify the cell.

Cell signalling

Cell signalling lets one cell bring about a specific response in another cell. The main stages are:

1. A cell secretes a specific chemical called a ligand (for example, a hormone).
2. The ligand is transported to target cells — often in the blood, so it can reach cells far from where it was made.
3. The ligand binds to a cell surface receptor on the target cell. The receptor is a glycoprotein with a shape complementary to the ligand, so only target cells with the matching receptor respond.
4. Binding triggers a specific response inside the target cell, such as switching on a process or changing the cell's activity.

For cell-surface signalling, the ligand binds to the outside of the receptor and does not cross the membrane — the message is passed into the cell by the receptor, not by the ligand entering. Because only cells with the correct receptor can bind a given ligand, the same signalling molecule can travel round the whole body yet affect only certain target cells.

Worked example

Exam-style question: A hormone is released into the blood and produces a response only in certain target cells, even though it reaches every cell in the body. Using your knowledge of cell signalling, explain how only the target cells respond. [3]

Model answer:

• The hormone acts as a ligand that is transported in the blood to all cells, but only target cells have a cell surface receptor with a shape complementary to the ligand.
• The ligand binds to these receptors (glycoproteins) on the target cells; cells without the matching receptor cannot bind it.
• Binding triggers a specific response inside the target cells, so only they react to the hormone.

Worked example

Exam-style question: In an experiment, the permeability of a cell surface membrane to ions was measured at several temperatures for two membranes: one containing cholesterol and one without. Both membranes leaked more as temperature rose, but the membrane without cholesterol leaked far more at high temperatures. Explain, in terms of the membrane components, why the membrane containing cholesterol leaks less at high temperatures. [4]

Model answer:

• As temperature rises, the phospholipids gain kinetic energy and move about more, so the bilayer becomes more fluid.
• A more fluid bilayer has larger gaps between the phospholipids, so more ions can leak across, increasing permeability.
• Cholesterol sits between the phospholipids and restrains their movement, keeping the membrane from becoming too fluid at high temperatures.
• By filling the spaces between phospholipids, cholesterol reduces leakage of ions, so the membrane with cholesterol stays less permeable.

Key Equations

This is a qualitative topic about membrane structure and function, so there are no equations to learn.

Common Mistakes to Avoid

• Calling a glycolipid a phospholipid with a sugar attached. A glycolipid is a lipid with a carbohydrate chain attached directly to it — it is a separate component from the phospholipids, not a phosphate head with a sugar added.
• Saying the phospholipids are bonded together. The bilayer is held together by hydrophobic and hydrophilic interactions with the surrounding water, not by bonds between the phospholipids; this is why the membrane stays fluid.
• Calling the double layer the "nuclear membrane" when describing membranes generally. Use precise terms: the cell surface membrane has a single bilayer, and the double membrane around the nucleus is the nuclear envelope.
• Putting carbohydrate chains on the inner surface. The carbohydrate parts of glycolipids and glycoproteins project only from the outer surface of the cell surface membrane.
• Confusing channel and carrier proteins. Channel proteins form fixed water-filled pores; carrier proteins change shape to move specific molecules across — name the correct one for transport questions.
• Saying cholesterol "makes the membrane fluid" without qualification. Cholesterol regulates fluidity in both directions: it reduces fluidity at high temperatures and prevents the membrane becoming too rigid at low temperatures.
• Describing cell signalling as "the hormone tells the cell what to do" with no mechanism. Name the stages: ligand secreted, transported, binds a complementary receptor (glycoprotein), specific response triggered.
• Assuming every membrane protein spans the whole membrane. Only transmembrane (intrinsic) proteins such as channel and carrier proteins cross the bilayer; peripheral proteins sit on one surface only, so do not describe all membrane proteins as channels or transporters.
• Saying the ligand "enters the cell" in cell-surface signalling. The ligand binds to the outside of a receptor; it does not cross the membrane, and the receptor is not an enzyme — it passes the signal on without breaking the ligand down.

Exam Tips

• Learn the membrane components as a checklist — phospholipids, cholesterol, proteins (channel and carrier), glycolipids, glycoproteins — and be ready to state the position and the role of each.
• Always link hydrophilic heads to the outside and hydrophobic tails to the inside when explaining why the bilayer forms; this single point answers most "describe the structure" questions.
• Use the word complementary for how a ligand fits its receptor, just as you would for an enzyme and its substrate.
• For "explain the role of cholesterol" questions, give the two-way effect (too fluid vs too rigid) to gain full marks rather than a single statement.
• When a question asks how only certain cells respond to a signal, the key idea is that only target cells carry the matching receptor — state this explicitly.
• Spell the components precisely: distinguish glycolipid (a lipid) from glycoprotein (a protein); mixing them up loses easy marks.