2.1.5(a)-(b) - Membrane roles and the fluid mosaic model

Why Membranes Matter

A biological membrane is a thin structure built mainly from a phospholipid bilayer with other molecules embedded in or attached to it. The cell-surface membrane, also called the plasma membrane, surrounds the cell. Eukaryotic cells also have membranes around organelles and membranes inside some organelles.

The first OCR role is as a partially permeable barrier. At the surface of a cell, the membrane separates the cytoplasm from the external environment. Around an organelle, a membrane separates the organelle contents from the cytoplasm. Within organelles, folded or stacked membranes can create separate reaction surfaces and spaces.

This barrier role is not the same as being impermeable. If a membrane were completely impermeable, cells could not take in required substances, remove waste products or respond to their surroundings. The key idea is controlled separation: the membrane keeps regions distinct while making selective movement possible. The exact movement mechanisms come in later lessons.

The second role is as a site of chemical reactions. Many reactions happen at membrane surfaces because enzymes and other proteins can be held in place there. For example, membranes inside mitochondria and chloroplasts provide organised surfaces for later energy-transfer processes; you do not need the details of those pathways in this lesson.

The third role is cell communication. A cell can detect chemical signals when molecules bind to membrane components on its surface. This is why membranes matter to coordination in a multicellular organism and to the action of some drugs.

Fluid Mosaic Model

The fluid mosaic model is the OCR model for membrane structure. It explains the membrane as a phospholipid bilayer with a mixture of other components, including proteins, glycoproteins, glycolipids and cholesterol.

The word "fluid" means the membrane is not a rigid sheet. Phospholipids and many membrane proteins can move sideways within the plane of the membrane. This gives the membrane flexibility and allows components to be arranged for different functions.

The word "mosaic" means the membrane contains a varied pattern of molecules. A membrane is not made from phospholipids alone. Proteins may span the bilayer or sit at one surface, carbohydrate chains may be attached to proteins or lipids, and cholesterol may sit among the phospholipid tails.

The model does not claim every membrane has the same proportions of components. In a correct bilayer, heads face the aqueous regions and tails face each other inside; carbohydrate chains project towards the extracellular or organelle-lumen side. The phospholipid bilayer gives the basic barrier. The embedded or attached components give more specific functions such as recognition, enzyme activity, signalling and regulated movement. Transport mechanisms are a later lesson; here the important point is that membrane proteins are part of the mosaic.
[DIAGRAM: fluid_mosaic_model: Fluid mosaic model of an animal cell-surface membrane; asset_slug: 041_m02_1_5_membrane_roles_and_the_fluid_mosaic_model__diagram_01; recommended_method: drawn_biology; description: Phospholipid heads face water and tails face inward, with cholesterol between tails, membrane proteins and extracellular carbohydrate chains.]

Component-Function Links

Strong OCR answers link each component to its role. A list of names is rarely enough; the mark often comes from explaining what the component does in the membrane.

Phospholipids are amphipathic: each molecule has a hydrophilic head and hydrophobic tails. In water, the hydrophilic heads face the aqueous solution outside the cell or in the cytoplasm, while the hydrophobic tails point away from water. This creates a bilayer with a hydrophobic core. The hydrophobic core restricts charged ions and many water-soluble molecules, whereas small lipid-soluble/non-polar molecules can cross more readily. It is therefore a selective barrier, rather than a free opening between the solutions.

Cholesterol should be described carefully. It is a membrane component required by OCR, and in animal cell membranes it helps regulate fluidity and permeability. It is not a separate layer, and it is not the same as a phospholipid. A safe answer is: cholesterol fits between phospholipid tails and helps stabilise membrane fluidity.

Proteins are the most varied membrane components. Some are intrinsic proteins, meaning they are embedded in the bilayer; a channel protein is one example. Other proteins are associated with one surface. Proteins can act as enzymes, receptors, channels or carriers. In this lesson, you only need the component-function idea, not the full mechanisms of transport across membranes.

Glycolipids and glycoproteins both involve carbohydrate chains projecting from the membrane. The difference is the molecule they are attached to: a glycolipid is carbohydrate attached to a lipid, while a glycoprotein is carbohydrate attached to a protein. These carbohydrate chains are useful for recognition between cells and for receptors at the cell surface.

Cholesterol buffers fluidity: at higher temperatures it restricts excessive phospholipid movement, while at lower temperatures it interferes with close packing of the tails. This prevents an oversimplified claim that cholesterol always increases, or always decreases, fluidity. Its small polar region lies near the heads and most of the molecule lies among the hydrophobic tails.

Receptors And Signalling

Cell signalling depends on a signal being detected by a target cell. A membrane-bound receptor is a protein, often a glycoprotein, with a binding site for a particular molecule.

Hormones are chemical messengers. For a hormone that acts through a membrane-bound receptor, only cells with a complementary receptor respond to it. Some other hormones act through intracellular receptors; this lesson focuses on the membrane-bound examples required by the specification. Binding to the receptor can start a response inside the target cell. You do not need the full signalling pathway here; the required idea is receptor binding at the membrane.

Some drugs act in the same general way. A drug molecule may bind to a membrane-bound receptor and change the activity of the target cell. This does not mean the drug must always enter the cell. It may act by binding to a receptor at the cell-surface membrane.

This is a useful place to avoid a common misconception: a receptor is not simply a hole in the membrane. A receptor is a specific binding site, usually on a protein. A channel may allow movement of substances across the membrane, but receptor binding is about detecting a signal.

Model, Scale And Precision

The fluid mosaic model is a scientific model. It simplifies a very small, dynamic structure so that it can explain several observations at once: membranes are flexible, they separate aqueous regions, they contain proteins and they allow specific communication through receptors.

When asked to describe the fluid mosaic model, include both parts of the phrase: "fluid" means components can move within the membrane, and "mosaic" means different components are scattered through the bilayer.

Membrane dimensions require very small units and standard form. A plasma membrane is often only a few nanometres thick. Since 1 nm is $1 \times 10^{-9}$ m, a membrane thickness of 7.5 nm is:

The biological interpretation is more important than the number itself: membranes are far too thin to resolve as detailed structures with a light microscope, so the model is an explanation of molecular organisation, not something you would see directly in an ordinary school light microscope.

A typical light microscope resolves structures about 200 nm apart. A 7.5 nm membrane is roughly 27 times thinner than that limit: magnifying a blurred image cannot recover its molecular detail. Electron microscopy and other experimental evidence inform the molecular model.