Structure of the Plasma Membrane

The fluid mosaic model

Content standard 3.1 introduces the fluid mosaic model, which describes the plasma membrane as a flexible, fluid double layer of phospholipid molecules with proteins scattered throughout it, like tiles in a mosaic. The phospholipids and many proteins can drift sideways within the layer, which is what makes the membrane fluid rather than rigid.

Components of the membrane

• Phospholipid bilayer, each phospholipid has a hydrophilic (water-attracting) phosphate head facing the watery environment on both sides, and two hydrophobic (water-repelling) fatty acid tails facing each other in the middle.
• Integral proteins, embedded within or spanning the whole bilayer; some form channels or carriers for transport, others act as receptors.
• Peripheral proteins, attached loosely to the inner or outer surface of the membrane rather than embedded in it.
• Cholesterol, sits between the phospholipids and helps regulate membrane fluidity and stability.
• Glycoproteins and glycolipids, proteins and lipids with attached carbohydrate chains on the outer surface, involved in cell recognition and signalling.

Why the arrangement matters

The hydrophilic heads and hydrophobic tails automatically arrange themselves into a bilayer in a watery environment, which is why the membrane forms spontaneously and reseals itself if punctured. Because the core of the membrane is hydrophobic, water-soluble substances and ions cannot pass through it freely, they need specific protein channels or carriers.

This selective barrier is what makes the membrane partially (selectively) permeable.

How it is examined

Exam questions often give a labelled or unlabelled diagram of the fluid mosaic model and ask you to name a component (such as the phospholipid head, tail, or a specific protein) and state its role. You may also be asked to explain why the membrane is described as 'fluid' and as 'partially permeable', or to link a membrane protein's function to how a substance crosses the membrane.

Worked exam-style question

Question. The diagram shows part of the plasma membrane based on the fluid mosaic model. Structure X spans the full width of the phospholipid bilayer and forms a channel through its centre.

Structure Y is a small molecule found scattered between the phospholipid tails. Structure Z is a protein on the outer surface with a branching carbohydrate chain attached to it.

(a) Name structure X, Y and Z. (b) State one function of structure X.

(c) Explain why structure Z is important for the cell. (d) Explain why the plasma membrane is described as 'fluid'.

Model answer. (a) X is an integral (channel) protein; Y is cholesterol; Z is a glycoprotein. (b) Structure X may act as a channel that allows specific ions or molecules to cross the membrane, or as a carrier protein involved in active transport.

(c) The carbohydrate chain on structure Z allows the cell to be recognised by other cells, which matters for cell signalling and for the immune system to identify the body's own cells. (d) The membrane is fluid because the phospholipids and many proteins are not fixed in place and can drift sideways within the layer, giving the membrane a flexible, constantly shifting structure rather than a rigid one.

Practice question

Try this. A plant cell placed in a concentrated (hypertonic) sugar solution loses water and its cytoplasm shrinks away from the cell wall, a condition called plasmolysis. Using the structure of the plasma membrane, explain (a) why water is able to leave the cell so readily while the dissolved sugar cannot enter freely, and (b) what would happen to the same cell if it were placed in a hypotonic solution instead.

Exam tip

Key terms

• Diffusion, net movement of particles from high to low concentration; small non-polar molecules can diffuse directly across the hydrophobic core of the membrane.
• Osmosis, the movement of water across a partially permeable membrane, made possible by the membrane's structure.
• Active transport, movement of substances against their concentration gradient using energy, carried out by specific carrier proteins embedded in the membrane.
• Hypotonic solution, a solution with a lower solute concentration than the cell, causing water to move into the cell across the membrane by osmosis.
• Hypertonic solution, a solution with a higher solute concentration than the cell, causing water to move out of the cell across the membrane and, in a plant cell, leading to plasmolysis.

Source:SRC-DSKP-EN

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