What is the molecular structure of the cell surface membrane, and how does the fluid mosaic model account for its properties?
Describe the fluid mosaic model of membrane structure and relate the roles of phospholipids, proteins, cholesterol and carbohydrates to membrane function
A focused answer to the H2 Biology Cell Biology outcome on membrane structure. The phospholipid bilayer, the proteins, cholesterol and carbohydrates embedded in it, and how the fluid mosaic model explains membrane fluidity, selective permeability and cell recognition.
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What this dot point is asking
SEAB wants you to describe the fluid mosaic model of membrane structure, to name and place each component (phospholipids, proteins, cholesterol, glycoproteins and glycolipids), and to relate these components to membrane functions such as forming a selective barrier, allowing transport, maintaining fluidity, and enabling cell recognition. This is the structural basis for the transport and signalling dot points that follow.
The answer
The phospholipid bilayer
The core of every membrane is a bilayer of phospholipids. Each phospholipid is amphipathic, with a hydrophilic phosphate head and two hydrophobic fatty acid tails. In the watery environment of the cell the heads face outwards toward the water on both sides and the tails turn inwards, forming a non-polar core. This arrangement is self-assembling and gives a stable, two-layer sheet.
Why "fluid" and why "mosaic"
The model is called fluid because the phospholipids are not bonded together and can move laterally within their own layer, and the embedded proteins drift through the bilayer. It is called a mosaic because the proteins are scattered through the lipid sheet like tiles in a mosaic, varying in size and position.
The other components
- Proteins. Intrinsic (transmembrane) proteins span the bilayer and act as channels, carriers, receptors and enzymes. Extrinsic (peripheral) proteins sit on one surface and act in support or signalling.
- Cholesterol. Sits between phospholipids and regulates fluidity: at high temperatures it restricts movement and stabilises the membrane, at low temperatures it prevents the tails packing too tightly and keeps the membrane fluid.
- Glycoproteins and glycolipids. Carbohydrate chains attached to proteins or lipids on the outer surface; they act as recognition sites, receptors and adhesion points.
Relating structure to function
The non-polar core makes the membrane a selective barrier; transport proteins create routes for substances that cannot cross the core; cholesterol buffers fluidity against temperature change; and the surface carbohydrates let cells recognise one another and respond to signals.
Examples in context
Example 1. Membrane receptors in signalling. Glycoprotein receptors on the membrane surface bind specific signal molecules such as hormones. Because these receptors are part of the mosaic, the membrane is not just a barrier but the cell's interface for receiving information, which the cell signalling dot point develops.
Example 2. Adapting to cold. Organisms living at low temperatures incorporate more unsaturated phospholipids and adjust cholesterol content to keep their membranes fluid. Without this, the membrane would solidify and transport would stop, showing how the model explains real physiological adaptation.
Try this
Q1. State why the phospholipid bilayer is impermeable to charged ions. [1 mark]
- Cue. The hydrophobic, non-polar core repels charged particles, so ions cannot pass through it freely.
Q2. Explain the role of cholesterol in the cell surface membrane. [2 marks]
- Cue. Cholesterol sits between phospholipids and regulates fluidity, restricting movement at high temperatures and preventing tight packing at low temperatures, stabilising the membrane across a range of temperatures.
Q3. Explain why the model is described as a mosaic. [1 mark]
- Cue. The proteins are scattered through the lipid bilayer in varied positions and sizes, resembling the tiles of a mosaic.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksUsing the fluid mosaic model, explain how the structure of the cell surface membrane accounts for its selective permeability and its fluidity.Show worked answer →
The answer should connect named components to the two named properties.
The membrane is a phospholipid bilayer with the hydrophilic heads facing the aqueous environments and the hydrophobic tails forming a non-polar core. This non-polar interior makes the membrane a barrier to charged and large polar molecules, which cannot cross freely, while small non-polar molecules and small uncharged molecules can pass. Selective permeability comes from this barrier together with specific channel and carrier proteins that allow particular ions and polar molecules through.
Fluidity arises because the phospholipids are not bonded to one another and can move laterally within their layer; the unsaturated fatty acid tails have kinks that prevent tight packing, keeping the membrane fluid. Cholesterol modulates this fluidity, restricting movement at higher temperatures and preventing tight packing at lower temperatures. Proteins also drift within the bilayer, which is the mosaic aspect of the model.
Markers reward the bilayer arrangement, the non-polar barrier explaining selectivity, the role of transport proteins, lateral movement explaining fluidity, and the role of cholesterol.
Original3 marksDescribe the roles of glycoproteins and glycolipids in the cell surface membrane.Show worked answer →
Both are membrane components carrying carbohydrate, and both have recognition roles.
Glycoproteins are proteins with carbohydrate chains attached, and glycolipids are lipids with carbohydrate chains attached. Their carbohydrate chains project from the outer surface of the membrane.
They act as recognition sites and as receptors. They allow cells to recognise one another (for example in tissue formation and in the immune system distinguishing self from non-self), and some act as binding sites for hormones, neurotransmitters or other signalling molecules. They also contribute to cell adhesion.
Markers reward correct identification of each molecule, the outward-facing carbohydrate, and at least two roles such as cell recognition and acting as receptors.
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