Cell Membrane Structure
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of the basic components of cells and the roles of lipids and proteins. You can test your knowledge on these below.
What are the main components of a phospholipid molecule?
Glycerol, two fatty acid tails (hydrophobic), and a phosphate group (hydrophilic).
Why do phospholipids form a bilayer in aqueous environments?
The hydrophobic tails face inwards away from water, while the hydrophilic heads face outwards into the aqueous surroundings.
What substances can diffuse through the phospholipid bilayer?
Substances that are lipid soluble, small and non-polar, such as oxygen and carbon dioxide. Water can also diffuse, even though it is polar, because it is so small.
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains cell membrane structure or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Cell Membrane Structure
The Fluid Mosaic Model
The fluid mosaic model describes the structure of the plasma membrane as a dynamic and flexible barrier composed of phospholipids, proteins, cholesterol, glycoproteins, and glycolipids.
- Fluid: The phospholipid molecules are moving laterally, allowing flexibility.
- Mosaic: The proteins embedded in the membrane appear scattered like a mosaic.
The simplified structure of a phospholipid.
The head consists of glycerol and a hydrophilic phosphate group which attracts water.
The tails are 2 fatty acids, these are hydrophobic and so repel water.
Components Cell Membrane
| Component | Function |
| Phospholipids | Form the bilayer, providing a partially-permeable barrier that allows lipid-soluble molecules to pass but prevents water-soluble molecules. |
| Intrinsic (Integral) Proteins | Span the bilayer and act as channel or carrier proteins for facilitated diffusion and active transport. |
| Extrinsic (Peripheral) Proteins | Found on one side of the membrane only; involved in cell signalling and structural support. |
| Glycoproteins | Carbohydrate chains attached to proteins; involved in cell recognition and acting as receptors. |
| Glycolipids | Carbohydrate chains attached to lipids; play a role in cell recognition and cell-to-cell adhesion. |
| Cholesterol | Maintains membrane fluidity and stability, preventing it from becoming too rigid at low temperatures or too fluid at high temperatures. |
Membrane Permeability
- Partial permeability: The membrane allows certain molecules to pass based on size, polarity, and charge.
- Non-polar, small molecules (e.g., oxygen, carbon dioxide) pass freely by simple diffusion through the phospholipid bilayer.
- Large, charged, or polar molecules (e.g., glucose, amino acids and ions) require channel or carrier proteins for transport.
Key Terms
- Phospholipid Bilaye: A double layer of phospholipids forming the primary structure of the cell membrane.
- Fluid Mosaic Model: The widely accepted model of membrane structure where proteins embed within a dynamic phospholipid bilayer.
- Intrinsic Protein: A protein embedded through the entire bilayer, involved in transport.
- Extrinsic Protein: A protein attached to the surface of the membrane, only spanning one layer of phospholipids, involved in communication and structure.
- Glycoprotein: A protein with a carbohydrate chain attached, involved in cell signalling and recognition.
- Glycolipid: A lipid with a carbohydrate chain attached, playing a role in cell recognition.
- Cholesterol: A lipid molecule that regulates membrane fluidity and stability.
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Exam Tip
Common mistake: Students often confuse channel proteins (only used in facilitated diffusion) with carrier proteins (used in both active transport & facilitated diffusion).
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The plasma membrane is described as a fluid mosaic. Explain what this means and describe the roles of proteins and cholesterol in the membrane. (5 marks)
- The membrane is fluid because phospholipids can move laterally, making it flexible.
- It is a mosaic because of the scattered proteins embedded within the phospholipid bilayer.
- Intrinsic proteins allow transport of molecules via channel proteins (for facilitated diffusion) or carrier proteins (for facilitated diffusion and active transport).
- Extrinsic proteins function in cell signalling and recognition.
- Cholesterol helps regulate membrane fluidity, preventing it from becoming too rigid or too fluid.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!