Phospholipids
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of triglycerides. You can test your knowledge on these below.
What are the elements found in triglycerides?
Carbon, hydrogen and oxygen.
Describe the structure of a triglyceride.
Triglycerides have a molecule of glycerol and three fatty acids. Joined by three ester bonds.
How are the ester bonds formed?
Via condensation reactions. This involves the removal of water.
Explainer Video
Structure of Phospholipids
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Composed of glycerol, two fatty acid chains, and a phosphate group.
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The fatty acid chains are hydrophobic (water-repelling), while the phosphate group is hydrophilic (water-attracting).
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This dual nature allows phospholipids to arrange into a bilayer in cell membranes.
Note, for A-level Biology you do not have to be able to draw the full chemical structure as shown below.
Simplified structure of a phospholipid:
The ‘head’ consists of glycerol and a phosphate group. The ‘tails’ are fatty acids which consist of a carboxyl group and a hydrocarbon chain.
The two fatty acid tails are joined to glycerol via ester bonds made in condensation reactions (which involve the removal of water).
Diagram showing an unsaturated fatty acid:
This is unsaturated as there is a C=C in the hydrocarbon chain of the fatty acid
Diagram showing a saturated fatty acid:
This is saturated as there are no C=C in the hydrocarbon chain. (Each carbon is fully saturated with hydrogen.
Properties & Functions of Phospholipids
Amphipathic nature
- Hydrophilic phosphate head interacts with water.
- Hydrophobic fatty acid tails avoid water and face inward in a bilayer.
Forms a bilayer in aqueous solutions
- This is crucial for the structure of the plasma membrane.
Partially permeability
- Small, non-polar molecules (e.g., oxygen, carbon dioxide) can diffuse through easily.
- Large, polar molecules require transport proteins to cross the membrane.
- Water is polar but is small enough that it can diffuse through the phospholipid bilayer.
Function of Phospholipids
- Major component of cell membranes (phospholipid bilayer).
- Provides fluidity to the membrane, allowing flexibility and movement. This is because the phospholipids can move laterally.
- Acts as a barrier to large, water-soluble molecules, helping regulate entry and exit from the cell.
Phospholipids vs. Triglycerides
|
Feature |
Triglycerides |
Phospholipids |
|
Basic Structure |
1 glycerol + 3 fatty acid chains |
1 glycerol + 2 fatty acid chains + 1 phosphate group |
|
Bond Type |
3 ester bonds (glycerol to fatty acids) |
2 ester bonds (to fatty acids) + 1 bond to phosphate group |
|
Hydrophilic/Hydrophobic |
Entirely hydrophobic |
Amphipathic: hydrophilic phosphate head + hydrophobic fatty acid tails |
|
Solubility in Water |
Insoluble; forms droplets |
Insoluble as a whole; self-organises into bilayers |
|
Biological Role |
- Energy storage - Insulation - Protection |
- Major component of cell membranes - Involved in cell signalling |
|
Orientation in Water |
Do not interact with water – clump together |
Form bilayers, heads attract water, tails repel it |
|
Presence in Membranes |
Not part of membranes |
Main structural component of membranes (phospholipid bilayer) |
|
Effect of Unsaturation |
More unsaturated = more fluid/oily |
Unsaturated tails increase membrane fluidity |
|
Hydrolysis Products |
1 glycerol + 3 fatty acids |
1 glycerol + 2 fatty acids + 1 phosphate group |
Key Terms
- Phospholipid: A lipid containing a phosphate group in its molecule, essential for cell membrane structure.
- Hydrophobic: Water-repelling; describes the fatty acid tails of phospholipids.
- Hydrophilic: Water-attracting; describes the phosphate head of phospholipids.
- Bilayer: A double layer of phospholipids that forms the structure of the cell membrane.
- Amphipathic: A molecule that has both hydrophobic and hydrophilic regions.
- Selective Permeability: The ability of the cell membrane to allow certain molecules to pass while blocking others.
No answer provided.
Exam Tips
When describing phospholipids in an exam, always mention their amphipathic nature and how this allows them to form a bilayer. Examiners often expect students to explain the role of hydrophilic heads and hydrophobic tails in membrane formation.
Make sure you are able to compare and contrast the structures, properties and functions of triglycerides and phospholipids.
No answer provided.
Describe how the structure of a phospholipid contributes to the formation of the cell membrane. (4 marks)
- Phospholipids have a hydrophilic phosphate head and hydrophobic fatty acid tails.
- They arrange themselves into a bilayer with heads facing outward towards water and tails facing inward, away from water.
- This bilayer forms the basic structure of the cell membrane.
- Allows selective permeability, enabling small, non-polar molecules to diffuse through while preventing large, polar molecules from passing freely.