2.1.7Inquiry Process
Phospholipids — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 42 of 160
Section 1 of 5Phospholipid Structure
A phospholipid is built like a triglyceride, but one fatty acid is replaced by a phosphate-containing group.
Glycerol backbone
- A three-carbon glycerol molecule forms the framework that holds the other parts together.
Two fatty acid tails
- Two hydrocarbon chains are attached by ester bonds; they are hydrophobic and can be saturated or unsaturated.
Phosphate head
- A phosphate group, often with a small attached group, replaces the third fatty acid and is polar and hydrophilic.
Amphipathic
- Because it has a hydrophilic head and hydrophobic tails, the molecule has both water-loving and water-hating regions.
| Feature | Triglyceride | Phospholipid |
|---|---|---|
| Fatty acids | Three | Two |
| Third position on glycerol | Fatty acid | Phosphate group |
| Polarity | Non-polar overall | Amphipathic |
Section 2 of 5Self-Assembly in Water
No energy input or enzyme is needed because the arrangement is the most stable one.
1
Water molecules form hydrogen bonds with the phosphate heads, which therefore face outwards.
2
The hydrophobic tails are excluded from water and cluster together in the interior.
3
A small group of phospholipids can form a micelle, a sphere with the tails hidden in the centre.
4
Double layers form where the tails of two sheets point toward each other, producing a bilayer.
5
Bilayer edges curl round and seal into a closed sphere because exposed tails would contact water.
Section 3 of 5Properties of the Bilayer
Properties of the Bilayer
Barrier
- The hydrophobic core blocks ions and polar molecules, so the membrane controls what enters and leaves.
Fluidity
- Phospholipids move sideways within their own layer, so the membrane is flexible and can reseal when damaged.
Selective passage
- Small non-polar molecules like oxygen can dissolve in the core and diffuse across directly.
Vesicle formation
- Bilayers can bend and fuse, enabling endocytosis, exocytosis and the movement of membrane between organelles.
Section 4 of 5Effect of Fatty Acid Saturation
Unsaturated tails
- Kinks prevent close packing, so the membrane is more fluid, especially at low temperature.
Saturated tails
- Straight chains pack tightly, giving a more rigid and less permeable membrane.
Adaptation
- Organisms in cold climates often have more unsaturated phospholipids to keep membranes working.
Link to bilayers
- The balance of tail types helps cells maintain a suitable fluidity.
Section 5 of 5Exam Checklist & Tips
2.1.7 – Key Points for the Exam
- Phospholipid A lipid with a glycerol, two fatty acids and a phosphate-containing head group.
- Amphipathic Having both hydrophilic and hydrophobic regions.
- Hydrophilic Attracted to water; polar or charged.
- Phospholipid bilayer Two layers of phospholipids with tails inward and heads outward, forming the basis of membranes.
Common Mistakes
- Drawing the hydrophilic heads pointing inwards; they face the water.
- Saying phospholipids have three fatty acids; they have two.
- Stating that phospholipid molecules never move; they move laterally and occasionally flip.
- Confusing hydrophobic with hydrophilic regions when labelling diagrams.
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