2.1.7Additional lessons
Phospholipids — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 16 of 142
Part 1 of 4Structure of a Phospholipid
Glycerol backbone
- A three-carbon glycerol molecule forms the backbone to which the other parts are attached.
Two fatty acid tails
- Two fatty acids are joined by ester bonds; these hydrocarbon tails are non-polar and hydrophobic.
Phosphate head
- A phosphate group, often linked to a small polar group, forms a charged head that is hydrophilic.
Amphipathic
- The molecule has a hydrophilic region and a hydrophobic region, so it behaves differently at each end.
Difference from triglyceride
- A triglyceride has three fatty acids, whereas a phospholipid has two fatty acids and a phosphate group.
Part 2 of 4Behaviour in Water
1
In water the hydrophilic heads form hydrogen bonds with water molecules and so face the aqueous surroundings.
2
The hydrophobic tails are not attracted to water and are pushed away from it, in a process called the hydrophobic effect.
3
At a surface the molecules can form a monolayer, with heads in the water and tails pointing into the air.
4
In bulk water they assemble into a bilayer in which tails face each other and heads face outwards on both sides.
5
Small spherical micelles or closed vesicles can also form, because exposed edges of a sheet are unstable.
Part 3 of 4Importance
Importance
Cell membranes
- Phospholipid bilayers form the basic structure of every cell membrane and separate the cell interior from its surroundings.
Barrier
- The hydrophobic core blocks most ions and polar molecules, which allows the cell to control what enters and leaves.
Self-assembly
- Bilayers form spontaneously without energy input, because arranging the tails away from water is the most stable state.
Fluidity
- Phospholipids can move sideways within a layer, which allows membranes to change shape and fuse.
Fluidity and fusion
- Because phospholipids can move laterally, membranes can bend, bud off vesicles and fuse with other membranes.
| Part | Polarity | Position in a bilayer |
|---|---|---|
| Phosphate head | Polar, hydrophilic | Outer surfaces facing water |
| Fatty acid tails | Non-polar, hydrophobic | Inner core of the bilayer |
Part 4 of 4Exam Checklist & Tips
2.1.7 – Key Points for the Exam
- Amphipathic Having both a hydrophilic and a hydrophobic region.
- Hydrophilic Attracted to water, usually because the molecule is polar or charged.
- Hydrophobic Not attracted to water and tending to be excluded from it.
- Phospholipid bilayer A double layer of phospholipids with tails inside and heads outside.
- Micelle A small sphere of lipid molecules with hydrophilic heads outside and tails inside.
Common Mistakes
- Placing the hydrophobic tails facing outward toward the water in a bilayer diagram.
- Calling the phosphate head hydrophobic; it is polar and hydrophilic.
- Saying phospholipids have three fatty acids; that describes triglycerides.
- Claiming bilayers need a source of energy to form, when they assemble spontaneously.
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