The big idea: A phospholipid is a lipid with a water-loving head and two water-fearing tails.
Because one part likes water and the other part avoids it, a phospholipid is described as amphipathic.
This single property is the reason phospholipids can build the membranes around every cell.
A single phospholipid: a hydrophilic phosphate head joined through glycerol to two hydrophobic fatty-acid tails.
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- Phospholipid
- A lipid built from one glycerol, two fatty acids and a phosphate-containing head group.
- Amphipathic
- Having both a hydrophilic (water-loving) part and a hydrophobic (water-fearing) part in the same molecule.
- Hydrophilic
- Water-loving — mixes with / is attracted to water. (The phosphate head is hydrophilic because it is polar/charged.)
- Hydrophobic
- Water-fearing — does not mix with water. (The fatty-acid tails are hydrophobic because they are non-polar.)
Picture a tadpole: Draw a phospholipid as a circle with two wavy tails, like a tadpole:
round head = the phosphate group → hydrophilic (likes water)
two tails = the fatty acids → hydrophobic (avoid water)
The glycerol is the small joint in the middle that holds the head and tails together.
A phospholipid has three parts joined to one glycerol backbone: a phosphate group at one end and two fatty-acid chains at the other.
The two ends behave in opposite ways towards water — and that split personality is exactly what makes the molecule useful.
Head — hydrophilic
- Made of the phosphate group (plus glycerol)
- Is polar / charged
- Attracted to water — dissolves towards it
- Points out towards the watery surroundings
Tails — hydrophobic
- Made of two fatty-acid chains
- Are non-polar (long hydrocarbon chains)
- Repelled by water — turn away from it
- Point in / together, away from water
Amphipathic = both, in one molecule: The whole word is the answer: 'amphi' = both, 'pathic' = feeling.
A phospholipid feels both ways about water — its head is hydrophilic and its tails are hydrophobic at the same time.
A molecule that is only water-loving (like glucose) or only water-fearing (like a triglyceride) is not amphipathic.
How a phospholipid is built — condensation: A phospholipid is assembled by condensation reactions — the same bond-making, water-releasing reaction that builds other macromolecules.
Each fatty acid joins the glycerol by a condensation reaction that forms an ester bond and removes one water molecule (H₂O). The phosphate group joins the third position of the glycerol in the same water-releasing way.
So building one phospholipid releases several water molecules — condensation builds, hydrolysis (adding water) would break it apart again.
- Glycerol
- A small 3-carbon molecule that acts as the backbone; the fatty acids and the phosphate head all attach to it.
- Fatty acid
- A long hydrocarbon chain; non-polar, so it is hydrophobic. A phospholipid has two of them as its tails.
- Condensation reaction
- A reaction that joins two molecules and releases a water molecule (H₂O). It builds the ester bonds that link the fatty acids to glycerol.
- Ester bond
- The bond formed by condensation between a fatty acid and the glycerol backbone.
Phospholipid vs triglyceride — the one swap: A triglyceride (the energy-storage lipid from 2.1.4) has the same glycerol backbone, but three fatty-acid tails and no phosphate.
A phospholipid swaps one of those tails for a phosphate head. That single change is what turns an all-hydrophobic storage molecule into an amphipathic membrane molecule.
| Feature | Phospholipid | Triglyceride |
|---|---|---|
| Backbone | 1 glycerol | 1 glycerol |
| Fatty-acid tails | TWO fatty acids | THREE fatty acids |
| Third position on glycerol | a phosphate-containing head | a third fatty acid |
| Overall charge spread | amphipathic (head ≠ tails) | all non-polar (hydrophobic) |
| Soluble part? | the head dissolves in water | none — fully water-repelling |
| Main job | builds cell membranes | stores energy |
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How this is tested: On Paper 1A you are often shown several structural formulae and asked to select the amphipathic molecule — the one with both a hydrophilic part and a hydrophobic part. You may also be shown molecular diagrams and asked to pick the phospholipid (glycerol + two fatty acids + a phosphate head — not three tails).
On Paper 2 a phospholipid is often drawn with its head labelled X and its tails labelled Y, and you must state the property of each region — head = hydrophilic, tails = hydrophobic.
IB-style question — state the properties of a phospholipid's regions
A phospholipid is shown with its head region labelled X and its two tail regions labelled Y. State one property of region X and one property of region Y, and use these to explain why the molecule is described as amphipathic. [3]
How to score all three marks
- Region X (the head). State that the head is hydrophilic — it is attracted to / mixes with water because it contains the polar phosphate group.
- Region Y (the tails). State that the tails are hydrophobic — they are repelled by / do not mix with water because the fatty-acid chains are non-polar.
- Link to amphipathic. Because the same molecule has both a hydrophilic part (X) and a hydrophobic part (Y), it is amphipathic. (Mark 1: X hydrophilic. Mark 2: Y hydrophobic. Mark 3: amphipathic = both parts in one molecule.)
Final answer
X (head) is hydrophilic — it is attracted to water (polar phosphate). Y (tails) are hydrophobic — they are repelled by water (non-polar fatty acids). Having both in one molecule makes the phospholipid amphipathic.
A single phospholipid: a hydrophilic phosphate head joined through glycerol to two hydrophobic fatty-acid tails.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
✓ How to spot the phospholipid in a 'select' question: Reject any molecule that is only water-loving or only water-fearing.
The phospholipid is the 'tadpole': one round (phosphate) head + two tails. If a diagram shows three tails and no head, that is a triglyceride, not a phospholipid.