Lesson 3 of 6 / Biological molecules
Triglycerides and phospholipids
Why does one lipid store energy while another builds a membrane?
In this lesson: Relate glycerol/fatty-acid chemistry and ester bonds to lipid properties.
About 6 min
The key ideaTriglycerides have three fatty acids and are largely hydrophobic; phospholipids have a polar head and two non-polar tails.
Explore the idea
Compare a store with a membrane unit
Three ester links join three fatty acids to glycerol. Forming all three releases three water molecules.
Coloured links represent ester bonds, not additional atoms. The phosphate-containing head is simplified; lipid molecules are not polymers of repeated monomers.
Recognise the starting molecules
Glycerol: CH2OH-CHOH-CH2OH, a three-carbon molecule with three hydroxyl groups.
Fatty acid: a hydrocarbon chain ending in -COOH. Saturated chains have no carbon-carbon double bonds; a cis double bond introduces a bend in an unsaturated chain. The carboxyl group reacts with a glycerol hydroxyl group to form an ester link.
Explanation
Glycerol is a three-carbon molecule with three hydroxyl groups. A fatty acid has a carboxyl group and a hydrocarbon chain. A saturated chain has no carbon-carbon double bonds; unsaturated chains contain one or more, often producing bends that reduce tight packing.
Condensation between a glycerol hydroxyl and fatty-acid carboxyl group forms an ester bond. A triglyceride has three fatty acids esterified to glycerol, releasing three water molecules during formation. Hydrolysis of its three ester bonds uses three water molecules to release glycerol and fatty acids.
Triglycerides are hydrophobic and insoluble in water. Their many reduced carbon-hydrogen bonds make them energy-rich respiratory substrates; their compact storage does not contribute as many dissolved particles as soluble small molecules. Stored fat also provides insulation and cushioning.
A phospholipid typically has two fatty-acid tails and a phosphate-containing polar head attached to glycerol. It is amphipathic: the head interacts with water while tails avoid it. In water, this arrangement favours a bilayer with tails inward and heads facing aqueous environments, providing the basis of cellular membranes.
Step by step
- 1
Count attachments
Distinguish three fatty acids from two plus a phosphate-containing head.
- 2
Identify polarity
Locate hydrophilic and hydrophobic regions.
- 3
Predict behaviour in water
Relate amphipathic structure to bilayer organisation.
Worked example
Work through the evidence
Why is a triglyceride unsuitable as the main amphipathic molecule of a bilayer?
One way to explain it
It lacks the prominent polar phosphate-containing head of a phospholipid and is largely hydrophobic, so it does not arrange with a hydrophilic face on each side in the same way.
Why this answer works
- A membrane must interact with water on both surfaces.
- Its interior can exclude many polar solutes.
Is this true? "A phospholipid is completely insoluble because every part is non-polar."
Its tails are non-polar, but its polar head interacts with water; this contrast drives bilayer formation.