Lesson 3 of 7 / Biological molecules
Triglycerides and phospholipids
Why does one lipid store energy while another builds a membrane?
I can relate glycerol/fatty-acid chemistry and ester bonds to lipid properties.
I can describe glucose isomers, glycerol, fatty acids and amino acids.Syllabus
Syllabus 9477, Core 1(g). Describe glucose isomers, glycerol, fatty acids and amino acids.
I can explain condensation and hydrolysis of glycosidic, ester and peptide bonds.Syllabus
Syllabus 9477, Core 1(h). Explain condensation and hydrolysis of glycosidic, ester and peptide bonds.
I can relate starch, cellulose, glycogen, triglyceride and phospholipid structure to properties and roles.Syllabus
Syllabus 9477, Core 1(i). Relate starch, cellulose, glycogen, triglyceride and phospholipid structure to properties and roles.
About 6 min
Make a guess. You are not marked.
A phospholipid is a lipid. Is the whole molecule hydrophobic?
Show the answer
No. Its phosphate head is hydrophilic.
This mixed nature, called amphipathic, is why phospholipids form a bilayer in water.
The key idea
Triglycerides 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.
Watch out for this
A student says: "A phospholipid is completely insoluble because every part is non-polar." What is wrong with this?
Show the answer
Its tails are non-polar, but its polar head interacts with water; this contrast drives bilayer formation.
Check your understanding
Complete hydrolysis of one triglyceride requires breaking how many ester bonds?
Show the answer
Three, one for each fatty acid
Three ester bonds attach the three fatty acids to glycerol.
Quick recall
Why does one lipid store energy while another builds a membrane?