Lesson 3 of 7 / Biomolecules: structure explains function
One glycerol, two different lipid designs
Why does a phospholipid form a bilayer?
I can describe glycerol, fatty acids, ester bonds, triglycerides and phospholipids in relation to their roles.
I can describe alpha/beta-glucose, glycerol, fatty acids and amino acids.Syllabus
Syllabus 8876, Core 1(e). Describe alpha/beta-glucose, glycerol, fatty acids and amino acids.
I can explain formation and hydrolysis of glycosidic, ester and peptide bonds.Syllabus
Syllabus 8876, Core 1(f). Explain formation and hydrolysis of glycosidic, ester and peptide bonds.
I can relate starch, cellulose, triglyceride and phospholipid structures to roles.Syllabus
Syllabus 8876, Core 1(g). Relate starch, cellulose, triglyceride and phospholipid structures to roles.
About 6 min
Make a guess. You are not marked.
Is a phospholipid just a triglyceride with an extra phosphate attached?
Show the answer
Phosphate replaces one fatty acid.
A phospholipid has two fatty-acid tails and a polar phosphate head. That shape lets it form a bilayer.
The key idea
A triglyceride has three fatty acids joined to glycerol. A phospholipid has two fatty-acid tails and a phosphate-containing head, giving it both hydrophobic and hydrophilic regions.
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.
Explanation
Glycerol is a small three-carbon molecule with three hydroxyl groups. A fatty acid has a carboxyl group and a hydrocarbon chain. The chain is largely non-polar and hydrophobic. Saturated chains have no carbon-carbon double bonds; unsaturated chains contain one or more, often producing bends.
An ester bond forms by condensation between a glycerol hydroxyl group and a fatty-acid carboxyl group. Forming a triglyceride joins three fatty acids to one glycerol and releases three water molecules. Hydrolysis of the three ester bonds uses water to release the components.
Triglycerides are hydrophobic and insoluble in water. Their many carbon-hydrogen bonds provide a concentrated energy store when oxidised, and storage without much associated water makes them compact. Stored fat can also provide insulation and cushioning.
A phospholipid has two fatty-acid tails and a phosphate-containing polar head attached to glycerol. In water, the hydrophilic heads face water while hydrophobic tails avoid it. This amphipathic structure favours bilayers, making phospholipids suitable for cell membranes. A triglyceride does not have this head-and-two-tail arrangement.
Three further properties of triglycerides earn marks. Because they are insoluble, storing them does not lower the water potential of cells, so it does not draw water in by osmosis, unlike storing the same energy as glucose. Oxidising them in respiration releases a large amount of water, called metabolic water, which some desert animals rely on. And fat is less dense than water, so a layer of fat helps aquatic animals such as whales stay buoyant.
Unsaturated fatty-acid tails in phospholipids have kinks at their double bonds. The kinks stop the phospholipids packing tightly, so a membrane rich in unsaturated tails is more fluid. Saturated tails are straight and pack closely, making the membrane less fluid.
Step by step
- 1
Count the fatty acids
Three in a triglyceride; two in the phospholipid model.
- 2
Identify the bond
Ester links connect fatty acids to glycerol.
- 3
Locate the polar region
The phospholipid head interacts with water; its tails occupy the bilayer interior.
Worked example
Three ester bonds
How many water molecules are used to hydrolyse one triglyceride fully into glycerol and fatty acids?
One way to explain it
Three water molecules, one for each ester bond broken. The products are one glycerol molecule and three fatty-acid molecules.
Why this answer works
- Count the bonds being broken.
- Apply one hydrolysis event to each ester bond.
Watch out for this
A student says: "A phospholipid is a triglyceride with an extra phosphate added as a fourth tail." What is wrong with this?
Show the answer
In the standard model, a phosphate-containing head replaces one fatty-acid position, leaving two hydrophobic tails.
Check your understanding
What makes a phospholipid suited to a membrane bilayer?
Show the answer
A hydrophilic head and hydrophobic tails orient differently in water.
Heads face aqueous environments and tails form the interior.
Quick recall
Why does a phospholipid form a bilayer?