Triglycerides

A triglyceride is a lipid made from one glycerol molecule joined to three fatty acid molecules. Glycerol has three hydroxyl groups, so it can form a bond with each of the three fatty acids.

Each fatty acid has a carboxyl group and a hydrocarbon group. Its general formula is $$\ce{RCOOH}$$, where $$\ce{R}$$ represents the hydrocarbon group and $$\ce{-COOH}$$ represents the carboxyl group.

Formation of a triglyceride

A hydroxyl group of glycerol reacts with the carboxyl group of a fatty acid. This is a condensation reaction: it joins the molecules, forming an ester bond, and releases a molecule of water.

Each of glycerol’s three hydroxyl groups reacts with a fatty acid in this way. The resulting triglyceride therefore has three ester bonds, and three water molecules are released in total.

Triglycerides and water

The fatty acid hydrocarbon chains make triglycerides predominantly non-polar. Triglycerides are hydrophobic and insoluble in water. Unlike phospholipids, they have no phosphate-containing hydrophilic head.

Fatty Acid Saturation

A fatty acid has a carboxyl group attached to a hydrocarbon group: a chain made of carbon and hydrogen. It can be represented as $$\ce{R-COOH}$$, where $$\ce{R}$$ represents the hydrocarbon group and $$\ce{COOH}$$ is the carboxyl group.

A saturated fatty acid has no carbon–carbon double bonds in its hydrocarbon chain. Its carbon atoms carry the maximum possible number of hydrogen atoms. An unsaturated fatty acid has one or more carbon–carbon double bonds. Because these bonds use more of the carbon atoms’ bonding capacity, the chain carries fewer hydrogen atoms.

Identifying fatty acid saturation

In a structural diagram, examine the bonds between carbon atoms in the hydrocarbon chain. A carbon–carbon double bond, $$\ce{C=C}$$, shows that the fatty acid is unsaturated. If there are no carbon–carbon double bonds, it is saturated.

The carboxyl group contains a carbon–oxygen double bond, $$\ce{C=O}$$. This bond does not make the fatty acid unsaturated: saturation is determined by carbon–carbon bonds, not the carbon–oxygen bond.

Phospholipids

A phospholipid has a glycerol-based structure joined to two fatty acid tails and a phosphate-containing group. In a triglyceride, the glycerol is joined to three fatty acids; in a phospholipid, the phosphate-containing group takes the place of the third fatty acid.

The phosphate-containing group forms a polar, hydrophilic head: it interacts with water. The fatty acid tails are non-polar and hydrophobic: they do not interact favourably with water. Because the molecule has both a hydrophilic region and a hydrophobic region, it is described as amphipathic.

Phospholipid bilayers

When phospholipids are in water, they can form a bilayer: two layers of phospholipids arranged tail to tail. The hydrophilic heads face the water on either side, while the hydrophobic tails point inwards, away from the water. This arrangement allows the heads to interact with water while shielding the tails from it.

Triglycerides lack the phosphate-containing hydrophilic head, so they do not form the same bilayer arrangement.

Emulsion Test for Lipids

The emulsion test detects lipids in a sample. It uses the fact that lipids dissolve in ethanol but do not dissolve in water.

Carrying out the emulsion test

1. Shake the sample with ethanol. Any lipid present dissolves in the ethanol.
2. Add the ethanol mixture to water.
3. Observe the mixture: a milky-white, cloudy emulsion is a positive result for lipids; a clear mixture is a negative result.

Why a lipid emulsion looks cloudy

When the ethanol mixture is added to water, the lipid cannot remain dissolved in the water. It forms tiny droplets suspended throughout the mixture. These droplets scatter light, giving the emulsion its cloudy appearance.