Lipids

Lipids are a diverse group of molecules with various functions. Two important types to understand are triglycerides and phospholipids.

Triglycerides are formed through a condensation reaction between:

• One glycerol molecule (a 3-carbon carbohydrate).

• Three fatty acid molecules.

Each condensation reaction creates an ester bond between glycerol and a fatty acid.

These ester bonds can be broken by hydrolysis, which requires a water molecule.

Triglycerides are entirely hydrophobic:

• They repel water because they are non-polar and do not interact with water molecules.

One triglyceride.

Fatty acids

Fatty acids are ;ong hydrocarbon chains with a carboxyl group (COOH) at one end.

They are represented as RCOOH, where ‘R’ represents the hydrocarbon chain.

Fatty acids can be saturated or unsaturated:

• Saturated fatty acids:

  • No carbon-carbon double bonds.

  • Saturated with hydrogen atoms.

  • Usually solid at room temperature (e.g., animal fats).

• Unsaturated fatty acids:

  • One or more carbon-carbon double bonds.

  • Kinked structure due to double bonds.

  • Usually liquid at room temperature (e.g., plant oils).

The structure of a saturated (top) and unsaturated (bottom) fatty acid.

Phospholipids

Phospholipids have a structure similar to triglycerides, but one fatty acid is replaced by a phosphate group (PO₄³⁻).

This creates two distinct regions:

• Head region (phosphate group):

  • Hydrophilic (water-attracting) due to the negative charge.

  • Forms hydrogen bonds with water.

• Tail region (fatty acids):

  • Hydrophobic (water-repelling) because the fatty acids are non-polar.

  • Avoids interaction with water.

Phospholipids form a bilayer:

• In water, phospholipids spontaneously arrange into a bilayer.

• Hydrophobic tails face inwards, avoiding water.

• Hydrophilic heads face outwards, interacting with water.

This structure is crucial for cell membranes, providing a barrier between the cell and its environment.

See Topic 2: Structure of Cell Membranes for more details on phospholipids.

One phospholipid.

Comparing triglycerides and phospholipids

Assessed Explanation: "Explain the different properties of triglycerides and phospholipids."

• Phospholipids have both hydrophilic and hydrophobic regions:

  • The phosphate group is charged (hydrophilic, interacts with water).

  • The fatty acid tails are uncharged (hydrophobic, repels water).

  • This makes phospholipids polar molecules.

• Triglycerides are entirely hydrophobic:

  • The whole molecule repels water as it is non-polar.

Biochemical test

1. Homogenise the sample to release lipids.

2. Shake the sample with ethanol (lipids dissolve in ethanol but not water).

3. Add water and shake again.

4. A positive result = white, milky emulsion, indicating the presence of lipids.

Definitions