Lipids (3.1.3)
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Triglycerides are non-polar, hydrophobic molecules consisting of one glycerol molecule (an alcohol) and three fatty acid chains. They are the main components of fats and oils and are insoluble in water, but soluble in organic solvents, e.g., ethanol.
Fatty acids have a methyl group () at one end of a hydrocarbon chain (R group) and a carboxyl group () at the other end.
Triglycerides contain a high ratio of hydrogen:oxygen atoms, which means that they release water when oxidised. This is called metabolic water and can be a source of hydration, especially for animals living in arid environments.
Triglycerides are a source of energy owing to the high number of bonds which store energy.
Triglycerides are formed through a condensation reaction where each hydroxyl group () of glycerol reacts with a carboxyl group () of a fatty acid to form an ester bond.
Three fatty acids are attached to one glycerol molecule, three water molecules are released per triglyceride formed.
Ester bonds are broken during the digestion of the triglyceride molecule through hydrolysis reactions consuming three water molecules per triglyceride hydrolysed.
Fatty acids can differ in hydrocarbon chain length (R group) and in their saturation.
Saturated fatty acids have no double bonds. They form a straighter structure and pack tightly together, resulting in higher melting points and being in a solid state at room temperature. They are primarily found in animal fats.
Unsaturated fatty acids contain one (monounsaturated) or more (polyunsaturated) double bonds, leading to ‘kinks’ in the hydrocarbon chains that prevent tight packing. This lowers the melting point, so they are usually liquids at room temperature. They are typically found in vegetable oils.
Phospholipids are essential components of cell membranes.
The structure of phospholipids is similar to triglycerides: glycerol plus two fatty acid chains and one phosphate. This results in a hydrophilic polar ‘head’ and two hydrophobic fatty acid ‘tails’.
Phospholipids are amphipathic. In aqueous environments, phospholipids spontaneously arrange into a bilayer with the hydrophilic heads facing outwards, facing the surrounding water and the hydrophobic tails facing inward. This arrangement forms the basic structure of cell-surface membranes.
The phospholipid bilayer is an essential part of the cell membrane:
- The bilayer has a hydrophobic core that prevents water-soluble substances from passing through easily. This enables control over substances entering and exiting the cell.
- The hydrophilic heads form a barrier at the surface of the plasma membrane, separating the inside and outside of a cell.
- This bilayer is flexible and can enclose parts of the cell, pinching off to form membrane–bound organelles.
- The phospholipid head can be modified with carbohydrates, forming glycolipids which enable cell signalling.
The emulsion test is used to identify lipids in a sample.
Method:
- Add ethanol to the sample and shake to mix.
- Add the mixture to a test tube containing water.
Results:
A white emulsion (cloudy appearance) indicates the presence of lipids. The more lipid present, the more pronounced the milkiness/white colour.
The emulsion test is qualitative and cannot provide concentration values.
