Triglycerides and Phospholipids

A triglyceride forms from one glycerol molecule and three fatty acids. Each fatty acid’s carboxyl group reacts with a glycerol hydroxyl group in condensation, forming an ester bond and releasing water. A triglyceride therefore contains three ester bonds; hydrolysis can break them to release glycerol and fatty acids.

A saturated fatty acid has no carbon–carbon double bonds in its hydrocarbon chain. An unsaturated fatty acid has at least one. A cis double bond usually introduces a bend, reducing close packing of adjacent chains. Identify the bond from a structural diagram rather than assuming all plant lipids or all liquid lipids must fit one simple category.

Triglycerides contain many carbon–hydrogen bonds and can release a large amount of energy when oxidised. They are hydrophobic and insoluble in water, so stored droplets have little direct osmotic effect. They also have a high energy store relative to mass, useful when an organism must carry its reserves.

A phospholipid has a glycerol-based structure with two fatty-acid tails and a phosphate-containing polar head group. Its head is hydrophilic and its tails hydrophobic. In water, phospholipids can form a bilayer with heads facing aqueous solutions and tails shielded inside. The bilayer provides a barrier to ions and most polar solutes; it is not an impenetrable barrier to every molecule.

Mix a sample with ethanol so any lipid can dissolve, then add the ethanolic mixture to water according to the approved method. A white cloudy emulsion supports lipid being present: tiny lipid droplets scatter light because lipid is less soluble in the final water-rich mixture.

Include a negative control with no lipid and avoid confusing an initially cloudy food suspension with the test outcome. Ethanol is flammable, so heating with a naked flame is inappropriate. A white emulsion identifies a broad lipid property; it does not by itself establish the number of double bonds, identify a particular fatty acid, or quantify total lipid accurately.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.