Two lipids we need to know about are triglycerides and phospholipids. They are organic as they contain carbon (they also contain hydrogen and oxygen). Lipids are insoluble in water but soluble in organic substances (e.g acetone/alcohols). The proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.
Triglycerides are composed of three fatty acids and a glycerol. They are formed from the condensation of these molecules (three fatty acids and a glycerol) forming an ester bond between the glycerol and each triglyceride. If the R-group of the fatty acid (RCOOH) is said to be unsaturated it just means it has carbon-carbon double bonds (monounsaturated means one carbon-carbon double bond). Fatty acids are hydrophobic.
Phospholipids are composed of two fatty acids, a glycerol, and a phosphate group. The phosphate ‘head’ is hydrophilic and the fatty acid ‘tail’ is hydrophobic. In this way, phospholipids are polar.
Triglyceride structure and function:
High ratio of energy-storing carbon-hydrogen bonds to carbon atoms so are an excellent store of energy
Low mass-energy ratio (much energy can be stored in a small volume)
Large and non-polar = insoluble so does not affect the water potential of cells
Release water when oxidised = provide an important source of water
Phospholipid structure and function:
Polar so form a bilayer in aqueous solutions/environments
hydrophilic heads hold at the surface of the cell-surface membrane
They form glycolipids by combining with the cell-surface membrane which are important in cell recognition
The emulsion test:
Take a dry/grease-free test tube
Add 2cm^3 of the sample and 5cm^3 of ethanol
Shake thoroughly - this dissolves any liquid present
Add 5cm^3 of water and shake gently
Cloudy-white emulsion/colour indicates a lipid is present (clear = no lipid). As a control, repeat with water as the sample.
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