• Lipids contain; carbon, hydrogen and oxygen
  • They are not made up of monomers that link together to form polymers but instead consist of two different molecules; glycerol and fatty acids
  • Lipids are formed by one molecule of glycerol and 3 molecules of fatty acids. Therefore they are know as triglycerides

Fatty Acid Structure

Fatty Acids consist of;

  • A carboxyl group (COOH)
  • A variable length hydrocarbon chain (CH2)n
  • A methyl group

There are always an even number of hydrogen atoms (between 14 and 2) 

Fatty acids can be;

  • Saturated (Fats mainly found in animals)
  • Unsaturated (Oils mainly found in plants
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The difference between them is in their hydrocarbon chains

Saturated Fatty Acids

  • Only single bonds between the carbon atoms in the hydrocarbon chain. Every bond has a hydrogen attached, so the lipid is saturated with hydrogen
  • They have high melting points and are therefore solid at room temperature e.g. animal fats
  • A high intake of saturated fats is a contributory factor in heart disease

Unsaturated Fatty Acids

  • Have one or more double bonds in their hydrocarbon chain
  • This gives the molecule a kink and they tend not to pack together well - giving an unsaturated fat a more open and therefore oily nature
  • They have low melting points,so are liquid at room temperature e.g. plant oils

Three fatty acids bond with a glycerol molecule to form a triglyceride. There are three condensation reactions which occur, releasing 3 water molecules and forming three ester bonds. Triglycerides are non-polar so they do not attract water, therefore they are hydrophobic molecules

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Function of Lipids

  • To act as an energy store in cells. A lipid stores twice as much energy as the same mass of carbohydrates. This makes is a very efficient store. Lipids are used in seeds to store energy
  • Although lipids have a higher energy yield/gram than carbohydrates, they cannot be broken down as quickly for use in respiration so are used as medium/long term energy stores in animals
  • Provide thermal insulation against heat loss
  • To protect vital organs in animals e.g.heart and kidneys
  • Bouyancy in aquatic animals
  • As a source of metabolic water for organisms which live in low water environments e.g. Camels
  • Provide a waterproofing layer

Emulsion test for lipids/triglycerides

  • Mix the lipid with ethanol
  • Pour/decant the mixture into water 
  • A white emulsion shows that lipid is present (if it stays clear - negative result)
  • Lipids do not dissolve in water but do dissolve in some organic solvents e.g. ethanol
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  • One fatty acid is replaced by a phosphate group (PO4 3-) 
  • The phosphate group is ionised which makes it polar, so it attracts water molecules
  • Part of the phospholipid is therefore hydrophilic and the rest (fatty acid tails) are hydrophobic
  • A phospholipid has 2 fatty acids and 1 glycerol instead of 3 and a phosphate
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