3.1.3 Lipids

• Lipids


  commonly known as fats or oils and are found in all cells, with a variety of different properties
• Lipids contain about twice as much energy per gram as


  carbohydrates and are insoluble in water — don’t affect the water potential of the cell and cause water to enter the cells by osmosis
• Lipids are different from proteins and carbohydrates because


  they are not polymers formed from long chains of monomers
• Lipids have many uses, including as

  ● as certain hormones ● respiratory substrates
• Lipids are made from


  a variety of different components, all of which contain hydrocarbons
• The difference between saturated and unsaturated fatty acids


  is in their hydrocarbon tails (R groups)

  
• The R-group of a fatty acid may be


  ● saturated ● unsaturated

  
• There are two kinds of fatty acids

  ● saturated ● unsaturated

  
• Unsaturated fatty acids


  have double bonds between carbon atoms, which cause the chain to kink

  
• Saturated fatty acids


  don’t have any double bonds between their carbon atoms— the fatty acid is ‘saturated’ with hydrogen.

  
• All fatty acids consist of the same basic structure

  but the hydrocarbon tail varies.

  
• Hydrocarbons


  a type of molecule that contains only hydrogen and carbon atoms
• There are two types/groups of lipid

  ● triglycerides● phospholipids
• The part of the fatty acid molecule that reacts with glycerol

  carboxylic acid group

  
• Structure of Triglycerides

  formed by the condensation of one molecule of glycerol and three fatty acid chains

  
• Bonds in Triglycerides

  Ester bond formed by condensation reaction between :● each of the three OH groups on the glycerol● the OH group of each fatty acid chainThey are non-polar​

  
• Properties of Triglycerides

  ● insoluble in water — fatty acid chains are hydrophobic● bundle together as insoluble droplets — because the tails face inwards, and the glycerol heads shield them from the water ● hydrocarbon fatty acid chains can be saturated or unsaturated​

  
• Uses of Triglycerides


  energy store — as there is a lot of energy released when the fatty acid chains are broken down

  
• Structure of Phospholipids


  lipids found in the cell membranes● are different from triglycerides — as one of the fatty acid molecules is replaced by a phosphate group

  
• Bonds in Phospholipids

  Ester bond formed by condensation reaction between :● two OH groups on the glycerol● OH group of each fatty acid chain

  
• Properties of Phospholipids

  ● phosphate group is hydrophilic (attracts water)● the fatty acid chains are hydrophobic (repel water)— allows phospholipid to form bilayers which make up membranes in and around cells

  
• Uses of Phospholipids

  ● Membranes ● Hormones — testosterone
• Triglycerides are used as energy storage molecules because


  they are easy to store and transport
• In phospholipids, one of the fatty acids of a triglyceride is substituted by

  a phosphate-containing group

  
• The phosphate group in phospholipids is

  hydrophilic while the fatty acid tails are hydrophobic

  
• Hydrophilic


  attracts water
• The structure of phospholipids makes them able

  to form the bilayer of cell membranes

  
• Phospholipid heads are hydrophilic and their hydrophobic so


  they form a double layer with their heads facing outwards the water either side— the centre of the bilayer of cell membranes is hydrophobic so water soluble substances can't easily pass through it

  
• Centre of the bilayer of cell membranes is hydrophobic so water soluble substances can't easily pass through it this


  ● creates a barrier● allows separation of solutions● can create different conditions — either side of the membrane

  
• Phospholipids make up the

  bilayer of cell membranes — control what enters and leaves a cell

  
• Hydrophobic


  repel water
• Storage molecules need to be insoluble because


  otherwise they'd just dissolve and release whatever they were storing whenever they came into contact with water
• The properties of triglycerides and phospholipids relate to


  their structures
• The emulsion test is used to identify


  lipids
• The emulsion test involves


  ● shake the test substance with ethanol for about a minute● pour the solution into water

  
• The emulsion test can be used to test for


  the presence of lipids in a food sample
• The emulsion test for lipids involves shaking the test substance with ethanol for about a minute, then pouring the solution into water.


  involves:● shaking the test substance for a minute● then pouring the solution into water

  
• Ethanol is flammable, so


  make sure to do the emulsion test away from open flame
• Any lipid will show up as

  a milky emulsion in the emulsion test

  
• Result of the emulsion test is positive

  the student should expect to see a milky emulsion