Lipids

• What is a lipid?


  Contains carbon, hydrogen and oxygen (although the portion of oxygen is smaller than in carbohydrates) • Are insoluble in water • Are soluble in organic solvents such as alcohols and acetone. • The main groups of lipids are triglycerides and phospholipids.
• What makes up triglycerides?


  One glycerol molecules and 3 fatty acids
• How do glycerol and fatty acids join together?


  The glycerol and fatty acids join together in a condensation reaction (3 water molecules are released per triglyceride as 3 bonds are formed.
• Describe how standard solutions could be used to estimate the concentration of reducing sugar in an unknown sample. (5 marks)

  • Sugar solutions of known / specific concentrations; • Test each concentration with Benedict’s solution; • use equal volumes of solutions / variables controlled; • Use a colorimeter to measure absorbance • plot this on a calibration curve. • Read off the curve to estimate concentration of unknown sample.
• What bonds are formed between glycerol and fatty acids?


  Ester bonds
• How can Ester bonds be broken down?

  By hydrolisis reactions
• What are the roles of lipids?

  • Source of energy – When oxidised provide more than twice the energy as the same mass of carbohydrate. They also release valuable water.
  • Waterproofing- Plants and insects have waxy lipid cuticles. Mammals produces oily lipid secretions from glands in the skin
  • Insulation – slow conductors of heat so retain body heat when stored under the skin. Also act as electrical insulators around nerve cells
  • Protection – fat is often stored round delicate organs and act as shock absorbers (eg kidneys)
• What are saturated fatty acids?

  They contain only C-C single bonds

  Every carbon atom is bonded to as many hydrogen atoms as possible no more can be added, hence ‘saturated (with hydrogen)’

  Triglycerides consisting of saturated fatty acids can pack together to form solid fat at room temperature

  Mainly found in animal fats from meat and dairy products
• What are unsaturated fatty acids?

  • They contain 1 or more C=C double bonds
  • Not every carbon atom is bonded to as many hydrogen atoms as it could be – hence ‘unsaturated (with hydrogen)’ – and so there are double bonds present
  • Double bonds introduce a definite ‘kink’ in the carbon atom chain. The more double bonds the more kinks there will be.
  • Triglycerides consisting of ‘ kinky’ unsaturated fatty acids do not pack together easily and form liquid oils at room temperature.
  • Mainly found in vegetable oils, nuts and fish
• How do you test for Lipids?

  Emulsion test
• How do you carry out an emulsion test?


  Crush food sample and add ethanol and shake. • THEN add water. • If positive a cloudy white emulsion will form.
• What is the structure of phospholipids?

  • One of the fatty acid molecules is replaced by a phosphate molecule
  • A Hydrodrophilic head: interacts with water but not with fat
  • A Hydrophobic tail: orients away from water but mixes with fat.
  • When exposed to water form micelle or bilayer
  • Hydrophilic heads face water, hydrophobic tails face inward
  • Key to phospholipid role in membranes
• What is the structure of phospholipids in relation to their Functions?

  • POLAR (hydrophilic head, hydrophobic tail) – Form a bilayer within cell membranes and thus a hydrophobic barrier is formed between the inside and the outside of the cell.
  • POLAR - allow lipid soluble substances to enter and leave the cell, prevent water soluble substances entering and leaving and to make the membrane flexible
  • Can combine with carbohydrates – forms glycolipids which are important in cell recognition.
• Fill in the gaps

  Fats and oils make up a group of liquids which is called lipids which, when hydrolysed, form glycerol and fatty acids. A fatty acid with more than 1 carbon-carbon double bond is described as unsaturated. In a phospholipid the number of fatty acids is two, these are described as hydrophobic because they repel water.