Lipids

• What characteristics do all lipids share?

  >contain carbon, hydrogen and oxygen

  >proportion of oxygen to carbon and hydrogen is smaller then in carbohydrates

  >insoluble in water

  >soluble in organic solvents such as alcohols and acetone
• What are the 5 main roles of lipids?

  cell membrane, source of energy, waterproofing, insulation, protection
• What is the role of lipids in the cell membrane and as a source of energy?

  cell membrane - phospholipids contribute to the flexibility of membranes and the transfer of lipid-soluble substances.

  source of energy - when oxidised, they provide twice the amount of energy as the same mass of carbohydrate and release water.
• What is the role of lipids in waterproofing?

  Waterproofing - insoluble in water, plants and insects have a waxy lipid cuticle that conserves water and mammals produce an oily secretion in the skin.
• What is the role of lipids in insulation and protection?

  Insulation - fats are low conductors of heat so when stored beneath the body surface they help retain body heat, they are also used as electrical conductors in the myelin sheath.

  Protection - fat is often stored around delicate organs such as the liver.
• What is the emulsion test for lipids?

  1.add 2cm$^3$ of the sample to a test tube

  2. add 5cm$^3$ of ethanol to the test tube

  3. shake the test tube thoroughly to dissolve any lipid in sample.

  4.add 5cm$^3$ of distilled water and gently shake

  positive - formation of a cloudy white emulsion

  negative - clear
• What is are fatty acids?

  Fatty acids contain a carboxyl group and a hydrocarbon tail, which is hydrophobic making them insoluble in water.
• What is the definition of hydrophobic?

  non-polar molecules that do not dissolve well in water and repel it (orients itself away from water), interacts with fats e.g. fatty acid.
• What is the definition of hydrophilic?

  polar molecules that dissolve in water and attract it does not interact with fat e.g. phosphate.
• What is the general diagram of a fatty acid?

  R group - hydrocarbon chain can be saturated or unsaturated

  carboxyl group

  
• How are triglycerides formed?

  A molecule of glycerol and 3 fatty acids bond together in a condensation reaction. The H$^+$ions from glycerol bond with the OH$^-$ions of the fatty acids to form 3 ester bonds, 3 molecules of water and a triglyceride molecule.
• What is the diagram for the synthesis of triglycerides?

  Formation of triglyceride:

  The hydrocarbon chain can also be drawn as a zig zag line.

  
• How does the structure of triglycerides relate to its properties?

  Good source of energy - high ratio of energy-storing carbon-hydrogen bonds to carbon atoms.

  Low mass to energy ratio - can be stored in small amounts, animals have to carry around less mass.

  Insoluble in water - storage does not affect water potential in cells.

  High ratio of hydrogen : carbon atoms - release water when oxidised, provide excellent source of water, especially for dessert organisms.
• What are saturated fatty acids and their properties?

  They have no double bonds between carbons, meaning every carbon atom is bonded to maximum number of hydrogen atoms, chain is straight with no kinks, as they are straight they pack together and form solids at room temperature, mainly found in animal fats from meat and dairy products.
• What are unsaturated fatty acids and their properties?

  They have one or more double bonds between carbons (mono or polysaturated), carbons are not bonded to as many hydrogens as possible, double bonds introduce kinks to chain, more double bonds = more kinks, form liquid at room temperature as don't easily pack together, found in vegetable oils, nut and fish.
• What is the structure of phospholipids?

  They have one glycerol molecule that forms 2 ester bonds with 2 fatty acids. They also contain a phosphate group. The phosphate end is hydrophilic and the fatty acids end is hydrophobic.
• What is the diagram for the structure of a phospholipid?


  The diagram is as shown.

  
• How do the different behaviours on each end of a phospholipid affect how they act as a whole?

  The opposite ends of phospholipids behave differently meaning they are polar molecules, when placed in water the molecule orients itself so the hydrophilic phosphate group is close to water and the hydrophobic side is furthest away from water.
• How is the structure of phospholipids related to its properties?

  In an aqueous environment they form a bilayer due to there polarity. This is what a cell membrane is made up of. Hydrophilic head orients itself to outside of barrier. Hydrophobic tail orients to centre of membrane. Due to phospholipid structure carbohydrates can bond on the surface to form glycolipids, which are important in cell recognition.