lipids

• LIPIDS:

  • elements: carbon, hydrogen, oxygen
  • insoluble in water
  • soluble in organic solvents (e.g. alcohols, acetone)
  • main groups: triglycerides and phospholipids
• roles of lipids:

  • in cell membranes, phospholipids contribute to flexibility of membranes and transfer lipid-soluble substances across
  • source of energy: oxidised= release energy and water
  • waterproofing: insoluble in H2O. plants & insects waxy, lipid cuticles, conserve H2O, mammals oily secretion from sebaceous glands in skin
  • insulation: fats slow conductors of heat, and retain body heat. act as electrical insulators in myelin sheath around nerve cells
  • protection: fat stored around delicate organs
• triglycerides: a type of lipid that is a mixture of three fatty acids and one glycerol, forms ester bond in condensation reaction

  the -H is lost from the glycerol molecule & the -OH group lost in the fatty acids group= form ester bond between molecules

  • tails: made of hydrocarbons, hydrophobic, make lipids insoluble in water.
  • hydrocarbon tail varies in triglycerides
• fatty acids varies in triglycerides & can be saturated or unsaturated

  • saturated: no double bonds between carbon atoms
  • mono-unsaturated: single C=C
  • poly-unsaturated: more than one C=C
• structure of triglycerides relating to their function:

  • high ratio of energy storing carbon-hydrogen bonds to carbon atoms= good source of energy
  • low mass to energy ratio, good storage molecules, since much energy is stored in a small volume
  • large, non-polar molecule= insoluble in H2O, storage not affect osmosis in cell or H2O potential
  • triglycerides clump together to form insoluble droplet in cells, fatty acid tails= hydrophobic so face inwards away from water with glycerol heads
• PHOSPHOLIPIDS: A class of lipids that are made up of a glycerol backbone and one or more phosphate groups and two fatty acid tails, found in cell membranes

  • hydrophilic head: phosphate head
  • hydrophobic tail: two fatty acid tails
  • polar molecules, hydrophillic heads close to H2O as possible, hydrophobic tails far away as possible
• Structure of phospholipids related to their functions:

  • are polar molecules, have hydrophilic phosphate head and hydrophobic tail of two fatty acids, in aqueous environment: forms a phospholipid bilayer within the cell-surface membrane. the hydrophobic barrier formed between the inside & outside cell
  • center of bilayer= hydrophobic: water-soluble molecules can't pass
  • phospholipid structure can form glycolipids: which important for cell recognition
  • hydrophilic head help hold the surface of cell-surface membrane
• TEST FOR LIPIDS:

  emulsion test

  process:

  1. use dry, grease free test tube
  2. add 2cm3 of sample being tested, and 5cm3 of ethanol
  3. shake tube thoroughly to dissolve lipids in sample
  4. add 5cm3 of water and shake gently
  5. cloudy-white emulsion forms if lipids present
  6. as control, repeat procedure with H2O instead of sample, final solution should remain clear