Lipids

• Lipids:

  • the elements contained in lipids are carbon, hydrogen and oxygen. Lipids are individual molecules, not polymers. They're not made up of repeating monomers. They're made of different individual molecules
  • lipids are a large and varied group of non-polar molecules that are insoluble in water, but dissolve easily in organic solvents, like alcohols and acetone
  • fats and oils are lipids. Fats are solid at room temperature and oils are liquid at room temperature
• Functions of lipids:

  • they form the main part of cell membranes, are a source of energy (they store twice the amount of energy per gram compared to carbohydrates). They can also provide a waterproof layer, insulation and protection around major organs
• Triglycerides:

  • triglycerides are made up of one molecule of glycerol and 3 fatty acids attached to it
  • glycerol has 3 hydroxyl groups (-OH), each of which can combine with a fatty acid forming a triglyceride. The formation of a triglyceride is a condensation reaction resulting in the production of 3 molecules of water
  • the bonds formed between the -OH groups of the glycerol and the -COOH of the fatty acids are called ester bonds (strong covalent bonds)
  • triglycerides have many biological roles such as energy storage, insulation (electrical and heat) and protection
• Structure of triglycerides:

  • have a high ratio of energy storing carbon-hydrogen bonds to carbon atoms (long fatty acid tails); contain lots of chemical energy - this makes it good for energy storage
  • have a low mass to energy ratio - this makes them good storage molecules because so much energy can be stored in a small volume (they contain about twice as much energy per gram as carbs)
  • large non-polar molecules that are insoluble in water - they have no osmotic effect and do not effect the water potential in cells. They can clump together in insoluble droplets (hydrophobic tails facing inwards)
• Fatty acids:

  • fatty acids are organic compounds that have the general formula CH3(CH2)nCOOH, where n usually ranges from 2 to 28 and is always an even number. The general formula can be written as RCOOH
  • fatty acids are made of a carboxylic acid group (-COOH) and a long hydrocarbon chain of varying length
  • fatty acids are hydrophobic so repel water. All fatty acids have the same basic structure, but their hydrocarbon tails can vary (variable 'R' group). The hydrocarbon chain of a fatty acid can be saturated or unsaturated
• Saturated fatty acid:

  • the thing that fatty acids are saturated with is hydrogen; in a saturated fat, as many hydrogen atoms as possible are attached to the carbon skeleton
  • there are only single bonds between carbon atoms (no double bonds)
  • the molecules can align readily so saturated fats are solid at room temperature. They remain semi-solid at body temperature so they're good for storage in mammals
  • the general formula is CnH2nO2
• Unsaturated fatty acid:

  • have at least one double bond between carbon atoms. The double bond in the unsaturated fatty acids creates a kink in the chain, whereas single bonds will leave a straight chain
  • unsaturated fatty acid molecules cannot align uniformly and the more double bonds that are in the chain, the more unsaturated the fatty acid is, the less straight the chain and the more fluid it will be
  • unsaturated fatty acids means the lipid will not solidify readily. Unsaturated lipids are oils and remain liquid at room temperature
• Phospholipids:

  • phospholipids are specialised lipids that are are similar to triglycerides but one of the fatty acids is substituted for a phosphate group
  • a phospholipid has a hydrophobic and hydrophilic part. They're composed of: a hydrophilic head (attracts water), which consists of the glycerol and phosphate and 2 hydrophobic fatty acid tails (repel water). These can be saturated or unsaturated and are insoluble in water because the fatty acids are non-polar
  • when multiple phospholipids come together in water, they form a bilayer (or micelle). Phospholipid bilayers make up cell membranes
• Structure and function of phospholipids:

  • they have a hydrophilic head and a hydrophobic tail - this allows them to form a double layer (bilayer) with heads facing outwards towards the water on each side
  • the centre of the cell membrane bilayer is hydrophobic - water-soluble substances can't pass through easily (forms a barrier)
• Test for lipids:

  the test for lipids is known as the ethanol emulsion test;

  • using a completely dry clean test tube add ethanol to the sample to be tested
  • shake thoroughly to dissolve any lipid in the sample
  • add water to the sample and shake gently

  a positive result for the presence of lipids using this test is a milky white colour/emulsion. This happens because the ethanol (alcohol) dissolves lipids