1.3 lipids

• Name two groups of lipid ? Describe the structure of a fatty acid (RCOOH)? and Describe the difference between saturated and unsaturated fatty acids?

  • Triglycerides and phospholipids
  • Variable R-group - hydrocarbon chain (saturated or unsaturated)
  • -COOH = carboxyl group
  • Saturated: no C=C double bonds in hydrocarbon chain; all carbons fully saturated with hydrogen
  • Unsaturated: one or more C=C double bond in hydrocarbon chain (creating bend / kink)

  
• Describe how triglycerides form?

  ● 1 glycerol molecule and 3 fatty acids undergo a Condensation reaction ● Removing 3 water molecules ● Forming 3 ester bonds.

  
• Explain how the properties of triglycerides are related to their structure?

  Function: energy storage

  ● High ratio of C-H bonds to carbon atoms in hydrocarbon chain

  ○ So used in respiration to release more energy than same mass of carbohydrates

  ● Hydrophobic / non-polar fatty acids so insoluble in water (clump together as droplets)

  ○ So no effect on water potential of cell (or can be used for waterproofing)
• Describe structure and function of phospholipids?

  One of the fatty acids of a triglyceride is substituted by a phosphate-containing group

  Function: form a bilayer in cell membrane, allowing diffusion of lipid-soluble (non-polar) or very small substances and restricting movement of water-soluble (polar) or larger substances

  ● Phosphate heads are hydrophilic

  ○ Attracted to water so point to water (aqueous environment) either side of membrane

  ● Fatty acid tails are hydrophobic

  ○ Repelled by water so point away from water / to interior of membrane

  
• Describe the test for lipids?

  1. Add ethanol, shake (to dissolve lipids), then add water

  2. Positive = milky white emulsion
• Exam insight: common mistakes ❌- (1/6)

  ❌-“Phospholipids have a phosphorus group.”

  ✅-Phospholipids have a phosphate group (PO4 3- ).
• Exam insight: common mistakes ❌- (2/6)

  ❌-“Phospholipids don’t contain glycerol.”

  ✅-Both phospholipids and triglycerides contain glycerol.
• Exam insight: common mistakes ❌- (3/6)

  ❌-“To test for lipids, add water then ethanol.”

  ✅-Ethanol is added first and this is crucial to pick up the mark
• Exam insight: common mistakes ❌- (4/6)

  ❌-“A positive test for lipids is cloudy

  ✅-This is too vague. You need to use the term ‘emulsion’.
• Exam insight: common mistakes ❌- (5/6)

  ❌-“Phospholipids have phosphodiester bonds.

  ✅-Phospholipids have ester bonds. Phosphodiester bonds are found in nucleic acids such as DNA and RNA.
• Exam insight: common mistakes ❌- (6/6)

  ❌-“In the test for lipid, ethanol is heated.”

  ✅-Only Benedict's solution (for sugars) requires heating.