Lipids

Question 1

Lipids

Answer 1

• Molecules made of C,H, O (P).
• Only soluble in organic solvents like alcohols and acetone.
• Main lipid groups are triglycerides and phospholipids.
• When oxidised, lipids provide more energy than the same mass of carbohydrate due to long C-H chains, also releases water when oxidised.
• Insoluble in water therefore provide waterproofing.
• Slow conductors of heat so help to retain body heat. Also act as electrical insulators in myelin sheaths around nerve cells.
• Stored around delicate organs for protection.

Question 2

Triglycerides

Answer 2

• Comprised of 3 fatty acids and one glycerol molecule.
• Each fatty acid forms an ester bond with the glycerol in a condensation reaction.
• Differences come from fatty acid hydrocarbon chains. (Over 70 fatty acids) All fatty acids contain a carboxyl group.
• Saturated fatty acids have no C=C double bonds as all carbon atoms are linked to the maximum possible hydrogen atoms.
• If there are any C=C double bonds, the acid is unsaturated. (For one double bond, monounsaturated. More than one, polyunsaturated.)

Question 3

Structure of triglycerides

Answer 3

• High ratio of energy storing C-H bonds to carbon atoms so excellent energy source.
• Low mass to energy ratio making them a good storage molecule.
• Large and non-polar so insoluble in water so has no effect on water potential in cells.
• High ratio of H to O atoms so release water when oxidised so important source of water.

Question 4

Phospholipids

Answer 4

• Two fatty acid chains and one phosphate molecule attached to a glycerol.
• Fatty acids are hydrophobic, phosphate is hydrophilic meaning phospholipid forms a hydrophilic head and a hydrophobic tail making it an amphipathic polar molecule.
• Phospholipids form micelles when in contact with water forming a phospholipid bilayer.

Question 5

Structure of phospholipids

Answer 5

• As they are amphipathic, they form a phospholipid bilayer in an aqueous environment allowing a hydrophobic barrier to be made between the inside and outside of cells.
• The phosphate heads hold at the surface of the cell-surface membrane.
• Their structure allows them to form glycolipids when combined with carbohydrates within the cell membrane. These glycolipids are important in cell recognition.

Question 6

Emulsion test for lipids

Answer 6

• Dry, grease free test tube.
• Add 2cm^3 of sample and 5cm^3 of ethanol.
• Shake the tube with a bung to dissolve lipid that is in the sample.
• Add 5cm^3 of water and shake gently.
• A cloudy white emulsion indicates the presence of a lipid.
• To control, repeat using water instead of the sample, control should remain clear.