biological molecules - 3.13 - (lipids)

Question 1

what are the two groups of lipid?

Answer 1

triglycerides and phospholipids

Question 2

what are triglycerides and phospholipids made of?

Answer 2

triglycerides - glycerol and 3 fatty acids
phospholipids - glycerol, 2 fatty acids and a phosphate group

Question 3

what are the properties of lipids?

Answer 3

-contain hydrogen. oxygen and carbon
-proportion of oxygen to carbon/hydrogen is smaller than in carbohydrates
-insoluble in water
- soluble in organic solvents such as alcohols and acetone.

Question 4

what are the roles of lipids?

Answer 4

• source of energy - when oxidised, lipids provide more than twice the energy as the same mass of carbohydrate and valuable water
• waterproofing- insoluble so useful for waterproofing. Plants/insects have waxy, lipid cuticles that conserve water, while mammals produce oily secretions from skin glands
  -insulation- fats are slow heat conductors so help retain body heat. Also act as electrical insulators around the myelin sheath around neurones.
  -protection - fats is stored around delicate organs, e.g. kidneys

Question 5

what is the structure of a fatty acid?

Answer 5

a methyl group CH3, followed by a long hydrocarbon chain, ending with a carboxylic acid chain (-COOH)

Question 6

what are saturated/unsaturated fatty acids?

Answer 6

saturated - no double bonds (all carbon bonds linked to maximum possible number of hydrogen bonds)
unsaturated - one or more double bond between carbon bond

Question 7

what is the structure of a glycerol molecule?

Answer 7

CH2OH
CH OH
CH2OH

Question 8

how are triglycerides formed?

Answer 8

Each fatty acid forms an ester bond with glycerol in a condensation reaction. Hydrolysis of a triglyceride therefore produces 3 fatty acids and glycerol.

Question 9

How does the structure of triglycerides relate to their function?

Answer 9

-low mass : energy ratio - good storage molecules as much energy can be stored in a small volume
- large and non-polar molecules - insoluble in water so doesn’t affect water potential or osmosis in cells
-high ratio of hydrogen : oxygen - triglycerides release water when oxidised and therefore provide an important source of water, especially for organisms living in dry deserts
- high ratio of energy-storing-carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy

Question 10

what 2 parts are phospholipids made of?

Answer 10

a hydrophilic ‘head’ (the glycerol and phosphate group) which attract water, and a hydrophobic ‘tail’ (the 2 fatty acids) which repel water but mix with fat.

Question 11

how is the structure of a phospholipid related to its function?

Answer 11

The polar molecules form a bilayer in cell membranes. the hydrophilic heads are positioned as close to the water as possible, and the hydrophobic tails positioned so they are as far away from water as possible. as a result, a hydrophobic barrier is formed between the inside and outside of the cell.

Question 12

what is the test for lipids?

Answer 12

-take a completely dry and grease free tube
- to 2cm^3 of the sample being tested, add 5cm^3 of ethanol.
-shake the tube thoroughly to dissolve any lipid in the sample
-add 5cm^3 of water and shake gently
-a cloudy-white emulsion indicates the presence of a lipid
-as a control, repeat the procedures using water instead of the sample; the final solution should remain clear.