Lipids

Question 1

Characteristics of Lipids

Answer 1

• Contain carbon, hydrogen and oxygen.
• Proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.
• Insoluble in water.
• Soluble in organic solvents such as alcohols and acetone.

Question 2

What are the two main groups of lipids?

Answer 2

Triglycerides (fats and oils) and phospholipids.

Question 3

What is the role of Lipids in cell membranes?

Answer 3

Phospholipids contribute to the flexibility of membranes and transfer of lipid-soluble substances across them.

Question 4

Roles of lipids

Answer 4

• source of energy
• waterproofing (insoluble in water)
• insulation (fats are slow conductors of heat), act as electrical insulators in the myelin sheath around nerve cells.
• protection (stored around delicate organs)

Question 5

What are the components of triglycerides?

Answer 5

Three fatty acids - (tri)
Glycerol - (glyceride)
Fatty acids form an ester bond with glycerol in a condensation reaction.

Question 6

What are the properties of a saturated fatty acid?

Answer 6

Chain has no carbon-carbon double bonds, all carbon atoms are links to maximum possible number of hydrogen atoms.

Question 7

What are the properties of a mono-unsaturated fatty acid?

Answer 7

Single carbon-carbon double bond in the chain.

Question 8

What are the properties of a poly-unsaturated fatty acid?

Answer 8

More than one carbon-carbon double bond present in the chain.

Question 9

Why is it significant that tri-glycerides have a high ratio of carbon-hydrogen bonds to carbon atoms?

Answer 9

Carbon-hydrogen bonds are energy storing, therefore this property makes them an excellent source of energy.

Question 10

Why is the low mass to energy ratio of triglycerides important?

Answer 10

They are good storage molecules, this means much energy can be stored in small volume. Especially important to animals as it reduces the mass they carry as they move around.

Question 11

What is the significance of the molecular properties of triglycerides?

Answer 11

Large, non-polar molecules, and so are insoluble in water. Storage doesn’t affect osmosis in cells or the water potential of them.

Question 12

How do triglycerides provide an important source of water?

Answer 12

They have a high ratio of hydrogen to oxygen atoms, therefore they release water when oxidised, especially important for organisms living in dry deserts.

Question 13

What are phospholipids?

Answer 13

Phospholipids are similar to lipids except one fatty acid tail is replaced by a phosphate molecule.

Question 14

What are the two parts of a phospholipid molecule?

Answer 14

Hydrophilic ‘head’ - attracts water but not fat.

Hydrophobic ‘tail’ - mixes with fat but not water.

Question 15

What does it mean if a molecule is polar?

Answer 15

The molecule has two ends that behave in a different way.

Question 16

How do phospholipid molecules position themselves in water?

Answer 16

Hydrophilic heads are as close to the water as possible and hydrophobic tails are as far away as possible.

Question 17

What do phospholipids form in an aqueous environment?

Answer 17

A bilayer within cell surface membranes, therefore a hydrophobic barrier is formed between the inside and outside of a cell.

Question 18

How can the hydrophobic phosphate heads help with the structure of membranes?

Answer 18

They help to hold at the surface of the cell-surface membrane.

Question 19

How are glycolipids formed from phospholipids?

Answer 19

Phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane.

Question 20

Why are glycolipids important?

Answer 20

They are important in cell recognition.

Question 21

Test for Lipids : The Emulsion Test

Answer 21

• Use a completely dry and grease free test tube.
• Add 5cm3 of ethanol to 2cm3 of sample.
• Shake thoroughly to dissolve any lipid in sample.
• Add 5cm3 of water and gently shake.
• A cloudy white colour indicates presence of lipid.

Question 22

What can be done as a control of the emulsion test?

Answer 22

Repeat procedure using water instead of the sample, the final solution should remain clear.

Question 23

Why does a cloudy precipitate form as a result of the emulsion test?

Answer 23

Lipid in the sample is finely dispersed in the water to form an emulsion. Light passing through the emulsion is refracted from oil droplets to water droplets, making it appear cloudy.