Lipids - 1.5

Question 1

What characteristics do lipids share?

Answer 1

• They contain carbon, hydrogen and oxygen.
• The proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.
• They are insoluble in water.
• They are soluble in organic solvents such as alcohols and acetone.

Question 2

What are the main groups of lipids?

Answer 2

Triglycerides and Phospholipids.

Question 3

What are the roles of lipids?

Answer 3

• Cell membranes (flexibility and transfer of lipid-soluble substances across them).
• Source of energy.
• Waterproofing.
• Insulation.
• Protection.

Question 4

How do lipids provide a source of energy?

Answer 4

When oxidised, lipids provide more than twice the energy as the same mass of carbohydrates, and release valuable water.

Question 5

How do lipids help in waterproofing?

Answer 5

Lipids are insoluble in water and therefore useful as a waterproofing. Both plants and insects have waxy, lipid cuticles that conserve water, while mammals produce an oily secretion from the sebaceous glands in the skin.

Question 6

How do lipids provide insulation?

Answer 6

Fats are slow conductors of heat and, when stored beneath the body surface, help to retain body heat. They also act as electoral insulators in the myelin sheath around nerve cells.

Question 7

How do lipids provide protection?

Answer 7

Fat is often stored around delicate organs, such as the kidney.

Question 8

What is the difference between fats and oils?

Answer 8

Fats are solid at room temperature (10-20 degrees Celsius) where as oils are liquid.

Question 9

What are triglycerides made up of?

Answer 9

Three fatty acids combined with glycerol.

Question 10

What type of bond does each fatty acid from with glycerol?

Answer 10

ESTER BOND

Question 11

When is a fatty acid described as saturated?

Answer 11

When the chain has no carbon-carbon double bonds.

Question 12

When is a fatty acid described as mono-unsaturated?

Answer 12

When there is a single carbon-carbon double bond.

Question 13

When is a fatty acid said to be polyunsaturated?

Answer 13

When there is more than one carbon-carbon double bond present.

Question 14

How is the structure of triglycerides related to their properties?

Answer 14

• Triglycerides have a high ratio of energy-storing carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy.
• Triglycerides have low mass to energy ratio, making them good storage molecules because much energy can be stored in a small in a small volume. This is especially beneficially to animals as it reduces the mass they have to carry as the move around.
• Being large, non-polar molecules, triglycerides are insoluble in water. As a result, their storage does not affect osmosis in cells or the water potential of them.
• As they have a high ratio of hydrogen to oxygen atoms, triglycerides release water when oxidised and therefore provide an important source of water, especially fro organisms living in dry deserts.

Question 15

What is the difference between triglycerides and phospholipids?

Answer 15

In phospholipids, one fatty acid is replaced by a phosphate molecule.

Question 16

What are the two parts of a phospholipid?

Answer 16

• A hydrophilic ‘head’ which interacts with water but not fat (phosphate attracts water).
• A hydrophobic ‘tail’ which orients itself away from water but mixes readily with fat (fatty acids repel water).

Question 17

How is the structure of phospholipids related to their properties?

Answer 17

• Phospholipids are polar molecules, having a hydrophilic phosphate head and a hydrophobic tail of two fatty acids. This means that in an aqueous environment phospholipid molecules form a bilayer within cell-surface membranes. As a result, a hydrophobic barrier is formed between the inside and outside of a cell.
• The phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell-recognition.

Question 18

What is the name for the test for lipids?

Answer 18

The emulsion test.

Question 19

How is the emulsion test carried out?

Answer 19

• Take a completely dry and grease-free test tube.
• To 2cm3 of the sample being tested, add 5cm3 of ethanol.
• Shake the tube throughly to dissolve any liquid in the sample
• Add 5cm3 of water and shake gently
• A cloudy-white colour indicates the presence of a lipid
• As a control, repeat the procedures using water instead of the sample; the final sample should remain clear.

Question 20

Why does a cloudy colour form if lipids are present when conducting the emulsions test?

Answer 20

The cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion. Light passing though this emulsion is refracted as it passes from oil droplets to water droplets, making it appear cloudy.