1.5 Lipids

Question 1

What are the characteristic of lipids?

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 three elements do lipids contain?

Answer 2

Carbon, hydrogen and oxygen

Question 3

What are the main groups in a lipid?

Answer 3

Triglycerides (fats and oils) and phospholipids

Question 4

What are the roles of lipids?

Answer 4

• source of energy → when oxidised, lipids provide more than double the energy as the same mass of carbohydrate
• waterproofing → lipids are insoluble in water, both plants and insects have waxy, lipid cuticles that conserve water, while mammals produce an oily secretion from sebaceous glands in the skin
• insulation → fats are slow conductors of heat and when stored beneath the body surface help to retain body heat, they also act as electrical insulators in the myelin sheath around nerve axons
• protection → fat is often stored around delicate organs, such as the kidney

Question 5

What is a triglyceride comprised of?

Answer 5

• three fatty acids

* glycerol

Question 6

What bond is present in lipids?

Answer 6

Ester bonds

Question 7

How is an ester bond formed?

Answer 7

Condensation reaction

Question 8

How do triglycerides vary?

Answer 8

Variations in the fatty acids, all glycerol molecules are the same.

Question 9

mono-unsaturated

Answer 9

a molecule with one carbon-carbon double bond

Question 10

poly-unsaturated

Answer 10

a molecule with more than one carbon-carbon double bond

Question 11

How does the structure of triglycerides relate to its properties?

Answer 11

• high ratio of energy-storing carbon-hydrogen bonds to carbon atoms → so excellent store of energy
• low mass to energy ratio → good storage molecules, lots of energy can be stored in a small volume, beneficial to animals as less mass to carry around
• large, non-polar molecules, insoluble → don’t affect water potential or osmosis
• high ratio of hydrogen to oxygen atoms → triglycerides release water when oxidised and so provide an important source of water

Question 12

How does a phospholipid differ from a lipid?

Answer 12

One fatty acid molecule is replaced by a phosphate molecule

Question 13

hydrophobic

Answer 13

repel water

Question 14

hydrophilic

Answer 14

attract water

Question 15

Is a phosphate molecule hydrophobic or hydrophilic?

Answer 15

Hydrophilic

Question 16

What are the two parts that make a phospholipid?

Answer 16

• hydrophilic ‘head’ → phosphate, interacts with water but not with fat
• hydrophilic ‘tail’ → fatty acid, orients itself away from water but mixes readily with fat

Question 17

How is the structure of phospholipids related to its properties?

Answer 17

• polar molecules → hydrophilic phosphate head and hydrophobic tail of two fatty acids, in an aqueous environment, phospholipid molecules form a bilayer within cell-surface membranes
• hydrophilic phosphate heads → help to hold at the surface of the cell-surface membrane
• phospholipid structure allows them to form glycolipids → by combining with carbohydrates within cell-surface membrane, these glycolipids are important for cell recognition

Question 18

Test for lipids

Answer 18

Emulsion test

Question 19

Emulsion test

Answer 19

1. Take a dry and grease-free test tube
2. Add 5cm³ of ethanol to 2cm³ of the sample being tested
3. Shake the tube thoroughly to dissolve any lipid in the sample
4. Add 5cm³ of water and shake gently
5. A cloudy-white colour indicates the presence of a lipid
6. As a control, repeat the procedures using water instead of the sample, final solution should remain clear

Question 20

Why will a lipid turn cloudy in the emulsion test?

Answer 20

Cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion.