3.1.3 Lipids

Question 1

2 groups of lipid

Answer 1

• Triglycerides (fats and oils)
• Phospholipids

Question 2

Characteristics of lipids

Answer 2

• Water insoluble
• Soluble in organic solvents (e.g. alcohols)
• Contain C, H and O. Contain less O compared to carbohydrates

Question 3

How are triglycerides formed?

Answer 3

Condensation of 3 fatty acid molecules and 1 glycerol molecule
* Condensation reaction between glycerol and fatty acid (RCOOH) = ester bond

Question 4

Roles of lipids

Answer 4

• Cell-surface membranes
• Source of energy
• Insulation
• Protection

Question 5

Roles of lipids: Cell-surface membranes

Answer 5

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

Question 6

Roles of lipids: Source of energy

Answer 6

When lipids are oxidised:
* lipids produce more than twice the amount of energy that the same amount of carbohydrate produces
* releases valuable water

Question 7

Roles of lipids: Insulation

Answer 7

• Slow conductors of heat
• so when beneath the body surface, help to retain heat
• electric insulator in the myelin sheath around nerve cells

Question 8

Roles of lipids: Protection

Answer 8

• Stored around delicate organs (e.g. kidney)

Question 9

Triglycerides

Answer 9

• Made up of 3 fatty acids and 1 glycerol

Question 10

What causes the differences in the properties of different triglycerides (fats and oils)?

Answer 10

• Variations in the fatty acids (many different ones)

Question 11

Structure of fatty acid

Answer 11

• Carboxyl (-COOH) group attached to hydrocarbon

Question 12

Different types of fatty acid

Answer 12

• Saturated
• Mono-unsaturated
• Polyunsaturated

Question 13

Fatty acids: Saturated

Answer 13

No double bonds between adjacent C atoms in a hydrocarbon chain

Question 14

Fatty acids: mono-unsaturated

Answer 14

1 double bond between adjacent C atoms in a hydrocarbon chain

Question 15

Fatty acids: polyunsaturated

Answer 15

more than 1 double bond between adjacent C atoms in a hydrocarbon chain

Question 16

Adaptations of triglycerides

Answer 16

• High energy ratio stored in C-H bonds: C atoms = good source of energy
• Low mass: energy ratio = good storage molecules (beneficial to animals as less mass to carry as they move around)
• Large and non-polar, so water insoluble = storage of lipids doesn’t affect osmosis or water potential
• High H: O ratio = triglycerides release water when oxidised

Question 17

Phospholipids

Answer 17

• 1 fatty acid in lipids is replaced by a phosphate molecule

Question 18

Polarity of triglycerides

Answer 18

Non-polar

Question 19

Polarity of phospholipids

Answer 19

polar

Question 20

Parts of a phospholipid

Answer 20

• Hydrophilic phosphate head
• Hydrophobic fatty acids tail
• The fatty acids face away from water, and the phosphate faces towards water

Question 21

Hydrophilic

Answer 21

Attracted to water

Question 22

Hydrophobic

Answer 22

Repels water

Question 23

Adaptations of phospholipids

Answer 23

• Polar = in aqueous environments form a bilayer in cell-membranes (forms hydrophobic barrier between inside and outside of cell)
• Hydrophilic phosphate heads = help to hold
• Structure allows them to form glycolipids (by combining with carbohydrates within the cell-surface membrane)

Question 24

Glycolipids

Answer 24

• Phospholipids + carbohydrates
• Important for cell recognition

Question 25

Test for lipids

Answer 25

1. To a dry and grease-free test tube, add the sample and ethanol
2. Shake thoroughly (to dissolve the lipid)
3. Add water
4. Shake gently
5. If lipid is present, a cloudy white emulsion will form

• As a control, repeat with water instead of sample (should remain clear)

Question 26

Why does a cloudy-white emulsion form?

Answer 26

Dispersion of lipid in the water

Question 27

How to test a solid sample for lipids?

Answer 27

1. Dissolve in alcohol
2. Add water