1.3 Lipids

Question 1

What are lipids?

Answer 1

Lipids are biological molecules made of carbon, hydrogen and oxygen which are only soluble in organic solvents such as alcohols

Question 2

What are the main types of lipids?

Answer 2

Trigylcerides and phospholipids

Question 3

How do triglycerides form?

Answer 3

A condensation reaction between 1 molecule of glycerol and 3 fatty acids forms ester bonds

Question 4

What are the two types of fatty acids?

Answer 4

Saturated and unsaturated

Question 5

Describe saturated fatty acids

Answer 5

• Contain only single bonds
• Straight-chain molecules have many contact points
• Higher melting point = solid at room temperature
• Found in animal fats

Question 6

Describe unsaturated fats

Answer 6

• Contains C=C double bonds
• ‘Kinked’ molecules have fewer contact points
• Lower melting point = liquid at room temperature
• Found in plant oils

Question 7

Relate the structure of triglycerides to their functions

Answer 7

• High ratio of energy storing carbon-hydrogen bonds to carbon atoms = excellent energy store
• Low mass : energy ratio = good storage molecule
• Insoluble hydrocarbon chain = no effect on water potential of cells and used for waterproofing
• High ratio of hydrogen-oxygen atoms = release water when oxidised
• Less dense than water = buoyancy of aquatic animals

Question 8

Explain the difference of a phospholipid

Answer 8

One of the fatty acids of a triglyceride is substituted by a phosphate-containing group

Question 9

Describe the structure of a phospholipid

Answer 9

Phosphate heads are hydrophilic (loves water) and the tails are hydrophobic (hates water) and as a result phospholipids form micelles when they are in contact with water. The molecule is therefore polar

Question 10

Phospholipids structure

Answer 10

Amphipathic molecule: glycerol backbone attached to 2 hydrophobic fatty acid tails and 1 hydrophilic polar phosphate head

Question 11

Describe the function of phospholipids

Answer 11

• Forms phospholipid bilayer in water = component in membranes, the hydrophilic heads can be used to hold at the surface of the cell surface membrane
• Their structure allows them to form glycolipids with carbohydrates which are important on the cell surface membrane for cell recognition
• Tails can splay outwards = waterproofing

Question 12

Compare phospholipids and triglycerides

Answer 12

• Both have glycerol backbone
• Both may be attached to a mixture of saturated, monosaturated and polysaturated fatty acids
• Both contain the elements C, H, O
• Both formed by condensation reactions

Question 13

Contrast phospholipids and triglycerides

Answer 13

Phospholipids:
- 2 fatty acids and 1 phosphate group attached
- Hydrophilic head and hydrophobic tail
- Used primarily in membrane formation

Triglycerides:
- 3 fatty acids attached
- Entire molecule is hydrophobic
- Used primarily as a storage molecule

Question 14

Describe how to test for lipids in a sample

Answer 14

1. Dissolve solid samples in ethanol
2. Add an equal volume of water and shake
3. Positive result: milky white emulsion forms