BM: Lipids

Question 1

Name two types of lipids:

Answer 1

1. Triglycerides
2. Phospholipids

Question 2

What characteristics do all lipids share?

Answer 2

• Contain hydrogen, carbon and oxygen
• Proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates
• Insoluble in water
• Soluble in organic substances like alcohols and acetone

Question 3

What are triglycerides made of?

Answer 3

One molecule of glycerol and three fatty acid tails.

Question 4

Draw the structure of a triglyceride:

Answer 4

Question 5

Describe the structure of fatty acid molecules.

How does this relate to the characteristics of a lipid?

Answer 5

Molecules have tails made of hydrocarbons.

Basic structure of fatty acid is constant - but hydrocarbon tail varies.

Tails are hydrophobic - they repel water molecules, meaning lipids are insoluble in water.

Question 6

How are triglycerides formed?

Answer 6

Through three condensation reactions - each one results in a fatty acid bonding to the glycerol molecule.

Question 7

What joins the fatty acid to the glycerol molecule in triglycerides?

What is released when this happens?

Answer 7

An ester bond.

Water molecule is released when ester bond forms.

Question 8

Draw the basic structure of a fatty acid:

Answer 8

• carbon atom links fatty acid to glycerol
• variable ‘R’ group indicates the hydrocarbon tail.

Question 9

Draw the condensation reaction that leads to the formation of a triglyceride:

Answer 9

Question 10

What does the hydrolysis of a triglyceride produce?

Answer 10

A glycerol molecule and three fatty acids.

Question 11

What are the two types of fatty acids?

Where does the difference between these two types occur?

Answer 11

Saturated and unsaturated.

The differences between these two types occurs in their hydrocarbon tails.

Question 12

What is the difference between saturated and unsaturated fatty acids?

Answer 12

Saturated = don’t have any double bonds between their carbon atoms.

Unsaturated = have at least one double bond between carbon atoms - this causes the chain to kink.

Question 13

What is the main use of triglycerides?

Answer 13

Energy storage molecules.

Question 14

What is it called when there is only one double bond present in a fatty acid?

What is it called when there is more than one double bond present in a fatty acid?

Answer 14

Mono-unsaturated.

Polyunsaturated.

Question 15

How does the structure of triglycerides relate to their function?

Answer 15

• High ratio of energy-storing carbon-hydrogen bonds present in fatty acid tails - this chemical energy is released when broken down.
• Insoluble as they are large and non-polar - so their storage does not affect the water potential of the cell/cause water to enter by osmosis.
• Low mass to energy ratio - a lot of energy can be stored in a small volume, which is beneficial to animals as they then have less mass to carry.
• High ratio of hydrogen to oxygen atoms - so water is released when they are oxidised, meaning they are a good water source for animals living in a dry, desert habitat.

Question 16

What do triglycerides do when in cells?

Answer 16

They clump together as insoluble droplets (as tails are hydrophobic) - tails face inwards, shielding themselves from water with glycerol heads.

Question 17

What lipids are found in cell membranes?

Answer 17

Phospholipids.

Question 18

What is the difference between a phospholipid and a triglyceride?

Answer 18

In a phospholipid, one of the fatty acid molecules is replaced by a phosphate group.

Question 19

Draw the structure of a phospholipid:

Answer 19

Question 20

Describe the hydrophilic head and the hydrophobic tail of a phospholipid:

Answer 20

Hydrophilic head = interacts with water (is attracted to it), but not with fat.

Hydrophobic tail= orients itself away from water, but mixes readily with fat.

Question 21

Molecules that have two poles that act in different ways, are said to be what?

Answer 21

Polar

Question 22

Describe the polarity of phospholipids.

What does this mean when they enter water?

Answer 22

Phospholipids are polar.

• Therefore, when they are placed in water, the hydrophilic heads position themselves as close tot he water as possible and the hydrophobic tails get as far away from the water as they can.*
• This happens in a double layer - forming a bilayer.*

Question 23

How does the structure of phospholipids relate to their function?

Answer 23

• Polar molecules - hydrophobic tails and hydrophilic heads mean that, in aqueous environments, they form a bilayer - a double layer with heads facing outwards on either side.
• Hydrophobic tails - means the centre of the bilayer is hydrophobic, so water-soluble substances can’t easily pass through it and the membrane acts as a barrier.
• Can easily form glycolipids by combining with carbohydrates within the cell-surface membrane - these are important in cell recognition.

Question 24

What is the role of phospholipids?

Answer 24

They make up the bilayer of cell membranes.

Question 25

What is the test for **lipids**?

Answer 25

The emulsion test.

Question 26

What is the positive result in the **emulsion test?** *What causes this?*

Answer 26

Cloudy-white emulsion. ## Footnote * Caused as lipid sample is finely dispersed in the water to form an emulsion.* * Light passing through is refracted as it passes from oil droplets to water droplets, making it appear cloudy.*

Question 27

Describe the **emulsion test**:

Answer 27

1. Add 2cm³ sample to dry, grease free test tube. 2. Add 5cm³ ethanol and shake thoroughly - ensures any lipid in sample is dissolved. 3. Add 5cm³ of water and shake gently. 4. Cloudy-white emulsion indicates the presence of lipid. ## Footnote *The intensity of the emulsion colour depends on amount of lipid present.*

Question 28

Give **5 roles** of lipids:

Answer 28

* **Cell membranes -** flexibility of membranes and the transfer of lipid-soluble molecules across them. * **Source of energy -** more than twice the energy as the same mass of carbohydrates. * **Waterproofing -** as they are insoluble, many plants/insects have waxy, lipid cuticles to conserve water and mammals produce oily secretion from sebaceous glands in skin. * **Insulation -** slow conductors of heat so when stored beneath body surface they help maintain body heat. * **Protection -** fat is often stored around vital organs like kidneys.

Question 29

What is the difference between fats and oils?

Answer 29

Fats are solid at room temperature (10-20°C) whereas oils are liquid.