1A - Lipids

Question 1

What 3 elements do lipids contain?

Answer 1

Hydrogen, carbon and oxygen.

Question 2

How do the amounts of each element vary in lipids compared to carbohydrates?

Answer 2

The proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.

Question 3

Are lipids soluble or insoluble?

Answer 3

Insoluble in water.

Soluble in organic solvents such as alcohols and acetone.

Question 4

What are the 2 main groups of lipids?

Answer 4

Triglycerides (fats and oils) and phospholipids.

Question 5

What are the main roles of lipids?

Answer 5

• Used in cell membranes.
• They are a source of energy.
• They are useful for waterproofing.
• Insulation.
• Protection.

Question 6

How are lipids used in cell membranes?

Answer 6

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

Question 7

How are lipids a source of energy?

Answer 7

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

Question 8

How are lipids used in waterproofing?

Answer 8

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 9

How are lipids used in insulation?

Answer 9

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 cells.

Question 10

How are lipids used in protection?

Answer 10

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

Question 11

What is the main difference between fats and oils?

Answer 11

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

Question 12

What are triglycerides?

Answer 12

Three fatty acids combined with a glycerol.

Question 13

What type of bond combines fatty acids and glycerol?

Answer 13

An ester bond.

Question 14

How is glycerol made and broken down?

Answer 14

Through a condensation reaction forming an ester bond then through a hydrolysis reaction which breaks this.

Question 15

What does the differences in the properties of different fats and oils come from?

Answer 15

Variations in the fatty acids.

Question 16

How many different fatty acids are there?

Answer 16

Over 70.

Question 17

What do all fatty acids have?

Answer 17

A carboxyl group (-COOH) with a hydrocarbon chain attached.

Question 18

What does it mean if a fatty acid is saturated and why?

Answer 18

There are no carbon-carbon double bonds in the hydrocarbon chain because all of the carbon are linked to the maximum possible number of hydrogen atoms (4). They are saturated with hydrogen atoms.

Question 19

What does it mean if a fatty acid is mono-unsaturated and why?

Answer 19

There is a single double bond between carbon atoms in the hydrocarbon tail.

Question 20

What does it mean if a fatty acid is poly-unsaturated and why?

Answer 20

There is there is more than one double bond between carbon atoms in the hydrocarbon tail.

Question 21

What is the structure/benefits of triglycerides?

Answer 21

• High ratio of energy-storing carbon-hydrogen bonds to carbon atoms.
• Low mass to energy ratio.
• Large, non-polar and insoluble in water.
• High ratio of hydrogen to oxygen atoms and so release water when oxidised.

Question 22

How does triglycerides having a high ratio of energy-storing carbon-hydrogen bonds to carbon atoms help their function?

Answer 22

This means that they are an excellent source of energy.

Question 23

How does triglycerides having a low mass to energy ratio help their function?

Answer 23

It makes them good storage molecules because much energy can be stored in a small volume. This is especially beneficial to animals as it reduces the mass they have to carry as they move around.

Question 24

How does triglycerides being large, non-polar and insoluble in water help their function?

Answer 24

This means that their storage does not affect osmosis in cells or the water potential of them.

Question 25

How does triglycerides having a high ratio of hydrogen to oxygen atoms and so release water when oxidised help their function?

Answer 25

This release of water provides an important source of water, especially for organisms living in dry deserts.

Question 26

Describe the shape/build of a phospholipid:

Answer 26

Similar to lipids except that one of the fatty acid molecules is replaced by a phosphate molecule.

They are made up of two parts: a hydrophilic ‘head’ which interacts with water but not with fat and a hydrophobic tail which orients itself away from water but mixes readily with fat.

Question 27

Do fatty acid molecules attract or repel water?

Answer 27

Repel water (hydrophobic).

Question 28

Do phosphate molecules attract or repel water?

Answer 28

Attract water (hydrophilic).

Question 29

What does hydrophobic mean?

Answer 29

Repel water.

Question 30

What does hydrophilic mean?

Answer 30

Attract water.

Question 31

What does it mean if a molecule is polar?

Answer 31

The molecule has two ends (poles) that behave differently in response to water for example.

Question 32

What happens when a polar phosphate molecule is placed in water?

Answer 32

It will position itself so that its hydrophilic head is as close to the water as possible and the hydrophobic tails are as far away from the water as possible.

Question 33

What is the structure of phospholipids?

Answer 33

• Polar molecules (hydrophilic phosphate head and hydrophobic tail of 2 fatty acids).
• Hydrophilic phosphate ‘heads’ made of phospholipid molecules.

Question 34

How does phospholipids being polar help their function?

Answer 34

In aqueous environments, phospholipid molecules form a bilayer within cell-surface membranes. As a result, a hydrophobic barrier is formed between the inside and the outside of a cell.

Question 35

How does phospholipids having hydrophilic phosphate ‘heads’ made of phospholipid molecules help their function?

Answer 35

They help to hold at the surface of the cell-surface membrane.

Question 36

How does phospholipids structure in general help their function?

Answer 36

It allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell recognition.

Question 37

What is the test for lipids?

Answer 37

Emulsion test.

Question 38

What is a positive result in the emulsion test?

Answer 38

A cloudy-white colour (emulsion).

Question 39

How do you carry out the emulsion test?

Answer 39

1. Take a completely dry and grease-free test tube.
2. To 2cm3 of the sample being tested, add 5cm3 of ethanol.
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; the final solution should remain clear.

Question 40

What is the cloudy colour in the emulsion test due to?

Answer 40

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