Lipids

Question 1

Describe 4 characteristics of lipids.

Answer 1

• Contains carbon, hydrogen, and oxygen.
• The proportion of C, H, O2, is smaller than the proportion in carbohydrates.
• Insoluble in water.
• Soluble in organic solvents e.g. alcohol, acetone.

Question 2

What are the main groups of lipids?

Answer 2

Triglycerides (fats and oils) and phospholipids.

Question 3

What are the roles of lipids in the cell membrane?

Answer 3

• In the cell membranes (cell-surface membranes and membranes around the organelles) It contributes towards the membrane’s flexibility and transfer of lipid-soluble substances across them.

Question 4

List 4 more roles of lipids.

Answer 4

• Source of energy - When oxidised, lipids release x2 the energy as the same mass of carbohydrate and it releases water.
• Waterproofing - Lipids are insoluble in water so it can be waterproof. Plants and insects have waxy lipid cuticles to conserve water but mammals produce an oily secretion from the sebaceous gland in the skin.
• Insulation - Fats are a slow conductor of heat so when it’s stored under the body’s surface, it helps the body to retain heat. They also act as electrical insulators in the myelin sheath around nerve cells.
• Protection - They often stored around delicate organs e.g. kidney.

Question 5

What is the difference between fats and oils?

Answer 5

Fats are solid at RT, but oils are liquid.

Question 6

What is a triglyceride?

Answer 6

A molecule with 3 fatty acids combined with glycerol.

Question 7

How are the fatty acids attach themselves to the glycerol?

Answer 7

Each fatty acid forms an ester bond with glycerol in a condensation reaction. Therefore hydrolysis produces glycerol and 3 fatty acids.

Question 8

Draw the formation of a triglyceride.

Answer 8

Draw on a piece of paper.

Question 9

What is saturated fat?

Draw the structure

Answer 9

There are over 70 different fatty acids and they all have carboxyl (-COOH) with a hydrocarbon chain attached, if the chain has no carbon-carbon double bonds, it’s SATURATED. Because all of the carbon atoms are linked to the maximum number of hydrogen atoms.

Question 10

What is it called when it only has a single double bond between the carbon atoms? And draw the structure.

Answer 10

Mono-unsaturated.

Question 11

What is it called when more than one double bond between the carbon atoms? Draw structure.

Answer 11

Polyunsaturated.

Question 12

Describe the structure of triglyceride and how it is related to their functions.

Answer 12

• They are a great source of energy because they have a high ratio of energy- storing carbon-hydrogen bonds to carbon atoms.
• Low mass to energy ratio so they are a good storage molecule because more energy can be stored in a small volume. This benefits animals because it allows them to carry around a lot of energy at a smaller mass.
• Large, non-polar molecules, triglycerides are insoluble in water. So their storage does not affect osmosis in cells or the water potential of them.
• High ratio of hydrogen to oxygen atoms, triglyceride release water when oxidised and therefore it provides source water, good for organisms living in dry deserts.

Question 13

What is the difference between phospholipids and lipids?

Answer 13

It’s similar to lipids but one of the fatty acids is replaced by phosphate molecule.
Fatty acid molecules repel water - hydrophobic.
Phosphate molecules attract water - hydrophilic.

Question 14

What is phospholipid made up of? Draw the structure of a phospholipid.

Answer 14

• A hydrophilic ‘head’- attracted to water but not with fat.

- A hydrophobic ‘tail’ - away from water but readily mixed with fat.

Question 15

What does polar mean?

Answer 15

Molecules that have 2 ends (poles) that behaves differently are polar.

Question 16

How does the polar phospholipid molecules work?

Answer 16

The hydrophilic heads are close to the water and hydrophobic tails are as far as possible from the water.

Question 17

How do phospholipids structure relate to their properties?

Answer 17

• Phospholipids are polar molecules, hydrophilic head and hydrophobic tail of 2 fatty acids. So in an aqueous environment, they form a bilayer within cell-surface membranes. So a hydrophobic barrier is formed between the inside and outside of a cell.
• The hydrophilic phosphate head helps to hold at the surface of the cell-surface membrane.
• The phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane- those glycolipids are important in cell recognition.

Question 18

What is the name for the test for lipids?

Answer 18

An emulsion test.

Question 19

How is the emulsion test carried out?

Answer 19

1) Take a dry and grease free test tube.
2) Mix 2cm3 of the sample with 5cm3 of ethanol.
3) Shake and make sure all lipids are dissolved.
4) Add 5cm3 of water and shake gently.
5) A cloudy white color indicates the presence of lipid.
6) As a control, use water instead of sample and the solution should stay as clear.

Question 20

Why is the result of a cloudy color?

Answer 20

Because the lipid is finely dispersed in the water to form an emulsion and as light passes through the emulsion, the light gets refracted from oil to water droplet making it to appear cloudy.

Question 21

What bond does a condensation reaction form between glycerol and a fatty acid?

Answer 21

An ester bond. (RCOOH)

Question 22

Draw the structure of triglyceride and phospholipid.

Answer 22

Draw on a piece of paper.

R group of a fat is basically the long chain of hydrocarbons.

Question 23

Why do animals intend to have saturated fat whereas plants intend to have unsaturated fat?

Answer 23

If fatty acids are unsaturated, their shape is altered from a saturated molecule so the molecules in the Lipid push apart, thus making it more fluid and oily.

Animals tend to have more saturated, and consequently solid at room temperature lipids whereas plants tend to have more unsaturated and so fluid at room temperature lipids.