Lipids

Question 1

What are characteristics of lipids?

Answer 1

Lipids are a varied group of substances that share the following characteristics:

• They contain carbon, hydrogen and oxygen.
• The proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.

-They are insoluble in water.

-They are soluble in organic solvents such as alcohols and acetone.

• The main groups of lipids are triglycerides (fats and oils) and
  phospholipids.

Question 2

What are the roles of Lipids?

Answer 2

• source of energy.
• waterproofing.
• insulation.
• protection.

Question 3

What are triglycerides?

Answer 3

Triglycerides are so called because they have three (tri) fatty acids combined with glycerol (glyceride).

Each fatty acid forms an ester bond with glycerol in a condensation reaction.

Hydrolysis of a triglyceride therefore produces glycerol and three fatty acids.

Question 4

What are the two types of lipids?

Answer 4

Triglycerides and phospholipids are two groups of lipid.

Question 5

How are triglycerides formed?

Answer 5

Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid

Question 6

What does a condensation reaction between a GLYCEROL and a FATTY ACID (RCOOH) form?

(What type of bond?)

Answer 6

A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond.

Question 7

What could the R grp of a fatty acid be?

Answer 7

The R-group of a fatty acid may be saturated or unsaturated.

Question 8

What is one of the fatty acids in a phospholipid?

Answer 8

In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group.

Question 9

Are the glycerol molecule in all triglycerides is the same?

If so, how can we see different properties of different fats?

Answer 9

As the glycerol molecule in all triglycerides is the same, the differences in the properties of different fats and oils come from variations in the fatty acids.

There are over 70 different fatty acids and all have a carboxyl (-COOH) group with a hydrocarbon chain attached

Question 10

What is a saturated fatty acid?

Answer 10

If this chain has no carbon-carbon double bonds,

the fatty acid is then described as saturated,

because all the carbon atoms are linked to the maximum possible number of hydrogen atoms, in other words they are saturated with hydrogen atoms.

Question 11

What is a monosaturated fatty acid?

Answer 11

If there is a single double bond, it is mono-unsaturated.

Question 12

What is a polyunsaturated fatty acid?

Answer 12

if more than one double bond is present, it is polyunsaturated.

Question 13

How is the structure of triglycerides related to their properties?

Answer 13

• Triglycerides have a high ratio of energy-storing carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy.
• Triglycerides have low mass to energy ratio, making 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.
• Being large, non-polar molecules, triglycerides are insoluble in water. As a result their storage does not affect osmosis in cells or the water potential of them.
• As they have a high ratio of hydrogen to oxygen atoms, triglycerides release water when oxidised and therefore provide an important source of water, especially for organisms living in dry deserts.

Question 14

What is different in phospholipids to lipids?

Answer 14

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

Whereas fatty acid molecules repel water (are hydrophobic),
phosphate molecules attract water (are hydrophilic).

Question 15

What are the two parts a phospholipid is made from?

Answer 15

• a hydrophilic ‘head’, which interacts with water (is attracted to it) but not with fat
• a hydrophobic ‘tail’, which orients itself away from water but mixes readily with fat.

Question 16

What does Polar mean?

What does this mean for phospholipid molecules?

Answer 16

Molecules that have two ends (poles) that behave differently in this way are said to be polar.

This means that when these polar phospholipid molecules are placed in water they position themselves so that the hydrophilic heads are as close to the water as possible and the hydrophobic tails are as far away from the water as possible.

Question 17

How is the structure of PHOSPHOLIPIDS related to their properties?

Answer 17

• Phospholipids are polar molecules, having a hydrophilic phosphate head and a hydrophobic tail of two fatty acids. This means that in an aqueous environment, phospholipid molecules form a bilayer within cell-surface membranes. As a result, a hydrophobic barrier is formed between the inside and outside of a cell.
• The hydrophilic phosphate ‘heads’ of phospholipid molecules help 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. These glycolipids are important in cell recognition.

Question 18

Describe the test for lipids.

Answer 18

The test for lipids is known as the emulsion test and is carried out as follows:

1) Take a completely dry and grease-free test tube.

2) To 2 cm3 of the sample being tested, add 5 cm3 of ethanol.

3) Shake the tube thoroughly to dissolve any lipid in the sample.

4) Add 5 cm3 of water and shake gently.

5) A milky-white emulsion 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.

The cloudy colour is 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.

Question 19

What does Hydrophilic and Hydrophobic mean?

Answer 19

Hydrophilic - attracted to water

Hydrophobic - repels water

Question 20

What is a role of phospholipids?

Answer 20

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

Question 21

How do lipids act as a source of energy?

Answer 21

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

Question 22

How are lipids waterproof?

Give an example of this in nature and humans.

Answer 22

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 23

How do lipids provide insulation?

Answer 23

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 24

How do lipids offer protection?

Answer 24

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

Question 25

How do we differentiate fats from oils?

Answer 25

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