1.5 lipids

Question 1

What are lipids (4 properties)

Answer 1

Substances that share the following:

. Contain carbon, hydrogen, oxygen
. proportion of oxygen to C and H is smaller than in carbohydrates
. They are insoluble in water
. soluble in organic compounds such as alcohols and acetone.

Question 2

role of phospholipids in cell membranes

Answer 2

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

Question 3

How are lipids a good source of energy compared to carbohydrates

Answer 3

. When oxidised, they provide double the energy as the same mass of carbohydrate, and also release water

Question 4

Why are lipids used for waterproofing

Give examples of where they’re used

Answer 4

. Insoluble in water
. Plants and insects have waxy lipid cuticles that conserve water
. Mammals produce an oily secretion from sebaceous glands in skin

Question 5

Why are lipids used in insulation

Give 2 examples of in the body

Answer 5

. Fats are slow conductors of heat, and when stored beneath the body surface, they help retain body heat

. Also act as electrical insulators in the myelin sheath around nerve cells

Question 6

How are lipids used for protection

Answer 6

. Fat is stored around delicate organs eg kidneys

Question 7

What are triglycerides made from

What are the bonds holding it together

Answer 7

. Three fatty acids and a glycerol molecule

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

This means hydrolysis of a triglyceride produces 3 fatty acids and 1 glycerol

Question 8

Structure of triglyceride

Answer 8

CH2OH HOOC—FA1
CHOH + HOOC—FA2
CH2OH HOOC—FA3

Glycerol + 3 fatty acids

—> CH2OOC—FA1
CHOOC—-FA2 +3H2O
CH2OOC–FA3

Question 9

Why are there different properties of different triglycerides

Answer 9

. In every triglyceride the glycerol molecule is the same
. Different fats and oils come from variations in the fatty acids (there are over 70 types of fatty acids)

Question 10

Properties of fatty acids

What are saturated ones?
What are unsaturated ones?

Answer 10

-All have a carboxyl group (COOH) with a hydrocarbon chain attached

saturated ones have no carbon double bonds, so all carbon atoms are linked to the max possible number of hydrogen atoms.

Unsaturated ones have one carbon double bond (mono-unsaturated) or more than one (poly-unsaturated)

Question 11

Why are triglycerides a good source of energy

Answer 11

. Excellent source of energy: High ratio of energy storing carbon-hydrogen bonds to carbon atoms

Question 12

Why are triglycerides good storage molecules in terms of mass

Answer 12

. Low mass to energy ratio so lots of energy can be stored in a small volume

Beneficial to animals as it reduces the mass they have to carry around

Question 13

Why are triglycerides insoluble in water

Answer 13

. Large, non polar molecules so their storage doesn’t affect osmosis in cells or the water potential of them

Question 14

Why do triglycerides release water when oxidised

Answer 14

High ratio oxygen to hydrogen atoms

They provide an important source of water, especially for organisms in desert

Question 15

What are phospholipids

Answer 15

Same as triglycerides but one of the fatty acids
is replaced by a phosphate molecule
So still contain a glycerol molecule

Question 16

What two parts of phospholipids are there

Answer 16

Head and tail

Head is hydrophilic whereas tail is hydrophobic

The head is a phosphate whilst the tail is the fatty acids

Question 17

What part of the phospholipid is hydrophilic

Answer 17

The phosphate molecules are hydrophillic and attract water.

This forms a hydrophilic head which interacts with water

Question 18

What part of phospholipids are hydrophobic

Answer 18

The tail orients itself away from water, but mixes with fat

Question 19

How do phospholipids position themselves in water

Answer 19

They are polar so have two ends that behave differently.

When in water, they position themselves so that the hydrophilic heads are as close to water as possible, and hydrophobic tails are as far from water as possible

Question 20

How do phospholipids form a bilayer

Answer 20

. Phospholipids are polar molecules, with a hydrophilic phosphate head and hydrophobic tail of two fatty acids.

This means that in an aqueous environment, phospholipid molecules form a bilayer within cell-surface membrane

As a result, a hydrophobic barrier is formed between the inside and outside of a cell.

Question 21

How are glycolipids formed

Answer 21

A phospholipid can combine with carbohydrates within cell surface membrane.

These glycolipids are important in cell recognition

Question 22

How do you test for lipids

Answer 22

. Take a completely dry and grease free test tube
. To 2cm of sample being tested, add 5cm ethanol
. Shake tube thoroughly
. Add 5cm water
. Shake again
. A cloudy white colour indicates the presence of a lipid
. As a control,

Question 23

Why do you shake the test tube in a lipid test

Answer 23

To make sure all liquid dissolves in the sample

Question 24

What can you do as a control when testing for lipids

Answer 24

. As a control, repeat the procedures using water instead of sample: final solution should remain clear

Question 25

Why is a cloudy sample formed from lipid test

Answer 25

. Due to any liquid 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