Lipids

Question 1

characteristics of lipids

Answer 1

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

Question 2

what are the main groups of lipids

Answer 2

triglycerides

phospholipids

Question 3

explain the role of the lipids in the cell membrane

Answer 3

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

Question 4

state all the roles of lipids

Answer 4

source of energy

water proofing

insulation

protection

cell membranes

Question 5

explain how lipids can be used as a source of energy

Answer 5

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

Question 6

explain how lipids are used for waterproofing

Answer 6

Lipids are insoluble in water and therefore useful as water proofing

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 7

explain how lipids are used for insulation

Answer 7

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 8

explain how lipids are used for protection

Answer 8

fat is often stored around delicate organs, such as the Kidney

Question 9

Describe triglycerides

Answer 9

they have three fatty acids combined with glycerol

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 10

how many fatty acids are there

Answer 10

70 different fatty acids

Question 11

what do all fatty acids have

Answer 11

carboxyl group with a hydrocarbon chain attached

Question 12

when are the fatty acids described as saturated

Answer 12

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

Question 13

when are the fatty acids described as mono-unsaturated

Answer 13

if the is a single double bond, it is mono-unsaturated

Question 14

when are the fatty acids described as polyunsaturated

Answer 14

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

these differences are illustrated

Question 15

how are high ratio energy storing, carbon-hydrogen good for storing energy

Answer 15

triglycerides have a ratio of energy-storing carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy

Question 16

explain the benefits of triglycerides having a low mass-to-energy ratio

Answer 16

triglycerides have a low mass to storage 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

Question 17

explain the benefits of triglycerides being large

Answer 17

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

Question 18

explain the benefits of triglycerides having a high ratio of hydrogen to oxygen atoms

Answer 18

triglycerides release water when oxidised and therefore provide an important source of water, especially for organisms living in dry deserts

Question 19

how are phospholipids different to triglycerides

Answer 19

one of the fatty acid molecules is replaced by a phosphate molecule.

whereas, fatty acid molecules are hydrophobic, phosphate molecules are hydrophillic

Question 20

what are phospholipids made up of

Answer 20

a hydrophilic ‘head’, which interacts with water but not with fat

A hydrophobic ‘tail’ which orients itself away from water but mixes readily with fat

Question 21

what does it mean when molecules are described as polar

Answer 21

molecules that have two ends that behave differently in this way are said to be polar

Question 22

explain what happens when these polar phospholipid molecules are placed in water

Answer 22

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

Question 23

why are polunsaturated triglycerides liquid at room temperature

Answer 23

the double bonds cause the molecule to bend.

they cannot therefore pack together closely making them liquid at room temp

Question 24

explain the result of phospholipids being polar molecules

Answer 24

they have 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 mambrances

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

Question 25

benefits of the hydrophilic phosphate heads

Answer 25

the hydrophilic phosphate heads of phospholipid molecules help to hold at the surface of the cell-surface membrane

Question 26

what does the phospholipid structure allow them to do

Answer 26

allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane

these glycolipids are important in cell recognition

Question 27

describe the test for lipids

Answer 27

take a completely dry and grease-free test tube

to 2cm3 of the sample being tested, add 5cm3 of ethanol

shake the tube thoroughly to dissolve any lipid in the sample

add 5 cm3 of water and shake gently

a milky white color indicates the presence of a lipid

as a control repeat the procedures using water instead of the sample; the final solution should remain clear

Question 28

what is the test for lipids known as

Answer 28

the emulsion test

Question 29

why is there a cloudy color during the test

Answer 29

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