lipids (biological molecules)

Question 1

the diagram below shows the structure of a triglyceride molecule, name molecules A and B

Answer 1

A: glycerol
B: fatty acids

Question 2

name the type of chemical bond that holds glycerol and fatty acids together in a triglyceride molecule

Answer 2

ester

Question 3

describe why the molecule shown above is classified as a triglyceride rather than a diglyceride or a monoglyceride

Answer 3

contains 3 fatty acid tails

Question 4

describe how you could test a food sample for the presence of a lipid (2)

Answer 4

• shake food sample with ethanol and water
• a white milky emulsion (will form if lipids are present)

Question 5

describe the difference between the structure of phospholipids and triglycerides (1)

Answer 5

a phospholipid has a phosphate group and only two fatty acid tails

Question 6

describe how ester bonds are formed in the synthesis of a phospholipid (2)

Answer 6

• condensation reaction or reaction involves the loss of water
• hydroxyl group or OH of the glycerol reacts
• with the carboxyl group or COOH of a fatty acid

Question 7

describe how phospholipids arrange themselves within the cell-surface membrane (2)

Answer 7

• they form a bilayer
• hydrophobic fatty acid tails point away from water
• or hydrophobic fatty acid tails are repelled from water
• or hydrophilic phosphate heads point towards water
• or hydrophilic phosphate heads are attracted to water

Question 8

compare the similarities and differences in terms of the structure and properties of triglycerides and phospholipids (5)

Answer 8

similarities:
- both contain ester bonds (between glycerol and fatty acid)
- both contain glycerol
- both contain fatty acids
- both insoluble in water
differences:
- only phospholipids contain phosphorous
- triglycerides have three fatty acid tails, phospholipids have two fatty acid tails and a phosphate group
- triglycerides are hydrophobic/non-polar
- phospholipids have hydrophobic and hydrophilic parts
- phospholipids form a bilayer in water, triglycerides do not

Question 9

describe how a triglyceride is synthesised (3)

Answer 9

• one glycerol molecule and three fatty acids
• condensation reaction to remove three water molecules
• forms three ester bonds

Question 10

explain why triglycerides are not described as being polymers (1)

Answer 10

• they are not made up of monomers
• or they are not made up of many repeating units

Question 11

explain how a triglyceride is broken down into its components (2)

Answer 11

• hydrolysis reaction, or uses water
• breaks three ester bonds
• splits the fatty acids from the glycerol

Question 12

explain how the structure of a triglyceride is related to its function (3)

Answer 12

• insoluble so does not affect water potential, or does not affect osmosis
• long hydrocarbon tails so bonds can be broken to release energy
• low mass:energy ratio, stores lots of energy in a small volume
• high ratio of hydrogen:oxygen so will release water when oxidised

Question 13

name the group consisting of COOH

Answer 13

carboxyl group

Question 14

does the diagram above show a saturated or unsaturated fatty acid, explain your answer (2)

Answer 14

• unsaturated
• there is a double bond (between the carbon atoms)

Question 15

describe the structure of a saturated fatty acid (1)

Answer 15

single bonds between the carbon atoms

Question 16

fatty acids are less soluble in blood compared to glucose and sodium, explain why (2)

Answer 16

• water is a polar solvent
• fatty acids have hydrophobic tails
• fatty acids repel polar liquids

Question 17

on the diagram of the triglyceride, circle one ester bond

Answer 17

Question 18

list three other functions of lipids within living organisms, other than as a store of energy

Answer 18

insulation and protection:
- electrical insulation of neurones
- thermal insulation
- organ protection
- waterproofing
absorption and production of other substances:
- absorption of fat-soluble vitamins
- synthesis of steroid hormones
- synthesis of cholesterol
- source of vitamin D
other functions:
- cell-surface membranes
- buoyancy

Question 19

two types of fatty acid tail are shown in the diagram above, name these types and explain the difference between them (2)

Answer 19

• saturated and unsaturated
• saturated fatty acids have only single (carbon-carbon) bonds, unsaturated fatty acids have at least one double (carbon-carbon) bond

Question 20

the table below shows some examples of fatty acids, their chemical formulae, and their melting points, describe and explain the pattern shown in the results table above (2)

Answer 20

• the less hydrogens
• or the more double bonds, the more kinks the molecule has
• molecules dont pack together as tightly, so have a lower melting point

Question 21

a researcher investigated the breakdown of phospholipids in egg yolk using the enzyme phospholipase, it hydrolyses phospholipids, the researcher used a pH meter to monitor the pH of the egg yolk sample before adding the phospholipase, and then every 20 seconds describe and explain the trend shown in the results table (2)

Answer 21

• as the reaction progresses, the pH decreases
• phospholipids are hydrolysed to produce fatty acids (which lower the pH)

Question 22

after 180 seconds, the pH did not change, explain why (2)

Answer 22

• all phospholipids had been used up/hydrolysed
• no more fatty acids were produced

Question 23

suggest why the researcher used a pH meter rather than a pH indicator (1)

Answer 23

• to give numerical readings
• colour change is subjective

Question 24

the researcher did not add a buffer to the egg yolk and phospholipase solution, suggest why (1)

Answer 24

• researcher was measuring the change in pH
• buffer would maintain a constant pH