Carbs and Lipids

Question 1

The structure of cellulose is related to its role in plant cell walls. Explain how. (3)

Answer 1

• Long, straight, unbranched chains of Beta glucose;
• (Joined by) many WEAK hydrogen bonds;
• Form microfibrils / microfibrils;
• Provide rigidity/strength/support;

Question 2

What is a polymer?

Answer 2

Polymers are large molecules made from bonding 3 or more similar or identical monomers together in a condensation reaction.

Question 3

What is Condensation?

Answer 3

Condensation is the addition of two molecules producing a bigger molecule through the removal of H2O

Question 4

What is hydrolysis?

Answer 4

Breaking bonds between a larger molecule to produce a smaller molecules through the addition of H2O

Question 5

Glucose + glucose?

Answer 5

Maltose

Question 6

Galactose + glucose?

Answer 6

Lactose

Question 7

Glucose + fructose?

Answer 7

Sucrose

Question 8

Benedict’s test for reducing sugar?

Answer 8

Add (equal volume/2cm^3) of Benedict’s reagent to sample

Heat to 95

If positive a colour change from blue to green/yellow/orange/red precipitate

If negative no colour change

Question 9

What is a monomer?

Answer 9

A monomer is a smaller / repeating) unit / molecule from which larger molecules / polymers are made

Question 10

A starch molecule has a spiral shape. Explain why this shape is important to its function in cells.

Answer 10

Compact/occupies small space/tightly packed;

Question 11

Give one feature of starch and explain how this feature enables it to act as a storage substance.

Answer 11

• Helical / spiral So compact / tightly packed / can fit (lots) into a small space;
• Insoluble So no osmotic effect / does not leave cell / does not affect water potential;
• Large molecule / long chain (So) does not leave cell / contains large number of glucose units;
• Branched chains (So) rapid hydrolysis to remove glucose for respiration;

Question 12

Hydrogen bonds are important in cellulose molecules. Explain why.

Answer 12

• Holds chains/cellulose molecules together/forms cross links between chains/cellulose molecules/forms microfibrils;
• Providing strength/rigidity (to cellulose/cell wall);
• Weak Hydrogen bonds provide strength in large numbers;

Question 13

Describe how you would test a sample of food for the presence of starch.

Answer 13

1. Add potassium iodide (KI) solution to the food sample;
2. Blue/black/purple indicates starch is present;

Question 14

Describe how you would test a liquid sample for the presence of a lipid and how you would recognise a positive result.

Answer 14

1. (Mix / shake sample) with ethanol, then water and shake;
2. Cloudy White / milky (emulsion);

Question 15

Omega-3 fatty acids are unsaturated. What is an unsaturated fatty acid?

Answer 15

Double bond(s);
(Bonds) between carbon atoms within the hydrocarbon chain;

Question 16

Describe how an ester bond is formed in a phospholipid molecule.

Answer 16

1. Condensation (reaction) OR Loss of water;
2. Between of glycerol and fatty acid;

Question 17

Some seeds contain lipids. Describe how you could use the emulsion test to show that a seed contains lipids. (3)

Answer 17

1. Crush/grind;
2. With ethanol/ alcohol, to dissolve the lipid;
3. Then add water then shake;
4. Forms a white emulsion / goes white;

Question 18

What are the differences between a triglyceride and a phospholipid?

Answer 18

A triglyceride has 3 fatty acid and 1 glycerol, whereas a phospholipid had 2 fatty acids, 1 glycerol and 1 phosphate group

Question 19

Compare and contrast the structure and properties of triglycerides and phospholipids

Answer 19

• Both contain ester bonds
• Both contain glycerol
• Fatty acids on both may be saturated or unsaturated
• Both are insoluble in water
• Both contain C, H and O but phospholipids are also contain P
• Triglyceride has three fatty acids and phospholipids have two fatty acids plus phosphate group
• Triglycerides are hydrophobic/non-polar and phospholipids have hydrophilic and hydrophobic region
• Phospholipids form monolayer/micelle/bilayer but triglycerides don’t.

Question 20

Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample. [6]

Answer 20

Lipid
1. Add ethanol/alcohol then add water and shake/mix OR Add ethanol/alcohol and shake/mix then pour into/add water;
2. White/milky emulsion OR emulsion test turns white/milky;

Non-reducing sugar
3. Do Benedict’s test and stays blue/negative;
4. Boil with acid then neutralise with alkali;
5. Heat with Benedict’s and becomes red/orange (precipitate);

Amylase
6. Add biuret (reagent) and becomes purple/violet/mauve/lilac;
7. Add starch, (leave for a time), test for reducing sugar/absence of starch;

Question 21

A student carried out the Benedict’s test. Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution.

Answer 21

1. Filter and dry (the precipitate);
2. Find mass/weight;

Question 22

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers.

Give two named examples of polymers and their associated monomers to illustrate your answer (5 marks)

Answer 22

1. A condensation reaction joins monomers together and forms a (chemical) bond and releases water
2. A hydrolysis reaction breaks the (chemical) bond between monomers and uses water
3. A suitable example of polymers and monomers from which they are made
4. A second suitable example of polymers and the monomers from which they are made
5. Reference to a correct bond within a named polymer

Suitable Example =

Amino Acids –> Polypeptide/Protein/Enzyme/Antibody

Nucleotides –> Polynucleotide/DNA/RNA

Alpha Glucose –> Starch/Glycogen

Beta Glucose –> Cellulose

Question 23

Explain how the active site of an enzyme causes a high rate of reaction (3 marks)

Answer 23

1. Lowers activation energy;
2. Induced fit causes active site (of enzyme) to
   change shape;
3. (So) enzyme-substrate complex causes bonds
   to form/break;