Section 2: c) Biological Molecules

Question 1

What chemical elements do carbohydrates contain?

Answer 1

Carbon, hydrogen and oxygen

Question 2

What chemical elements do lipids (fats and oils) contain?

Answer 2

Carbon, hydrogen and oxygen

Question 3

What chemical elements do proteins contain?

Answer 3

Carbon, hydrogen, oxygen, nitrogen and sulphur

Question 4

What are the basic units of carbohydrates (e.g. starch and glycogen) that make long chains?

Answer 4

Simple sugars

Question 5

What is the chemical name for a simple sugar?

Answer 5

A monosaccharide

Question 6

Give 3 examples of monosaccharides.

Answer 6

Glucose, fructose and galactose

Question 7

What is formed when two simple sugars join together?

Answer 7

A disaccharide

Question 8

Give 3 examples of disaccharides.

Answer 8

Maltose, sucrose and lactose

Question 9

What is formed when lots of simple sugar molecules join together?

Answer 9

A polysaccharide

Question 10

Give 3 examples of polysaccharides.

Answer 10

Starch, glycogen and cellulose (in plants)

Question 11

What are the basic units of proteins that make long chains?

Answer 11

Amino acids

Question 12

How many different types of amino acids are there?

Answer 12

20

Question 13

What can amino acids make by joining together in any order?

Answer 13

They can make all the different proteins (hundreds of thousands) within the plant or animal body.

Question 14

A lipid is a fat or oil. What is the difference between fats and oils?

Answer 14

Fats (e.g. butter) are solid at room temperature whereas oils (e.g. sunflower oil) are liquid at room temperature.
However they have a similar structure.

Question 15

What is the most common type of lipid in the body called?

Answer 15

A triglyceride

Question 16

What are the basic units of lipids?

Answer 16

Fatty acids and glycerol

Question 17

How do you prepare a food sample before doing a food test?

Answer 17

1) Grind up food-pestle and mortar.
2) Transfer ground up food to beaker & add distilled water.
3) Stir (using glass rod) to dissolve some food.
4) Filter solution (using filter funnel lined with filter paper)-remove suspended food particles.

Question 18

Name and describe the food test for reducing sugars such as glucose.

Answer 18

Benedict’s test:
1) Prepare food sample and transfer 5cm^3 to test tube.
2) Add about 10 drops of Benedict’s solution to test tube-pipette.
3) Place in water bath (around 80°C) using test tube holder and leave for 5 minutes. Make sure tube pointing away from you.
4) Negative result=blue
Low concentrations of glucose=green/yellow
High concentrations of glucose=brick-red

Question 19

What should you always do during the Benedict’s test and why?

Answer 19

Always use an excess of Benedict’s solution to make sure that all the glucose reacts.

Question 20

Why does sucrose not produce a brick-red colour with Benedict’s solution?

Answer 20

It is a non-reducing sugar.

Question 21

Give two safety precautions for the Benedict’s test.

Answer 21

• Wear safety goggles.

- Wear a lab coat.

Question 22

Name and describe the test for starch.

Answer 22

Iodine Solution:
1) Prepare food sample and transfer 5cm^3 to test tube.
2) Add few drops of iodine solution and gently shake tube to mix contents.
3) Positive result=blue-black
Negative result=orange

Question 23

Name and describe the food test for proteins.

Answer 23

Biuret Test:
1) Prepare food sample and transfer 2cm^3 to test tube.
2) Add 2cm^3 of biuret solution and mix by gently shaking it
3) Positive result=purple
Negative result=blue

Question 24

Name and describe the food test for lipids.

Answer 24

Emulsion test:
1) Prepare food sample (as before but do not filter solution-lipid molecules can stick to filter paper) and put it into a test tube
2) Add few drops of ethanol, shake test tube and leave for 1 minute.
3) Add water and look for colour change.
4) Positive result=cloudy white layer forms
Negative result=clear solution

Question 25

Why does the cloudy white layer form in the emulsion test?

Answer 25

The cloudy layer forms because, although the lipids will dissolve in ethanol, they will not dissolve in water and so form an emulsion (milky-white liquid).

Question 26

What is a catalyst?

Answer 26

A substance that helps to speed up a reaction, without being used up or chemically changed in the reaction.

Question 27

What are enzymes?

Answer 27

Enzymes are biological catalysts that control metabolic reactions (processes that keep a living organism alive).

Question 28

To which group of chemical compounds do enzymes belong?

Answer 28

Proteins

Question 29

Why is an enzyme specific for a particular substrate?

Answer 29

The substrate fits the active site of the enzyme as they are complementary shapes. This is called the lock and key hypothesis.

Question 30

Why are enzymes only needed in small amounts?

Answer 30

They are only needed in small amounts as they can be re-used; they do not get used up and remain chemically unchanged by the reaction.

Question 31

What types of reactions do enzymes catalyse?

Answer 31

• Synthetic reactions (where large molecules are built up from small molecules) e.g. muscle
• Breakdown reactions (splitting large molecules into smaller ones) e.g. lipase break down lipids into fatty acids and glycerol

Question 32

What is the optimum temperature of an enzyme?

Answer 32

The temperature at which the rate of reaction of an enzyme-controlled reaction is the fastest (40°C)

Question 33

Describe the effect of temperature on enzymes.

Answer 33

At lower temperatures, they work more slowly because the molecules have less kinetic energy and move around more slowly.
Up to 40°C, as the temperature increases, the rate of reaction increases because the enzyme and substrate molecules have more kinetic energy and collide more frequently.
Above 40°C, the enzymes start to change so the active site no longer fits the substrate. At this point, the enzyme is said to be denatured.

Question 34

What is the optimum pH of an enzyme?

Answer 34

A particular pH which enzymes work best at/require to work efficiently.

Question 35

Describe the effect of pH on enzymes.

Answer 35

The shape of the enzyme will depend on the surrounding pH. If the pH changes too much from the optimum pH, the shape of the active site of the enzymes will change. So the substrate will not fit as well and the rate of reaction will decrease.

Very high or low pH can slow down the rate of reaction of enzymes and can even denature them.

Question 36

Describe a method for a practical that could be used to investigate the effect of temperature on amylase activity.

Answer 36

1. Take 4 test tubes and add 5cm^3 starch.
2. Take 4 more test tubes and add 1cm^3 amylase.
3. Label and put one of each into a water bath. There are 5 different temperatures to choose from (20, 30, 40, 50 or 60ºC).
4. Put the two spotting tiles next to each other with two drops of iodine in each.
5. Add the amylase to the starch - shake to mix - start stopwatch
6. Immediately take a pipette of the starch/amylase from a water bath.
7. Add a drop to the first well for that temperature.
8. Keep adding drops until there is no colour change and record time taken for starch solution to stop turning the iodine black/stay the same colour.
9. Repeat for a different temperature, (try to do at least 3 repeat temperatures for each one) and make sure that all 5 temperatures have been tested.

Question 37

What pattern do you see from your results (effect of temp change)? Explain why.

Answer 37

As temperature increases, time taken for iodine to stay same colour decreases with the exception of 60ºC where the time taken is much longer.

• Rate of reaction increases as temperature increases up to 40ºC because the enzyme and substrate molecules have more kinetic energy.
• Above 50ºC - the enzyme starts to change shape. Active site doesn’t fit the substrate. Enzyme denatured.
• At 60ºC - no reaction between starch and denatured amylase.

Question 38

Suggest 2 improvements that could be made to the design of the experiment to make the results more reliable.

Answer 38

• Repeat the experiment and calculate the average time

- Use a thermostatically controlled water bath - keeps temp. constant

Question 39

What temperature would you expect to be the optimum temperature for amylase in the mouth? Explain.

Answer 39

Around 37.5ºC as it is the body temperature.