Chapter 4

Question 1

What is one way water is important?

Answer 1

as a solvent

Question 2

Describe the structure of DNA

Answer 2

they are two strands coiled together to form a double
helix and each strand contains chemicals called bases. cross-links between the strands are formed by pairs of bases and the bases always pair up in the same way: A with T, and C with G.

Question 3

Describe the roles of water as a solvent in organisms with respect to digestion, excretion and transport

Answer 3

1.

Question 4

List the chemical elements that make up carbohydrates

Answer 4

Carbon, hydrogen and oxygen

Question 5

List the chemical elements that make up fats

Answer 5

Carbon, hydrogen and oxygen

Question 6

List the chemical elements that make up proteins

Answer 6

Carbon, hydrogen, oxygen, nitrogen and sulfur

Question 7

Large molecules are made up of …

Answer 7

….smaller molecules

Question 8

Explain that different sequences of amino acids give different shapes to protein molecules

Answer 8

The individual amino acids connect via a peptide bond. The different sequences of amino acids give different shapes to protein molecules.

Question 9

Relate the shape and structure of protein molecules to their function, limited to the active site of enzymes and the binding site of antibodies

Answer 9

The different sequences of amino acids give different shapes to protein molecules. These different shapes are related to their function. Enzymes are proteins which provide a surface called a active site. Antibodies are proteins that have a surface of binding sites etc.

Question 10

Examples of large molecules and their smaller molecules

Answer 10

– starch and glycogen from glucose
– cellulose from glucose
– proteins from amino acids
– fats and oils from fatty acids and glycerol

Question 11

Describe the test for starch

Answer 11

1. Half fill a test tube with the food extract
2. add 2 or 3 drops of iodine solution (starts as yellow or light brown)
3. A positive result for starch is if the iodine solution turns blue-black. If it stays the same it does not contain starch.

Question 12

Describe the test for reducing sugars

Answer 12

1. fill a test tube with the food extract
2. place the beaker on a heat-proof mat
3. carefully half fill with boiling water from a kettle
4. add the same volume (of the food extract) of Benedict’s solution (colour is bright blue) into the test tube with the food extract and place it in the beaker with hot water
5. A positive test would be the solution turning orange/red. If you leave it for long enough there is even a precipitate. Depending on how much reducing sugars there are, it can go from green, to yellow, to orange.
6. A negative test would be no change.

Question 13

Describe the test for proteins

Answer 13

1. Half fill a test tube with the food extract
2. Add 5 or 6 drops of blue biuret solution (copper sulphate and sodium hydroxide)
   WARNING: sodium hydroxide is highly corrosive
3. A positive test is if the biuret solution turns purple, violet or lilac.
4. If it remains blue, the extract does not contain the protein

Question 14

Describe the test for fats

Answer 14

1. chop up or grind the material
2. put the material in a clean test tube and then add enough ethanol to cover it
3. Put a stopper over the test tube and shake up the contents
4. add some distilled water to the test tube to make it half full
5. shake up the contents once more
6. A white emulsion that looks cloudy white or a milky colour is a positive test. If not, there is not fats.

Question 15

Describe the test for vitamin C

Answer 15

1. put a known volume of blue DCPIP solution in a test tube
2. Fill a syringe or dropping pipette with a solution of vitamin C or juice
3. add a drop of it at a time
4. if the colour of DCPIP disappears, it is a positive test for vitamin C. If it stays blue it is negative