What food is made of — answer key

1. BQ1 · Practice

Options as numbered on the pupil sheet: 1 — Carbon, hydrogen and oxygen only; 2 — Carbon, hydrogen, oxygen and nitrogen; 3 — Carbon, hydrogen and nitrogen only.

Model answer. Carbohydrates contain carbon, hydrogen and oxygen. Fats and oils contain carbon, hydrogen and oxygen too. Proteins contain carbon, hydrogen and oxygen and also nitrogen, so protein is the only one of the three with nitrogen.

A full answer includes

• Carbohydrate: carbon, hydrogen and oxygen.
• Fat or oil: carbon, hydrogen and oxygen.
• Protein: carbon, hydrogen, oxygen and nitrogen.

Also accept

• carbohydrate: C, H, O; fat or oil: C, H, O; protein: C, H, O and N

Watch for

• “Proteins are made of carbon, hydrogen and oxygen, just like carbohydrates.” — Match protein to the option with four elements: every amino acid contains nitrogen as well as carbon, hydrogen and oxygen.
• “Fats contain nitrogen.” — Match fat or oil to carbon, hydrogen and oxygen only: fatty acids and glycerol contain no nitrogen.

2. BQ2 · Practice

Options as numbered on the pupil sheet: 1 — Glucose; 2 — Amino acids; 3 — Fatty acids and glycerol.

Model answer. Starch, glycogen and cellulose are all made from glucose. Proteins are made from amino acids. Fats and oils are made from fatty acids and glycerol.

A full answer includes

• Starch: glucose.
• Glycogen: glucose.
• Cellulose: glucose.
• Protein: amino acids.
• Fat or oil: fatty acids and glycerol.

Also accept

• starch, glycogen, cellulose: glucose; protein: amino acids; fat or oil: fatty acids and glycerol

Watch for

• “Glycogen and cellulose are made from different sugars.” — All three carbohydrates are made from glucose; they differ in how the glucose units are joined and arranged.

3. BQ3 · Practice

Options as numbered on the pupil sheet: 1 — A carbon atom; 2 — A glucose molecule; 3 — A starch molecule.

Model answer. A carbon atom is smallest. A glucose molecule is made of 24 atoms, including 6 carbon atoms. A starch molecule is a chain of thousands of glucose units, so it is by far the largest.

A full answer includes

• A carbon atom → a glucose molecule → a starch molecule.

Also accept

• carbon atom, glucose molecule, starch molecule

Watch for

• “Glucose is bigger than starch, because sugar is what starch breaks down into.” — Glucose is the small unit; starch is thousands of glucose units joined together.

4. BQ4 · Practice

Model answer. (a) Carbohydrates are made of carbon, hydrogen and oxygen. (b) Fats are made of carbon, hydrogen and oxygen. (c) Proteins are made of carbon, hydrogen, oxygen and nitrogen, because every amino acid contains nitrogen.

A full answer includes

• (a) carbon, hydrogen, oxygen.
• (b) carbon, hydrogen, oxygen.
• (c) carbon, hydrogen, oxygen and nitrogen.

Also accept

• (a) C, H, O; (b) C, H, O; (c) C, H, O, N (symbols or names)

Watch for

• “Proteins are made of carbon, hydrogen and oxygen, just like carbohydrates.” — In (c), add a fourth element: proteins are built from amino acids, and every amino acid contains nitrogen.
• “Fats contain nitrogen.” — In (b), list only carbon, hydrogen and oxygen: fats are built from fatty acids and glycerol, which have no nitrogen.

5. BQ5 · Practice

Model answer. (a) Starch is made from glucose. (b) Glycogen is made from glucose. (c) Cellulose is made from glucose. (d) A protein is made from amino acids.

A full answer includes

• (a) glucose.
• (b) glucose.
• (c) glucose.
• (d) amino acids.

Also accept

• (a) glucose; (b) glucose; (c) glucose; (d) amino acids

Watch for

• “Cellulose is made from fibre.” — Cellulose is the fibre, and it is made from glucose units.
• “Glycogen is made from glycerol.” — Glycerol is part of a fat. Glycogen is made from glucose.

6. BQ6 · Practice

Model answer. a. Glycogen is made from glucose units joined in branched chains. Animals and fungi store it, for example in your liver and muscles.

A full answer includes

• I chose a.

Also accept

• a

Watch for

• “Glycogen and cellulose are made from different sugars.” — Glycogen, starch and cellulose are all made from glucose.

7. BQ7 · Practice

Model answer. a, b and e: a fat molecule is one glycerol joined to three fatty acids, and it contains carbon, hydrogen and oxygen. It is not a long chain of fatty acids (c), and it contains no nitrogen (d).

A full answer includes

• I chose a, b and e.

Also accept

• a, b, e

Watch for

• “A fat is a long chain of fatty acids.” — The three fatty acids are joined to the glycerol, not to each other.

8. BQ8 · Practice

Model answer. A protein is a long chain of amino acids. Egg white protein and keratin use the same kinds of amino acid, but in a different order and number along the chain, so each chain folds into a different shape and is a different protein.

A full answer includes

• Each protein is a chain of amino acids joined in its own order.
• A different order (and number) of amino acids makes a different protein, which folds into a different shape.

Also accept

• chain of amino acids; different order (and number) of amino acids; different shape

Watch for

• “A protein is one kind of unit repeated, like starch.” — A protein chain is made of many kinds of amino acid in an order that belongs to that protein.

9. BT1 · Apply

Model answer. (a) It is a protein. (b) The powder must be one of the three. Carbohydrates and fats contain only carbon, hydrogen and oxygen, so neither could contain nitrogen. Proteins contain nitrogen as well, because every amino acid contains nitrogen. So the nitrogen shows that, of the three, the powder is the protein.

A full answer includes

• (a) A protein.
• (b) Of the three, only proteins contain nitrogen (in their amino acids); carbohydrates and fats contain only carbon, hydrogen and oxygen.

Also accept

• (a) protein; (b) of the three, only proteins contain nitrogen (from amino acids); carbohydrates and fats are C, H, O only

Watch for

• “The powder is a carbohydrate, because it contains carbon, hydrogen and oxygen.” — All three nutrients contain those; nitrogen is the clue, and of the three only protein has it.

10. BT2 · Apply

Model answer. (a) Inulin is most like starch (glycogen or cellulose would also do): like them, it is a large molecule made of many units of one small sugar joined in a chain. Starch repeats glucose; inulin repeats fructose. (b) A fat molecule is not a chain. It is always one glycerol joined to three fatty acids, which stick out side by side; the fatty acids are joined to the glycerol, not to each other.

A full answer includes

• (a) Starch (or glycogen or cellulose): a long chain of one kind of sugar unit, repeated many times.
• (b) A fat molecule is one glycerol joined to three fatty acids; the fatty acids are not joined to each other, so it is not a chain of repeating units.

Also accept

• (a) like starch, glycogen or cellulose: many units of one sugar in a chain; (b) fat = one glycerol + three fatty acids, not a chain

Watch for

• “A fat is a long chain of fatty acids.” — A fat molecule is one glycerol with three fatty acids.
• “Inulin is like a protein because it is a chain.” — A protein chain uses many kinds of amino acid; inulin repeats one kind of sugar, like starch.