Why we chew — answer key

1. CQ1 · Practice

Model answer. Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. The crumbs of egg white are still made of the same protein molecules as the whole cube.

A full answer includes

• Physical digestion breaks food into smaller pieces.
• There is no chemical change to the food molecules.

Also accept

• (1) smaller; (2) chemical

Watch for

• “Physical digestion makes new, smaller molecules.” — Only enzymes break molecules down; that is chemical digestion. Physical digestion only makes the pieces smaller.

2. CQ2 · Practice

Model answer. The whole cube has 6 faces of 1 cm × 1 cm, so 6 cm². Each small cube has 6 faces of 0.5 cm × 0.5 cm = 0.25 cm², so 1.5 cm², and the eight together have 8 × 1.5 = 12 cm²: twice the surface for the same mass of egg white.

Checked automatically

• (a) Whole cube: 6 cm² (exact value)
  • Common wrong answer 1 : That is the area of one face. A cube has 6 faces.
• (b) Eight small cubes together: 12 cm² (exact value)
  • Common wrong answer 6 : That is the whole cube again. Each small cube has 1.5 cm², and there are eight of them.
  • Common wrong answer 1.5 : That is one small cube. Multiply by 8.

A full answer includes

• (a) 6 × (1 cm × 1 cm) = 6 cm².
• (b) One small cube: 6 × (0.5 × 0.5) = 1.5 cm²; eight cubes: 8 × 1.5 = 12 cm².

Also accept

• (a) 6 cm²; (b) 12 cm²

Watch for

• “The eight small cubes have the same surface as the whole cube, because it is the same egg white.” — The mass is the same, but every cut makes new faces, so the total surface area grows.

3. CQ3 · Practice

Model answer. The protease can act only on the surface of a solid piece. Breaking the same mass into smaller pieces makes much more surface, so more protease molecules work at the same time and the egg white is digested faster. Mashing did not change the protein itself.

A full answer includes

• The crumbs had the largest total surface area, so more protease molecules could act on them at once.

Also accept

• the crumbs had the largest surface area for the enzyme

Watch for

• “The crumbs were lighter.” — All three samples had the same mass: one cube of egg white each.
• “Mashing changed the protein into a different substance.” — Mashing is physical digestion: the crumbs are made of the same protein molecules as before.

4. CQ4 · Practice

Model answer. The pupil is wrong. Mashing breaks the potato into smaller pieces, but the starch and the other molecules are the same as before: no new substance is made. That is physical digestion. Chemical digestion needs enzymes to break the large molecules into smaller ones.

A full answer includes

• The pupil is wrong: this is physical digestion, not chemical.
• The potato is only broken into smaller pieces; its molecules, such as starch, are not changed and no new substance is made.

Also accept

• wrong; physical digestion: smaller pieces, same molecules, no new substance

Watch for

• “A change in how food looks is a chemical change.” — A change of shape or size is physical. A chemical change makes new substances.

5. CQ5 · Practice

Model answer. The big cube has 6 faces of 3 cm × 3 cm = 9 cm², so 54 cm². Each small cube has 6 cm², so the 27 cubes have 27 × 6 = 162 cm². The surface area is 162 ÷ 54 = 3 times larger, for the same mass of potato.

Checked automatically

• (a) Big cube: 54 cm² (exact value)
  • Common wrong answer 9 : That is one face. A cube has 6 faces.
  • Common wrong answer 27 : That is its volume in cm³, not its surface area.
• (b) 27 small cubes together: 162 cm² (exact value)
  • Common wrong answer 6 : That is one small cube. Multiply by 27.
• (c) How many times larger: 3 (exact value)
  • Common wrong answer 27 : There are 27 pieces, but the surface is not 27 times larger: divide 162 by 54.

A full answer includes

• (a) 6 × 9 = 54 cm².
• (b) 27 × 6 = 162 cm².
• (c) 162 ÷ 54 = 3 times.

Also accept

• (a) 54 cm²; (b) 162 cm²; (c) 3

Watch for

• “Cutting a cube into 27 pieces makes 27 times as much surface.” — Only the new faces are added. Here the surface becomes 3 times larger: the cubes are one third as wide.

6. CQ6 · Practice

Options as numbered on the pupil sheet: 1 — Chemical digestion; 2 — Physical digestion.

Model answer. Chewing, cutting and churning only make the pieces smaller, with the same molecules: physical digestion. They increase the surface area for enzymes. The protease and the enzyme that breaks down starch change large molecules into smaller, new ones: chemical digestion.

A full answer includes

• Chewing bread: physical.
• Cutting meat into small pieces: physical.
• The stomach churning food: physical.
• A protease breaking protein molecules into smaller molecules: chemical.
• An enzyme breaking starch down into sugar: chemical.

Also accept

• chewing, cutting, churning: physical; protease on protein, enzyme on starch: chemical

Watch for

• “Churning in the stomach is chemical, because it happens inside the body.” — Where it happens does not decide it. Churning only breaks food into smaller pieces, so it is physical.

7. CQ7 · Practice

Model answer. The protease can act only where egg white touches the solution, at the surface. The eight small cubes have 12 cm² of surface, twice the 6 cm² of the whole cube, for the same mass. So twice as many protease molecules can work on the egg white at the same time, and it is digested in about half the time.

A full answer includes

• The eight small cubes have twice the surface area of the whole cube (12 cm² instead of 6 cm²).
• The protease acts only at the surface of a solid piece, so twice as many protease molecules can work at once and the egg white is digested about twice as fast.

Also accept

• twice the surface area; enzyme acts at the surface; twice as many enzyme molecules at work

Watch for

• “The small cubes are digested faster because the protease gets inside them.” — The protease cannot get inside a solid piece: it works on the surface, and the small cubes have more of it.

8. CT1 · Apply

Model answer. Mincing and mashing do the physical digestion that chewing would do: they break the food into small pieces. Small pieces have a large total surface area, so the enzymes in the gut can act on more of the food at once and digest it faster and more completely. Soft food is also easy to swallow.

A full answer includes

• Mincing and mashing break the food into small pieces, as chewing would.
• This increases the surface area of the food, so enzymes can digest it faster (and more of it is digested).

Also accept

• physical digestion done before eating; smaller pieces; larger surface area for enzymes

Watch for

• “Mashed food has more nutrients in it.” — Mashing does not add nutrients: it only makes the pieces smaller, so enzymes digest them faster.

9. CT2 · Apply

Model answer. The control must show that the protease, and nothing else, makes the egg white disappear. A whole cube in water shows that a big piece does not dissolve. But someone could still say that tiny crumbs might break up or dissolve on their own in warm, shaken water. Crumbs in water, kept like the other tubes, would rule that out: if they are still there after the crumbs in protease have gone, the protease did it.

A full answer includes

• The crumbs are the sample that disappeared fastest, so they are the one to test without protease.
• If crumbs in water stayed, it would show that they disappear because of the protease, not because small pieces break up or dissolve in warm, shaken water.

Also accept

• crumbs are the sample that vanished; crumbs in water would show the protease, not the water or shaking, made them disappear

Watch for

• “A control is not needed, because everybody knows water does not digest egg white.” — A fair test checks it: the control shows that the change is caused by the enzyme and not by something else in the tube.

10. CT3 · Apply

Model answer. Each edge of the big cube holds 10 ÷ 2.5 = 4 small cubes, so there are 4 × 4 × 4 = 64. Each small cube has sides of 0.25 cm, so its surface area is 6 × 0.25 × 0.25 = 0.375 cm², and the 64 cubes have 64 × 0.375 = 24 cm²: four times the whole cube. They disappear after 2.5 ÷ 0.1 = 25 minutes, a quarter of the whole cube's 100 minutes.

Checked automatically

• (a) Number of small cubes: 64 (exact value)
  • Common wrong answer 4 : That is the number along one edge. Multiply 4 × 4 × 4.
  • Common wrong answer 16 : That is one layer. There are four layers.
• (b) Total surface area: 24 cm² (exact value)
  • Common wrong answer 6 : That is the whole cube. Smaller pieces have more surface in total.
• (c) Time to disappear: 25 min (exact value)
  • Common wrong answer 100 : That is the whole 10 mm cube. Use s = 2.5 mm.

A full answer includes

• (a) 4 × 4 × 4 = 64 cubes.
• (b) Each cube: 6 × 0.25 × 0.25 = 0.375 cm²; 64 × 0.375 = 24 cm².
• (c) 2.5 ÷ 0.1 = 25 minutes.

Also accept

• (a) 64; (b) 24 cm²; (c) 25 minutes

Watch for

• “Cutting into cubes of 2.5 mm makes 4 cubes.” — There are 4 along each edge, in three directions: 4 × 4 × 4 = 64.

11. CT4 · Apply

Model answer. The snake does no chewing, so the mouse is not broken into small pieces. A whole mouse has a small surface area for its size, and the snake's enzymes can act only at its surface. So it is digested from the outside inwards, like the whole cube of egg white, and this takes days. Chewed food, in small pieces, has a much larger surface area and is digested much faster.

A full answer includes

• The mouse is not broken into smaller pieces: there is no physical digestion by chewing.
• So its surface area is small, and the enzymes can act only on its outside; it is digested from the outside in, slowly.

Also accept

• no chewing, no physical digestion; small surface area; enzymes act only at the surface; slow

Watch for

• “The snake digests slowly because its enzymes are weaker than ours.” — Even with the same enzymes, a whole mouse has little surface for its size: the enzymes reach only its outside.