Why we chew — questions

1. define

Complete the description of physical digestion.

Physical digestion is the breakdown of food into … pieces

without … change to the food molecules.

Nudge (only if you are stuck): Think of the egg white after the scalpel and the glass rod: what changed, and what stayed the same?

2. calculate

A cube of egg white has sides of 1 cm. It is cut into eight cubes with sides of 0.5 cm. Calculate (a) the surface area of the whole cube and (b) the total surface area of the eight small cubes.

(a) Whole cube: ______________ unit ________

(b) Eight small cubes together: ______________ unit ________

Nudge (only if you are stuck): A cube has 6 faces. Work out the area of one face first, then multiply.

3. explain

In the experiment, the crumbs of egg white disappeared first, the eight small cubes next and the whole cube last. Which is the best explanation?

Circle one answer.

• The crumbs were lighter, so they sank less and dissolved faster
• Mashing changed the protein into a different substance that the protease digests faster
• The crumbs were in warmer protease solution
• The crumbs had the largest total surface area, so more protease molecules could act on them at once

Nudge (only if you are stuck): Every tube had the same mass of egg white, the same protease and the same temperature. What was different?

4. explain

A pupil mashes a boiled potato with a fork and says: "I have digested it chemically, because it looks completely different now." Explain whether the pupil is right.

Nudge (only if you are stuck): Ask: are there any new substances in the mashed potato?

5. calculate

A cube of potato with sides of 3 cm is cut into 27 cubes with sides of 1 cm. Calculate (a) the surface area of the big cube, (b) the total surface area of the 27 small cubes and (c) how many times larger the surface area has become.

(a) Big cube: ______________ unit ________

(b) 27 small cubes together: ______________ unit ________

(c) How many times larger: ______________ unit ________

Nudge (only if you are stuck): Area of one face × 6 for each cube. For (c), divide (b) by (a).

6. classify

Is each of these physical digestion or chemical digestion?

Write the number of the matching option.

1. Chemical digestion
2. Physical digestion

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

Nudge (only if you are stuck): Ask each time: is a new substance made, or are the pieces only smaller?

7. explain

The eight small cubes of egg white disappeared in about half the time of the whole cube. Explain why, in terms of the protease and the surface area.

Nudge (only if you are stuck): Compare the surface areas you worked out: 6 cm² and 12 cm².

8. suggest

After an operation on the jaw, a patient cannot chew for a few weeks. The hospital gives them minced and mashed food. Suggest how this helps the digestion of their food.

Nudge (only if you are stuck): The kitchen does a job that the teeth normally do.

9. evaluate

In the experiment, the control tube held a whole cube of egg white in water. A pupil says that crumbs in water would have been a better control. Explain why the pupil is right.

Nudge (only if you are stuck): Which sample disappeared? What could someone say about tiny crumbs in warm, shaken water?

10. calculate

In the experiment's model the protease eats into every face of a piece by 0.05 mm each minute. So a piece gets 0.1 mm narrower each minute (0.05 mm from each side), and a piece of side s mm disappears after s ÷ 0.1 minutes. A pupil cuts a 1 cm cube (10 mm) into cubes with sides of 2.5 mm. Calculate (a) how many small cubes there are, (b) their total surface area and (c) how long they take to disappear.

(a) Number of small cubes: ______________ unit ________

(b) Total surface area: ______________ unit ________

(c) Time to disappear: ______________ unit ________

Nudge (only if you are stuck): Along each edge, 10 mm ÷ 2.5 mm = 4 cubes. One face of a small cube is 0.25 cm × 0.25 cm.

11. explain

A snake has no teeth for chewing: it swallows a mouse whole. Digesting the mouse takes the snake several days. Use the ideas of physical digestion and surface area to explain why.

Nudge (only if you are stuck): What would chewing have done to the mouse? Where do the snake's enzymes work?