The gut and physical digestion — final check key

1. CK1 · Final check

Model answer. Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. Chewing by the teeth and the churning of the stomach are examples.

A full answer includes

• Physical digestion breaks food into smaller pieces without chemical change to the food molecules.
• An example: chewing by the teeth, or churning by the stomach.

Also accept

• smaller pieces, no chemical change to the molecules; chewing or churning

Watch for

• “Physical digestion is done by enzymes.” — Enzymes do chemical digestion. Physical digestion is done by the teeth and by the muscles of the stomach.

2. CK2 · Final check

Model answer. Chewing breaks food into smaller pieces. The same mass of food then has a much larger surface area, and enzymes act on the food at its surface, so more enzyme molecules can work on it at the same time and it is digested faster.

A full answer includes

• Chewing breaks food into smaller pieces, which increases its surface area.
• Enzymes act at the surface of the food, so more enzyme molecules can work at once.

Also accept

• smaller pieces; larger surface area; more enzyme molecules act at once

Watch for

• “Chewed food is digested faster because it contains more enzyme.” — Chewing adds no enzyme to the food; it gives the enzymes more surface.

3. CK3 · Final check

Model answer. The big cube has 6 faces of 4 cm × 4 cm = 16 cm², so 96 cm². Each small cube has 6 cm², so the 64 have 64 × 6 = 384 cm², four times as much. Cutting is physical digestion: the cheese is broken into smaller pieces without any chemical change to its molecules.

Checked automatically

• (a) Big cube: 96 cm² (exact value)
  • Common wrong answer 16 : That is one face.
  • Common wrong answer 64 : That is its volume in cm³.
• (b) 64 small cubes together: 384 cm² (exact value)
  • Common wrong answer 6 : That is one small cube.

A full answer includes

• (a) 6 × 16 = 96 cm².
• (b) 64 × 6 = 384 cm².
• Physical: the pieces are smaller, but the molecules are not changed.

Also accept

• (a) 96 cm²; (b) 384 cm²; physical: smaller pieces, no chemical change

Watch for

• “Cutting the cheese is chemical, because the surface area changes.” — Surface area is about the size of the pieces. A chemical change makes new substances, and cutting makes none.

4. TK1 · Final check

Options as numbered on the pupil sheet: 1 — Premolars; 2 — Molars; 3 — Incisors; 4 — Canine.

Model answer. R, the two narrow teeth at the middle of the jaw with straight edges, are incisors. P, the one tooth with a single point, is the canine. S, the two teeth with two cusps each, are premolars. Q, the three large square teeth with four cusps at the back, are molars.

A full answer includes

• P: canine.
• Q: molars.
• R: incisors.
• S: premolars.

Also accept

• P canine; Q molars; R incisors; S premolars

Watch for

• “The single pointed tooth is a premolar.” — The single pointed tooth is the canine. Premolars have two cusps.

5. TK2 · Final check

Model answer. Enamel covers the crown. Under it is dentine, which makes up most of the tooth. The pulp in the middle contains nerves and blood vessels. The root is covered by cement and embedded in the jaw bone, and the gum covers the bone around the neck of the tooth.

A full answer includes

• Enamel on the outside of the crown.
• Dentine under the enamel.
• Pulp in the middle, with nerves and blood vessels.
• Cement covers the root, which is embedded in the jaw bone; the gum surrounds the neck.

Also accept

• enamel, dentine, pulp with nerves and blood vessels; cement; jaw bone and gum

Watch for

• “Teeth are held in place only by the gums.” — The roots are embedded in the jaw bone and fixed by cement; the gum covers the bone around the neck.

6. TK3 · Final check

Model answer. The incisors cut into the apple and bite off a piece. The molars, with the premolars, crush and grind the piece into small pieces. This physical digestion increases the surface area of the apple for the enzymes of chemical digestion.

A full answer includes

• Incisors cut and bite off pieces of the apple.
• Molars crush and grind the pieces into smaller pieces.
• Smaller pieces have a larger surface area for enzymes.

Also accept

• incisors cut and bite; molars crush and grind; larger surface area

Watch for

• “Molars cut the apple and incisors grind it.” — It is the other way round: incisors cut, molars grind.

7. AK1 · Final check

Model answer. The duodenum (Q) is the short first part of the small intestine, leaving the stomach. The rectum (P) is the last, short part of the large intestine, and the anus (S) is the opening at its end. The gall bladder (V) stores bile; the stomach (N) churns food; the colon (L) absorbs water from undigested food.

A full answer includes

• (a) Q, the duodenum.
• (b) P, the rectum.
• (c) S, the anus.
• (d) V, the gall bladder.
• (e) N, the stomach.
• (f) L, the colon.

Also accept

• (a) Q; (b) P; (c) S; (d) V; (e) N; (f) L

Watch for

• “The liver stores bile.” — The liver makes bile; the gall bladder stores it.

8. AK2 · Final check

Model answer. The oesophagus carries food from the mouth to the stomach. The pancreas makes pancreatic juice, which contains enzymes, and sends it into the duodenum. The rectum stores faeces, which leave the body through the anus: egestion.

A full answer includes

• (a) Carries food from the mouth to the stomach.
• (b) Makes pancreatic juice, containing enzymes, which flows into the duodenum.
• (c) Stores faeces.
• (d) Egestion.

Also accept

• (a) carries food to the stomach; (b) juice with enzymes into the duodenum; (c) stores faeces; (d) egestion

Watch for

• “The rectum absorbs digested food.” — Digested food is absorbed in the ileum. The rectum stores faeces.

9. AK3 · Final check

Model answer. The muscular wall of the stomach contracts and relaxes, churning the food. This breaks it into smaller pieces and mixes it with gastric juice, without chemical change, which increases the surface area for enzymes.

A full answer includes

• Its muscular wall churns the food.
• This breaks the food into smaller pieces and mixes it with gastric juice, increasing its surface area for enzymes.

Also accept

• muscular wall churns; smaller pieces; mixing; surface area

Watch for

• “The stomach does physical digestion with acid.” — Acid is a chemical. The physical digestion is done by the muscles churning the food.

10. AK4 · Final check

Model answer. Bile, from the liver and gall bladder, emulsifies fats in the duodenum: it breaks large drops into many tiny droplets without chemical change. This increases the surface area of the fat for the enzyme, so the fat is digested faster.

A full answer includes

• Bile emulsifies fats: it breaks large drops into tiny droplets.
• This increases the surface area of the fat for enzymes, so it is digested faster.

Also accept

• emulsifies; tiny droplets; larger surface area for enzymes

Watch for

• “Bile contains the enzyme that digests fat.” — Bile emulsifies the fat; an enzyme from the pancreas digests it.