Along the gut — answer key

1. AQ1 · Practice

Options as numbered on the pupil sheet: 1 — Liver; 2 — Colon; 3 — Oesophagus; 4 — Salivary glands; 5 — Gall bladder; 6 — Ileum; 7 — Pancreas; 8 — Stomach.

Model answer. A, beside the mouth, are the salivary glands. B, the tube to the stomach, is the oesophagus. C, the large organ at the top of the abdomen, is the liver, and D, the small bag under it, is the gall bladder. E, the muscular bag, is the stomach, and F, the long, flat organ below it, is the pancreas. G, the coiled tube, is the ileum, and H, the wide tube around it, is the colon.

A full answer includes

• A: salivary glands.
• B: oesophagus.
• C: liver.
• D: gall bladder.
• E: stomach.
• F: pancreas.
• G: ileum.
• H: colon.

Also accept

• A salivary glands; B oesophagus; C liver; D gall bladder; E stomach; F pancreas; G ileum; H colon

Watch for

• “The coiled tube in the middle is the large intestine.” — The coiled tube is the small intestine (the ileum). The large intestine, the colon, is wider and runs around it.

2. AQ2 · Practice

Options as numbered on the pupil sheet: 1 — The uptake and use of nutrients by cells; 2 — The removal of undigested food from the body as faeces; 3 — Taking substances, such as food and drink, into the body; 4 — The breakdown of food; 5 — The movement of nutrients from the intestines into the blood.

Model answer. Ingestion takes food and drink into the body; digestion breaks the food down; absorption moves nutrients from the intestines into the blood; assimilation is the uptake and use of nutrients by cells; egestion removes undigested food as faeces.

A full answer includes

• Ingestion: taking substances, such as food and drink, into the body.
• Digestion: the breakdown of food.
• Absorption: the movement of nutrients from the intestines into the blood.
• Assimilation: the uptake and use of nutrients by cells.
• Egestion: the removal of undigested food from the body as faeces.

Also accept

• ingestion: taking in; digestion: breakdown; absorption: into the blood; assimilation: use by cells; egestion: faeces out

Watch for

• “Absorption and assimilation mean the same.” — Absorption is into the blood; assimilation is the uptake and use of nutrients by cells.

3. AQ3 · Practice

Options as numbered on the pupil sheet: 1 — Stores bile; 2 — Stores faeces; 3 — Carries food from the mouth to the stomach; 4 — Makes bile, and takes up nutrients from the blood and uses or stores them; 5 — Absorbs water from undigested food; 6 — Makes a juice containing enzymes and sends it into the duodenum; 7 — Absorbs digested food into the blood.

Model answer. The oesophagus carries food to the stomach. The pancreas makes pancreatic juice, with enzymes, for the duodenum. The liver makes bile and deals with nutrients from the blood; the gall bladder stores the bile. The ileum absorbs digested food into the blood, the colon absorbs water from what is left, and the rectum stores the faeces.

A full answer includes

• Oesophagus: carries food from the mouth to the stomach.
• Pancreas: makes a juice containing enzymes and sends it into the duodenum.
• Liver: makes bile, and takes up nutrients from the blood and uses or stores them.
• Gall bladder: stores bile.
• Ileum: absorbs digested food into the blood.
• Colon: absorbs water from undigested food.
• Rectum: stores faeces.

Also accept

• oesophagus: carries food; pancreas: juice with enzymes; liver: bile, nutrients; gall bladder: stores bile; ileum: absorbs digested food; colon: absorbs water; rectum: stores faeces

Watch for

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

4. AQ4 · Practice

Model answer. The wall of the stomach contains thick layers of muscle. They contract and relax, churning the food: it is broken into smaller pieces and mixed with gastric juice. This increases the surface area of the food for enzymes.

A full answer includes

• (1) muscle
• (2) churns (or squeezes and mixes)
• (3) smaller
• (4) gastric juice
• (5) surface area

Also accept

• (1) muscle; (2) churns; (3) smaller; (4) gastric juice; (5) surface area

Watch for

• “The stomach does only chemical digestion.” — Its muscular wall also churns the food, which is physical digestion.

5. AQ5 · Practice

Options as numbered on the pupil sheet: 1 — Bile emulsifies fats into tiny droplets; 2 — The enzyme digests the many tiny droplets faster; 3 — Bile is stored in the gall bladder; 4 — Bile flows along the bile duct into the duodenum; 5 — Bile is made in the liver.

Model answer. Bile is made in the liver and stored in the gall bladder. After a meal it flows along the bile duct into the duodenum. There it emulsifies fats and oils into tiny droplets, which have a much larger surface area, so the enzyme that digests fat works faster.

A full answer includes

• Made in the liver; stored in the gall bladder; flows along the bile duct into the duodenum; emulsifies fats into tiny droplets; the enzyme digests the droplets faster.

Also accept

• liver, gall bladder, bile duct to duodenum, emulsifies, faster digestion

Watch for

• “Bile is made in the gall bladder.” — The gall bladder only stores bile; the liver makes it.

6. AQ6 · Practice

Options as numbered on the pupil sheet: 1 — Stomach; 2 — Rectum; 3 — Ileum; 4 — Anus; 5 — Colon; 6 — Duodenum; 7 — Mouth; 8 — Oesophagus.

Model answer. Food passes from the mouth along the oesophagus to the stomach, then through the small intestine (first the duodenum, then the ileum) and the large intestine (the colon, then the rectum), and leaves through the anus.

A full answer includes

• Mouth, oesophagus, stomach, duodenum, ileum, colon, rectum, anus.

Also accept

• mouth, oesophagus, stomach, duodenum, ileum, colon, rectum, anus

Watch for

• “Food passes through the liver after the stomach.” — The liver is an associated organ: food never passes through it.

7. AQ7 · Practice

Options as numbered on the pupil sheet: 1 — Assimilation; 2 — Egestion; 3 — Ingestion; 4 — Digestion; 5 — Absorption.

Model answer. Sipping water takes it into the body: ingestion. Churning breaks food down: digestion. Nutrients moving from the intestine into the blood: absorption. A cell taking up and using a nutrient: assimilation. Undigested fibre leaving as faeces: egestion.

A full answer includes

• A pupil sips water: ingestion.
• The stomach churns a meal: digestion.
• Vitamins and mineral ions pass from the small intestine into the blood: absorption.
• A bone cell uses calcium to build new bone: assimilation.
• Fibre left over from a meal leaves the body: egestion.

Also accept

• ingestion; digestion; absorption; assimilation; egestion

Watch for

• “Fibre leaving the body is excretion.” — Fibre was never digested or absorbed, so it never entered the cells: its removal is egestion.

8. AQ8 · Practice

Model answer. The pancreas is one of the associated organs of the digestive system. It makes pancreatic juice, which contains enzymes, and sends it along a duct into the duodenum. There the enzymes continue the digestion of the food, so the pancreas helps digestion without food passing through it.

A full answer includes

• The pancreas is an associated organ: it makes pancreatic juice, which contains enzymes.
• The juice flows along a duct into the duodenum, where it digests food.

Also accept

• associated organ; makes juice with enzymes; flows into the duodenum

Watch for

• “Only organs that food passes through are part of the digestive system.” — The associated organs, the salivary glands, pancreas, liver and gall bladder, make or store juices for the canal.

9. AQ9 · Practice

Model answer. The stomach wall has thick layers of muscle, which contract and relax again and again. This churns the food, breaking it into smaller pieces and mixing it with gastric juice, with no chemical change. The smaller pieces have a larger surface area, so enzymes digest them faster.

A full answer includes

• The muscles of the stomach wall contract and relax, churning the food.
• This breaks it into smaller pieces and mixes it with gastric juice, without changing the food molecules.
• Smaller pieces have a larger surface area for enzymes.

Also accept

• muscular wall churns; smaller pieces, mixed with gastric juice; larger surface area for enzymes

Watch for

• “The stomach breaks up food only with its acid.” — The muscles of the wall churn it too: that is physical digestion.

10. AQ10 · Practice

Model answer. Bile breaks large drops of fat into many tiny droplets, but the fat molecules are not changed, so this is physical digestion, not chemical. The tiny droplets have a much larger total surface area, so the enzyme that digests fat can act on much more of the fat at once, and the fat is digested faster.

A full answer includes

• Bile breaks large drops of fat into many tiny droplets, but does not change the fat molecules: physical.
• The tiny droplets have a much larger surface area, so the enzyme that digests fat can act on more of it at once.

Also accept

• tiny droplets, no chemical change; larger surface area for the enzyme

Watch for

• “Bile digests fat.” — Bile contains no enzyme for fat; it emulsifies the fat so that the enzyme can digest it faster.

11. AT1 · Apply

Model answer. The ileum is where digestion is completed and the small, soluble molecules are absorbed into the blood. With a shorter ileum, less of the digested food is absorbed, so less reaches the cells to be assimilated, and more is lost in the faeces. The patient may lose weight although they eat normally.

A full answer includes

• Absorption is affected most: the ileum absorbs digested food into the blood.
• Less of the digested food reaches the blood and the cells, so less is assimilated, and more leaves in the faeces.

Also accept

• absorption; less digested food reaches the blood and cells

Watch for

• “The patient loses weight because they cannot ingest food.” — Ingestion happens at the mouth and is not affected. The food is taken in but less of it is absorbed.

12. AT2 · Apply

Model answer. Weak muscles churn the food less. It stays in larger pieces and is less well mixed with gastric juice, so it does not become a smooth liquid. Larger pieces have a smaller surface area, so the enzymes in the stomach and small intestine digest the food more slowly.

A full answer includes

• Less churning: the food stays in larger pieces and is less well mixed with gastric juice.
• Larger pieces have a smaller surface area, so enzymes digest the food more slowly.

Also accept

• less churning; larger pieces, less mixing; smaller surface area; slower chemical digestion

Watch for

• “Weak stomach muscles stop chemical digestion.” — The enzymes still work, but on larger pieces with less surface, so more slowly.

13. AT3 · Apply

Model answer. The droplets are 100 times narrower, so 100 × 100 × 100 = 1 000 000 of them hold the same volume of oil. The total surface area grows by the same factor as the width shrinks: 100 times larger. This is why emulsifying speeds up the chemical digestion of fat so much.

Checked automatically

• (a) Number of droplets (digits without spaces, or as 1 × 10⁶): 1000000 (exact value)
  • Common wrong answer 100 : That is how many times narrower they are. Multiply 100 × 100 × 100.
  • Common wrong answer 1000 : That is for droplets one tenth as wide.
• (b) How many times larger the surface area: 100 (exact value)
  • Common wrong answer 1000000 : That is the number of droplets. Each one is much smaller.
  • Common wrong answer 10 : That is for droplets one tenth as wide.

A full answer includes

• (a) 100 × 100 × 100 = 1 000 000 droplets.
• (b) 100 times larger.

Also accept

• (a) 1 000 000; (b) 100

Watch for

• “The surface area grows by the same factor as the number of droplets.” — A million droplets, but each is much smaller: the total surface grows 100 times, not a million times.

14. AT4 · Apply

Model answer. The capsule goes where food goes: mouth, oesophagus, stomach, duodenum, ileum, colon, rectum and anus. It never enters the salivary glands, the pancreas, the liver or the gall bladder: these associated organs only send their juices into the canal along ducts. It leaves with the faeces, by egestion.

A full answer includes

• (a) Mouth, oesophagus, stomach, duodenum, ileum, colon, rectum, anus.
• (b) Any two of: salivary glands, pancreas, liver, gall bladder; they are associated organs, joined to the canal only by ducts, so it never enters them.
• (c) Egestion.

Also accept

• (a) canal in order; (b) two associated organs, joined only by ducts; (c) egestion

Watch for

• “The capsule passes through the liver, where substances are processed.” — Food and capsules stay in the canal. The liver works on nutrients brought by the blood.