Free Digestion MCQs with Answers

1. Salivary amylase begins the digestion of

• A. protein
• B. starch
• C. fat
• D. cellulose

Explanation: Salivary amylase hydrolyses starch to the disaccharide maltose, so carbohydrate digestion is the only kind that starts in the mouth. It works best near neutral pH and is destroyed once the food reaches the acid of the stomach, which is why chewing time matters. Humans secrete no cellulase at all, so cellulose passes through as roughage.

2. Hydrochloric acid in gastric juice is important because it

• A. digests proteins directly into amino acids
• B. emulsifies the fat in the food
• C. kills most swallowed bacteria and gives pepsin the acid pH it needs
• D. neutralises the acid arriving from the mouth

Explanation: The acid brings the stomach contents to about pH 2, which is the optimum for pepsin and also converts inactive pepsinogen into pepsin, while destroying most of the microbes swallowed with the food. The acid itself is not an enzyme, so it does not hydrolyse protein, and emulsification is the work of bile in the duodenum. The stomach lining is protected from the acid by a thick layer of mucus.

3. Pepsin is secreted in an inactive form as pepsinogen because

• A. an active enzyme would digest the protein of the cells that make it
• B. pepsinogen is easier to store in the blood
• C. pepsin cannot be transported across a membrane
• D. pepsinogen digests fat before it is converted

Explanation: Pepsin is a protease, and the cells that secrete it are themselves made of protein, so it is released as an inactive precursor and converted to the active enzyme by hydrochloric acid only after it is safely inside the stomach lumen. Trypsin is handled in the same way, being secreted as trypsinogen and activated by enterokinase in the duodenum. This is a general safety principle for digestive proteases.

4. Bile helps in the digestion of fat because it

• A. contains the enzyme lipase
• B. converts fatty acids into glycerol
• C. makes the duodenum acidic
• D. emulsifies fat into tiny droplets, greatly increasing the surface area for lipase

Explanation: Bile salts are detergents that break large fat globules into a suspension of tiny droplets, and since lipase can only act at the surface of a droplet this multiplies the area available and speeds digestion enormously. Bile contains no enzyme of its own, which is the point students most often get wrong. It is also alkaline, so it neutralises the acid chyme arriving from the stomach.

5. The enzyme that digests fats into fatty acids and glycerol is secreted mainly by the

• A. gall bladder
• B. pancreas
• C. gastric glands
• D. salivary glands

Explanation: Pancreatic juice contains lipase along with amylase and trypsinogen, and it enters the duodenum through the pancreatic duct, which is also where bile arrives to emulsify the fat first. The gall bladder only stores and concentrates bile and secretes no enzymes at all. A small amount of lipase is made in the stomach but it contributes very little in an adult.

6. Trypsinogen released by the pancreas is converted into active trypsin by

• A. enterokinase secreted by the wall of the duodenum
• B. hydrochloric acid from the stomach
• C. bile salts from the gall bladder
• D. salivary amylase

Explanation: Enterokinase, an enzyme of the intestinal lining, removes a short peptide from trypsinogen and unmasks its active site, after which trypsin can activate more trypsinogen and other pancreatic precursors as well. Hydrochloric acid is the activator of pepsinogen in the stomach, which is why it is the tempting answer, but it is already neutralised by the time chyme meets pancreatic juice.

7. Most of the products of digestion are absorbed in the

• A. stomach
• B. large intestine
• C. small intestine
• D. oesophagus

Explanation: The small intestine is long, folded and covered in villi and microvilli, so its absorptive surface is measured in hundreds of square metres, and by the time chyme reaches it digestion is essentially complete. The stomach absorbs only alcohol and a few drugs, and the large intestine takes up water and mineral ions rather than nutrients. Damage to the villi, as in coeliac disease, causes malnutrition even on a good diet.

8. Which feature of a villus is NOT an adaptation for absorption?

• A. A dense network of blood capillaries
• B. A single layer of epithelial cells covering it
• C. Microvilli on the free surface of its cells
• D. A thick covering of muscle around each villus

Explanation: A villus is adapted by being thin walled, richly supplied with blood and lymph and covered in microvilli, all of which shorten the diffusion path or enlarge the surface. A thick muscular coat would do the opposite by lengthening the distance nutrients must travel. Muscle lies in the wall of the intestine as a whole, where it produces peristalsis, not around individual villi.

9. The main function of the large intestine is

• A. the digestion of protein
• B. the absorption of water and mineral salts
• C. the secretion of bile
• D. the absorption of amino acids

Explanation: Undigested material entering the colon is very fluid, and the reabsorption of water and salts turns it into semi solid faeces, which is why disease that speeds transit causes diarrhoea and dehydration. Bacteria living there also synthesise vitamin K and some B vitamins. No digestive enzymes are secreted by the colon, so nutrient absorption is finished before this point.

10. Peristalsis is

• A. a wave of contraction of circular muscle behind the food that pushes it along the gut
• B. the churning of food by the teeth
• C. the return of food from the stomach to the mouth
• D. the absorption of nutrients into the blood

Explanation: Circular muscle contracts behind the bolus and longitudinal muscle contracts in front of it, so a wave of narrowing travels along the tube and moves the contents in one direction. Because the muscle does the work, food reaches the stomach even when a person is lying down or standing on their head. The same mechanism moves material through the whole alimentary canal.