Digestion and Absorption in the Gastrointestinal Tract

Physiology, MBBS Second Year. This chapter is split into 8 topics and has 234 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Hydrolysis & digestion of carbohydrates 31 questions
2. Digestion of proteins 28 questions
3. Digestion of fats 27 questions
4. Principles of GI absorption & surface area 28 questions
5. Absorption of water, sodium & chloride 35 questions
6. Absorption of bicarbonate, calcium & iron; cholera 28 questions
7. Absorption of carbohydrates, proteins & fats 31 questions
8. Absorption in the large intestine, bacterial action & faeces 26 questions

Sample questions: Digestion and Absorption in the Gastrointestinal Tract

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. Which sequence correctly describes fat digestion?

• A. Fat → amylase → glycerol
• B. Fat → pepsin → fatty acids
• C. Fat → lingual lipase in mouth (less than 10%) → agitation in stomach → emulsification by bile salts and lecithin → pancreatic lipase → glycerol and 3 fatty acids, then fatty acids and 2-monoglycerides → micelle formation → absorption(correct answer)
• D. Fat → trypsin → cholesterol

Answer: C.Fat digestion begins with lingual lipase in the mouth, continues with gastric agitation, and is completed in the small intestine after emulsification by bile salts and lecithin, when pancreatic lipase produces fatty acids and 2-monoglycerides that form micelles.

Q2. Why is fat digestion more dependent on bile than carbohydrate digestion is?

• A. Because bile contains amylase
• B. Because fat is hydrophobic and must be emulsified and carried in micelles, whereas carbohydrate is water-soluble and needs no emulsification(correct answer)
• C. Because bile activates lingual lipase
• D. Because carbohydrate requires micelles

Answer: B.Carbohydrates are water-soluble and directly accessible to enzymes, whereas fat is hydrophobic: it must be emulsified by bile salts to expose surface area for lipase and its products must be carried in micelles across the aqueous phase.

Q3. In the duodenum and jejunum, bicarbonate is absorbed indirectly by:

• A. Simple diffusion of bicarbonate
• B. Sodium-hydrogen exchange: hydrogen secreted into the lumen reacts with bicarbonate to form carbonic acid, which breaks down to water and CO₂(correct answer)
• C. Direct active transport of bicarbonate
• D. Chloride-bicarbonate exchange

Answer: B.In the duodenum and jejunum, sodium is absorbed in exchange for hydrogen secreted into the lumen; the hydrogen reacts with luminal bicarbonate to form H₂CO₃, which dissociates to water and CO₂, so bicarbonate is effectively absorbed — the so-called active absorption of bicarbonate ions.

Q4. Which sequence correctly describes bicarbonate absorption in the duodenum and jejunum?

• A. Bicarbonate actively pumped whole into the blood
• B. Bicarbonate converted to chloride
• C. Bicarbonate precipitated with calcium
• D. Sodium absorbed in exchange for secreted hydrogen → hydrogen + luminal bicarbonate → carbonic acid → water + CO₂ → CO₂ absorbed into blood and expired(correct answer)

Answer: D.Sodium absorption is coupled to hydrogen secretion; the hydrogen combines with luminal bicarbonate to form carbonic acid, which dissociates into water and CO₂, and the CO₂ is absorbed and expired — an indirect 'active absorption' of bicarbonate.

Q5. Which statement about bicarbonate handling in the gut is TRUE?

• A. It is absorbed in the duodenum and jejunum indirectly, via hydrogen ion secretion, and is also secreted in exchange for chloride in the ileum and colon(correct answer)
• B. It is only absorbed
• C. It is never transported
• D. It is only secreted

Answer: A.Bicarbonate is absorbed in the duodenum and jejunum indirectly (via Na-H exchange and carbonic acid formation, with CO₂ expired), while in the ileum and large intestine it is secreted into the lumen in exchange for absorbed chloride.

Q6. Why is oral rehydration therapy effective in cholera even though intestinal secretion is enormous?

• A. Because it blocks chloride channels
• B. Because sodium-glucose co-transport remains functional, so providing glucose and sodium in the right proportion drives sodium (and water) absorption(correct answer)
• C. Because it increases cAMP breakdown in the lumen
• D. Because it neutralises cholera toxin

Answer: B.Despite the massive secretory drive of cholera, the brush-border sodium-glucose co-transporter is preserved; giving glucose with sodium stimulates sodium entry and water follows osmotically, making oral rehydration therapy highly effective.

Q7. Which sequence correctly describes sodium-coupled glucose absorption?

• A. Sodium and glucose diffuse independently
• B. Glucose enters by pinocytosis
• C. Glucose is pumped actively into the cell → sodium follows
• D. Na-K ATPase on the basolateral membrane depletes intracellular Na+ → Na+ enters from the lumen bound to a transport protein → glucose binds to the same transport protein → cotransport of Na+ and glucose across the brush border(correct answer)

Answer: D.Active sodium transport at the basolateral membrane (Na-K ATPase) keeps intracellular sodium low; sodium then enters the cell from the lumen coupled to a transport protein, and glucose binds to the same protein, giving secondary active cotransport at the brush border.

Q8. Which statement correctly contrasts the absorptive routes of long-chain fats and short-chain fatty acids?

• A. Long-chain fats are resynthesised into triglycerides and travel via the lymphatics as chylomicrons; short- and medium-chain fatty acids pass directly into the portal blood(correct answer)
• B. Both travel via the lymphatics
• C. Both pass directly into the portal blood
• D. Long-chain fats pass into the portal blood directly

Answer: A.Long-chain fatty acids and monoglycerides are resynthesised into triglycerides, packaged as chylomicrons and carried by the lymphatics, whereas short- and medium-chain fatty acids are water-soluble enough to diffuse directly into the portal blood.

Q9. Assertion: Carbohydrates and proteins enter the portal blood, whereas most fat enters the lymphatics. Reason: Most absorbed fat is packaged into chylomicrons that are too large to enter the blood capillaries directly.

• A. Assertion is false but Reason is true
• B. Both are true, but the Reason does not explain the Assertion
• C. Assertion is true but Reason is false
• D. Both Assertion and Reason are true, and the Reason correctly explains the Assertion(correct answer)

Answer: D.Water-soluble monosaccharides and amino acids pass directly into the villus capillaries and portal blood, whereas absorbed fat is packaged into chylomicrons too large for capillaries and is therefore carried by the central lacteal and thoracic duct.

Q10. Why must bile salts be present for efficient absorption of long-chain (but not short-chain) fats?

• A. Because short-chain fats require chylomicrons
• B. Because long-chain fats are insoluble and must be solubilized in micelles to reach the brush border, whereas short-chain fatty acids are water-soluble enough to diffuse directly into the portal blood(correct answer)
• C. Because long-chain fats are absorbed in the colon
• D. Because bile salts digest long-chain fats

Answer: B.Long-chain fats are highly insoluble and depend on bile salt micelles for transport to the brush border and on chylomicron formation for exit; short- and medium-chain fatty acids are sufficiently water-soluble to be absorbed directly into the portal blood without micelles.