The small intestine.

Question 1

Describe the functions of villi and microvilli in the small intestine.

Answer 1

Amplify the surface area available for interaction with food.

Question 2

Where would you find brush border enzymes?

Answer 2

Integral membrane proteins on the luminal surface of the intestinal microvilli.

Question 3

What is the function of brush border enzymes?

Answer 3

To break down materials in contact with brush border.

Question 4

Define peristalsis.

Answer 4

Waves of muscular contractions which move contents along the length of the GI tract.

Question 5

Describe the action of circular and longitudinal muscles during peristalsis.

Answer 5

Circular muscles contract behind bolus, while circular muscles ahead of bolus relax. Longitudinal muscles ahead of bolus contract,shortening adjacent segments. Wave of contraction in circular muscles forces bolus forward

Question 6

What is segmentation and what is its function?

Answer 6

Alternate contraction of neighbouring segments. Churn and fragment the bolus. Mix contents with intestinal secretions

Question 7

What is the migrating motor complex and what is its function?

Answer 7

When the gut is relatively empty, MMCs are generated roughly every 90 minutes. Stimulated by motilin, secreted by M cells (and by erythromycin). Suppressed by feeding. For a few minutes, a series of strong, slow, peristaltic waves will sweep down, mostly from the stomach, and along the small bowel. Pyloric sphincter is relaxed, allowing larger things to pass These waves are thought to help keep the gut clean, prevent reflux, and reduce bacterial growth.

Question 8

Which nutrients must be digested before they can be absorbed?

Answer 8

Carbohydrates, lipids and proteins.

Question 9

Which bonds within the starch polymer can be hydrolysed by amylase?

Answer 9

Internal α1,4 bonds.

Question 10

Following initial starch digestion by amylase, where are the remaining short-chain carbohydrates digested and how?

Answer 10

At the brush border by brush border enzymes.

Question 11

How are glucose and galactose absorbed in the small intestine?

Answer 11

Actively via SGLT.

Question 12

How is fructose absorbed in the small intestine?

Answer 12

Passively via GLUT5.

Question 13

What percentage of proteolysis takes place in the stomach?

Answer 13

15%

Question 14

From where is enterokinase secreted and what is its function?

Answer 14

Secreted from crypts of Lieberkühn. Converts trypsinogen to trypsin.

Question 15

What are the functions of trypsin?

Answer 15

Endopeptidase (serine protease), activates further trypsinogen via autoactivation, activates other pancreatic zymogens via hydrolysis.

Question 16

Name the pancreatic zymogens secreted into the small intestine actived by trypsin.

Answer 16

Chymotrypsinogen, proelastase, procarboxypeptidase A & B

Question 17

Why is it important that these enzymes are produced as zymogens and only become active once entering the small intestine?

Answer 17

To prevent autodigestion of the pancreas - can occur in pancreatitis.

Question 18

Which of the main peptidases are endopeptidases?

Answer 18

Trypsin, chymotrypsin, elastase .

Question 19

Which of the main pepsidases are exopeptidases and what does this mean?

Answer 19

Carboxypeptidases A & B. They cut at the terminal peptide bond.

Question 20

What happens to the small peptides produced by the soluble peptidases?

Answer 20

They diffuse to the brush border where they are digested by membrane-bound proteases to mono and dipeptides.

Question 21

How are amino acids taken up by the small intestine?

Answer 21

Sodium-linked secondary active transporters.

Question 22

How are di- and tri- peptides sometimes taken up?

Answer 22

By proton-linked secondary active transporters.

Question 23

How is the proton gradient across small intestinal luminal cells created?

Answer 23

Luminal Na⁺/H⁺ antiporters are linked to the sodium gradient, maintained by the basal 3K⁺/2Na⁺ ATPase.

Question 24

What is the function of bile salts?

Answer 24

The emulsification of lipids to increase digestive surface area.

Question 25

How does pancreatic lipase work?

Answer 25

Cleaves the ‘outside’ fatty acids leaving 2 free fatty acids and one monoglyceride.

Question 26

What name is given to the complex of bile salts and free fatty acids/monoglycerides?

Answer 26

Micelle

Question 27

What happens to micelles?

Answer 27

They diffuse close to the brush border, delivering their contents to the cell membrane.

Question 28

What happens to lipids once inside the cell?

Answer 28

Inside the cell, triglycerides are resynthesised and packaged into chylomicra, which are exocytosed into the interstitium.

Question 29

What happens to chylomicra in the interstitium?

Answer 29

They can’t get into capillaries, but can get into lymphatic lacteals.

Question 30

Where/how are conjugated bile salts reabsorbed?

Answer 30

Actively in the distal ileum.

Question 31

What happens to bile salts that reach the colon?

Answer 31

They are deconjugated by bacteria, rendering them lipophilic and allowing passive reabsorption.

Question 32

What percentage of bile salts are lost in the faeces?

Answer 32

5%

Question 33

How is Fe³⁺ reduced to Fe²⁺?

Answer 33

Duodenal cytochrome B (Dcytb) at the luminal surface.

Question 34

How does Fe²⁺ enter the luminal cell?

Answer 34

DMT co-transports Fe²⁺ with H⁺

Question 35

What happens to non-heme Fe²⁺ once it has entered the luminal cell?

Answer 35

Tranfers to mobilferrin.

Question 36

What happens to heme - Fe²⁺ once it has entered the luminal cell?

Answer 36

Heme oxygenase oxidises heme Fe²⁺ and releases Fe³⁺. Heme is broken down to bilirubin.

Question 37

What happens to Fe²⁺ at the basal surface of the cell?

Answer 37

Leaves the cell via ferroportin. Fe²⁺ is then oxidised to Fe³⁺ by hephaestin. Iron binds to tranferrin in plasma.

Question 38

How is Ca²⁺ absorbed in the gut?

Answer 38

Ca2+ is actively reabsorbed in the duodenum

Regulated by Vit. D

In the rest of the small bowel, it is reabsorbed paracellularly.

Question 39

How are fat-soluble vitamins absorbed?

Answer 39

Via micelles and chylomicrons.

Question 40

Which vitamins are fat soluble?

Answer 40

A, D, E and K

Question 41

How are water-soluble vitamins absorbed from the gut lumen?

Answer 41

Require special transport proteins – usually Na+-linked

Question 42

How is vitamin B12 absorbed?

Answer 42

Absorbed only when bound to intrinsic factor (secreted by gastric parietal cells).

Question 43

In what part of the small intestine are glucose, galactose and fructose absorbed?

Answer 43

The duodenum.

Question 44

In what part of the small intestine are calcium, magnesium and iron absorbed?

Answer 44

Duodenum.

Question 45

Where in the small intestine are lipids and fat-soluble proteins absorbed?

Answer 45

Duodenum and ileum.

Question 46

Where in the small intestine are Na⁺, K⁺ and Cl^- absorbed?

Answer 46

Jejunum

Question 47

In what part of the small intestine are amino acids and peptides absorbed?

Answer 47

Jejunum and ileum.

Question 48

In what part of the gut are bile acids reabsorbed?

Answer 48

Colon