GI tract absorption

Question 1

What are the 3 main sections of the small intestine ?

Answer 1

• duodenum
• jejunum
• ileum

Question 2

Describe the structure of the small intestine

Answer 2

• it has circular folds which increase the surface area and allow the spiralling of chyme (this allows chyme to come into contact with membrane bound enzymes)
• it contains lymph nodules known as Peyer’s patches

Question 3

Describe the structure and function of the duodenum

Answer 3

• 20 - 25 cm long
• absorbs 9 litres of water per day

Question 4

Describe the structure and function of the jejunum

Answer 4

• 1 m long
• absorbs 3 - 4 litres of water per day

Question 5

Describe the structure and function of the ileum

Answer 5

• 2 - 3 m long
• absorbs 1 litre of water per day

Question 6

What are the main cell types found in the small intestine ?

Answer 6

• enterocytes
• goblet cells
• paneth cells
• I cells
• S cells
• D cells

Question 7

What is the structure and function of enterocytes ?

Answer 7

• columnar epithelial cells
• have microvilli on their ends which creates a brush border

Question 8

What is the function of goblet cells ?

Answer 8

they secrete mucus to reduce friction

Question 9

What is the function of paneth cells ?

Answer 9

they have an immunological function and produce lysozymes

Question 10

What is the function of I cells ?

Answer 10

release CCK

Question 11

What is the function of S cells ?

Answer 11

release secretin

Question 12

What is the function of D cells ?

Answer 12

release somatostatin

Question 13

Describe the structure of intestinal villi

Answer 13

• they have microvilli creating a brush border
• contain lacteals which are involved in the absorption of fats
• there is a capillary network which has a high blood flow and a countercurrent flow which maximises absorption
• found over intestinal crypts
• brush border enzymes are anchored on the apical membrane of enterocytes

Question 14

Describe the state of the food in the duodenum

Answer 14

• carbohydrates have been partly converted by salivary amylase
• proteins have been hydrolysed by pepsin
• fats are now mostly in the form of triglycerides

Question 15

How is food release from the stomach regulated ?

Answer 15

• fats have an important role in the regulation of food release from the stomach
• they increase the motility of the stomach
• fats are the last to leave the stomach

Question 16

Describe the digestion of carbohydrates

Answer 16

• carbohydrates are hydrolysed by salivary and pancreatic amylase into disaccharides
• brush border enzymes hydrolyse the disaccharides into monosaccharides

Question 17

Which brush border enzymes are involved in the digestion of disaccharides ?

Answer 17

• maltase hydrolyses maltose into 2 glucose molecules
• lactase hydrolyses lactose into glucose and galactose
• sucrase hydrolyses sucrose into glucose and fructose

Question 18

Describe the effects of a mutation in the lactase enzyme

Answer 18

a mutation in the lactase enzyme means there will be no production of glucose and galactose and so the person will be lactose intolerant

Question 19

Describe how glucose and galactose is absorbed

Answer 19

1) glucose and galactose will enter the enterocyte via SGLT1
2) Na+ will enter simultaneously
3) the Na+ concentration is maintained via Na+/K+ ATPase
4) a high glucose concentration will result in the insertion of GLUT 2 into the brush border membrane
5) GLUT 2 is also present on the basolateral surface
6) glucose will diffuse into the blood

Question 20

Describe how fructose is absorbed

Answer 20

1) fructose will enter the enterocyte via a GLUT 5 carrier - this is a Na+ independent transporter
2) a high glucose concentration will result in the insertion of GLUT 2 into the brush border membrane and fructose can also enter the enterocyte via GLUT 2

Question 21

Which enzymes are involved in the hydrolysis of proteins ?

Answer 21

• pancreatic enzymes : trypsin, chymotrypsin and carboxypeptidase
• brush border enzymes : aminopeptidases
• stomach : pepsin

Question 22

Describe how proteins are digested

Answer 22

• pancreatic enzymes break down proteins into small peptides
• aminopeptidases break down proteins into amino acids

Question 23

Describe how proteins are absorbed

Answer 23

1) small peptides enter enterocytes via a H+ dependent carrier
2) amino acids enter via a Na+ dependent carrier
3) small peptides are then hydrolysed into amino acids by intracellular peptidases
4) amino acids exit the cells via imports and Na+ linked antiports
5) they diffuse into the blood

Question 24

Describe how fats are digested and absorbed

Answer 24

1) fats form large fat globules
2) they get coated with bile salts to break them into smaller globules
3) phospholipids bind to smaller globules
4) this forms an emulsion
5) pancreatic lipase will act on this emulsion and form micelles
6) the monoglycerides and fatty acids within the micelles will enter the enterocytes via diffusion
7) the smooth endoplasmic reticulum will resynthesise the fats
8) this forms chylomicrons which diffuse into lacteals

Question 25

Describe the function of the liver

Answer 25

• largest organ in the abdomen
• it has a dual blood supply : oxygen rich through the hepatic artery and nutrient rich through the portal vein
• it secretes bile salts to emulsify fats
• 94% of bile slats are reabsorbed back into the liver through the portal vein

Question 26

Describe the function of the gallbladder

Answer 26

• it is a reservoir for the storage of bile
• concentrates bile
• CCK will cause contraction of the gallbladder to release bile

Question 27

How much bile is lost from the body each day ?

Answer 27

0.8g of bile is lost through faeces each day

Question 28

How is bile regulated ?

Answer 28

• through the return of bile
• secretion of bicarbonate rich watery fluid enhanced by secretin