Session 6 - The Intestines

Question 1

What is the role of the intestines?

Answer 1

Absorbs nutrients

Absorbs water/electrolytes

Question 2

How is a large surface area created in the intestines?

Answer 2

Mucosa folded into villi

Surface covered in microvilli

Question 3

What are plicae circulares?

Answer 3

Permanent folds of the intestines

Question 4

What is the main epithelial cell of the intestines?

Answer 4

Enterocytes

Question 5

Glucose can only enter enterocytes when it is co-transported along with what ion?

Answer 5

Na+

Question 6

Where does the final digestion of carbohydrates occur?

Answer 6

The brush border of intestines

Question 7

What are the common dietary carbohydrates?

Answer 7

Starch (polysaccharide)
Lactose (disaccharide)
Sucrose (dissacharide)

Question 8

What bond holds glucose molecules together in starch?

Answer 8

Alpha 1-4 bonds

Question 9

What enzyme converts alpha dextrins to glucose?

Answer 9

Isomaltase

Question 10

What does lactase convert lactose into?

Answer 10

Glucose and galactose

Question 11

What does sucrase convert sucrose into?

Answer 11

Glucose and fructose

Question 12

Explain how monosaccharide absorption occurs in an enterocyte.

Answer 12

Glucose (or galactose) co-transported into cell with sodium through SGLT-1 transporter.
Fructose moves in through GLUT5 channel by facilitated diffusion.
GLUT2 channel allows glucose and fructose out of the cell, diffuses down concentration gradient into capillary blood.
Na+/K+ ATPase on basolateral membrane maintain low intracellular Na+

Question 13

How is water taken up in the intestines?

Answer 13

Uptake of Na+ generates an osmotic gradient, water follows.
Glucose uptake stimulates further Na+ uptake, therefore a mixture of glucose and salt will stimulate maximum water uptake.

Question 14

What cells release pepsinogen?

Answer 14

Chief cells

Question 15

What converts pepsinogen to pepsin?

Answer 15

HCl

Question 16

What is the action of pepsin?

Answer 16

Breaks down proteins to oligopeptides and amino acids (occurs in stomach)

Question 17

What enzyme converts trypsinogen to trypsin?

Answer 17

Enteropeptidase

Question 18

What is the action of trypsin?

Answer 18

Breaksdown proteins

Activates other proteases (e.g. chymotrypsin)

Question 19

What is the difference between exopeptidases and endopeptidases?

Answer 19

Exopeptidases break bonds at the ends of polypeptide chains to produce dipeptides or individual amino acids.
Endopeptidases break bonds in the middle of polypeptide chains to produce shorter polypeptides.

Question 20

Give examples of endopeptidases and exopeptidases.

Answer 20

Endopeptidases:
- trypsin
- chymotrysin 
- elastase
exopeptidases:
- carboxypeptidase

Question 21

How are amino acids transported into cells?

Answer 21

Na+-amino acid co-transporters
Different ones (neutral, acidic, basic, imino)

Question 22

Are most protein products absorbed as amino acids or dipeptides/tripeptides?

Answer 22

Most protein products are ingested as dipeptides/tripeptides, not as amino acids.

Question 23

How are dipeptides/tripeptides absorbed?

Answer 23

Peptide transporter 1 (PepT 1)

Question 24

What happens to dipeptides/tripeptides once they have entered the enterocyte?

Answer 24

they are converted to amino acids inside the cell by cytosolic peptidases

Question 25

What is the difference in Na+ absorption between the small and large intestines?

Answer 25

In the small intestine Na+ is co-transported. | in the large intestine Na+ moves in through Na+ channels (which are induced by aldosterone).

Question 26

Calcium absorption is stimulated by what hormone?

Answer 26

Parathyroid hormone

Question 27

How does the absorption of calcium in the intestines differ between when calcium levels are low and when calcium is normal/high?

Answer 27

When there are low levels of calcium in the blood: - active transcellular absorption - enters cells via facilitated diffusion - Ca+ ATPase removes Ca+ from the basolateral membrane - this process requires vitamin D (calbindin) - stimulated by parathyroid hormone When there are normal/high levels of calcium in the blood: - passive paracellular absoprtion

Question 28

Why do PPIs decrease iron absorption?

Answer 28

Gastric acid is important in the uptake of iron in the intestines as iron is co-transported across the apical membrane with H+. Use of PPIs means less H+ so less iron absorption.

Question 29

how is iron distributed in body?

Answer 29

Approximately half of iron is in haemoglobin, the other half is stored in ferritin complexes in bone marrow, liver and spleen.

Question 30

Once iron has been taken up by enterocytes, what happens to it next depending on whether iron levels in the body are high or low?

Answer 30

When iron levels are low, iron binds to transferrin and is transported to iron stores (in bone marrow, spleen or liver). When iron levels are high, iron remains contained in ferritin complexes within the enterocytes, meaning it is trapped there and then lost when the enterocyte is replaced.

Question 31

How are water soluble vitamins absorbed?

Answer 31

Na+ co-transport

Question 32

How is vitamin B12 absorbed?

Answer 32

Vitamin B 12 is absorbed in the terminal ileum bound to intrinsic factor.

Question 33

What cells secret intrinsic factor?

Answer 33

Gastric parietal cells

Question 34

Give two causes of vitamin B12 deficiency other than lack of dietary intake.

Answer 34

Gastritis (less intrinsic factor produced) | Terminal ileum removal (less vit B12 absorbed)

Question 35

What would the blood test of someone with coeliac disease show?

Answer 35

``` Electrolyte imbalance (due to reduced ion absorption) Anaemia (due to reduced iron absorption) ```

Question 36

Give some symptoms of coeliac disease.

Answer 36

Diarrhoea Flatulence Weight loss Sensory loss

Question 37

What is coeliac disease?

Answer 37

Intolerance to the gliadin fraction of gluten resulting in an immune response.

Question 38

What damage does the immune response in coeliac disease cause to the intestines?

Answer 38

Damages mucosa of intestines: - absence of intestinal villi - lengthening of intestinal crypts - infiltration of lymphocytes into epithelium all result in impaired digestion and malabsorption

Question 39

What investigations can be used to help diagnose coeliac disease?

Answer 39

Upper GI endoscopy and biopsies - identifies mucosal pathology and whether villi are reduced or absent. Bloods - serology, electrolyte imbalances, anaemia.

Question 40

How is coeliac disease treated?

Answer 40

Cut gluten out of diet.