10. The Intestines

Question 1

What size of carbohydrates can be absorbed and what are the main ones that are absorbed?

Answer 1

Only monosaccharides (glucose, fructose, galactose)

Question 2

What happens to carbohydrates of plant origin that cannot be digested in SI?

Answer 2

• These are utilised and partially digested by bacteria in the colon (providing nutrients for colonic mucosa)

Question 3

Which ion does glucose enter with?

Answer 3

Na+

Question 4

Give 3 examples of common dietary carbohydrates

Answer 4

Starch, lactose and sucrose

Question 5

What chains does starch consist of and what bonds are present?

Answer 5

• Straight chains of glucose- Amylose (alpha 1-4)

* Branched chains of glucose-Amylopectin (alpha 1-6)

Question 6

Which enzymes are required to break the bonds in amylose and amylopectin?

Answer 6

Amylose (alpha 1-4): salivary and pancreatic amylase

Amylopectin (alpha 1-6): isomaltase (brush border)

Question 7

What is produced from amylopectin when the alpha 1-4 bonds are broken?

Answer 7

alpha Dextrin (shorter but still branched chains of glucose)

Question 8

What are the products from digestion of starch?

Answer 8

• Glucose (fine)
• Maltose (maltase) = Glucose + Glucose
• Alpha dextrins (isomaltase) = Glucose

Question 9

Which enzymes digests the disaccharides?

Answer 9

Lactose (lactase) = glucose + galactose
Sucrose (sucrase) = glucose + fructose
maltose (maltase) = glucose + glucose
(brush border enzymes)

Question 10

What maintains the Na+ ions concentration gradient in the enterocytes?

Answer 10

Na+/K+ ATPase (3Na+ out, 2K+ in) on basolateral membrane

• Maintains low intracellular Na+

Question 11

How do glucose enter enterocytes?

Answer 11

SGLT -1 binds Na+
• This allows glucose binding
• Na+ & glucose moves into cell

Question 12

How does glucose get from the enterocytes into the blood?

Answer 12

GLUT2 transports glucose out of enterocyte

• Diffuses down gradient into capillary blood

Question 13

How does fructose enter enterocytes?

Answer 13

Fructose uses GLUT5 transporter to enter enterocyte

• Facilitated diffusion

Question 14

Which 4 places do proteins get digested and by what?

Answer 14

1. Stomach- (H+/pepsin)
2. Intestinal lumen- (Trypsin…)
3. Brush border
4. Cytosol (cytosolic peptidases)

Question 15

Where is pepsinogen released from and what causes its activation?

Answer 15

Chief cells in the stomach, converted to pepsin by HCL

Question 16

What are the products of protein digestion by pepsin?

Answer 16

• Oligopeptides/amino acids

* Move to small intestine

Question 17

In what form does the pancreas release proteases?

Answer 17

Pancreas releases proteases as zymogens –(move into intestinal lumen to be activated)
• Trypsinogen is important

Question 18

What activates trypsinogen?

Answer 18

Enteropeptidase (enterokinase)

Question 19

What does trypsin lead to activation of?

Answer 19

Various other proenzymes:

• Trypsinogen
• chymotrypsinogen
• proelastase
• procarboxypeptidase A
• procarboxypeptidase B

Question 20

What is the difference between endo and exopeptidase?

Answer 20

Endo: breaks bonds in the middle of the polypeptide to produce shorter polypeptides
Exo: Breaks bonds at the end of polypeptides to produce dipeptides and amino acids

Question 21

Give examples of endo and exopeptidases.

Answer 21

Endo: trypsin, chymotrypsin, elastase
Exo: carboxypeptidase A&B

Question 22

What does the brush border contain that aids protein digestion?

Answer 22

Brush border- also contains proteases
•The enterocytes express peptidases in their brush border
•Sometimes cannot completely digest proteins down to amino acids

Question 23

What size peptides can be absorbed?

Answer 23

short peptides and amino acidds

Question 24

Which transporter absorbs di/tripeptides?

Answer 24

PepT1 (peptide transporter 1)

Question 25

How are amino acids transported into enterocytes?

Answer 25

• Na+-amino acid co-transporters

Question 26

What happens to the small peptides in enterocytes?

Answer 26

* The small peptides are then acted on by cytosolic peptidases (broken down to amino acids) * Certain di- and tri-peptides can also be absorbed into blood

Question 27

How is water absorbed?

Answer 27

Absorption driven by movement of sodium into enterocytes •Na+ moved by active transport out of cell on basolateral membrane • Na+ diffuses into epithelial cells • Water then moves by osmosis • Movement occurs through para-cellular and transcellular paths *Osmotic gradient from all absorption leads to uptake of water ◦ Fluid absorbed is isosmotic

Question 28

What is the difference in Sodium uptake between small and large intestine?

Answer 28

• Both have Na+-k+ ATPase on basolateral membrane • Apical membrane - Small intestine- Na+ is co-transported with (glucose, amino acids…) - Large intestine- Na+ channels

Question 29

What hormone induces Na+ channels in the large intestine?

Answer 29

Aldosterone, increases Na+ uptake when blood pressure is low

Question 30

What is the principle of oral rehydration therapy?

Answer 30

Mixture of glucose and salt will stimulate maximum water uptake: - glucose stimulates Na+ uptake - Na+ uptake creates osmotic gradient so water follows

Question 31

What is secretion of water driven by?

Answer 31

Chloride movement (predominantly)

Question 32

Describe the mechanism for Chloride ion secretion.

Answer 32

* Chloride enters crypt epithelial cell, Co-transported with Na and K * Cyclic AMP levels increase inside cells * Increased cAMP activate CFTR * Cl ions are secreted * Na is drawn into lumen across tight junctions * NaCl secretion creates osmotic gradient * Water moves into lumen

Question 33

What does B12 deficiency cause?

Answer 33

Megaloblastic anaemia and neurological symptoms

Question 34

What are the causes of B12 deficiency?

Answer 34

•Lack of intrinsic factor (IF is released by parietal cells) - B12 is bound to IF in the small intestine and transports this to distal ileum - Most reabsorbed •Hypochlorhydria (inadequate stomach acid) - Acid is important in initially releasing cobalamin - Gastric atrophy, PPIs •Inadequate intake in food (strict vegetarians) •Inflammatory disorders of ileum (where it is absorbed) - Crohn’s disease

Question 35

What is lactose intolerance?

Answer 35

Caused by deficiency of the enzyme lactase (brush border enzyme) •After the age of 2 years the enzyme is expressed less

Question 36

Why do symptoms of lactose intolerance occur?

Answer 36

When lactose is consumed in quantity it cannot be absorbed • Remains in gut lumen created high osmotic effect • Water is not absorbed resulting in diarrhoea • Lactose is fermented in gut producing flatus/bloating

Question 37

What are the symptoms of irritable bowel syndrome?

Answer 37

* Abdominal pain (often cramping, sometimes relieved by defaecation) * Bloating * Flatulence * Diarrhoea/constipation (sometimes alternating) * Rectal urgency

Question 38

Who are IBS more common in?

Answer 38

* More common in females vs males (2:1) * 20s-40s most affected age range * More common in association with psychological disorders

Question 39

What is the cause of IBS?

Answer 39

Causes are multifactoria

Question 40

What is coeliac disease?

Answer 40

Immunological response to the gliadin fraction of gluten (found in wheat, rye, barley)

Question 41

What damage does coeliac disease cause?

Answer 41

Damages mucosa of intestines • Absence of intestinal villi • Hypertrophy/Lengthening of intestinal crypts • Lymphocytes infiltrate epithelium and lamina propria • Impaired digestion/malabsorption

Question 42

What are the symptoms of coeliac disease?

Answer 42

* Majority related to malabsorption (diarrhoea, weight loss, flatulence, abdo pain) * Anaemia (impaired iron absorption), neurological symptoms (hypocalcaemia)

Question 43

How do genetic factors affect coeliac disease?

Answer 43

``` Genetic factors (high concordance monozygotic twins (75%) • Very common (underdiagnosed) estimates of 1% of western population affected ```

Question 44

What are the investigations for coeliac disease?

Answer 44

• Bloods- Immunoglobulin A (IgA) antibodies to smooth muscle endomysium and tissue transglutaminase • Upper GI endoscopy + biopsies (duodenum) • Mucosal pathology • Villi are reduced or absent

Question 45

What is the treatment for coeliac disease?

Answer 45

* Diet- strict gluten free diet * Clinical improvement quite quick (days/ weeks) * Histological improvement (weeks/months