Lecture 9 - Absorption & Digestion

Question 1

How are polysaccharides such as starch broken down?

What are the breakdown products?

Answer 1

• Amylase breaks a-1,4 bonds, isomaltase breaks branched a-1,6 chains

1) Glucose
2) Maltose - broken by maltase into 2 x glucose
3) Alpha-dextrin - broken by isomaltase in amylose, amylase breaks into glucose

Question 2

How are lactose & sucrose broken down?

What are the 3 primary monosaccharides absorbed?

Answer 2

Lactose broken down by lactase (brush border enzyme) into glucose & galactose
Sucrose broken down by sucrase (brush border enzyme) into glucose & fructose

• Glucose, Galactose & Fructose

Question 3

How are glucose, galactose & fructose absorbed by enterocytes?

Answer 3

1) Glucose & Galactose absorbed via SGLT1 by secondary AT set up by Na/K ATPase on BL membrane
2) Fructose absorbed via GLUT5. All 3 exit via GLUT2 transporter on BL membrane into blood.

Question 4

Describe the full process of protein digestion in the GIT

Answer 4

1) Chief cells in gastric glands release pepsinogen, converted in stomach to pepsin due to acidic conditions. Breaks proteins into oligopeptides.
2) Oligopeptides enter duodenum. Pancreas releases pro-enzymes in zymogen granules, they are converted into active form when trypsinogen coveted into trypsin by enteropeptidase. Endopeptidases break middle of protein chain, enteropeptidases break end of chain to form smaller polypeptides.
3) AA’s + Di/tripeptides, absorbed by enterocytes which express PepT1 (similar to glucose transport - SAT).
4) Cytosolic peptidases in enterocytes break down remaining peptides into AA’s. AA’s absorbed into blood.

Question 5

How can water move across the intestinal epithelium? (2)
How does this form the basis for oral rehydration therapy?
How is water secreted into the gut lumen?

Answer 5

1) Transcellularly (through epithelial cell) or 2) paracellularly (through tight junctions). Na/K ATPase pump out Na to form hyper-osmotic solution in intercellular spaces which provide water gradient.

• Lots of electrolytes in rehydration sachets, drives uptake of water
• Chloride ions move in from blood through NaK2Cl, secreted through CFTR into lumen, drags water with it.

Question 6

What are the symptoms of Vit B12 deficiency?

What are the main causes?

Answer 6

• B12 is a co-factor in DNA synthesis, important for nervous system functioning and synthesis of myelin. Deficiency causes neurological problems and megaloblastic anaemia.
• Lack of intrinsic factor (released by parietal cells)
• Hypochlorhydria (lack of stomach acid) - so parietal cells not stimulated
• Dietary deficiency of B12

Question 7

What are the consequences of lactose intolerance?

What are the consequences of IBS?

Answer 7

• Lactase insufficiency, lactose moved into colon, fermented by bacteria, releases gas causing bloating & flatulence. Lactose downs water into lumen causing diarrhoea.
• Cause not well understood, causes abdominal pain, bloating, flatulence, diarrhoea. F>M (2:1), 20-40 Y.O’s more affected.

Question 8

What is the pathophysiology of Coeliacs disease?
What are the symptoms?
How is it diagnosed & treated?

Answer 8

• Immunological response to gluten, causes flattening of villi, hypertrophy of crypts etc.
• Majority of symptoms relate to malabsorption, e.g.: diarrhoea, weight loss, anaemia (reduced Fe2+ absorption)
• IgA in blood tests or GI endoscopy looking for reduced/absent villi. Treated with gluten free diet.