Digestion and Absorption of Carbohydrate and Protein

Question 1

What are the 3 forms of dietary carbohydrate?

Answer 1

monosaccharides, oligosaccharides, polysaccharides

Question 2

Is dietary fibre soluble or insoluble?

Answer 2

insoluble and soluble fibres
insoluble - non water soluble, not easily fermented, cellulose and hemicellulose, aid digestion
soluble - water solb=uble, pectins, gums, mucilage, easily digested by microbiome

Question 3

What is starch made up of?

What are the linkages and subsequent structure of these components?

Answer 3

amylose - linear glucose polymer, alpha-1,4 linkages

amylopectin - branched glucose polymer, alpha-1,4 and alpha 1,6 linkages

Question 4

What is the structure of glycogen?

Answer 4

like amylopectin as it is the ‘animal starch’.

contains alpha-1,4 and alpha 1,6 linkages but is more highly branched

Question 5

What are the 3 common disaccharides and what sugars are they made of?

Answer 5

sucrose = glucose and fructose
lactose = glucose and galactose
maltose = glucose and glucose

Question 6

In what form can carbohydrates be absorbed in the small intestine?

Answer 6

monosaccharides only

everything else must be broken down

Question 7

What are the two stages of digesting dietary carbohydrates?
What are the class of enzymes involved?

Answer 7

1. intraluminal hydrolysis - starch to oligosaccharides by salivary and pancreatic amylases
2. membrane digestion - oligosaccharides to monosaccharides by brush border disaccharidases

Question 8

1. Both salivary and pancreatic ……… cells synthesise and secrete active .. ………..
2. Salivary amylase in the mouth initiates starch digestion but is inactivated by ……. ………
3. Pancreatic alpha amylase which is induced by …….. completes starch digestion in the lumen of the ……. …….

Answer 8

1. acinar, alpha amylases
2. gastric acid
3. CCK
4. small instestine

Question 9

1. What kind of an enzyme is amylase?
2. What does it do?
3. What are the possible end products?

Answer 9

1. endoenzyme
2. breaks internal alpha-1,4 linkages but cannot cleave terminal linkages, branch points or alpha-1,4 linkages immediately adjacent to branch points
3. mlatose, maltoriose, alpha-limit dextrins, further breakdown into monosaccharides required

Question 10

Membrane digestion

1. What are the 4 brush border oligosaccharidases?
2. What breaks down alpha-limit dextrin?
3. Where is peak oligosaccharidase distribution and activity in the GI tract?

Answer 10

1. lactase, sucrase, isomaltase, maltase
2. isomaltase
3. proximal jejunum, less in duodenum and distal ileum , none in large intestine

Question 11

Glucose, galactose and fructose are all absorbed by the small intestine in a two-step process. Describe these steps and the transporters involved.
1.
2.

Answer 11

1. uptake across the apical membrane into the epithelial cell
   - glucose/galactose use SGLT-1, electrochemical gradient maintained by Na-K pump
   - fructose uses GLUT-5
2. exit across the basolateral membrane
   - all effluxed by GLUT-2

Question 12

Lactase deficiency

1. What are the symptoms?
2. The ingestion of lactose leads to a much smaller rise in plasma …….. and breath ….. increases because lactose is metabolised by ……….. …………… This is absorbed into the blood and subsequently excreted by the …….

Answer 12

1. GI symptoms, diarrhoea, cramps

2. glucose, H2, colonic bacteria, lungs

Question 13

What are the 4 major pathways of digestion/absorption of protein?

Answer 13

1. luminal proteases from stomach/pancreas hydrolyse proteins to peptides and to amino acids, which are then absorbed
2. luminal proteases digest proteins to peptides, but enzymes present at the brush border digest the peptides to amino acids which are then absorbed
3. Luminal enzymes diest proteins to peptides which are taken up as oligopeptides. Further digestion done by cytosolic animus to yield intracellular amino acids
4. Luminal enzymes digest dietary proteins to oligopeptides, which are take-up by enterocytes and moved directly to blood.

Question 14

1. Both gastric and pancreatic proteases are secreted as ……..
2. Gastric chief cells secrete …….. which is activate to pepsin by …….. ……
3. Pepsin is an ……..peptidase.
4. Although pepsin in the stomach partially digested 10-15% of dietary protein, pepsin hydrolysis is not absolutely ………..

Answer 14

1. proenzymes
2. pepsinogen, low pH
3. endo
4. necessary

Question 15

1. What is the most important pancreatic zymogen?
2. What is it converted to? What activates this?
3. What does this do?

Answer 15

1. trypsinogen
2. trypsin, brush border peptidases called enterokinase
3. initiate exponential cascade for activation of all other zymogens as well as more trypsin

Question 16

1. What brush border transporter can absorb oligopeptides?

Answer 16

1. PepT1

Question 17

How does the charge of the amino acid change its uptake into enterocytes?
What are the 4 routes?

Answer 17

1. Glycine and proline, Na+ dependent, IMINO
2. AA+, antiport with AA0, B0+
3. AA0, Na+ dependent, B0
4. AA-, Na+ dependent, EAAT3

Question 18

1. Which route of transport is affected in Hartnup disease?

2. What are the typical clinical signs most evident in children?

Answer 18

1. neutral amino acid uptake by B0

2. skin changes, cerebellar ataxia, pscyhiatric abnormalities

Question 19

1. Which route of transport is affected in Cystinuria?

2. What is the major clinical feature?

Answer 19

1. cysteine and basic amino acid uptake by B0+

2. formation of kidney stones

Question 20

By what 2 route can the adult intestine absorb finite amounts of intact protein? What is the most likely fate of this protein?

Answer 20

1. enterocyte-phagocytosis, mostly degraded in lysosomes
2. M cells (peyers patches), few microvilli, specialised for protein uptake, packaged in clathrin-coated vesicles, presented to immune cells and can illicit an immune response