GI Physiology of digestion and absorption 1 (Carbohydrate and protein)

Question 1

What are the main constituents of food?

Answer 1

Carbohydrates
Lipids
Proteins

Question 2

What different forms of carbohydrates can you get?

Answer 2

Starch
Cellulose (indigestible in humans - roughage)
Glycogen
Disaccharides (sucrose, lactose)

Question 3

What forms of starch do you get?

Answer 3

Amylose and amylopectin

Question 4

How much carbs a day do you usually get?

Answer 4

400g

Question 5

Describe the types of lipids?

Answer 5

Triacylglycerols (approximately 90% of total lipid ingested as fats and oils)
Phospholipids
Cholesterol and cholesterol esters
Free fatty acids 
Lipid vitamins

Question 6

what are free fatty acids important for?

Answer 6

Free fatty acids - important as a source of energy and membrane strict and signal transduction

Question 7

How much lipids do you get a day?

Answer 7

25-160g

Question 8

What id digestion?

Answer 8

enzymatic conversion of complex dietary substances to a form that can be absorbed

Breakdown of complex molecules into simpler smaller molecules

Question 9

Where does most digestion occur?

Answer 9

The small intestine

Question 10

Describe the 2 ‘types’ of digestion?

Answer 10

Luminal and membrane digestion

Question 11

Describe luminal digestion?(what is it mediated by?)

Answer 11

It is mediated by pancreatic enzymes secreted into the dueodenum

Question 12

Describe membrane digestion? (what is it mediated by?)

Answer 12

It is mediated by enzymes situated at the brush border of the epithelial cells

Question 13

What is absorption?

Answer 13

Absorption is the processes by which the absorbable products of digestion are transferred across both the apical and basolateral membranes of enterocytes (absorptive cells of the intestinal epithelium

Question 14

Transceulluar transport? (CHECK)

Answer 14

Transport through basolateral membrane, apical membrane (Brush border) etc

Question 15

What is the brush border ?

Answer 15

The apical membrane

Question 16

How much energy of the total energy requirement does digestible carbohydrate provide?

Answer 16

45%

Question 17

Describe the basic structure of amylose and amylopectin?

Answer 17

Amylose - unbranched chains, alpha 1,4-linkage

Amylopectin - branches, approx every 30 glucose molecule, alpha 1,6-linkages.

Question 18

Describe Glycogen compared to amylopectin

Answer 18

More highly branched than amylopectin - branches every 10 glucose molecules

Question 19

What provides roughage in our diet?

Answer 19

Cellulose - not digestable in humans, as they are linked by beta-1,4 linkages, thus it passes directly into the colon

Question 20

Where might you find sucrose, what is it made of?

Answer 20

In table sugar

Glucose and fructose

Question 21

Where might you find lactose, what is it made of?

Answer 21

Milk sugar

Glucose and galactose

Question 22

What is alpha amylase, endoenzyme?

Answer 22

An endoenzyme, or intracellular enzyme, is an enzyme that functions within the cell in which it was produced.

Question 23

What does alpha amylase do?

Answer 23

It breaks down linear internal alpha 1,4-linkages but NOT terminal alpha 1,4 linkages - therefore it cannot produce glucose

Question 24

Can alpha amylase cleave alpha 1,6 linkages at branch points in amylopectin?

Answer 24

NO

Question 25

What are Oligosaccharidases?

Answer 25

Oligosaccharidases are integral membrane proteins with a catalytic domain that faces the lumen of the GI tract ON/IN MEMBRANE

Question 26

What is an example of an Oligosaccharidases? (all brush border enzymes)

Answer 26

Lactase, Maltase, Sucrase, Isomaltase

Question 27

What is lactose intolerance?

Answer 27

Relatively common digestive problem resulting from the inability to adequately digest lactose: caused by lactase insufficiency

Question 28

What can lactose intolerance result from?

Answer 28

Primary lactase deficiency (primary hypolactasia) (MOST COMMON WORLDWIDE) Secondary lactase deficiency (MOST COMMON IN UK) Congenital lactase deficiency

Question 29

Describe Primary lactase deficient (primary hypolactasia)?

Answer 29

due to lack of the lactase persistence (LP) allele – most common cause world wide

Question 30

Describe secondary lactase deficiency?

Answer 30

caused by damage to/ infection of/ the proximal small intestine. Can develop later in life. (e.g. crohns, UC etc)

Question 31

Describe congenital lactase deficiency?

Answer 31

rare autosomal recessive disease – no ability to digest lactose from birth (Passed on by autosomal recessive) - can overcome this, by giving babies artificial milk, lactose replaced by sucrose or another similar?

Question 32

What happens as a consequence of lactase insufficiency?

Answer 32

Causes no disturbance unless lactose-containing food is consumed and activity of remaining enzyme is overwhelmed

Question 33

What happens if lactose is delivered to the colon from the ileum colonic microflora?

Answer 33

Short-chain fatty acids (which can be absorbed) Hydrogen (H2 - which can be detected in the breath of lactase deficient individuals following a lactose challenge) Carbon dioxide Methane

Question 34

What do the by products of lactose ingestion in lactose intolerance cause?

Answer 34

Bloating Abdominal pain Flatulence

Question 35

What can undigested lactose cause in lactose intolerance?

Answer 35

Acidification of the colon | An increased osmotic load – loose stools and diarrhoea

Question 36

Where does Absorption of the Final Products of Carbohydrate Digestion: Glucose, Galactose and Fructose occur?

Answer 36

In the duodenum and jejunum

Question 37

How does the absorption of the final products of carb digestion happen?

Answer 37

In a two-step process involving entry and exit from the enterocytes via the apical and basolateral membranes respectively

Question 38

What are glucose and galactose absorbed by, and mediated by?

Answer 38

Secondary active transport mediated by SGLT1

Question 39

What is fructose absorbed by, and mediated by?

Answer 39

By facilitated diffusion mediated by GLUT5

Question 40

What GLUT receptors are expressed in the small intestine?

Answer 40

GLUT 1, 2 and 5 are all expressed in cells lining the small intestine

Question 41

How is exit for all monosaccharides in the absorption of the final products of carbohydrate digestion mediated by?

Answer 41

facilitated diffusion by GLUT2

Question 42

What is an enterocyte?

Answer 42

A cell of the intestinal lining

Question 43

What must proteins be digested to to be efficient for absorption?

Answer 43

oligopeptides and amino acids

Question 44

Describe digestion of proteins in the stomach?

Answer 44

HCL begins to denature proteins | Pepsin cleaves proteins into peptides

Question 45

What cleaves proteins in the stomach?

Answer 45

Pepsin cleaves proteins into peptides

Question 46

Describe digestion in the duodenum?

Answer 46

The five pancreatic enzymes are secreted into the duodenum (in the inactive form) from the EXOCRINE pancreas and converted to the ACTIVE forms in the duodenum. They function as endopeptidases, or exopeptidase.

Question 47

Is pepsin an endopeptidas, or exopeptidase?

Answer 47

Endopeptidase

Question 48

How much of protein digestion is accounted for in the duodenum?

Answer 48

50% - compared to 15% in the stomach

Question 49

Describe the brush border peptidases?

Answer 49

They are numerous | They have an affinity for larger oligopeptides

Question 50

Describe cytoplasmic peptidases?

Answer 50

Less numerous | Primarily hydrolyse dipeptides or tripeptides

Question 51

What form of carbohydrate is indigestible in humans?

Answer 51

Cellulose

Question 52

What membrane faces the lumen and what one faces the interstitium in the small intestines?

Answer 52

Lumen - apical membrane (brush border) | Insterstitium - Basolateral membrane

Question 53

What is the overall process of digestion and absorption called?

Answer 53

Assimilation

Question 54

Give examples of polysaccharides?

Answer 54

Starch - amylose, amylopectin | Glycogen

Question 55

Give examples of oligosaccharides and what is it?

Answer 55

Sucrose Lactose Usually containing a few monosaccharide molecules together

Question 56

Give examples of monosaccharides?

Answer 56

Glucose | Fructose

Question 57

What must all dietary carbohydrates be converted to for absorption?

Answer 57

Monosaccharides

Question 58

What is the rough mode of operateion of SGLT1?

Answer 58

Sodium binds, increasing the affinity for glucose extracellularly, glucose and sodium then move intracellularly

Question 59

Roughly what is the pathway of protein digestion?

Answer 59

Protein Peptides Amino acids Amino acids in blood

Question 60

Describe protein absorption of amino acids at the brush border?

Answer 60

There are at least 7 different mechanisms present 5 - are Na+ dependent co transporters (secondary active transport) 2 - are Na+ independent

Question 61

Describe protein absorption of amino acids at the basolateral membrane?

Answer 61

There are at least 5 different mechanisms 3 - mediate efflux of amino acids and are NA+ independent 2 mediate influx of amino acids and are Na+ dependent

Question 62

Describe protein absorption of Di-, tri-, and tetra-peptides | at the brush border?

Answer 62

H+ dependent mechanism - they are further hydrolysed to amino acids within the enterocyte

Question 63

What is protein digested to in the lumen and by what?

Answer 63

Amino acids or oligopeptides by pepsin and the pancreatic proteases

Question 64

What does peptidases at the brush border to to oligopeptides?

Answer 64

They further hydrolyse oligopeptides to amino acids