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Gastrointestinal > Physiology of carbohydrate and protein absorption > Flashcards

Physiology of carbohydrate and protein absorption Flashcards

1
Q

What are the main constituents of food?

A

Carbohydrates, lipids and protein

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2
Q

What are the carbohydrates?

A

Starch - amylose and amylopectin
Celluose
Glycogen
Disaccharides (sucorse and lactose)

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3
Q

Which carbohydrate is indigestible in humans?

A

Cellulose

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4
Q

What are the lipids?

A 
Triacylglycerols 
Phospholipids 
Cholesterol and cholesterol esters
Free fatty acids
Lipid vitamins
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5
Q

What are the sources of proteins?

A

Ingest protein and from within the body

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6
Q

Where does the protein from within the body come from?

A

Cells shed, go into the lumen of the digestive tract and are digested

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7
Q

What is the definition of digestion?

A

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

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8
Q

What are the two overall process that digestion can be broken down into?

A

Luminal digestion and membrane digestion

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9
Q

What is luminal digestion?

A

The first stage of digestion. Mediated by pancreatic enzymes secreted into the duodenum

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10
Q

What is membrane digestion?

A

The products of luminal digestion are passed to the enzymes embedded int he apical membrane of the enterocytes. Mediated by enzymes situated at the brush border of epithelial cells

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11
Q

What is absorption?

A

The process by which the absorbable products of digestion are transferred across the apical and basolateral membranes of enterocytes

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12
Q

What are enterocytes?

A

Absorptive cells of the intestinal epithlium

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13
Q

What are the two general stages of absorption?

A

Digestive products must leave the lumen and enter the enterocytes (cross apical membrane)
Then they must exit the enterocyte and enter the bloodstream (cross the basolateral membrane)

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14
Q

What is the name for the overall process of digestion and absorption?

A

Assmiliation

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15
Q

What kind junctions are between enterocytes?

A

Tight junctions

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16
Q

What dietary component is so simple that is does not require digestion?

A

Monosaccharide glucose

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17
Q

By what type of digestion is protein broken down to enter the epithelium?

A

Luminal hydrolysis of polymer to monomers

Protein –> amino acids

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18
Q

By what type of digestion are disaccharides broken down to enter the epithelium?

A

Brushborder hydrolysis of oligomer to monomer disaccharide –> monosaccharides

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19
Q

What is an example of intracellular hydrolysis?

A

Short chain peptides not being broken down into amino acids until in the epithelium (pass from the lumen to the epithelium has whole peptides)

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20
Q

What is an example of luminal hydrolysis followed by intracellular resynthesis?

A

Triglycerides –> fatty acids + glycerol to enter the epithelium
Within the epithelium fatty acids + glycerol –> triglycerol which enters the interstitial space

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21
Q

What are the 3 different types of carbohydrate?

A

Polysaccharides
Oligosaccharides
Monosaccharides

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22
Q

What are the plant and animal polymers of glucose (polysaccharides)?

A

Starch (plant) and glycogen (animal)

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23
Q

What are the two different types of starch?

A

Amylose and amylopectin

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24
Q

What is the structure of amylose

A

Chain of a-glucose molecules

Joint by a-1,4-glycosidic bonds

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25
Q

What is the structure of amylopectin?

A

Branched structure
a-1,4-glycosidic bonds
Branches are connected by a-1,6-glycosidic bonds

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26
Q

What are two oligosaccharides?

A

Sucrose and lactose

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27
Q

What is sucrose composed of and what are the bonds?

A

Glucose and fructose

a-1,2 linkages

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28
Q

What is lactose composed of and what are the bonds?

A

Glucose and galactose

B-1,4 linkages

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29
Q

Give two examples of monosaccharides?

A

Glucose and fructose

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30
Q

What are the 3 different types of carbohydrate/

A

Polysaccharides
Oligosaccharides
Monosaccharides

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31
Q

What are the plant and animal polymers of glucose (polysaccharides)?

A

Starch (plant) and glycogen (animal)

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32
Q

What are the two different types of starch?

A

Amylose and amylopectin

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33
Q

What enzymes break down oligosaccharidases in the brush border)

A

Lactase
Maltase
Sucrase -isomaltase (2 enzymes linked together)

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34
Q

What is the structure of amylopectin?

A

Branched structure
a-1,4-glycosidic bonds
Branches are connected by a-1,6-glycosidic bonds

35
Q

What are two oligosaccharides?

A

Sucrose and lactose

36
Q

What are the products of a-amylase breakdown?

A

Linear glucose oligomers (maltotriose, maltose) and a-limit dextrins

37
Q

What is lactose composed of and what are the bonds?

A

Glucose and galactose

B-1,4 linkages

38
Q

Give two examples of monosaccharides?

A

Glucose and fructose

39
Q

What is the sequence of carbohydrate digestion?

A

Intracluminal hydolysis = polysaccharide –> oligosaccharides
Membrane digestion =
oligosaccharides –> monosaccharides
then absorbed

40
Q

What enzymes converts starch and glycerol to oligosaccharides?

A

a-amylase

salivary and pancreatic

41
Q

What oligosaccharides does a-amylase break the polysaccharides down into?

A

a-limit dextrins
Maltotriose
Maltose

42
Q

What enzymes break down oligosaccharidases?

A

Lactase
Maltase
Sucrae-isomaltase

43
Q

What type of enzyme is a-amylase?

A

Endoenzyme

44
Q

What is the action of a-amylase?

A

Breaks down linear internal a-1,4-linkages but not terminal a-1,4-linkages (no production of glucose)
Cannot cleave a-1,6-linkages or a-1,4-linkages adjacent to branch points.

45
Q

What are the products of a-amylase breakdown?

A

Linear glucose oligomers (maltotriose, maltose) and a-limit dextrins

46
Q

What is the role of individual oligosaccharidases?

A

They are enzymes that are integral membrane proteins with an active site that faces the lumen of the GI tract

47
Q

Name four oligosaccharidases

A

Lactase
Maltase
Sucrase
Isomaltase

48
Q

What does lactase act on?

A

Breaks down lactose to glucose and galactose

49
Q

What is the action of all oligosaccharides (other than lactase)?

A

They cleave the terminal a-1,4-linkages of maltose, maltotriose and a-limit dextrins to produce glucose

50
Q

What is the additional action of maltase?

A

Can degrade the a-1,4 -linkages in straight chain oligomers up to nine monomers in length is the action of sucrase?

51
Q

What is the action of sucrase?

A

Specifically responsible for hydrolysing sucrose to glucose and fructose

52
Q

What is the action of isomaltase?

A

It is the only enzyme that can split the branching a-1,6-linkages of a-limit dextrins

53
Q

What is the rate limiting step in maltase, sucrase and isomaltase assimilation?

A

The transport of the released monomers

hydrolysis is faster than transport of the released monomers

54
Q

What is the rate limiting step in lactase assimilation?

A

Hydrolysis

55
Q

What is lactose intolerance?

A

Digestive problem resulting from the inability to adequately digest lactose
It is caused by lactase insufficiency

56
Q

Explain the concept of lactase persistence?

A

Lactase is lost in other mammals following weaning but in humans there is a variable degree of lactase persistence.

57
Q

What is primary lactase deficiency (primary hypolactasia)?

A

Lack of lactase persistence (LP) allele (most common cause)

58
Q

What is secondary lactase deficiency?

A

Caused by damage to/infection of the proximal small intestine

59
Q

What is congenital lactase deficiency?

A

Rare autosomal recessive disease resulting in an inability to digest lactose from birth

60
Q

When does hypolactasia cause disturbance?

A

When lactose-contain food is consumed and the activity of the remaining enzyme is overwhelmed

61
Q

What happens when lactose enters digestion when there is poor lactase activity?

A

Lactose is delivered to the colon from the ileum colonic microflora. The lactose is hydrolysed by the bacteria to short-chain fatty acids which can be absorbed across the colon.
In the process of breakdown, hydrogen, carbon dioxide and methane are produced

62
Q

What are the clinical signs of lactose breakdown by the microflora?

A

H2 in the breath
Bloating (gas build up)
Abdominal pain (distension)
Flatulence (release of gas)

63
Q

What does undigested lactose cause?

A

Acidification of the colon

An increased osmotic load - loose stools and diarrhoea

64
Q

Where does absorption of the final products of carbohydrate digestion take place?

A

Duodenum and jejunum

65
Q

How are glucose and galactose absorbed?

A

Secondary active transport mediated by SGLT1
One glucose enters the cells along with 2 sodium ions. ENergy comes from the movement of Na+ down its concentration gradient

66
Q

How is fructose absorbed?

A

Facilitated diffusion mediated by GLUT5

67
Q

How is exit of all monosaccharides mediated?

A

By facilitated diffusion by GLUT2

68
Q

What is protein broken down into for absorption?

A

Oligopeptides and amino acids

69
Q

What beings to denature proteins in stomach digestion?

A

HCl

70
Q

What is the function and action of pepsin?

A

Cleaves proteins into peptides
Optimum pH of 1.8-3.5
It is an endopeptidase
Not essential for protein digestion

71
Q

Name the 5 pancreatic proteases that are secreted as proenzymes

A 
Trypsin 
Chymotrypsin
Elastase
Procaroxypeptidase A
Procarboxypeptidase B
72
Q

Name the endopeptidase proteases

A

Trypsin
Chymotrypsin
Elastase

73
Q

Name the exopeptidase proteases

A

Procaroxypeptidase A

Procarboxypeptidase B

74
Q

What are the products of endopeptidase digestion?

A

Oligopeptides (2-6 amino acids)

75
Q

What are the products of exopeptidase digestion?

A

Single amino acids

76
Q

What are endopeptidases?

A

Enzymes that break peptide bonds other than the terminal ones in a peptide chain

77
Q

What are exopeptidases?

A

An enzyme which breaks the terminal peptide bond in a peptide chain

78
Q

Where are additional proteases present?

A

Brush border and the cytoplasm of the enterocyte

79
Q

What do brush border peptidases break down?

A

Larger oligopeptides (3-8 amino acids)

80
Q

What do cytoplasmic peptidases have an affinity for?

A

Dipeptides and tripeptides

81
Q

By what mechanisms are amino acids absorbed across the brush border?

A

7 mechanisms
5 Na+ dependent co-transporters mediating ‘uphill’ movement (secondary active transport)
2 are Na+ independent

82
Q

How are amino acids absorbed across the basolateral membrane?

A

5 different mechanisms
2 mediate efflux of amino acids and are Na+ independent
2 mediate influx and are Na+ dependent

83
Q

How are di-, tri- and tetra-peptides absorbed?

A

Via H+ dependent mechanism at brush border (co-transport)

84
Q

What happens to di-, tri- and tetra-peptides once they have be absorbed into the enterocyte?

A

They are further hydrolysed to amino acids within the enterocyte
Leave by Na+ independent systems at the basolateral membrane (facilitated transport)

Gastrointestinal (11 decks)

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