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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
What is the structure of amylopectin?
Branched structure a-1,4-glycosidic bonds Branches are connected by a-1,6-glycosidic bonds
26
What are two oligosaccharides?
Sucrose and lactose
27
What is sucrose composed of and what are the bonds?
Glucose and fructose | a-1,2 linkages
28
What is lactose composed of and what are the bonds?
Glucose and galactose | B-1,4 linkages
29
Give two examples of monosaccharides?
Glucose and fructose
30
What are the 3 different types of carbohydrate/
Polysaccharides Oligosaccharides Monosaccharides
31
What are the plant and animal polymers of glucose (polysaccharides)?
Starch (plant) and glycogen (animal)
32
What are the two different types of starch?
Amylose and amylopectin
33
What enzymes break down oligosaccharidases in the brush border)
Lactase Maltase Sucrase -isomaltase (2 enzymes linked together)
34
What is the structure of amylopectin?
Branched structure a-1,4-glycosidic bonds Branches are connected by a-1,6-glycosidic bonds
35
What are two oligosaccharides?
Sucrose and lactose
36
What are the products of a-amylase breakdown?
Linear glucose oligomers (maltotriose, maltose) and a-limit dextrins
37
What is lactose composed of and what are the bonds?
Glucose and galactose | B-1,4 linkages
38
Give two examples of monosaccharides?
Glucose and fructose
39
What is the sequence of carbohydrate digestion?
Intracluminal hydolysis = polysaccharide --> oligosaccharides Membrane digestion = oligosaccharides --> monosaccharides then absorbed
40
What enzymes converts starch and glycerol to oligosaccharides?
a-amylase | salivary and pancreatic
41
What oligosaccharides does a-amylase break the polysaccharides down into?
a-limit dextrins Maltotriose Maltose
42
What enzymes break down oligosaccharidases?
Lactase Maltase Sucrae-isomaltase
43
What type of enzyme is a-amylase?
Endoenzyme
44
What is the action of a-amylase?
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
What are the products of a-amylase breakdown?
Linear glucose oligomers (maltotriose, maltose) and a-limit dextrins
46
What is the role of individual oligosaccharidases?
They are enzymes that are integral membrane proteins with an active site that faces the lumen of the GI tract
47
Name four oligosaccharidases
Lactase Maltase Sucrase Isomaltase
48
What does lactase act on?
Breaks down lactose to glucose and galactose
49
What is the action of all oligosaccharides (other than lactase)?
They cleave the terminal a-1,4-linkages of maltose, maltotriose and a-limit dextrins to produce glucose
50
What is the additional action of maltase?
Can degrade the a-1,4 -linkages in straight chain oligomers up to nine monomers in length is the action of sucrase?
51
What is the action of sucrase?
Specifically responsible for hydrolysing sucrose to glucose and fructose
52
What is the action of isomaltase?
It is the only enzyme that can split the branching a-1,6-linkages of a-limit dextrins
53
What is the rate limiting step in maltase, sucrase and isomaltase assimilation?
The transport of the released monomers | hydrolysis is faster than transport of the released monomers
54
What is the rate limiting step in lactase assimilation?
Hydrolysis
55
What is lactose intolerance?
Digestive problem resulting from the inability to adequately digest lactose It is caused by lactase insufficiency
56
Explain the concept of lactase persistence?
Lactase is lost in other mammals following weaning but in humans there is a variable degree of lactase persistence.
57
What is primary lactase deficiency (primary hypolactasia)?
Lack of lactase persistence (LP) allele (most common cause)
58
What is secondary lactase deficiency?
Caused by damage to/infection of the proximal small intestine
59
What is congenital lactase deficiency?
Rare autosomal recessive disease resulting in an inability to digest lactose from birth
60
When does hypolactasia cause disturbance?
When lactose-contain food is consumed and the activity of the remaining enzyme is overwhelmed
61
What happens when lactose enters digestion when there is poor lactase activity?
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
What are the clinical signs of lactose breakdown by the microflora?
H2 in the breath Bloating (gas build up) Abdominal pain (distension) Flatulence (release of gas)
63
What does undigested lactose cause?
Acidification of the colon | An increased osmotic load - loose stools and diarrhoea
64
Where does absorption of the final products of carbohydrate digestion take place?
Duodenum and jejunum
65
How are glucose and galactose absorbed?
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
How is fructose absorbed?
Facilitated diffusion mediated by GLUT5
67
How is exit of all monosaccharides mediated?
By facilitated diffusion by GLUT2
68
What is protein broken down into for absorption?
Oligopeptides and amino acids
69
What beings to denature proteins in stomach digestion?
HCl
70
What is the function and action of pepsin?
Cleaves proteins into peptides Optimum pH of 1.8-3.5 It is an endopeptidase Not essential for protein digestion
71
Name the 5 pancreatic proteases that are secreted as proenzymes
``` Trypsin Chymotrypsin Elastase Procaroxypeptidase A Procarboxypeptidase B ```
72
Name the endopeptidase proteases
Trypsin Chymotrypsin Elastase
73
Name the exopeptidase proteases
Procaroxypeptidase A | Procarboxypeptidase B
74
What are the products of endopeptidase digestion?
Oligopeptides (2-6 amino acids)
75
What are the products of exopeptidase digestion?
Single amino acids
76
What are endopeptidases?
Enzymes that break peptide bonds other than the terminal ones in a peptide chain
77
What are exopeptidases?
An enzyme which breaks the terminal peptide bond in a peptide chain
78
Where are additional proteases present?
Brush border and the cytoplasm of the enterocyte
79
What do brush border peptidases break down?
Larger oligopeptides (3-8 amino acids)
80
What do cytoplasmic peptidases have an affinity for?
Dipeptides and tripeptides
81
By what mechanisms are amino acids absorbed across the brush border?
7 mechanisms 5 Na+ dependent co-transporters mediating 'uphill' movement (secondary active transport) 2 are Na+ independent
82
How are amino acids absorbed across the basolateral membrane?
5 different mechanisms 2 mediate efflux of amino acids and are Na+ independent 2 mediate influx and are Na+ dependent
83
How are di-, tri- and tetra-peptides absorbed?
Via H+ dependent mechanism at brush border (co-transport)
84
What happens to di-, tri- and tetra-peptides once they have be absorbed into the enterocyte?
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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