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Digestive and Renal Sciences > Digestion and Absorption of Carbohydrate and Protein > Flashcards

Digestion and Absorption of Carbohydrate and Protein Flashcards

1
Q

What are the different types of dietary carbohydrates?

A
  • monosaccharides (monomers)
  • oligosaccharides (short polymers)
  • polysaccharides (long polymers)
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2
Q

How can carbohydrates be digested?

A

The small intestine can directly absorb the monomers but not the polymers.

  • Some polymers are digestible, that is, the body can digest them to form the monomers that the small intestine can absorb.
  • Other polymers are nondigestible
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3
Q

What are the dietary fibres?

A

(primarily present in fruits, vegetables, and cereals) consists of both soluble and insoluble forms of fibre.

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4
Q

What are the insoluble fibres?

A
  • Non water soluble
  • Not easily fermented
  • Cellulose and Hemicellulose
  • Aid the digestive system and ease elimination
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5
Q

What are the soluble fibres?

A
  • Water soluble
  • Pectins, gums and mucilage
  • Easily digested by bacteria in the colon
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6
Q

What is starch?

A

storage form for carbohydrates that is primarily found in plants, and consists of both amylose and amylopectin (about half of the carbohydrates absorbed in the diet)

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7
Q

What is amylose?

A

straight-chain glucose polymer that typically contains multiple glucose residues, connected by α-1,4 linkages.

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8
Q

What is amylopectin?

A

branched glucose polymer and thus contains both α-1,4 and α-1,6 linkages

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9
Q

What is Glycogen?

A

the “animal starch”-is like amylopectin (α-1,4 and α-1,6 linkages) but more highly branched

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10
Q

Other than starch what are the main other carbohydrates obtained from the diet?

A

30-40% from disaccharides (sucrose and lactose)

5-10% from monosaccharides (fructose and glucose)

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11
Q

What is the digestive process of carbohydrates?

A

Intraluminal hydrolysis - of starch to oligosaccharides by salivary and pancreatic amylases.

Membrane digestion - of oligosaccharides to monosaccharides by brush border disaccharides

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12
Q

What are the stages in luminal hydrolysis (digestion)?

A
  • Action of Salivary Amylase and Pancreatic Amylase
  • Both salivary and pancreatic acinar cells synthesize and secrete active α amylases.
  • Salivary amylase in the mouth initiates starch digestion, however this is inactivated by gastric acid.
  • Pancreatic α amylase which is induced by CCK completes starch digestion in the lumen of the small intestine.
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13
Q

What is amylase?

A

An endoenzyme that hydrolyses internal 1-4 linkages but does not cleave terminal ends of the 1-4 / 1-6 linkages. (cleaves internally and not at the ends or branches)

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14
Q

What does starch hydrolysis result in?

A

Culminates in maltose, maltotriose and alpha-limit dextrins.

Further breakdown is required into monosaccharides

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15
Q

What are the stages in membrane digestion?

A
  • Hydrolysis of Oligosaccharides to Monosaccharides by Brush-Border Disaccharidases
  • The human small intestine has 3 brush-border oligosaccharidases which are all membrane proteins :lactase, maltase and sucrase-isomaltase (two enzymessucrase and isomaltase)
  • Lactase has only one substrate -Lactose which is broken down into glucose and galactose
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16
Q

What does maltase do?

A

Cleave terminal 1-4 linages of maltose, maltotriose and alpha-limit dextrins

can also degrade the α-1,4 linkages in straight-chain oligosaccharides to yield glucose

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17
Q

What does Sucrase do?

A

can split sucrose into glucose and fructose.

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18
Q

What does Isomaltase do?

A

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

19
Q

How are monosaccharides absorbed?

glucose, galactose and fructose

A

Glucose, Galactose, and Fructose are all absorbed by the small intestine in a two-step process :
1. Uptake across the apical membrane into the epithelial cell

  1. Exit across the basolateral membrane.
20
Q

How are Glucose and galactose absorbed?

A

Na/glucose transporter SGLT1

SLGT 1 co-transporter uses the Na gradient to provide the energy to transport glucose and galactose against the conc gradient into the cell.

Once in the cell, Gucose and galactose enter the blood through the GLUT2 transporter.

21
Q

How is Fructose absorbed?

A

Apical fructose transport is via GLUT5 transporter (facilitated diffusion)

Then enters the blood through GLUT2 transporter.

22
Q

What does lactase deficiency cause?

A

causes gastrointestinal symptoms such as diarrhoea and cramps.

23
Q

What happens to people with lactase deficiency?

A

the ingestion of lactose leads to a much smaller rise in plasma [glucose], and an increase breath H2

Due to the non-absorbed lactose is metabolized by colonic bacteria to H2,

which is absorbed into the blood and subsequently excreted by the lungs

24
Q

How is lactase deficiency treated?

A

Treatment is reducing or eliminating milk and milk products, or using milk products treated with a commercial lactase preparation

25
Q

How are proteins digested?

A

In neonates dietary proteins are absorbed intact.

Proteins are digested into their constituent oligopeptides and amino acids before enterocyte uptake.

26
Q

What are the 4 main pathways for protein digestion and absorption?

A
  1. Intraluminal Hydrolysis - Proteases liberate amino acids which then get absorbed via Na/ AA transporter
  2. Intraluminal hydrolysis - liberates oligopeptides which are then broken down into amino acids by brush border peptidases. The amino acids are then taken up.
  3. Intraluminal Hydrolysis - oligopeptides are transported into the cell via a PEPT1 transporter. Peptidases then break down these oligopeptides into amino acids which are then moved into the blood across the basolateral membrane
  4. Luminal enzymes digest dietary proteins to oligopeptides which are taken up by enterocytes and exported directly into the blood in an intact state.
27
Q

How does intraluminal hydrolysis start?

A

Starts with pepsinogen which then gets activated to pepsin by acid in the stomach. Pepsin is an endopeptidase which then liberates some oligosaccharides in the stomach.

28
Q

What are the main 5 pancreatic enzymes (zymogens) that participate in protein digestion?

A

Trypsinogen - Trypsin
(activated by enterokinase)

Chymotrypsinogen - chymotrypsin (activated by Trypsin)

Proelastase - Elastase (activated by Trypsin)

Procarboxypep’ase A - Carboxypep’ase A (activated by Trypsin)

Procarboxypep’ase B - Carboxypep’ase B (activated by Trypsin)

29
Q

Of the 5 zymogens which are exopeptidases?

A

Carboxypep’ase A

Carboxypep’ase B

30
Q

What are the products produced by endo and exopeptidases?

A

Endopeptidases - Oligopeptides

Exopeptidases - Amino acids

31
Q

What are brush border peptidases?

A

Hydrolises Di and Tri peptides

32
Q

What is the importance of PepT1?

A

Responsible for the uptake of luminal oligopeptides into the cell.

33
Q

What are the various epithelial amino acid transport systems and their mediators?

A

IMINO
B^0,+
B^0
EAAT3

34
Q

What is hartnups disease?

A

An autosomal recessive hereditary disorder of apical membrane amino acid transport.

The absorption of neutral amino acids by system B+ in the small intestine is markedly reduced

35
Q

What is Cystinuria?

A

An autosomal recessive hereditary disorder of apical membrane amino acid transport.

Cysteine and basic amino acid import by system B0+ or b0+ is abnormal. The absorption of oligopeptides containing basic amino acids is normal

36
Q

How does the adult intestine absorb finite amounts of intact protein and polypeptides?

A
  1. Enterocyte-phagocytosis:

2. M cells (located in Peyer’s patches) have few microvilli and are specialized for protein uptake.

37
Q

Where does the protein being absorbed come from?

A

50% diet

50% endogenous sources (from lysed cells, hormones)

38
Q

What does the IMINO transporter move across the apical membrane of the brush border?

A

Glycine
Proline

Coupled per Na

39
Q

What does the B+ transporter move across the apical membrane of the brush border?

A

Cationic amino acids (positively charged) and cystine.

Antiporter for influx of +AA and cystine for efflux of neutral AA

40
Q

What does the B0 transporter move across the apical membrane of the brush border?

A

Neutral amino acid, sodium co-transporter

41
Q

What does the EAAT3 transporter move across the apical membrane of the brush border?

A

Anionic (negatively charged) amino acid sodium co-transporter

3Na+ ions

42
Q

What are the clinical signs of Hartnup disease?

A

Often in children:

skin changes of pellagra
Cerebellar ataxia
Psychiatric abnormalities

43
Q

What is the clinical sign of cystinuria?

A

Formation of kidney stones

Digestive and Renal Sciences (26 decks)

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