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DIG > Digestion and Absorption of Carbohydrates and Proteins > Flashcards

Digestion and Absorption of Carbohydrates and Proteins Flashcards

1
Q

Which chemical form must carbohydrates be in to be absorbed by the small intestine?

A

Monomers.

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2
Q

Are cellulose and hemicellulose soluble or insoluble fibres?

A

Insoluble.

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3
Q

Are pectins soluble or insoluble fibres?

A

Soluble.

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4
Q

What is starch composed of?

What is its function in the body?

A

Amylose and amylopectin.

Functions as a storage form for carbohydrates in plants.

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5
Q

How does amylose differ from amylopectin?

A

Amylose:

  • Straight chain.
  • Only α-1,4 linkages.

Amylopectin:

  • Branched chains.
  • Both α-1,4 and α-1,6 linkages.
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6
Q

How does glycogen differ from amylopectin?

What is its function?

A
  • More highly branched than amylopectin due to more α-1,6 linkages.
  • Functions as a storage form for carbohydrates in animals.
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7
Q

Which monomers compose sucrose?

A

Alpha glucose and beta fructose.

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8
Q

Which monomers compose lactose?

A

Beta glucose and beta galactose.

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9
Q

Which monomers compose maltose?

A

Alpha glucose x2.

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10
Q

What are the two steps involved in the conversion of polysaccharides to monosaccharides.?

A

1 - Intraluminal hydrolysis.

2 - Membrane digestion.

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11
Q

What occurs in intraluminal hydrolysis?

A
  • Starch is converted to oligosaccharides by salivary and pancreatic α amylases.
  • Salivary amylase initiates starch digestion, but is inactivated by gastric acid.
  • Pancreatic amylase (induced by CCK) completes starch digestion in the lumen of the small intestine.
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12
Q

What occurs in membrane digestion of carbohydrates?

A

Oligosaccharides are converted to monosaccharides by brush-border disaccharidases.

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13
Q

What is CCK?

What is its function?

Where is it secreted?

A

Cholecystokinin, a peptide hormone that:

  • Stimulates the release of bile into the intestine.
  • Stimulates the secretion of enzymes by the pancreas.
  • Is secreted by duodenal cells.
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14
Q

Why does starch hydrolysis terminate at maltose, maltotriose and α-limit dextrins?

A

Because amylase is and endoenzyme that hydrolyses internal α-1,4 links only (cannot hydrolyse external links).

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15
Q

What is an α-limit dextrin?

A

The remaining polymer with an α-1,6 bond, produced by hydrolysis of amylopectin with amylase, which cannot hydrolyse the alpha-1,6 bonds at branch points.

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16
Q

What are the 3 brush-border oligosaccharidases?

A

1 - Lactase.

2 - Maltase.

3 - Sucrase-isomaltase (two enzymes).

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17
Q

Which enzyme can split branching α-1,6 linkages of α-limit dextrins?

A

Isomaltase.

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18
Q

How does oligosaccharidase activity and distribution differ across the small and large intestine?

A
  • Highest in the jejunum.
  • Less in the duodenum and ileum.
  • None in the large intestine.
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19
Q

What are the two steps involved in the absorption of monosaccharides in the small intestine?

A

1 - Uptake across the apical membrane into the enterocyte.

2 - Exit across the basolateral membrane.

20
Q

What is SGLT1?

A
  • A Na/Glucose transporter that is responsible for glucose and galactose uptake at the apical membrane.
  • An example of secondary active transport.
21
Q

Why is glucose transport across the apical membrane an example of secondary active transport?

A

As sodium is actively pumped out of the enterocyte, but then passively cotransports glucose into the enterocyte back down sodium’s electrochemical gradient.

22
Q

What is GLUT5?

A

A protein that transports fructose across the apical membrane via facilitated diffusion.

23
Q

What is GLUT2?

A

A protein that transports all three monosaccharides (glucose, fructose and galactose) across the basolateral membrane via facilitated diffusion.

24
Q

Why is breath hydrogen increased in individuals with primary lactase deficiency?

A

Non-absorbed lactose is metabolised by colonic bacteria to hydrogen gas, which is absorbed into the blood and excreted by the lungs.

25
Q

Which chemical form must proteins be in to be absorbed?

A

Amino acids or oligopeptides.

26
Q

What are the 4 ways in which digestion and absorption of proteins occurs in the GIT?

A

1 - Luminal proteases from the stomach and pancreas: proteins -> peptides -> amino acids -> absorbed.

2 - Luminal proteases: protein -> peptides,
Enzymes at the brush border: peptides -> amino acids -> absorbed.

3 - Luminal enzymes: proteins -> oligopeptides -> uptake by enterocytes,
Cytosolic enzymes: peptides -> amino acids -> absorbed.

4 - Luminal enzymes: proteins -> oligopeptides -> absorbed.

27
Q

What activates the pro-enzyme (pepsinogen) secreted by chief cells of the stomach?

What is the product of activation of pepsinogen?

A
  • Low pH.

- Pepsin.

28
Q

Which linkages is pepsin specific for?

How much protein digestion does pepsin account for?

A
  • Linkages of aromatic and large neutral amino acids.

- Pepsin accounts for 10% - 15% of protein digestion.

29
Q

What is the activating agent for trypsinogen?

Is this an endoenzyme or exoenzyme in its active form?

A

Enterokinase.

Trypsin is an endoenzyme.

30
Q

For which 4 proenzymes is trypsin an activating agent?

Which of these proenzymes are exopeptidases in their active form?

A

1 - Chymotrypsinogen. - endo.

2 - Proelastase. - endo.

3 - Procarboxypeptidase A. - exo

4 - Procarboxypeptidase B. - exo

31
Q

How do endopeptidases and exopeptidases differ in function?

What about dipeptidases?

A
  • Endopeptidases hydrolyses peptide bonds between specific amino acids, resulting in oligopeptides with 2-6 amino acids.
  • Exopeptidases hydrolyse peptide bonds near the ends of the peptides, resulting in single amino acids.
  • Dipeptidases hydrolyse peptide bonds in dipeptides.
32
Q

What is the function of brush border peptidases?

A

To hydrolyse oligopeptides formed during luminal digestion to amino acids.

33
Q

How do brush-border peptidases differ from cytosolic peptidases?

A

Brush border peptidases have a gretaer affinity for relatively larger oligopeptides (3-8 AAs) compared to cytosolic peptidases.

34
Q

How does PEPT1 transport amino acids across the apical membrane?

A

It uses secondary active transport driven by a proton gradient.

35
Q

What is iminoglycinuria?

A

A mutation in the IMINO amino acid transport system, which is responsible for the cotransport of glycine and proline with Na+ into enterocytes.

36
Q

What is cystinuria?

A

A mutation in the antiporters responsible for the absorption of cationic amino acids such as cysteine and efflux of neutral amino acids, leading to increased cysteine in the urine.

37
Q

What is Hartnup disorder?

A

A mutation in the B0 transporters responsible for the cotransport of non-polar amino acids with Na+ into enterocytes.

38
Q

What is dicarboxylic aminoaciduria?

A

A mutation in the XAG transporters responsible for the cotransport of glutamate and aspartate with 3Na+ into enterocytes, leading to increased concentrations of these amino acids in the urine.

39
Q

What is lysinuric protein intolerance?

A

A mutation in the transporters at the basolateral membrane that are responsible for the transport of lysine, arginine and ornithine.

40
Q

Which transporter mediates the release of cationic amino acids at the basolateral membrane?

A

4F2hc/y+LAT1 antiporters.

41
Q

What property do all transporters of amino acids at the basolateral membrane have?

A

They are Na+ independent.

42
Q

Which transporter mediates the release of neutral amino acids at the basolateral membrane?

A

4F2hc-independent LAT transporters.

43
Q

List 3 signs of Hartnup disease.

A

1 - Skin changes of pellagra.

2 - Cerebellar ataxia (lack of fine control of voluntary movements).

3 - Psychiatric abnormalities.

44
Q

What are the two routes by which in tact proteins / polypeptides are absorbed in adults?

A

1 - Enterocyte phagocytosis.

2 - M cells (in Peyer’s patches).

45
Q

Where are Peyer’s patches?

A

In lymphatic tissue in the ileum.

46
Q

How do M cells facilitate protein uptake?

What is the importance of protein uptake in adults?

Where else is in tact protein uptake seen?

A
  • M cells take up proteins by phagocytosis and package them in clathrin-coated vesicles.
  • The vesicles are secreted at their basolateral membranes into the lamina propria.
  • Immunocompetent cells process the target antigens and transfer them to lymphocytes/
  • An immune response is initiated.
  • Important in disease processes of the mucosa.
  • In tact protein uptake also seen in neonates (from colostrum / breast milk).

DIG (20 decks)

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