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MD3002 Physiology > GI digestion and absorption > Flashcards

GI digestion and absorption Flashcards

1
Q

What is digestion?

A

The breakdown of nutrients into absorbable molecules

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2
Q

What is absorption?

A

Movement of nutrients, water and electrolytes from the gut lumen into the internal enviroment

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3
Q

What is the surface of the small intestine arranged in?

A

Circular folds of Keckring

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4
Q

What is the apical surface of epithelial cells covered in?

A

Microvilli - creates brush border

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5
Q

What are enterocytes?

A

Simple columnar epithelial cells which line the inner surface of the small and large intestines

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6
Q

What do lacteals transport?

A

Most fats to the lymph

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7
Q

What is meant by protein which comes from ‘endogenous sources’?

A

Protein which is not from diet, old gut cells (35-200g)

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8
Q

What can be absorbed in the mouth?

A

almost none (certain drugs can)

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9
Q

What is absorbed in the colon?

A

Almost nothing (apart from water), some “indigestible” substances used as fuel by gut flora

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10
Q

What kind of carbohydrates can be absorbed?

A

Only monosaccharides

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11
Q

How is glucose absorbed in the SI?

A
  • Na+ dependant cotransport
  • SGT 1 - sodium dependant glucose transporter 1 - located on the apical membrane
  • Secondary active transport
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12
Q

How is galactose absorbed in the SI?

A
  • Na+ dependant cotransport
  • SGT 1 - sodium dependant glucose transporter 1 - located on the apical membrane
  • Secondary active transport
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13
Q

How is fructose absorbed in the SI

A
  • Facilitated diffusion

- GLUT 5 (glucose transporter 5) transports fructose across apical membrane

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14
Q

What are starch (from plants and glycogen (from animals) joined by?

A

alpha 1-4 glycosidic bonds

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15
Q

What are alpha 1-4 glycosidic bonds (present in starch and glycogen) digested by?

A

Amylase

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16
Q

What is cellulose (from plants) made up from?

A

Linear chains linked by B 1-4 glycosidic bonds. No enzymes in humans to digest this

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17
Q

What does pancreatic amylase produce when long chain carbohydrates are broken down?

A
  • Maltotriose
  • Maltose
  • Alpha-limit dextrins
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18
Q

What are products of amylase and oligosaccharidases hydrolysed by?

A

Disaccharidases which are attached to the brush border membrane

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19
Q

What are some examples of disaccharidases?

A
  • Maltase - produces glucose
  • Sucrase - produces glucose and fructose
  • Lactase - produces galactose and glucose
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20
Q

Describe the structure of maltose?

A
  • 2 glucose molecules bound together by a alpha 1-4 glycosidic bond
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21
Q

Describe the structure of sucrose`

A
  • GLUCOSE-FRUCTOSE

- bound together by an alpha 1-2 glycosidic bond

22
Q

Describe the structure of lactose

A
  • GALACTOSE-GLUCOSE

- Bound together by a B 1-4 glycosidic bond

23
Q

What are the final products of carbohydrate digestion?

A
  • Fructose
  • Glucose
  • Galactose
24
Q

What does protein digestion begin with?

A

Stomach with pepsin

25
Q

Where is protein digestion completed?

A

SI with pancreatic and brush-border proteases

26
Q

How can the enzymes which digest proteins be classified?

A
  • Endopeptidases - hydrolyse the interior peptide bonds of proteins
  • Exopeptidases hydrolyse one amino acid at a time
27
Q

Give 4 examples of endopeptidases

A
  • Pepsin
  • Trypsin
  • Chymotrypsin
  • Elastase
28
Q

What are the products of stomach and pancreatic luminal enzymes?

A
  • Oligopeptides

- Amino acids

29
Q

What are the products of the action of brush border enzymes?

A
  • Amino acids
  • Dipeptides
  • Tripeptides
30
Q

What are protein-derived structures further broken down into in enterocytes

A

Amino acids

31
Q

How are amino acids transported into SI?

A

Through Na+ - dependant cotransport

32
Q

How are dipeptides transported into SI?

A

H+ - dipeptide cotransport

33
Q

How are tripeptides transported into SI?

A

H+-tripeptide cotransport

34
Q

How are lipids digested and absorbed?

A
  • Dietary lipids are hydrophobic (insoluble in water) so causes problems with di/absorption
  • Must be solubilised b
  • Digestion begins in stomach with the action of lingual and gastric lipases
  • Completed in the SI with the action of the pancreatic enzymes
35
Q

What are the 4 main types of lipids?

A
  • Fats/oils
  • Phospolipids
  • Cholesterol and cholesterol esters
  • Fatty acids
36
Q

What are the 3 main types of enzymes which digest lipids?

A
  • Lipases
  • Phospholipases
  • Cholesterol esterases
37
Q

Describe the process of lipid digestion in the mouth and stomach

A
  • Salivary / gastric lipases
  • Heat and movements in stomach mix food with lipases
  • Hydrolysis initially slow due to largely seperate aqueous/lipid interface
  • As hydrolysis procedes, rate increases due to fatty acids produced acting as surfactants breaking down lipid globules aiding emulsification
  • Emulsified fats ejectd from stomach and duodenum
38
Q

Describe lipid digestion in the duodenum

A
  • Bile salts, lysolecithin and products of lipid digestion emulsify dietary lipids
  • Emulsification produces small droplets of lipid dispersed in aqueous solution creating a large surface area for pancreatic enzyme digestion
  • Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase and phosphlipase A2) and the protein, colipase are secreted to complete digestion
39
Q

What’s the difference between chylomicrons and micelles?

A
  • Micelles - gut lumen

- Chylomicrons - in enterocytes and exocytosed into the lymphatic system

40
Q

Describe the structure of chylomicrons?

A
  • 100 nm in diameter
  • Core of triglycerides and cholesterol ester
  • Phospholipids and apoproteins on the outside (80%/20%)
41
Q

How do chylomicrons enter vascular capillaries?

A
  • Too big to enter themselves

- Enter lacteals by moving between endothelial cells that line lacteals

42
Q

How do chylomicrons enter the bloodstream?

A
  • Enter lacteals

- Lymphatic circulation carries chylomicrons to the thoracic duct which empties them into the bloodstream

43
Q

How does the body deal with not having enough bile salts for the average meal?

A
  • 3-4g in body but 3-15g required per meal

- Most reabsorbed from terminal ileum into the liver via the enterohepatic circulation (via hepatic portal veins)

44
Q

What percentage of bile salts are lost in feces?

A

5%

45
Q

How is water absorbed in the colon?

A
  • Na+ actively absorbed (in exchange for K+)
  • K+ reabsorbed in exchange for H+
  • Cl- absorbed (in exchange for HCO3-)
  • H2O follows due to osmosis
46
Q

What is water absorption controlled by?

A

Enteric nerve plexi and hormonal control

- Aldosterone (increased water absorption)

47
Q

What are formed in the colon

A
  • B vitamins (B12, thiamin, riboflavin)

- Formation of vit K (B and K absorbed passively, probably)

48
Q

How much bacteria is present in the feces?

A

10^11 bacteria / g of faeces

49
Q

What can be absorbed in digestion?

A
  • Certain bile constituents

- Other ‘indigestable’ matter

50
Q

How much fluid enters the GI tract each day?

A

~ 9L (approx 2.3L from ingestion, the rest from secretion)

51
Q

How much fluid is reabsorbed in the small intestine?

A

~8L

52
Q

What percentage of the last litre of fluid is reabsorbed by the colon?

A

~90%

Through osmosis through cell walls into vascular capillaries inside villi

MD3002 Physiology (24 decks)

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