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BMSC2225 Absorption and Excretion - GI Tract Physiology > Digestion and Absorption > Flashcards

Digestion and Absorption Flashcards

1
Q

What is caviteal digestion?

A

Digestion by enzymes that are secreted into the lumen

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2
Q

What is membranal digestion?

A

Digestion by enzymes bound to the intestinal wall

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3
Q

What are the five layers that the digested material has to pass through?

A 
Glycocalyx
Apical cell membrane
Intracellular space
Basolateral cell membrane
Basement membrane
Capillary endothelium
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4
Q

What is the make up carbohydrates that are intaken?

A 
50% starch
30% sucrose
6% lactose
2% maltose
13% mixture of other sugars
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5
Q

What two enzymes facilitate caviteal digestion of carbohydrates?

A

Salivary amylase

Pancreatic alpha amylase

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6
Q

What is the optimal pH for salivary amylase?

A

6.7-7.5

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7
Q

What is the substrate and product of salivary amylase?

A

Starch –> Tri and disaccharides

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8
Q

Where is salivary amylase denatured?

A

Stomach

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9
Q

Where does pancreatic alpha amylase work?

A

Small intestine

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10
Q

Which bonds does pancreatic alpha amylase break?

A

Alpha 1,4 bonds

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11
Q

What are the three enzymes that facilitate membranal carbohydrate digestion?

A

Maltase
Sucrase
Lactase

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12
Q

What are the substrate and products of maltase?

A

Maltose –> glucose + glucose

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13
Q

What are the substrate and products of lactase?

A

Lactose –> glucose + galactose

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14
Q

What are the substrate and products of sucrase?

A

Sucrose –> glucose + fructose

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15
Q

Describe the process of glucose/galactose absorption?

A

Na/K ATPase sets up concentration gradient of sodium
Sodium moves in from the gut via SGLT1 transporter and brings glucose/galactose with it
The sugar then diffuses across the basolateral membrane via GLUT2 channel

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16
Q

Describe the process of fructose absorption

A

Fructose diffuses across the apical membrane via GLUT5

Fructose diffuses out via GLUT2

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17
Q

Which cells release pepsinogen?

A

Chief cells

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18
Q

What converts pepsinogen to pepsin?

A

Hydrochloric acid

19
Q

What is the optimal pH for pepsin?

A

pH 1-3

20
Q

Where is pepsin active?

A

Stomach

21
Q

Where is pepsin denatured?

A

Duodenum

22
Q

What enzyme activates trypsin?

A

Enterokinase

23
Q

What enzymes does trypsin activate?

A

Chymotrypsin
Elastase
Carboxypeptidases

24
Q

Describe the process of protein absorption?

A

A sodium gradient is set up by Na/K/ATPase across the basolateral membrane
A hydrogen ion gradient is set up by Na/H exchanger
An amino acid, dipeptide or tripeptide is brought in via cotransport with a proton via PepT1 transporter
Any dipeptides or tripeptides are broken down by a peptidase intracellularly
The amino acids leave via the basolateral membrane via facilitated diffusion

25
Q

What enzymes are involved in caviteal digestion of lipids?

A 
Lingual lipase
Gastric lipase
Pancreatic lipase
Phospholipase
Cholesterol esterase
26
Q

What happens to the fatty acids after lipid digestion?

A

Formation of miscelles

27
Q

Describe the process of fatty acid absorption

A

Fatty acids enter the enterocyte by passive diffusion
The fatty acids are reformed into triglycerides at the smooth ER to maintain a concentration gradient
The triglycerides are modified at the Golgi apparatus with cholesterol, proteins and phospholipids to form chylomicrons
The chylomicrons are exocytosed into the interstitium and are taken up by lymphatic lacteals

28
Q

What are the two routes of fluid absorption and where do they occur?

A

Paracellular - duodenum and jejumum

Transcellular - Ileum

29
Q

Describe the process of transcellular fluid absorption

A

Na/K/ATPase sets up a sodium gradient across the basolateral membrane
Sodium enters across the apical membrane via Na/H antiporter of Na/Cl cotransporter
Osmolarity increases in the cells so water enters via osmosis from the lumen

30
Q

Which ions are secreted into the lumen by the enterocyte?

A

Carbonate and potassium

31
Q

Describe the role of carbonic anhydrase in ion secretion and reabsorption

A

Carbonic anhydrase converts water and carbon dioxide into a proton and carbonate
The proton leaves via the apical membrane via Na/H antiport
The carbonate leaves via the apical membrane via Cl/HCO3 antiport

32
Q

When is potassium secreted by the enterocytes?

A

When luminal concentration is <25mM

33
Q

How does calcium travel in the blood?

A

As free ions in the plasma or bound to albumin or citrate phosphate

34
Q

Describe the process of calcium absorption?

A

Taken up into the enterocyte by facilitated diffusion
Sequestered into the endoplasmic reticulum
Passes through the basolateral membrane via Ca/ATPase of Ca/Na transporter

35
Q

Describe the process of calcium absorption regulation

A

Parathyroid hormone is secreted in response to a low plasma calcium concentration
Parathyroid hormone stimulates calcitriol production from vitamin D
Calcitriol stimulates Ca/ATPase
Ca/ATPase reduces the intracellular concentration which increases absorption

36
Q

What three forms of iron may be found in the lumen?

A

Fe2+
Fe3+
Bound to haem

37
Q

How is Fe2+ absorbed?

A

Cotransport with Fe/H transporter

38
Q

How is Fe3+ absorbed?

A

Conversion to Fe2+ by membranal reductase

Cotransport with Fe/H transporter

39
Q

How is haem absorbed?

A

Endocytosis of the haem molecule

Digestion by haem oxygenase to release the iron ions

40
Q

Which vitamins are fat soluble?

A

A, D, E and K

41
Q

How are fat soluble vitamins absorbed?

A

Dissolved in miscelles

42
Q

Which vitamins are water soluble?

A

B and C

43
Q

How are water soluble vitamins absorbed?

A

Sodium dependent active transport

44
Q

Describe the process of vitamin B12 absorption

A

Vitamin B12 is ingested bound to proteins
The proteins are digested
Vitamin B12 binds to R proteins
R proteins are digested
Vitamin B12 binds to protease resistant intrinsic factor
IF/B12 complexes on the brush border trigger endocytosis
Carried in blood via transcobalamin

BMSC2225 Absorption and Excretion - GI Tract Physiology (8 decks)

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