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MD3002 > GI digestion and absorption > Flashcards

GI digestion and absorption Flashcards

1
Q

Define digestion.

A

Breakdown of nutrients into absorbable molecules.

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2
Q

Define absorption.

A

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

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3
Q

Describe the structure of small intestine mucosa.

A
  • Surface of small intestine is arranged in circular fold of Keckring.
  • Vili project from the folds
  • Surface of vili are covered with absorptive epithelial cells (enterocytes) and with mucus secreting cells (goblet cells)
  • The apical surface of the epithelial cells is covered by microvili (= brush border)
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4
Q

Identify the factors which increase the SA of the small intestine.

A
  • Circular folds of Keckring
  • Vili
  • Microvili
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5
Q

Identify the main constituents of an average western diet.

A

CARBS - 250 to 800 g/day
(starch, glycogen, cellulose, disaccharides)

LIPIDS - 25 to 160 g/day
(triglycerides, phospholipids, cholesterols and cholesterol esters, free FAs, lipid vitamins)

PROTEINS - 70 to 100 g/day ingested + 35 to 200 g/day from endogenous sources (e.g. digestive enzymes and dead cells from GI tract)

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6
Q

Distinguish between oils and fats.

A

Oils are lipids which are liquid at room temperature

Fats are lipids which are solid at room temperature

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7
Q

Identify other dietary requirements besides carbs, lipids and proteins.

A

VITAMINS
(fat soluble ones e.g. A, D, E, K and water soluble ones e.g. B12, folate)

MINERALS
Ca, Fe, Mn, Mg

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8
Q

What is the significance of the distinction between water soluble and fat soluble vitamins ?

A
  • If substance is water soluble (e.g. vit C), excess will be in the urine
  • If fat soluble vitamine, absorbed by same process as lipids, but they are stored, so easy to overdose on lipid soluble vitamins (cytotoxic in ↑ conc, therefore stick to RDA)
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9
Q

Why is it important we get vitamins and minerals in our diet ?

A

Because they are not produced by the body, or the amount produced endogenously is not sufficient (e.g. vit D – synthesised in liver in sunlight, but required in diet if not exposed))

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10
Q

What is the role of vitamins and minerals on our body ?

A

They may be required as co-factors, antioxydants, hormones

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11
Q

Identify pathologies resulting from vitamin deficiencies.

A

Rickets (D)
Scurvy (C)
Anaemia (B12)

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12
Q

Describe the amounts of digestion and absorption going on in each of:

  • Mouth
  • Stomach
  • Small intestine
  • Large intestine
A
  • Mouth: very little digestion (lipids, carbs), almost no absorption
  • Stomach: some digestion (proteins), almost no absorption
  • Small intestine: Lots of digestion (carbs, lipids, proteins) and absorption
  • Large intestine: Very little digestion (some indigestible substances used as fuel by gut flora) and little absorption (water)
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13
Q

Explain how carbs are absorbed.

A
  • Only monosaccharides can be absorbed so di or polysaccharides are broken down (starting with salivary alpha-amylase) into glucose, galactose and fructose.
  • These are absorbed in the small intestine thanks to the Na+ dependant cotransport (glucose and galactose) and thanks to facilitated diffusion (fructose).
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14
Q

Identify the places in the GI tract where polysaccharides are digested, and explain how they are initially digested in each of these parts.

A

1) Mouth
- Salivary α amylase (weak effect)

2) Stomach
- α amylase continues to act (salivary α amylase has been mixed in with bolus in the mouth, can continue to act for few moments even when it hits the stomach; material on the inside of the bolus will continue to be digested by alpha amylase, while α amylase on outside will be denatured)

3) Duodenum
- Pancreatic amylase
- Brush border enzymes (maltase, sucrase, and lactase) act on disaccharides, producing monosaccharides (glucose, galactose, fructose)

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15
Q

Explain what happens in lactose intolerance, in light of the digestion of carbs in the duodenum.

A

No enzymes (to convert lactose into a monosaccharide), so bacteria ferment sugar resulting in gas and diarrhea

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16
Q

Identify the digestible and indigestible polysaccharides (along with the type of bonds in each). For the digestible ones, identify the enzyme used to digest them.

A

DIGESTIBLE:
Starch (from plants) and glycogen (from animals), linear chains of glucose molecules joined by a α 1-4 glycosidic bonds initially digested by amylase

INDIGESTIBLE:
Cellulose (from plants), linear chains linked by β 1-4 glycosidic bonds. No enzymes in humans to digest cellulose

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17
Q

What type of bond do amylases digest ?

A

Internal α-1-4 glycosidic bonds

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18
Q

Identify the main kinds of amylases, and where they act.

A

Salivary amylases– secreted from mouth in (response to sight and smell of food)

Pancreatic amylases- secreted from pancreas into duodenum

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19
Q

What does amylase digest polysaccharides into ?

A

Produces maltotriose, maltose and α-limit dextrins (α-1-6 bonds form the cross links between polymeric chains)

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20
Q

What is special about the structure of α-limit dextrins, one of the products of the digestion of polysaccharides by amylase ?

A

α-1-6 bonds form the cross links between polymeric chains

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21
Q

How are the products of amylase digestion further digested ?

A

Products of amylase are now digested by oligosaccharidases:

α-glucosidase - cleaves α-1-4 glycosidic bonds to remove single glucose units from the non-reducing end of the polymer

Isomaltase - cleaves α-1-6 glycosidic bonds in the α-limit dextrin oligosaccharides

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22
Q

Where are oligosaccharides which further digest the products of amylase digestion located ?

A

They are attached to the enterocyte mucosal membrane of the brush border of epithelial cells:

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23
Q

How are the products of oligosaccharidases digested ?

A

Products of amylase and oligosaccharidases are hydrolysed by disaccharidases:

Maltase - produces glucose and glucose
Sucrase - produces glucose and fructose
Lactase - produces glucose and galactose

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24
Q

Where are disaccharidases which further digest the product of oligosaccharidase digestion located ?

A

Disaccharidases are attached to the bursh border membrane.

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25
Q

Identify the type of bond digested by each kind of disaccharidase.

A

Maltase - α 1-4 glycosidic bond
Sucrase - α 1-2 glycosidic bond
Lactase - β 1-4 glycosidic bond

26
Q

In light of digestive enzymes, how does lactose intolerance occur ?

A

Lactase lacking, results in lactose in large intestine, fermented by gut bacteria.

27
Q

What are the final products of carbohydrate digestion ?

A

Glucose
Galactose
Fructose

28
Q

Describe the process of carbohydrate digestion.

A

1) Secondary active transport
- SGLT1 (sodium dependent glucose transporter 1) located on the apical membrane transports glucose and galactose from the lumen to the epithelial cell

2) Facilitated diffusion
- GLUT5 (glucose transporter 5) transports fructose across the apical membrane from the lumen to the epithelial cell

Then, GLUT2 may transport each of fructose, glucose and galactose from epithelial cell into blood through facilitated diffusion where needed.

29
Q

Briefly summarise how proteins are digested.

A

1) Begins in stomach with pepsin
2) Completed in small intestine with pancreatic and brush border proteases (Trypsin, Chymotrypsin, Carboxypeptidase A and B, Elastase)

30
Q

Recall how pepsin is activated.

A

Pepsinogen in low pH

31
Q

Distinguish between endopeptidases and exopeptidases.

A

Endopeptidases - cleave large polypeptides in middle of chain → oligopeptides (2-8 AA long)

Exopeptidases - cleave amino acids one at a time from either end of protein

32
Q

Identify the main endopeptidases, and state where they act, and exactly what AA bonds they target.

A
  • Pepsin (stomach): hydrolyses links with tyrosine, D-alanine, and leucine (hydrolyses long polypeptide chains into shorter ones)
  • Trypsin (SI): hydrolyses links with arginine and lysine
  • Chymotrypsin (SI): hydrolyses links with tyrosine, tryptophan, D-alanine, methionine, and leucine
  • Elastase (SI): degrades elastin
33
Q

Identify any protections of the body against Trypsin.

A

Trypsin inhibitor is a small protein present in pancreatic juice to inhibit any trypsin found prematurely in the pancreatic cells or duct

34
Q

Identify the main exopeptidases, and state where they act.

A

Carboxypeptidase (C terminal)

Aminopeptidase (N terminal)

35
Q

What are the products of stomach and pancreatic luminal enzymes ?

A

Oligopeptides

AAs

36
Q

How are the oligopeptides and AAs produced by pepsin and proteases in the stomach/SI further digested ? Where does this occur ?

A

Oligopeptidase
Aminopeptidase

In the brush border

37
Q

What are the products of AA/oligopeptide digestion by oligopeptidases and aminopeptidases ? How and where are these further digested ?

A

AAs
Dipeptides
Tripeptides

They are further broken down into AAs in enterocytes (using dipeptidase and tripeptidase)

38
Q

List the main events of protein digestion according to the digestion phases.

A

CEPHALIC
Nothing

GASTRIC
HCl converts pepsinogen into pepsin
Pepsin hydrolyses long peptides chains into smaller ones

INTESTINAL (can be broken down into two phases)
1) Pancreatic phase
-Trypsin
-Chymotrypsin
-Carboxypeptidase
all break down proteins into AAs and oligopeptides (2-8 residues)

2) Mucosal phase
-Oligopeptidase
-Aminopeptidase
breaks oligopeptides down into AAs, dipeptides and tripeptides, which are then broken down into AAs using dipeptidase and tripeptidase

39
Q

Briefly explain how protein absorption occurs (including where it occurs).

A

All absorption occurs in small intestine.

  • AA absorption: Na+-dependant cotransport
  • Dipeptide absorption: H+-dipeptide cotransport
  • Tripeptide absorption: H+-tripeptide cotransport
40
Q

How is the problem of the hydrophobicity of lipids, with respect to digestion, solved ?

A

Enzymes like to live in aqueous environments, so lipids are solubilised before digestion and absorption.

41
Q

Which parts of the GI tract are involved in the digestion of lipids ?

A

Mouth (lingual lipases)
Stomach (lingual and gastric lipases)
Small intestine (pancreatic enzymes)

42
Q

Identify the main types of lipids.

A

Fats/oils (TAGs)
Phospholipids
Cholesterols/cholesterol esters
FAs

43
Q

Identify the main types of enzymes involved in lipid digestion. Why aren’t bile salts one of these ?

A

Lipases
Phospholipases
Cholesterol esterases

Bile salts do have an important impact on lipid digestion/absorption, but they are NOT enzymatic.

44
Q

Describe the digestion of lipids in the mouth/stomach.

A

SALIVARY/GASTRIC LIPASES (-relatively unimportant ~10% of ingested lipids
hydrolysed)
-Heat and movement (including retropulsion) in stomach mix food with lipases → lipid emulsion
-Hydrolysis initially slow due to largely separate aqueous/lipid interface
-As hydrolysis proceeds, rate increases due to FAs produced acting as surfactants breaking down lipid globules aiding emulsification
-Emulsified fats ejected from stomach to duodenum

45
Q

Describe the process of lipid digestion and absorption occurring in the small intestine.

A

PANCREATIC LIPASES (main lipid digestive enzyme)

1) Bile salts (including HCO3-, helps neutralise acid stomach), lysolecithin and products of lipid digestion emulsify dietary lipids (i.e. bile salts coat lipids)
2) Coenzymes grab enzymes and bring them in close proximity with lipid emulsion of particles (since emulsification produces little droplets of lipids dispersed in an aqueous solution creating a large surface area for pancreatic enzyme digestion)
3) Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase, and phospholipase A2) and the protein colipase are secreted to complete digestion. They digest fats into monoglycerides and FAs which are stored in micelles
4) (When come in contact with brush border), monoglycerides and FAs move out of micelles and absorbed into enterocytes by diffusion
5) Cholesterol is transported into cells by a membrane transporter
6) Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons
7) Chylomicrons are packaged into secretory vesicles on the Golgi membrane and are exocytosed across the basolateral membrane into the lymphatic system. The lymphatic circulation carries the chylomicrons to the thoracic duct which empties into the blood stream

Process of re-esterification within the cell of triglycerides and cholesterol also involved

46
Q

What are micelles ? What is their composition ?

A

Products of lipid digestion (cholesterol, monoglycerides, lysolecithin, free FAs) are solubilised in mixed micelles

  • Core contains the product of lipid digestion
  • Surface coating of bile acids which are amphipathic
47
Q

What is the composition of chylomicrons ?

A
  • Core of triglycerides and cholesterol ester

- Phospholipids and apoproteins on the outside (80% / 20%)

48
Q

What is the diameter of chylomicrons ? of micelles ?

A

Chylomicrons: 100 nm
Micelles: 5 nm

49
Q

Do chylomicrons enter vascular capillaries ?

A

No, chylomicrons are too big to enter vascular capillaries but enter the lymphatic capillaries (lacteals) by moving between the endothelial cells that line the lacteals.

50
Q

Is there enough bile salts to deal with the average meal ? Be specific. How does the body compensate for this ?

A

No:
~3-4 g in body, but ~3-15 g required per meal

After use, most bile salts are reabsorbed from terminal ileum into the liver via the enterohepatic circulation (via hepatic portal vein)

51
Q

Briefly describe the enterohepatic circulation of bile salts.

A
  • Bile salts produced in the liver
  • From the liver, going to be stored and concentrated in the gall bladder
  • In some circumstance (e.g. CCK, secretin), the gall bladder contracts and bile salts enter duodenum through sphincter of Odi
  • As ingesta goes through small intestine towards large intestine, micelle gradually loses breakdown products because they have been absorbed
  • By terminal ileum, no more breakdown product (because should have absorbed it all), but there should still be bile salts
  • Most of the bile salts are recycled and recirculated, back to liver
52
Q

What percentage of bile salts are still present in feces ? What percentage is recirculated ?

A

5% of bile salts are lost in feces

Remaining 95% of bile salts not excreted into feces, recirculated

53
Q

What is the main function of the large intestine ?

A

Absorption of water and NaCl

54
Q

Briefly describe the mechanism of water absorption form large intestine. Explain the nervous pathways involved in this.

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

Controlled by enteric nerve plexi and hormonal control (e.g. aldosterone increases water absorption)

55
Q

What happens if not enough water is absorbed in large intestine ?

A

Diarrhea

56
Q

Why should the large intestines not absorb ALL the water ?

A

Because must leave enough water to lubricate and soften feces to facilitate defecation

57
Q

Describe the bacterial actions in the large intestine.

A

Since movement through the LI is very slow, bacteria thrive (10^11 bacteria / g faeces)

  • Formation of certain B vitamins (B12, thiamin, riboflavin)
  • Formation of vitamin K (B and K absorbed passively)
  • Digestion of some bile constituents, and other indigestible matter
58
Q

What is the volume of fluid that:

  • enters the GI tract each day ? from secretions ? from ingestion ?
  • the small intestine reabsorbs ?
  • the large intestine reabsorbs ?
  • goes into feces ?
A
  • 9L of fluid enters GI tract each day (2.3 from ingestion, rest from secretions)
  • Small intestine reabsorbs 8 L
  • Large intestine reabsorbs 90% of the liter by osmosis through cell walls into vascular capillaries inside vili
  • 100 mL eliminated in feces
59
Q

Where do monosaccharides, AAs, short chain FAs, and chylomicrons go after being absorbed to the enterocytes ?

A

Monosaccharides → Blood capillary of a villus
AAs → Blood capillary of a villus
Short chain FAs → Blood capillary of a villus
Chylomicrons → Lacteal of a villus

60
Q

Where are lipids, carbs, and proteins absorbed ?

A

They are all absorbed in the small intestine

61
Q

State where each of the following is absorbed, and state the mechanism used for each absorption.

  • Fat soluble vitamins
  • Water soluble vitamins
A

FAT SOLUBLE VITAMINS

  • Small intestine
  • Bile salts form micelles, which diffuse into intestinal cells

WATER SOLUBLE VITAMINS

  • Small intestine
  • Na+ dependent co-transport
62
Q

State where each of the following is absorbed, and state the mechanism used for each absorption.

  • Vitamin B12
  • Bile salts
  • Ca++
  • Fe++
A

VITAMIN B12

  • Ileum
  • Intrinsic factor

BILE SALTS

  • Ileum
  • Na+ bile salt co-transport

CA++

  • Small intestine
  • Vitamin D-dependent Ca++ binding protein (Calbindin)

FE++

  • Small intestine
  • Binds to apoferritin in intestinal cell, and binds to transferrin in blood

MD3002 (95 decks)

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