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Digestion and Absorption in the GI tract Flashcards

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1
Q

Why are digestive enzymes localized to different areas of the gut?

A

The optimal pH of enzymes varies in different gut regions

Some enzymes are secreted as inactive precursors and require activation

Some enzymes are membrane-bound

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2
Q

What is the main goal of digestive functions?

A

To break macromolecules into monomer or dimer units for absorption

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3
Q

Where does most nutrient absorption occur?

A

Mainly in the small intestine, but water absorption also occurs in the colon

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4
Q

How do different molecules get absorbed in the intestine?

A

Different transport mechanisms are specific to different molecules

Transport may involve passive diffusion, facilitated diffusion, or active transport

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5
Q

When is energy required for absorption?

A

When transport is against a gradient, requiring primary or secondary active transport

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6
Q
A
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7
Q

What is the main role of enterocytes in the small intestine?

A

Enterocytes contain membrane transporters and enzymes that allow for digestion and absorption of nutrients from the lumen.

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8
Q

How do sodium pumps contribute to absorption?

A

maintains Na gradient (low Na inside cell) which is required by many transporters, and creates osmotic gradient in intercellular space, which drives water absorption from lumen

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9
Q

what are transporters used for?

A

required for absorption of many solutes (products of digestion)

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10
Q

Where does protein digestion begin?

A

Protein digestion starts in the stomach with the action of pepsin and HCl.

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11
Q

What is the pH of chyme when released from the stomach?

A

Chyme has a low pH due to the presence of gastric acid (HCl).

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12
Q

What 4 things does chyme contain when it enters the duodenum?

A

Chyme contains:

Solubilized, slightly digested carbohydrates
Solubilized, partly digested proteins
Slightly digested fats
Intrinsic factor, which enables Vitamin B12 absorption in the small intestine

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13
Q

What additional processes are needed for further digestion in the small intestine?

A

Other enzymes from the pancreas and enterocytes aid digestion
pH must be neutralized before enzymes function effectively
Bile salts are required for fat digestion

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14
Q

Where does protein digestion continue after the stomach?

A

duodenum

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15
Q

What enzyme on the brush border of duodenal enterocytes activates pancreatic trypsinogen?

A

Enterokinase activates pancreatic trypsinogen, converting it into trypsin, which then activates other proteolytic enzymes of the pancreas.

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16
Q

What are the functions of pancreatic trypsin, elastase, and chymotrypsin in protein digestion?

A

These enzymes are endopeptidases that cleave peptide bonds within proteins, producing short peptides.

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17
Q

What is the function of pancreatic carboxypeptidases?

A

Carboxypeptidases are exopeptidases that remove amino acids from the carboxyl (C-terminal) end of peptides.

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18
Q

How do aminopeptidases contribute to protein digestion?

A

Aminopeptidases, located on the brush border of enterocytes, remove amino acids from the amino (N-terminal) end of peptides.

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19
Q

How are free amino acids absorbed in the small intestine?

A

Na⁺-linked luminal transporters absorb free amino acids, which then exit via the basolateral side into the capillaries.

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20
Q

How are di- and tri-peptides absorbed in the small intestine?

A

They are absorbed via H⁺-linked luminal transporters, then hydrolyzed into amino acids before exiting on the basolateral side.

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21
Q

What happens to larger peptides in the small intestine?

A

Peptidases break down larger peptides into di-peptides, tri-peptides, or free amino acids for absorption.

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22
Q

Why is peptide absorption different in infants?

A

The small intestine wall is permeable to peptides, allowing absorption of growth factors & antibodies from colostrum (first milk).

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23
Q

what are the 3 polysaccharides

A

starch
glycogen
cellulose

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24
Q

what is starch made from

A

Glucose polymer, 
α(1→4) & α(1→6) links

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25
what is glycogen made of
Glucose polymer, α(1→4) & α(1→6) links
26
what is cellulose made of
Glucose polymer, β(1-4) links
27
what are the 3 dissacharides
sucrose lactose maltose
28
what is sucrose made of
Glucose + fructose
29
what is lactose made of
glucose + galactose
30
what is maltose made of
glucose + glucose
31
what are the 3 monosaccharides
glucose fructose galactose
32
What is the optimum pH for salivary amylase, and where does hydrolysis occur?
The optimum pH is 6.8, allowing some hydrolysis in the mouth before the bolus enters the acidic stomach.
33
What is the optimum pH for pancreatic amylase?
The optimum pH is 7.1.
34
What is the optimum pH range for small intestine membrane-bound disaccharidases?
Between 6 and 7.
35
Why is duodenal pH suitable for pancreatic amylase and disaccharidases?
Because the duodenal pH falls within the optimum range for both pancreatic amylase (7.1) and disaccharidases (6-7).
36
What type of bonds does pancreatic amylase cleave, and which bonds can it NOT cleave?
Pancreatic amylase cleaves straight-chain bonds but cannot cleave branch points in starch.
37
What are the 3 main products of pancreatic amylase digestion?
Short oligosaccharides, maltose (disaccharide), and maltotriose (trisaccharide).
38
How are disaccharides (such as lactose and sucrose) further digested?
They are digested to monosaccharides by brush border enzymes.
39
How are glucose and galactose absorbed on the apical (lumenal) side of enterocytes?
They are transported via SGLT (Na-linked transporter) using secondary active transport.
40
How is fructose transported into enterocytes?
GLUT5 transporter via facilitated diffusion.
41
How do all three hexoses (glucose, galactose, fructose) exit enterocytes into the bloodstream?
They exit via GLUT2 transporter using facilitated diffusion.
42
What maintains the Na gradient necessary for glucose and galactose transport?
The Na+/K+ pump on the basolateral membrane.
43
What is the primary enzyme responsible for fat digestion?
Pancreatic lipase, but some gastric lipase is also involved.
44
What three factors are necessary for fat digestion in the duodenal lumen?
Colipase Bile salts Right pH
45
What is the function of beta-lipoprotein in fat absorption?
Inside enterocytes, it packages lipids for export as chylomicrons.
46
What is the first step of fat digestion?
Emulsification of fat in the lumen, which requires bile salts.
47
What does pancreatic lipase do in fat digestion?
Converts triglycerides (TGs) into monoglycerides (MGs) and fatty acids (FAs).
48
What 4 things are micelles made of?
Monoglycerides (MGs) Fatty acids (FAs) Bile salts Other lipid components
49
How do micelles aid in fat absorption?
Micelles diffuse to the epithelial cell brush border, allowing fatty acids and monoglycerides to be absorbed.
50
How do free fatty acids (FAs) and monoglycerides (MGs) enter the cell?
They diffuse through the lipid core of the membrane via passive diffusion.
51
Where does triglyceride resynthesis occur inside the enterocyte?
In the smooth endoplasmic reticulum (SER).
52
What happens to triglycerides after resynthesis?
They are incorporated into chylomicrons along with beta-lipoprotein, phospholipids, and cholesterol.
53
How do chylomicrons exit the enterocyte?
They exit via the basolateral side into nearby lacteals (lymphatic capillaries).
54
Where are approximately 95% of bile salts reabsorbed?
Mainly in the terminal ileum.
55
How are reabsorbed bile salts transported back to the liver?
Via the portal vein, where they are re-extracted by hepatocytes for re-secretion into bile.
56
What percentage of bile salts are lost daily in feces, and how is this loss compensated?
5-10% is lost daily and compensated by de novo synthesis in the liver.
57
How often can bile salts be recycled during digestion of a single large meal?
They can be turned over 3-4 times.
58
How is water absorbed in the colon?
Water follows Na⁺ down the osmotic gradient into the cells.
59
Which hormone stimulates water and sodium reabsorption in the colon?
Aldosterone
60
How do resident microflora contribute to nutrient absorption in the colon?
They produce vitamin K and folic acid, which are absorbed in the colon.
61
Why don’t resident microflora cross the epithelial barrier?
The epithelial barrier prevents microbial translocation, ensuring they remain in the lumen.
62
Which mineral ions are specifically absorbed in the GI tract?
Ca²⁺ (calcium) and Fe²⁺ (iron) are absorbed through specific mechanisms.
63
Which vitamins have specific mechanisms for absorption?
B and C vitamins.
64
How are fat-soluble vitamins absorbed?
Fat-soluble vitamins are absorbed with lipids in the small intestine (SI).
65
Where are non-ionized drugs best absorbed?
In the small intestine (SI) and colon, where the right pH allows for better absorption.
66
Can drugs be absorbed across the gastric mucosa?
Limited absorption occurs for some drugs, such as aspirin.
67
What protects Vitamin B12 from the acidic stomach environment?
Haptocorrin
68
What is the function of Intrinsic Factor (IF) in B12 absorption?
Intrinsic Factor (IF) is a glycoprotein secreted by parietal cells that binds to Vitamin B12 for absorption.
69
Why is the Vitamin B12/IF complex important for absorption?
The Vitamin B12/IF complex is resistant to digestion, allowing it to reach the small intestine intact.
70
What happens if Intrinsic Factor (IF) is not present?
Without Intrinsic Factor, Vitamin B12 cannot be absorbed.
71
Where does Vitamin B12 absorption take place in the intestine?
The Vitamin B12/IF complex binds to the cubam receptor and is absorbed in the distal ileum.
72
Why must calcium be regulated within enterocytes?
Calcium is an active signaling molecule, so it must be regulated to prevent unwanted intracellular effects.
73
What protein helps regulate calcium within the enterocyte?
Calbindin binds to calcium to prevent it from acting as a signaling molecule inside the enterocyte.
74
Through which channel does calcium enter the enterocyte from the gut lumen?
TRPV6 calcium channel.
75
What are the two key transporters involved in calcium absorption into the blood?
NCX1 - Na+/Ca2+ exchanger PMCA1b - Plasma membrane Ca2+ ATPase
76
What are the two routes for calcium absorption?
Regulated Paracellular Transport (through tight junctions, influenced by Claudin proteins) Transcellular Transport (via TRPV6, calbindin, and PMCA1b)
77
Which form of iron can be absorbed in the intestine?
Fe²⁺ (ferrous iron) can be absorbed, whereas Fe³⁺ (ferric iron) cannot.
78
What enzyme converts Fe³⁺ to Fe²⁺ for absorption?
Dcytb (Duodenal Cytochrome b) converts Fe³⁺ to Fe²⁺ at the brush border of enterocytes.
79
How is heme iron from the diet absorbed?
Heme is transported into the enterocyte, where Heme Oxygenase (HO-1) releases Fe²⁺ from heme.
80
Which transporter exports Fe²⁺ from the enterocyte into the blood?
Ferroportin exports Fe²⁺ into the bloodstream
81
What happens to Fe²⁺ once exported into the blood?
Hephaestin or Ceruloplasmin converts Fe²⁺ back to Fe³⁺ so it can bind to Transferrin for circulation.
82
What are three causes of failure to digest macromolecules in chyme?
1. Interruption of the enterohepatic circulation of bile salts. 2. Failure to deliver pancreatic enzymes. 3. Poor coordination of gastric emptying with pancreatic & biliary secretions.
83
What happens when brush border lactase is absent?
Lactose remains in the lumen, leading to alactasaemia and digestive issues.
84
What are two reasons for lipid malabsorption?
1. Lack of beta-lipoprotein, required for chylomicron formation. 2. Failure of lipid digestion due to enzyme or bile salt deficiency.
85
How does malabsorption affect water balance in the gut?
Water remains in the lumen due to increased solute load, leading to diarrhea.
86
How does bacterial infection contribute to malabsorption?
Increases salt and water secretion, worsening diarrhea.
87
Which micronutrients are poorly absorbed in malabsorption syndromes?
Vitamin B₁₂, folate, and Fe²⁺ (iron), leading to anemia.
88
How is vitamin B₁₂ absorbed in the ileum?
It is only absorbed when complexed with intrinsic factor, which is recognized by a specific transporter in the ileum.
89
What are two causes of insufficient intrinsic factor?
Gastric atrophy. Autoimmune conditions targeting intrinsic factor or parietal cells.
90
What happens to erythropoiesis when vitamin B₁₂ is deficient?
Disturbed erythropoiesis, resulting in fewer but macrocytic red blood cells (RBCs).
91
Where is iron absorbed, and what helps its absorption?
Absorbed in the small intestine (SI). Acidic pH in the stomach maintains Fe³⁺ in a soluble form for later absorption.
92
What type of anemia results from iron deficiency?
Failure to produce sufficient hemoglobin, leading to microcytic RBCs.
93
How can dentists detect anemia?
Pale gums due to anemia. Glossitis (sore/burning tongue). Changes to the tongue surface.
94
What causes osmotic diarrhoea?
Malabsorption leads to an increased solute load in the small intestine (SI), causing water retention due to osmotic effects.
95
What are three key outcomes of osmotic diarrhoea?
Increased lumenal volume stimulates peristalsis. Undigested fats can have a laxative effect. Fermentation by microflora in the large intestine produces gases (e.g., H₂), causing distension and pain.
96
What is secretory diarrhoea and what causes it?
It occurs due to bacterial toxins (e.g., from Vibrio cholerae and Escherichia coli), which increase secretion by intestinal crypt cells.
97
Besides osmotic and secretory mechanisms, what is another cause of diarrhoea?
Intestinal mucosal damage can also lead to diarrhoea.
98
How does the small intestine structure change in coeliac disease?
Villi become flattened, enterocytes are damaged, and crypts show hyperplasia due to increased lymphocyte infiltration.
99
What causes tissue damage in the small intestine of coeliac patients?
Activated lymphocytes in the mucosa lead to local tissue damage, disrupting normal absorption.
100
How does coeliac disease contribute to diarrhoea?
Coeliac disease is an autoimmune condition affecting the small intestine (SI), triggered by an adverse reaction to gluten, leading to intestinal mucosal damage and malabsorption, causing diarrhoea.
101
What is Crohn’s disease and how does it affect the intestines?
Crohn’s disease is a chronic inflammatory condition mainly affecting the ileum and colon, leading to intestinal damage, malabsorption, and diarrhoea.

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