GI VIII Flashcards
Define digestion. In what juices are digestive enzymes secreted? Where are these enzymes present (What part of membrane of intestinal epithelial cells?)
The process by which ingested food is broken down chemically into absorbable molecules.
Digestive enzymes are secreted in salivary, gastric, pancreatic juices.
These enzymes are also present in the apical membrane of intestinal epithelial cells.
Define absorption.
What are the two paths? Define each.
Where is absorption of carbohydrates, proteins, and lipids highest?
Calcium, phosphate and iron?
Bile acids?
Is the movement of nutrients, water and electrolytes from the lumen of the intestine into blood.
There are two paths: Cellular and paracellular.
Cellular path- the substances enter the intestinal epithelial cells via the apical (luminal membrane) and are extruded via basolateral membrane to enter blood.
Paracellular path -substances move across the tight junctions, through the lateral interspaces into blood.
Where is absorption of carbohydrates, proteins, and lipids highest? - duodenum
Calcium, phosphate and iron? -duodenum
Bile acids? -ilieum
Describe the types of ingested carbohydrates.
Carbohydrates ingested are polysaccharides, disaccharides and very little amount of monosaccharides.
Which are the absorbed through intestinal epithelial cells?
polysaccharides, disaccharides, or monosaccharides?
monosaccharides
Thus all ingested carbohydrates must be digested to monosaccharides to be absorbed through the intestine.
Describe starch.
Major dietary carbohydrate is starch – is a mixture of both straight and branched-chain polymers of glucose.
Amylose – are the straight chain polymers.
Amylopectin - are the branched-chain polymers.
What are the disaccharides in food? What do they consist of?
The disaccharides in food are – trehalose, sucrose and lactose.
Trehalose – consists of two molecules of glucose.
Sucrose – consists of glucose and fructose.
Lactose – consists of glucose and galactose.
What are the monosaccharides?
Monosaccharides are – glucose, fructose and galactose.
Describe cellulose.
Cellulose (β1,4-linkage) no enzymes available to hydrolyze this linkage – cellulose is excreted.
Which sugar is excreted? Why?
Cellulose (β1,4-linkage) no enzymes available to hydrolyze this linkage – cellulose is excreted.
Which linkage is hydrolysed in alpha amylase?
hydrolysis of α 1,4 linkage only
What does digestion of a starch begin with?
Digestion of starch begins with alpha amylase.
Salivary amylase starts the process , but plays an insignificant role in a healthy individual, since amylases are inactivated by low gastric pH.
Which amylase is most significant? What bonds does it digest? What results?
Pancreatic amylase (most significant) digests internal alpha-1,4-bonds in starch – yielding a mixture of disaccharides, trisaccharides and oligosaccharides.
alpha-limit dextrins (oligosaccharide), maltose (disaccharides)and maltotriose (trisaccharide).
How are disaccharides further digested to monosaccharides? What is the product?
These disaccharides are further digested to monosaccharides by intestinal brush border enzymes:
alpha-dextrinase (isomaltase), maltase and sucrase.
The product of each of these is glucose (monosaccharide) – absorbed by the epithelial cells.
Draw a flow chart of carbohydrate digestion.
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The three disaccharides in food are – trehalose, sucrose and lactose. How is each digested? What is the end product?
These don’t require amylase digestion.
Trehalose – digested by trehalase – two molecules of glucose.
Lactose - digested by lactase – glucose and galactose.
Sucrose - digested by sucrase – glucose and fructose.
Glucose, galactose and fructose - the three end-products of carbohydrate digestion – are absorbed by intestinal epithelial cells.
Describe how glucose and galactose is transported in the cell on both apical and basolateral membranes . What transporter?
The symporter sodium/glucose transporter I (SGLT1) transports glucose and galactose against their concentration gradients by coupling their transport to that of Na+ (active process).
Glucose and galactose - extruded across basolateral membrane by facilitated diffusion by a transporter called GLUT2.
Fructose - transported across apical & basolateral membranes by facilitated diffusion
GLUT5 (apical) - fructose specific
GLUT2 (basolateral).
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What is lactose intolerance? What is lacking?
What happens if lactose is ingested?
Lack or deficiency of lactase in the brush border- lactose is not digested to glucose and galactose.
This reflects a normal developmental decline in the expression of lactase by enterocytes.
Present in 50% of adult and higher in some populations (Asians).
If lactose is ingested (in milk or milk products), lactose remains undigested and unabsorbed in the intestinal lumen – and retains water causing osmotic diarrhea.
Some patients benefit from taking bacterially derived lactase enzyme in a tablet form.
Describe congenital lactose intolerance. What is lacking?
(lack of jejunal lactase)– rare and very serious.
Replace lactose with a sucrose or fructose diet to avoid diarrhea and its consequences.
What causes glucose-galactose mal-absorption?
This is due to mutation of SGLT1 – very rare.
Can result in severe diarrhea and its consequences.
Fructose diet is recommended.
Describe protein digestion. Which aa are essential? (What does that mean?)
Of the 20 amino acids (AAs), some are called essential amino acids (in boxes)- which cannot be synthesized by the body - must be obtained from diet.
Where does protein digestion start and by what action? Where is it completed? How?
Protein digestion starts in the stomach with the action of pepsin.
It is completed in the small intestine with pancreatic and brush-border proteases.
Describe the two classes of proteases.
Endopeptidases – hydrolyze the interior peptide bonds of proteins- Example – pepsin, trypsin, chymotrypsin, elastase.
Exopeptidases – hydrolyze one amino acid at a time from the C-terminal ends of proteins and peptides - Example - carboxypeptidases A&B.
Describe pepsin. How is it derived? When is it active and inactive? (What places)
Can pepsin digest proteins?
Protein digestion starts in the stomach.
Pepsinogen (inactive precursor of pepsin) is released in response to a meal by gastric chief cells.
At low gastric pH, pepsinogen is activated to pepsin.
Pepsin is active at low pH and inactivated in the duodenum due to pancreatic HCO3-.
Pepsin cannot digest protein fully and yields a mixture of intact protein, large peptides and very little free amino acids.
Is pepsin necessary for normal protein digestion?
Pepsin is not essential for normal protein digestion, since in people whose stomach has been removed or who do-not secrete gastric H+, have normal protein digestion and absorption.
Protein digestion continues in the small intestine with combined actions of pancreatic and brush border proteases.
What are the five major pancreatic proteases that are secreted as inactive precursors?
trypsinogen chymotrypsinogen proelastase procarboxypeptidase A procarboxypeptidase B
What is the first step in intestinal protein digestion? By what enzyme?
First step in intestinal protein digestion is the activation of inactive trypsinogen to active trypsin – by brush border enzyme enterokinase.
What cleaves trypsinogen to yield active trypsin?
What does trypsin do?
Enterokinase cleaves trypsinogen to yield active trypsin.
Trypsin then catalyzes the conversion of all the other inactive precursors to their active enzymes.
It even autocatalyzes the remaining trypsinogen – to active trypsin.
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What enzymes does active trypsin yield?
What do they do?
trypsin chymotrypsin elastase carboxypeptidase A carboxypeptidase B
These pancreatic proteases digest dietary protein into amino acids, di- and tri-peptides and larger peptides called oligo-peptides.
