Physiology of carbohydrate and protein absorption Flashcards
What are the main constituents of food?
Carbohydrates, lipids and protein
What are the carbohydrates?
Starch - amylose and amylopectin
Celluose
Glycogen
Disaccharides (sucorse and lactose)
Which carbohydrate is indigestible in humans?
Cellulose
What are the lipids?
Triacylglycerols Phospholipids Cholesterol and cholesterol esters Free fatty acids Lipid vitamins
What are the sources of proteins?
Ingest protein and from within the body
Where does the protein from within the body come from?
Cells shed, go into the lumen of the digestive tract and are digested
What is the definition of digestion?
Enzymatic conversion of complex dietary substances to a form that can be absorbed
What are the two overall process that digestion can be broken down into?
Luminal digestion and membrane digestion
What is luminal digestion?
The first stage of digestion. Mediated by pancreatic enzymes secreted into the duodenum
What is membrane digestion?
The products of luminal digestion are passed to the enzymes embedded int he apical membrane of the enterocytes. Mediated by enzymes situated at the brush border of epithelial cells
What is absorption?
The process by which the absorbable products of digestion are transferred across the apical and basolateral membranes of enterocytes
What are enterocytes?
Absorptive cells of the intestinal epithlium
What are the two general stages of absorption?
Digestive products must leave the lumen and enter the enterocytes (cross apical membrane)
Then they must exit the enterocyte and enter the bloodstream (cross the basolateral membrane)
What is the name for the overall process of digestion and absorption?
Assmiliation
What kind junctions are between enterocytes?
Tight junctions
What dietary component is so simple that is does not require digestion?
Monosaccharide glucose
By what type of digestion is protein broken down to enter the epithelium?
Luminal hydrolysis of polymer to monomers
Protein –> amino acids
By what type of digestion are disaccharides broken down to enter the epithelium?
Brushborder hydrolysis of oligomer to monomer disaccharide –> monosaccharides
What is an example of intracellular hydrolysis?
Short chain peptides not being broken down into amino acids until in the epithelium (pass from the lumen to the epithelium has whole peptides)
What is an example of luminal hydrolysis followed by intracellular resynthesis?
Triglycerides –> fatty acids + glycerol to enter the epithelium
Within the epithelium fatty acids + glycerol –> triglycerol which enters the interstitial space
What are the 3 different types of carbohydrate?
Polysaccharides
Oligosaccharides
Monosaccharides
What are the plant and animal polymers of glucose (polysaccharides)?
Starch (plant) and glycogen (animal)
What are the two different types of starch?
Amylose and amylopectin
What is the structure of amylose
Chain of a-glucose molecules
Joint by a-1,4-glycosidic bonds
What is the structure of amylopectin?
Branched structure
a-1,4-glycosidic bonds
Branches are connected by a-1,6-glycosidic bonds
What are two oligosaccharides?
Sucrose and lactose
What is sucrose composed of and what are the bonds?
Glucose and fructose
a-1,2 linkages
What is lactose composed of and what are the bonds?
Glucose and galactose
B-1,4 linkages
Give two examples of monosaccharides?
Glucose and fructose
What are the 3 different types of carbohydrate/
Polysaccharides
Oligosaccharides
Monosaccharides
What are the plant and animal polymers of glucose (polysaccharides)?
Starch (plant) and glycogen (animal)
What are the two different types of starch?
Amylose and amylopectin
What enzymes break down oligosaccharidases in the brush border)
Lactase
Maltase
Sucrase -isomaltase (2 enzymes linked together)
What is the structure of amylopectin?
Branched structure
a-1,4-glycosidic bonds
Branches are connected by a-1,6-glycosidic bonds
What are two oligosaccharides?
Sucrose and lactose
What are the products of a-amylase breakdown?
Linear glucose oligomers (maltotriose, maltose) and a-limit dextrins
What is lactose composed of and what are the bonds?
Glucose and galactose
B-1,4 linkages
Give two examples of monosaccharides?
Glucose and fructose
What is the sequence of carbohydrate digestion?
Intracluminal hydolysis = polysaccharide –> oligosaccharides
Membrane digestion =
oligosaccharides –> monosaccharides
then absorbed
What enzymes converts starch and glycerol to oligosaccharides?
a-amylase
salivary and pancreatic
What oligosaccharides does a-amylase break the polysaccharides down into?
a-limit dextrins
Maltotriose
Maltose
What enzymes break down oligosaccharidases?
Lactase
Maltase
Sucrae-isomaltase
What type of enzyme is a-amylase?
Endoenzyme
What is the action of a-amylase?
Breaks down linear internal a-1,4-linkages but not terminal a-1,4-linkages (no production of glucose)
Cannot cleave a-1,6-linkages or a-1,4-linkages adjacent to branch points.
What are the products of a-amylase breakdown?
Linear glucose oligomers (maltotriose, maltose) and a-limit dextrins
What is the role of individual oligosaccharidases?
They are enzymes that are integral membrane proteins with an active site that faces the lumen of the GI tract
Name four oligosaccharidases
Lactase
Maltase
Sucrase
Isomaltase
What does lactase act on?
Breaks down lactose to glucose and galactose
What is the action of all oligosaccharides (other than lactase)?
They cleave the terminal a-1,4-linkages of maltose, maltotriose and a-limit dextrins to produce glucose
What is the additional action of maltase?
Can degrade the a-1,4 -linkages in straight chain oligomers up to nine monomers in length is the action of sucrase?
What is the action of sucrase?
Specifically responsible for hydrolysing sucrose to glucose and fructose
What is the action of isomaltase?
It is the only enzyme that can split the branching a-1,6-linkages of a-limit dextrins
What is the rate limiting step in maltase, sucrase and isomaltase assimilation?
The transport of the released monomers
hydrolysis is faster than transport of the released monomers
What is the rate limiting step in lactase assimilation?
Hydrolysis
What is lactose intolerance?
Digestive problem resulting from the inability to adequately digest lactose
It is caused by lactase insufficiency
Explain the concept of lactase persistence?
Lactase is lost in other mammals following weaning but in humans there is a variable degree of lactase persistence.
What is primary lactase deficiency (primary hypolactasia)?
Lack of lactase persistence (LP) allele (most common cause)
What is secondary lactase deficiency?
Caused by damage to/infection of the proximal small intestine
What is congenital lactase deficiency?
Rare autosomal recessive disease resulting in an inability to digest lactose from birth
When does hypolactasia cause disturbance?
When lactose-contain food is consumed and the activity of the remaining enzyme is overwhelmed
What happens when lactose enters digestion when there is poor lactase activity?
Lactose is delivered to the colon from the ileum colonic microflora. The lactose is hydrolysed by the bacteria to short-chain fatty acids which can be absorbed across the colon.
In the process of breakdown, hydrogen, carbon dioxide and methane are produced
What are the clinical signs of lactose breakdown by the microflora?
H2 in the breath
Bloating (gas build up)
Abdominal pain (distension)
Flatulence (release of gas)
What does undigested lactose cause?
Acidification of the colon
An increased osmotic load - loose stools and diarrhoea
Where does absorption of the final products of carbohydrate digestion take place?
Duodenum and jejunum
How are glucose and galactose absorbed?
Secondary active transport mediated by SGLT1
One glucose enters the cells along with 2 sodium ions. ENergy comes from the movement of Na+ down its concentration gradient
How is fructose absorbed?
Facilitated diffusion mediated by GLUT5
How is exit of all monosaccharides mediated?
By facilitated diffusion by GLUT2
What is protein broken down into for absorption?
Oligopeptides and amino acids
What beings to denature proteins in stomach digestion?
HCl
What is the function and action of pepsin?
Cleaves proteins into peptides
Optimum pH of 1.8-3.5
It is an endopeptidase
Not essential for protein digestion
Name the 5 pancreatic proteases that are secreted as proenzymes
Trypsin Chymotrypsin Elastase Procaroxypeptidase A Procarboxypeptidase B
Name the endopeptidase proteases
Trypsin
Chymotrypsin
Elastase
Name the exopeptidase proteases
Procaroxypeptidase A
Procarboxypeptidase B
What are the products of endopeptidase digestion?
Oligopeptides (2-6 amino acids)
What are the products of exopeptidase digestion?
Single amino acids
What are endopeptidases?
Enzymes that break peptide bonds other than the terminal ones in a peptide chain
What are exopeptidases?
An enzyme which breaks the terminal peptide bond in a peptide chain
Where are additional proteases present?
Brush border and the cytoplasm of the enterocyte
What do brush border peptidases break down?
Larger oligopeptides (3-8 amino acids)
What do cytoplasmic peptidases have an affinity for?
Dipeptides and tripeptides
By what mechanisms are amino acids absorbed across the brush border?
7 mechanisms
5 Na+ dependent co-transporters mediating ‘uphill’ movement (secondary active transport)
2 are Na+ independent
How are amino acids absorbed across the basolateral membrane?
5 different mechanisms
2 mediate efflux of amino acids and are Na+ independent
2 mediate influx and are Na+ dependent
How are di-, tri- and tetra-peptides absorbed?
Via H+ dependent mechanism at brush border (co-transport)
What happens to di-, tri- and tetra-peptides once they have be absorbed into the enterocyte?
They are further hydrolysed to amino acids within the enterocyte
Leave by Na+ independent systems at the basolateral membrane (facilitated transport)
