GI Physiology of digestion and absorption 1 (Carbohydrate and protein) Flashcards
What are the main constituents of food?
Carbohydrates
Lipids
Proteins
What different forms of carbohydrates can you get?
Starch
Cellulose (indigestible in humans - roughage)
Glycogen
Disaccharides (sucrose, lactose)
What forms of starch do you get?
Amylose and amylopectin
How much carbs a day do you usually get?
400g
Describe the types of lipids?
Triacylglycerols (approximately 90% of total lipid ingested as fats and oils) Phospholipids Cholesterol and cholesterol esters Free fatty acids Lipid vitamins
what are free fatty acids important for?
Free fatty acids - important as a source of energy and membrane strict and signal transduction
How much lipids do you get a day?
25-160g
What id digestion?
enzymatic conversion of complex dietary substances to a form that can be absorbed
Breakdown of complex molecules into simpler smaller molecules
Where does most digestion occur?
The small intestine
Describe the 2 ‘types’ of digestion?
Luminal and membrane digestion
Describe luminal digestion?(what is it mediated by?)
It is mediated by pancreatic enzymes secreted into the dueodenum
Describe membrane digestion? (what is it mediated by?)
It is mediated by enzymes situated at the brush border of the epithelial cells
What is absorption?
Absorption is the processes by which the absorbable products of digestion are transferred across both the apical and basolateral membranes of enterocytes (absorptive cells of the intestinal epithelium
Transceulluar transport? (CHECK)
Transport through basolateral membrane, apical membrane (Brush border) etc
What is the brush border ?
The apical membrane
How much energy of the total energy requirement does digestible carbohydrate provide?
45%
Describe the basic structure of amylose and amylopectin?
Amylose - unbranched chains, alpha 1,4-linkage
Amylopectin - branches, approx every 30 glucose molecule, alpha 1,6-linkages.
Describe Glycogen compared to amylopectin
More highly branched than amylopectin - branches every 10 glucose molecules
What provides roughage in our diet?
Cellulose - not digestable in humans, as they are linked by beta-1,4 linkages, thus it passes directly into the colon
Where might you find sucrose, what is it made of?
In table sugar
Glucose and fructose
Where might you find lactose, what is it made of?
Milk sugar
Glucose and galactose
What is alpha amylase, endoenzyme?
An endoenzyme, or intracellular enzyme, is an enzyme that functions within the cell in which it was produced.
What does alpha amylase do?
It breaks down linear internal alpha 1,4-linkages but NOT terminal alpha 1,4 linkages - therefore it cannot produce glucose
Can alpha amylase cleave alpha 1,6 linkages at branch points in amylopectin?
NO
What are Oligosaccharidases?
Oligosaccharidases are integral membrane proteins with a catalytic domain that faces the lumen of the GI tract
ON/IN MEMBRANE
What is an example of an Oligosaccharidases? (all brush border enzymes)
Lactase, Maltase, Sucrase, Isomaltase
What is lactose intolerance?
Relatively common digestive problem resulting from the inability to adequately digest lactose: caused by lactase insufficiency
What can lactose intolerance result from?
Primary lactase deficiency (primary hypolactasia) (MOST COMMON WORLDWIDE)
Secondary lactase deficiency (MOST COMMON IN UK)
Congenital lactase deficiency
Describe Primary lactase deficient (primary hypolactasia)?
due to lack of the lactase persistence (LP) allele – most common cause world wide
Describe secondary lactase deficiency?
caused by damage to/ infection of/ the proximal small intestine. Can develop later in life.
(e.g. crohns, UC etc)
Describe congenital lactase deficiency?
rare autosomal recessive disease – no ability to digest lactose from birth (Passed on by autosomal recessive)
- can overcome this, by giving babies artificial milk, lactose replaced by sucrose or another similar?
What happens as a consequence of lactase insufficiency?
Causes no disturbance unless lactose-containing food is consumed and activity of remaining enzyme is overwhelmed
What happens if lactose is delivered to the colon from the ileum colonic microflora?
Short-chain fatty acids (which can be absorbed)
Hydrogen (H2 - which can be detected in the breath of lactase deficient individuals following a lactose challenge)
Carbon dioxide
Methane
What do the by products of lactose ingestion in lactose intolerance cause?
Bloating
Abdominal pain
Flatulence
What can undigested lactose cause in lactose intolerance?
Acidification of the colon
An increased osmotic load – loose stools and diarrhoea
Where does Absorption of the Final Products of Carbohydrate Digestion: Glucose, Galactose and Fructose occur?
In the duodenum and jejunum
How does the absorption of the final products of carb digestion happen?
In a two-step process involving entry and exit from the enterocytes via the apical and basolateral membranes respectively
What are glucose and galactose absorbed by, and mediated by?
Secondary active transport mediated by SGLT1
What is fructose absorbed by, and mediated by?
By facilitated diffusion mediated by GLUT5
What GLUT receptors are expressed in the small intestine?
GLUT 1, 2 and 5 are all expressed in cells lining the small intestine
How is exit for all monosaccharides in the absorption of the final products of carbohydrate digestion mediated by?
facilitated diffusion by GLUT2
What is an enterocyte?
A cell of the intestinal lining
What must proteins be digested to to be efficient for absorption?
oligopeptides and amino acids
Describe digestion of proteins in the stomach?
HCL begins to denature proteins
Pepsin cleaves proteins into peptides
What cleaves proteins in the stomach?
Pepsin cleaves proteins into peptides
Describe digestion in the duodenum?
The five pancreatic enzymes are secreted into the duodenum (in the inactive form) from the EXOCRINE pancreas and converted to the ACTIVE forms in the duodenum.
They function as endopeptidases, or exopeptidase.
Is pepsin an endopeptidas, or exopeptidase?
Endopeptidase
How much of protein digestion is accounted for in the duodenum?
50% - compared to 15% in the stomach
Describe the brush border peptidases?
They are numerous
They have an affinity for larger oligopeptides
Describe cytoplasmic peptidases?
Less numerous
Primarily hydrolyse dipeptides or tripeptides
What form of carbohydrate is indigestible in humans?
Cellulose
What membrane faces the lumen and what one faces the interstitium in the small intestines?
Lumen - apical membrane (brush border)
Insterstitium - Basolateral membrane
What is the overall process of digestion and absorption called?
Assimilation
Give examples of polysaccharides?
Starch - amylose, amylopectin
Glycogen
Give examples of oligosaccharides and what is it?
Sucrose
Lactose
Usually containing a few monosaccharide molecules together
Give examples of monosaccharides?
Glucose
Fructose
What must all dietary carbohydrates be converted to for absorption?
Monosaccharides
What is the rough mode of operateion of SGLT1?
Sodium binds, increasing the affinity for glucose extracellularly, glucose and sodium then move intracellularly
Roughly what is the pathway of protein digestion?
Protein
Peptides
Amino acids
Amino acids in blood
Describe protein absorption of amino acids at the brush border?
There are at least 7 different mechanisms present
5 - are Na+ dependent co transporters (secondary active transport)
2 - are Na+ independent
Describe protein absorption of amino acids at the basolateral membrane?
There are at least 5 different mechanisms
3 - mediate efflux of amino acids and are NA+ independent
2 mediate influx of amino acids and are Na+ dependent
Describe protein absorption of Di-, tri-, and tetra-peptides
at the brush border?
H+ dependent mechanism
- they are further hydrolysed to amino acids within the enterocyte
What is protein digested to in the lumen and by what?
Amino acids or oligopeptides by pepsin and the pancreatic proteases
What does peptidases at the brush border to to oligopeptides?
They further hydrolyse oligopeptides to amino acids
