Flashcards in Physiology of Carbohydrate and Protein digestion and absorption Deck (38):
What is digestion?
Digestion – enzymatic conversion of complex dietary substances to a form that can be absorbed
How do most digestive processes occur in the small intestine?
As Luminal digestion - mediated by pancreatic enzymes secreted into the duodenum,
Or Membrane digestion - 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)
What is Amylose?
Glucose monomers joined together with alpha 1,4 linkages
What is amylopectin?
Glucose monomer joined together with alpha 1,4 linkages with branches by alpha -1,6 linkages
What are the componenets of starch?
Amylose and Amylopectin
What is the structure of glycogen?
Glycogen is more highly branched than amylopectin. It has alpha -1,4 and alpha -1,6 linkages
What is sucrose composed of and what is its structure?
Glucose and Fructose. Contains alpha 1,2 linkages
What is lactose composed of and what is its structure?
Glucose and Galactose. Contains Beta 1,4 linkages.
What are the oligosaccharides?
Lactose and Sucrose
What are the monosaccharides?
Glucose and fructose
Describe the breakdown of starch?
Starch first undergoes intaluminal hydrolysis. Which is carried out by alpha amylase (both salivary and pancreatic)
This breaks starch down into Oligosaccharides - these are not absorbed. These are acted on by oligosaccharidases, breaking them down into Monosaccharides
These are then absorbed.
What are the Oligosaccharidases?
What is an endoenzyme and what is an example?
An enzyme that works within the cell it was produced in.
e.g. Alpha Amylase
Describe how alpha amylase works?
breaks down linear internal α-1,4 linkages but not terminal α-1,4 linkages. Hence, no production of glucose
cannot cleave α-1,6 linkages at branch points (in amylopectin) or α-1,4 linkages adjacent to branch points
Products: linear glucose oligomers (maltotriose, maltose) and α-limit dextrins
Describe how lactase works?
Lactase has only one substrate – breaks down lactose to glucose and galactose
Describer how Maltase works?
Maltase can degrade the α-1,4 linkages in straight chain oligomers up to nine monomers in length
Describe how Isomatase works?
Splits the branching alpha-1,6 linkages of alpha limit dextrins
What can lactose internalise result from?
Primary lactase deficiency (Primary Hyolactasia)
Secondary lactase deficiency
Congenitcal lactase deficiency
What is lactose intolerance?
The inability to digest lactose.
What are the symptoms of lactase insufficiency/ Hypolactasia?
In lactase insufficiency, 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)
Where does is glucose, galactose and fructose absorbed?
Duodenum and Jejunum
How is glucose and galactose absorbed?
Absorbed by secondary active transport mediated by SGLT1
How is fructose absorbed?
Absorbed by facilitated diffusion mediated by GLUT5
How do all monosaccharides exit the cell?
Exit for all monosaccharides is mediated by facilitated diffusion by GLUT2
How does the SGLT 1 receptor work?
1. 2 Na+ binds
2. Affinity for glucose increases, glucose binds
3. Na+ and glucose translocate from extracellular to intracellular: 4. 2 Na+ dissociate, affinity for glucose falls
5. Glucose dissociates
6. Cycle is repeated
How does protein digestion occur?
Protein is digested to Oligopeptides and then to amino acids for absorption. There are 4 different methods of protein digestion, there is a role of end and exo-peptidases
How does digestion occur in the stomach?
HCl begins denature of proteins
Pepsin cleaves proteins into peptides
What are the optimum conditions for Pepsin?
Optimum pH for 1.8-3.5
It is inactive in alkaline pH
What are the digestive enzymes secreted by the duodenum?
These can function as an endopeptidase or a exopeptidase
Where are additional proteases located other than in the duodenum?
At the brush border
Within the cytoplasm of the enterocyte
By what mechanism does the brush border use for protein absorption?
Brush border – at least 7 different mechanisms are present
5 are Na+-dependent co-transporters mediating ‘uphill’ movement (secondary active transport)
2 are sodium independent.
What is an example of a Na+ sodium dependent brush border protein absorption pathway?
system B0AT1 (SLCA19) – mediates uptake of neutral amino acids (dysfunction results in Hartnup disease)
What is an example of a Na+ sodium independent brush border protein absorption pathway ?
example system b0+AT (SLC7A9/SCL3A1 dimer) – mediates uptake of cationic amino acids (dysfunction results in cystinuria)
How does the Basolateral membrane absorb protein?
Basolateral membrane – at least 5 different mechanisms: 3 mediate efflux of amino acids and are Na+-independent: 2 mediate influx
How are di, tri and tetra peptides absorbed?
via H+-dependent mechanism (PepT1, SLC15A) at brush border (co-transport)