Digestive and absorptive process of small intestine Flashcards
Explain the digestive functions of the GI tract
Localised to different areas of the gut where:
The optimal pH of enzymes differs
Some enzymes may be secreted as inactive precursors and need enzymatic activation (true of all proteases to prevent digesting yourself)
Some enzymes exist as membrane bound forms (sit on brush border)
Objective: to break macromolecules down into monomer or dimer units for absorption
What are the absorptive processes of the GI tract?
Mainly in small intestine, but water also in the colon
Requiring a variety of transport mechanisms specific for particular molecules
If transport is against a gradient, energy must be used (primary or secondary active transport)
Give a reminder of the structure of the small intestine
Lots of villi to increase SI.
Single epithelial layer so macromolecules have short diffusions distance.
Goblet cells to secrete mucus to lubricate food.
Enterocytes absorb the nutrients. Stem cells at base of crypt to differentiate at the trans amplifying zone. These cells renew every 3-4 days as they are exposed to pathogens and get damaged.
Lymph system is important for fat absorption. Fat does not go straight into the blood system, they are absorbed into lacteal and then transported out.
Brush border has microvilli where there are lots of transporters (for absorption of solutes) and digestive enzymes which are made by the enterocyte to break down food stuff.
What is the unstirred layer in the small intestine?
a layer which does not get churned about even with passing food stuff. This is an added protective layer as food has to diffuse across it
What cell junctions are there in the small intestine?
Tight junctions for a barrier function
What is the main maintenance function of the small intestine?
What are transporters required for here?
Maintains Na+ gradient which is required by many transporters and created osmotic gradient in intracellular space, which drives water absorption from lumen.
Required for absorption of many solutes (products of digestion).
Explain the start of protein digestion
Starts with pepsin and HCl in stomach.
Chief cell release inactive pepsinogen. Becomes active in an acidic environment and formed pepsin. This breaks the proteins into small peptides.
Acidic environment created by parietal cell.
What pH is chyme and what’s does it contain?
What else is required in the stomach for digestion of food?
Chyme has a LOW pH and contains
– Solubilised, slightly digested carbohydrates
– Solubilised, partly digested proteins
– Slightly digested fats
• other enzymes are supplied (from pancreas and made by enterocytes)
• pH has to be neutralised,
bile salts needed for fat digestion
What is the second stage of protein digestion?
Enterokinase on brush border of duodenal enterocytes activates pancreatic trypsinogen (a specific inactive enzyme) which in turn activates proteolytic enzymes of the pancreas.
- trypsinogen activates trypsin which then can break down the proteins
Give the 3 specific enzymes in protein digestion and explain what they do
1) Pancreatic trypsin, elastase & chymotrypsin (endo- peptidases) cleave peptide bonds in protein interior, producing short peptides (chunk up the proteins)
2) Pancreatic carboxypeptidases (exopeptidase) remove amino acids from the carboxyl ends
3) Aminopeptidases (exo- peptidase) on brush border of enterocytes remove amino acids from the amino terminal of peptides
How are proteins and amino acids absorbed?
End up near the brush borders with dipeptides. Di and tri peptides are absorbed using H+ transporters. Into enterocytes they are broken down into free amino acids with the enzymes found here to then be absorbed into blood stream.
Free amino acid form can go into the enterocyte via carriers with sodium cotransporter and into the blood stream. They are absorbed via Na+ lumen transporters and exit on the basolateral side.
What is infant SI permeable to peptides?
Allows absorption of growth factors and antibodies.
What form do we want carbohydrates to be absorbed?
Monosaccharides
Explain the 3 places where carbohydrates are digested
- Salivary amylase:
optimum pH is 6.8, so some hydrolysis in the mouth
Swallowed bolus then enters acidic stomach (salivary amylase then stops working here) - Pancreatic amylase:
optimum pH is 7.1 - Small intestine membrane-bound disaccharides:
optimum pH valves between 6 and 7
Duodenal pH therefore suitable for (2) and (3)
Explain the pancreatic amylase role in carbohydrate digestion
It cleaves straight chains but cannot cleave bonds at branch points.
Produces short oligosaccharides, maltose and maltriose.