Transport along and across the GI tract Flashcards
Define digestion
An event leading up to absorption (transport across the gut lumen)
What does the colon do?
Colon will not absorb nutrients to any significant degree; a lot of water is absorbed; fermentation also occurs here.
What are proteins broken down by?
By gastric acid and broken down by pepsin in the stomach.
What is the breakdown of proteins dependant on?
- Proteases in the food – are the proteases damaged/undamaged? – effect of stomach acid on the neutralisation of the proteases
- Amount of stomach acid produced to denature the protein – protein must be unwound so that pepsin can act on it.
- Pepsin (active; inactive enzyme is called pepsinogen which can only be converted upon high acidity)- breaks down the protein into its smaller component chunks.
What pancreatic enzymes breakdown protein?
Pancreatic enzymes [e.g., trypsin, chymotrypsin, and elastase (elastase chops elastin fibres) continue the digestion of protein in the small intestine. The enzymes all act at different places in the aa sequences of the proteins.
The brush border cells of the intestine secrete peptidases, e.g. aminopeptidase and dipeptidase, which breakdown as into much small units for transport across the brush border cells and into the bloodstream.
What 2 initial things digest lipids?
Salivary and gastric lipases cause some fat digestion, but much more takes place in the small intestine.
What does pancreatic lipase and colipase do?
Pancreatic lipase (secreted into duodenum): breaks fats into smaller, absorbable units (monoglyceride, with 2 lipids still attached to the glycerine). Colipase is known to facilitate the action of pancreatic lipase on the small emulsion droplets that results from the action of bile salts
Where does digestion of carbohydrates first start?
Chewing food allows the food to mix with amylase and allows some digestion of carbohydrates to occur in the mouth.
If you do not chew your food/do not take supplemental enzymes, carbohydrates may enter the duodenum intact.
What cleaves carbohydrates in the duodenum?
In the duodenum, pancreatic amylase cleaves carbohydrates.
What does lactase, sucrase and maltase act on?
- Lactase acts on lactose → galactose and glucose
- Sucrase acts on sucrose → fructose and glucose
- Maltase acts on maltose → glucose and glucose
What does insufficient lactase do?
Insufficient lactase cause lactose intolerance; bacteria ferment the unbroken lactose → excess gas generation, accompanied by intestinal discomfort.
What cells absorb the nutrients in the SI?
The enterocytes make up the epithelium layer that covers the villi - hair-like projections of the small intestine- thin, highly vascularised, moist, etc.).
How can nutrients move through the small intestine?
- The transcellular route - across the plasma membrane of the enterocytes.
- The paracellular route - across tight junctions between the enterocytes.
Large organic (e.g. aas, glucose) molecules cannot enter the blood stream via the tight junctions of the paracellular route. Thus the transcellular route transports such molecules.
How are fats handled by the gut?
Fats behave differently in that instead of diffusing into the capillaries, they go into the lacteals (i.e., the lymphatic vessels present in each villus). They then drain from the intestine and flow into the blood stream via the lymphatic system.
Note that the epithelial tissue of the villi is not uniform and thus there is some variability in nutrient transport from the epithelium into the villus.
Each villus has different receptor sites for the uptake of specific nutrients. There is evidence that each protein fragment and each type of carbohydrate fraction has its own particular receptor for its absorption.
Also some nutrients enter the blood stream via the paracellular route (via simple diffusion)
Describe the uptake of carbohydrates into the blood stream
Glucose and galactose are taken into receptor sites found on the villi by co-transport with sodium using the same transporter called SGLUT-1 (sodium-dependent hexose transporter, on the intestinal wall)
SGLUT-1 will only transport the combination of glucose and sodium ion or galactose and sodium ion into the cell together - it does not transport either molecule alone.
