GI 2: Digestion And Absorption: Small Intestine

Question 0

Luminal and membrane digestion
Describe luminal digestion
And membrane digestion

Answer 0

Luminal digestion is due to enzymatic secretion from the pancreas
-these enzymes cleave large polymers (protein polypeptides, starch) into smaller (smaller peptides and polysaccharidess/oligosaccharides)

Membrane digestion occurs in the enterocyte membrane.
Essentially these membrane enzymes cleave the smaller polymers into monomeric molecules (monosaccharides and aa) that can be transported into the cell.

Question 1

Describe the structure of the small intestine

Answer 1

• small intestine has folds on the surface
• small intestine is lined by finger like processes (villi) that are covered by epithelial cells (brush boarder cells) that in turn have micro villi in the external lumen membrane
• the combination of fold, villi and micro villi give huge surface area needed for absorption

Question 2

Pancreatic enzymes
What are its functions?
What are the enzymes secreted?

Answer 2

So what happens is when stomach contents enters duodenum, the acid tells the duodenum to release secretion and cholecystokinin into blood which tells the pancreas to release its pancreatic juice.
-pancreatic juice is alkaline and neutralises the HCL. This makes pH more alkaline so its suitable for pancreatic enzyme activity and terminates pepsin activity.

Pancreatic juice enzymes include 3 endopeptidases:

• trypsin
• chymotrypsin
• elastase.

The pancreatic enzymes enter small intestine as inactive proenzymes (eg tripsinogen).
-enteropeptidase- an enzyme secreted by the small intestine that cause the activation of trypsin.
This activation then causes a cascade which activates all proenzymes. This is designed so active enzymes are only present in intestinal lumen protection from autodigestion.
ie
Enteropeptidase triggers
Trypsinogen➡ trypsin
Chymotrypsinogen➡ chymotrypsin
Procarboxypeptidase➡ carboxypeptidase
Proelastase➡ elastase

Question 3

Protein digestion and absorption: complete

Answer 3

Protein digestion begins in stomach were HCL denatures proteins and pepsin starts to cleave the proteins

• the partially digested proteins passes into small intestine
• pancreatic juice secreted into small intestine contains endopeptidases AND exopeptidases
• the endopeptidases cleave the middle of the polypeptide
• an exopeptidases is attached to lumen side of small intestinal , they cleave of the end aa
• aa are then transported across the cell membranes of the brush boarder. There is a transport pump which uses the Na gradient to drive the uptake of amino acids

Question 4

Carbohydrate digestion

Answer 4

Aim of digestion is to hydrolyse carbs to monosaccharides

Lactose is cleaved by enzyme lactase to galactose and glucose
Sucrose is cleaved by the intestinal enzyme sucrase to glucose and fructose which are absorbed

Glucose units are linked by glycosidic bonds

• a-amylase (from salivary and pancreatic juice) is an endoenzyme that attacks glycosidic bonds.
• absorption of glucose into epithelial cells is driven by co-transport using the Na+ gradient
• fructose is absorbed by the Na+ independent passive transport system

Question 5

Digestion and absorption of fat

Answer 5

The main site of fat digestion is in small intestine
-limited activity of gastric lipases in adults coz of the low pH

Triacolglycerol (TAG) isn’t soluble in water and can’t be absorbed so needs to be digested into NEFA and monoacylglycerol (MAG)

• main digestion of fat occurs in small intestine by pancreatic lipase which cleaves the primary ester bonds
• the avilable SA is increased by the emulsification of fat by bile acids
• MAG and NEFA in the presence of bile salts form micelles
• micelles allow lipid digestion products to diffuse into the epithelial cell
• the bile salts in the micelle are partially reabsorbed and transported back to the liver.
• in the cell, the MAG and fatty acids are re-ester field to produce TAG
• inside the enterocyte, TAG and cholesterol are arranged in membrane vesicles called chylomicrons.
• chylomicrons are absorbed into the lymphatic ducts (the lacteals) draining the villi