Digestion and Absorption Flashcards
Define digestion
The chemical breakdown of food by enzymes secreted by glandular cells in the mouth, chief cells in the stomach and the exocrine cells of the pancreas, or enzymes bound to the apical membranes of enterocytes.
Although some digestion of carbohydrates, proteins and fats occurs in the stomach, most occurs in the small intestine
What are cavital enzymes?
Enzymes secreted from the salivary glands, stomach and pancreas, have signifiant chemical degradation of food.
What are hydrolytic enzymes?
Enzymes associated with the small intestinal brush border. Hydrolysis by these enzymes is termed contact or membrane digestion
Where in the GIT does most digestion occur?
Small intestine
Define absorption
The movement of material from the intestinal lumen to the blood. Secretion implies movement in the opposite direction. The brush border is the barrier of movement
What is the glycocalyx layer?
Layer of glycoproteins on the outside of the microvilli
What is the unstirred layer?
Layer of intestinal secretions that sit on top of the microvilli, the first barrier
What is an enterocyte?
Cell of the intestinal lining
What percentage of calories digested are carbohydrates?
50%
Composition of carbohydrates
50% starch 30% sucrose 6% lactose 1-2% maltose trehalose, glucose, fructose, cellulose, sorbitol, hemicellulose and pectins make up the remainder
What does the starch compound consist of?
Two polysaccharides, amylose and amylopectin
What is amylose?
Straight chain polymer of glucose linked by a-1,4-glycolytic bonds. The repeating disaccharide unit is maltose
What is amylopectin?
Plant starch, major form of carbohydrate in the diet and is similar to amylose; however, in addition to the 1,4-glycosidic bonds, there is a 1,6-linkage for every 20-30 glucose units
What is glycogen?
High-molecular weight polysaccharide similar to amylopectin in molecular structure but with considerably more 1,6-linkages
How many glucose residues does the average alpha-limit dextrin contain?
5-10
Percentages of membrane enzymes that break down alpha-1,6 bonds in alpha-dextrin
95% isomaltase
5% glucoamylase
Percentages of membrane enzymes that break down alpha-1,4 bonds in alpha-dextrin
60% glucoamylase
20% sucrase
20% isomaltase
Percentages of membrane enzymes that break down maltotriose
50% isomaltase
25% sucrase
25% glucoamylase
Percentages of membrane enzymes that break down maltose
50% isomaltase
25% sucrase
25% glucoamylase
What is the enzyme that breaks down lactose into glucose and galactose?
Lactase
What is the enzyme that breaks down sucrose into glucose and fructose?
Sucrase
What is the enzyme that breaks down trehalose into glucose?
Trehalase
Is digestion or absorption the rate limiting step of carbohydrate digestion?
Absorption
What are the transporters involved in carbohydrate absorption?
GLUT2, GLUT5 and SGLT1
What does GLUT5 allow to enter the cell?
Fructose
What does GLUT2 allow to leave the cell?
Fructose, glucose and galactose
What does SGLT1 allow to enter the cell?
Glucose and galactose, is Na+ dependent, Na+ has to move out of the basolateral membrane via a Na+/K+ ATPase to maintain concentration gradient
Where is pepsinogen secreted from?
Chief cells of the crypt pits of the stomach
How is pepsinogen converted to pepsin?
Converted in the presence of H+, HCl secreted from the parietal cells in the stomach
What percentage of protein digestion occurs in the stomach?
Around 15%
What is the function of pepsin?
Breaks down proteins into smaller peptide fragments - optimum pH 1-3
At what pH does pepsin stop working?
pH 5
Where is the main site of protein digestion?
Duodenum
What does an endopeptidase do?
Splits proteins into smaller fragments by breaking bonds
What does an exopeptidase do?
Cleaves single amino acids off the end of proteins
What size peptides can be absorbed in the small intestine?
Small peptides - di and tripeptides
Single amino acids
What transporters are involved in the absorption of proteins?
PepT1, 3Na+/2K+ ATPase and NaH exchanger
What are most fats in the diet in the form of?
Triglycerides - 3FAs linked with glycerol
What is TAG hydrolysed by in the digestive tract?
Pancreatic lipase
What do bile acids to do to fats?
Solubilises larger fat molecule, increasing surface area for the action of lipase enzyme
What are micelles?
Monoglycerides and fatty acids associate with bile salts and phopholipids to form micelles. Cannot be absorbed
What types of fat can be absorbed?
Free fatty acids or monoglycerides pass through the cell membrane via diffusion
What happened to free fatty acids when they are in the cell?
Meet the smooth endoplasmic reticulum, which turns them back into triglycerides in order to maintain a concentration
