Digestion And Absorption Flashcards
During the gastric and cephalic phases of pancreatic secretion, mainly enzymatic with little aqueous secretion is stimulated. T/F?
True
Cholecystokinin (CCK) from I cells stimulates acinar cells to secrete pancreatic enzymes. What factors stimulate the release of CCK?
Amino acids, particularly Phenylalanine, methionin, tryptiphan
Small peotides
Fatty acids
ACh
Hydrogen ions, CCK and ACh can stimulate the ductal cells of the pancreas to secrete bicarbonate ions through the stimulation of which hormone?
Secretin
Erepsin is a brush border enzyme. What does it do?
Converts peptones and polypeptides to amino acids
Maltase is a brush border enzyme. What does it do?
Coverts maltose to glucose
Lactase is a brush border enzyme. What does it do?
Converts lactose to glucose and galactose
Sucrase is a brush border enzyme. What does it do?
Coverts sucrose to glucose and fructose
Why are sugars only broken down into monosaccharides in the brush border and not by other digestive enzymes?
If sugars were broken down into monosaccharides elsewhere in the digestive tract then this would create a huge osmotic pressure and draw water into the GI tract. By only having enzymes which can produce monosaccharides situated in the brush border, this prevents hypersecretion of water
Which enzyme converts trypsinogen into its active form?
Enterokinase
What is the action of the pancreatic enzyme trypsin?
Cleaves peptide bond on carboxyl side of amino acids
What is the action of the pancreatic enzyme chemotrypsin?
Breaks down proteins at their aromatic amino acids
What is the action of the pancreatic enzyme elastase?
Degrades elastin proteins
What is the action of the pancreatic enzyme lipase?
Degrades triglycerides into fatty acids and glycerol
What is the action of the pancreatic enzyme amylase?
Breaks down starch into maltose, maltitriose and dextrins
Describe carbohydrate digestion.
Most digested sugars are in the form of polysaccharides. These are broken down into monosaccharides for absorption. Salivary amylase breaks down starch into maltose, maltitriose and alpha limit dextrins. Pancreatic amylase continues this breakdown of polysaccharides into disaccharides. Sugars are only digested into monosaccharides by the enzymes of the brush border.
Describe the absorption of carbohydrates.
Sugars are absorbed as monosaccharides. Fructose is absrobed by facilitated diffusion with a GLUT transporter.
Glucose and galactose undergo secondary active transport coupled to sodium ions.
These are then transported out of the enterocyte via facilitated diffusion and they diffuse into the surrounding blood capillaries
Describe the digestion of proteins
Pepsin in the stomach first breaks down proteins into peptide fragments, trypsin and chymotrypsin (among other enzymes) will then further break down proteins in the small intestine in to small peptide fragments. Some of these digestive enzymes are capable of digesting proteins into individual amino acids. These include enzymes in the brush border - there are many different peptidases in the brush birder as each is specific to the amino acid that it cleaves
Describe the absorption of proteins.
Small peptide chains can be absorbed via secondary active transport coupled to hydrogen ions.
Individual amino acids can be absorbed via secondary active transport coupled to sodium ions.
Hydrolysis inside the enterocyte breaks down any peptide chains into individual amino acids which leave the cell via facilitated diffusion and are absorbed into the capillaries
Describe the digestion of fats
Lipase is the main digestive enzymes of fat but as it is water soluble it can only work on the surface of fat molecules. Thus, fat is broken down by mechanical digestion into smaller droplets and is emulsified by bile to aid digestion. The formation of micelles further aids this process. Lipase also uses the co enzyme colipase.
Describe the absorption of fat
Fats are absorbed via diffusion as free fatty acids. Once inside the cell fatty acids are produced into chylomicrons and this helps maintain the concentration gradient to allow more fatty acid absorption into the enterocyte. Chylomicrons are then released from the cell and diffuse into the surrounding lacteals where the travel in the lymphatic system to the thoracic duct where they join the bloodstream
Describe the absorption of vitamins
Fat soluble vitamins - A, D, E and K follow the absorption pathway for fats
Water soluble vitamins other than B12 are absorbed by diffusion.
Vitamin B12 is very large so must be bound to intrinsic factor in order to be absorbed and most of its absorption usually occurs in the terminal ileum
