Unit 2 - Digestion/ Absorption Flashcards
How are carbohydrates digested?
- polysaccharides digested by salivary amylase
- majority digested by pancreatic amylase - occurs in duodenum (breaks down into oligosachharides by splitting 1,4 glycosidic bond)
- brush border enzymes - breaks down into monosaccharides
give example of brush border enzymes
maltase
sucrase
lactase
dextrinase
if there is a low concentration of glucose, how are carbohydrates absorbed?
absorbed into enterocytes across apical surface by SGLT1 then across basolateral surface by GLUT2
if there is a high concentration of glucose, how are carbohydrates absorbed?
SGLT1 are saturated so absorbed via GLUT2 through apical and basolateral surface
how are monosaccharides transported to liver?
in blood via hepatic portal vein
how are proteins broken down?
- gastric digestion by proteolytic enzymes which are secreted in their inactive form: pepsinogen and then activated in acidic environment (lumen of stomach) to pepsin
- pancreatic digestion: trypsinogen/ chymotrypsinogens which are activated in response to alkaline environment - converted by enterokinase
- further digestion by exopeptidases
how are proteins absorbed?
by enterocytes at brush border via carrier protein then exported into extracellulr space across basolateral membrane
- M cells package proteins into vesicles
how are lipids digested?
- emulsification by bile salts initially: hydrophilic regions of bile salts repel so droplets are formed
- lipolytic enzymes from pancreas digest lipids - secreted in their inactive form but activated by trypsin in duodenum
give two examples of lipolytic enzymes
- phospholipase A2- cleaves fatty acids
- colipase - anchors lipase to micelles= maximal SA
how are lipids absorbed?
micelles pass across brush border - re-esterified in endoplasmic reticulum and form chylomicrons - released by exocytosis and join lacteal duct
what are the three stages of digestion?
cephalic
gastric
intestinal
describe the cephalic phase
neurogenic signals from cortex/ appetite centres relayed to stomach via parasympathetic efferent responses - vagal stimulation increases salivary secretion and vasodilation
describe the gastric phase
distension of stomach walls activates mechanoreceptors which activates local enteric reflexes and vago-vagal reflexes -which leads to release of gastrin from G cells -
= causing release of HCl acid by parietal cells
= release of pepsinogen by chief cells
= increased motility
in the intestinal phase why is secretin released?
HCl acid from stomach passes into small intestine - secretin inhibits acidity and stimulates release of bicarbonate rich solution by acinar cells of duodenum
what causes the release of CCK? and what is the effect of CCK?
fat in chyme
= inhibits stomach motility and release of gastrin
= stimulates release of pancreatic enzymes (lipase)
= causes contraction of gall bladder