11G. Physiology of digestion and absorption Flashcards
where does digestion mostly occur
small intestine
what is digestion
Enzymatic conversion of complex dietary substances to a form that can be absorbed
what are the 2 types of digestion
luminal and membrane digestion
describe luminal digestion
mediated by pancreatic enzymes secreted into the duodenum
describe membrane digestion
mediated by enzymes situated at the brush border of epithelial cells
where is the brush border on an epithelial cell
apical membrane
what junction type is between two epithelial cells
tight
explain carbohydrate digestion fully
Start of Digestion: Intraluminal Hydrolysis
Carbohydrates like starch undergo hydrolysis by enzymes such as salivary amylase (in the mouth) and pancreatic amylase (in the small intestine).
These enzymes break starch into smaller units called oligosaccharides (such as maltose and maltotriose).
Oligosaccharides Stage
At this point, the oligosaccharides are not absorbed yet. Other dietary carbohydrates like lactose and sucrose join this pool of undigested molecules.
Membrane Digestion
At the brush border of the small intestine, enzymes called oligosaccharidases break down oligosaccharides into monosaccharides.
Specific enzymes like lactase, maltase, and sucrase-isomaltase act on lactose, maltose, and sucrose, respectively. This step ensures carbohydrates are ready for absorption.
Formation of Monosaccharides
The result of membrane digestion is the production of glucose, fructose, and galactose—the simplest forms of carbohydrates.
Absorption into the Body
These monosaccharides are absorbed into the bloodstream via specific transport mechanisms in the intestinal cells.
what is the main enzyme in carbohydrate digestion
pancreatic amylase
what does pancreatic amylase target in starch
alpha 1-4 glycosidic bonds and breaks them down to produce disaccharides
where does carbohydrate absorption occur
duodenum and jejunum
can you explain carbohydrate absorption
so at the end of digestion, carbs are now glucose and galactose.
Glucose and galactose enter the enterocytes apical membrane via SGLT2 secondary transport (jumping in with Na+ to go on sodiums gradient)
Then it exits the enterosite on the basolateral side via GLUT2 receptors into the blood
explain fructose in absorption please
fructose enters apical side via facilitated diffusion mediated by GLUT5 and then exits into blood via GLUT2
where does protein digestion begin
stomach
what is the process of protein digestion
pepsin breaks protein down into amino acids and oligopeptides
where is protein digestion completed
in the small intestine with pancreatic and brush border enzymes which split oligopeptides into amino acids, dipeptides and tripeptides
explain protein absorption
absorbed same way as glucose= SGLT then exit via facilitated diffusion
how are di and tripeptides absorbed
separate H+ dependent cotransporters and once inside cell they hydrolysed into amino acids
what is optimum pH for pepsin
1.8-3.5
name the 5 pancreatic proteases
trypsin, chymotrypsin, elastase, carboxypeptidase A and B
how many substrates does lactase have
Lactase has only one substrate – breaks down lactose to glucose and galactose
what provides 55-60 % of daily energy requirements
lipids
what are the major types of lipids
cholesterol, triglycerides (TAGs) and phospholipids
what are the challenges faced with lipid digestion and absorption
lipids are insoluble or poorly soluble in water
what makes absorption and digestion easier for lipids
when the are emulsified
explain the emulsification process for lipids
they need to be suspended in water
through : chewing, stomach, small intestine- bile salts
why do bile salts help emulsify
they are amphipathic and help stabilize these tiny droplets
