digestion and absorption Flashcards
what is the definition of digestion?
the enzymatic conversion of complex dietary substances to a form that can be absorbed
where does most digestion occur?
small intestine
what does digestion in the small intestine occur as?
luminal digestion
membrane digestion
what mediates luminal digestion?
pancreatic enzymes secreted from the duodenum
what mediates membrane digestion?
enzymes at the brush border of epithelial cells
what is absorption?
the process by which the absorbable produces of digestion are transferred across both apical and basolateral membranes of enterocytes
what do carbohydrates need be broken down into to be sufficiently absorbed?
monosaccharides
what percentage of total energy intake comes from carbs?
45%
what percentage of carbs energy comes from polysaccharides?
45-60
examples of polysaccharides from diet
plant = starch - amylose and amylopectin
animal = glycogen
what percentage of carbs energy comes from oligosaccharides?
30-40
examples of oligosaccharides from diet
sucrose - table sugar
lactose - milk sugar
what is sucrose broken down into?
glucose nd fructose
what is lactose broken down into?
glucose and galactose
what percentage of carbs energy comes from monosaccharides?
5-10
examples of monosaccharides from diet
glucose and fructose
how is starch broken down to oligosaccharides?
intraluminal hydrolysis by alpha-amylase (salivary and pancreatic)
how are oligosaccharides broken down to monosaccharides?
by membrane digestion at the brush border by oligosaccharidases
examples of oligosaccharidases
lactase
malatse
sucrase-isomalatse
examples of oligosaccharides not from diet
alpha-limit dextrins
maltotiose
maltose
examples of monosaccharides not from diet
galactose
what does alpha-amylase do?
can break the linear internal alpha 1-4 linkages
what is alpha-amylase unable to do?
break down terminal linkages
- can’t produce glucose
can’t cleave alpha 1-6 linkages at branches
can’t cleave alpha 1-4 linkages adjacent to branch points
what are the products of intraluminal hydrolysis of starch by alpha-amylase?
linear glucose oligomers
alpha-limit dextrins
what is alpha-amylase?
an endoenzyme
what are oligosaccharidases?
integral membrane proteins with catalytic domain facing the lumen of the GI tract
lactase only has 1 substrate, what does this mean?
it can only convert lactose into glucose + galactose
what do oligosaccharidases, except lactase, do?
cleave terminal alpha 1-4 linkages of maltose, mlatotriose and alpha-limit dextrins
yields glucose
what does maltase do?
degrades alpha 1-4 linkages in straight chain oligomers up to 9 monomers in length
what does sucrase do?
hydrolysis sucrose to form glucose and fructose
what makes isomaltase special?
only one that ca split the branching at alpha 1-6 linkages of alpa-limit dextrins
what is the hydrolysis rate of maltase, sucrase and isomaltase?
faster than the rate of transport of released monomers
what is the hydrolysis rate of lactase?
the rate of hydrolysis is rate limiting is assimilation
where does absorption occur?
duodenum and jejunum
what are the 2 steps of absorption?
entry and exit from enterocytes
via the apical and basolateral membranes
how are glucose and galactose absorbed?
by secondary active transport
mediated by SGLT 1
how is fructose absorbed?
by facilitated diffusion
mediated by GLUT-5
how do all monosaccharides exit enterocytes?
