Absorption in the GI Tract Flashcards
describe the roles of the sites of absorption
mouth, esophagus, stomach:
- minimal absorption
small intestine:
- main site of absorption (90% of all water and sodium, all nutrients)
- bulk abosrption in ‘leaky’ epithelium
large intesine:
- 9% of water and sodium
- regulated/fine tuning absorption in ‘tight’ epithelium
what are the 5 factors that affect absorption
- reduction nutrient size by chemical digestion
- motility
- transport across GIT epithelium
- paracellular pathway: tight junctions
- transcellular pathway: membrane channels and transporters - surface area available for absorption
- removal from interstitial fluid
describe the second factor that affect absorption - motility
food and chyme just travel at the correct rate through the GIT to allow:
- mechanical digestion, chemical digestion, absorption
The rate of food/chyme travels through the GIT depends on:
- storage: stomach and large intestine
- peristalsis: stomach, small and large intestines
motility also affects the exposure of the products of digestion (nutrients) to absorptive surfaces
- segmentation
describe the third factor that affect absorption - transport across GIT epithelium in the PARACELLULAR PATHWAY
between the epithelial cells:
- particles do not cross cell membranes
- the only barrier is the tight junctions
Relatively non-selective:
- if the tight junctions are ‘leaky’ and the particle is small enough it can move via the paracellular pathway
- if the tight junctions are ‘tight’ particles are unable to move via the paracellular pathway
Passive movement by diffusion:
- requires a gradient as the diving force: osmotic gradient (for movement of water), electrical, chemical (concentration) or both gradient
describe the third factor that affect absorption - transport across GIT epithelium in the TRANSCELLULAR PATHWAY
Through the epithelial cells:
- particles must cross TWO cell membranes
Lipid soluble particels:
- diffuse through the membrane
- requires a gradient as the driving force: chemical/concentration gradient
Water soluble particles:
- require a channel or transporter brane protein to cross the membrane
- requires a driving force
- ATP: primary active transport
- osmotic gradient: for movement of water
- electrical, chemical or electrochemical gradient
describe the fourth factor that affect absorption - affect of surface area
the rate of absorption of a particle is proportional to the surface area
- greater the surface area the faster the rate of absorption
anatomical adaptions maximise surface area
to maximise absorption within the surface area available
- reduce nutrients into their smallest unit via chemical digestion (proteins to AAs, starch to glucose, triglcyerides to FFAs and monoglycerides)
- use specific transport proteins to absorb particles
- allows active transport: movement of particles against a gradient
describe the fifth factor that affect absorption - removal of interstitial fluid
prevent buildup of particles in the interstitial fluid after absorption
- high blood flow to intsestines
- blood vessles and lacteals close to the basolateral side of epithelial cells
describe the mechanisms of absorption for carbohydrates
all absorbed in the small intesine
1. passive absorption bia the paracellular pathway
- monosaccharides
- driving force: glucose molecules diffuse down their concentration gradient (driving force) across the ‘leaky’ tight junctions
2. active absorption via the transcellular pathway
- monosaccharides
- driving force: sodium gradient generated by Na+/K+ ATPase (on basolateral membrane)
transported across the cell membranes
- apical membrane: secondary active transport via sodium-glucose cotransporter
- basolateral membrane: facilitated diffusion bis glucose carrier/transporter
describe the mechanisms of absorption for proteins: amino acids
all absorbed in the small intestine
1. passive absorption via the paracellular pathway
- amino acids
- driving force: amino acids diffuse down their concentration gradient (driving force) across the ‘leaky’ tight junctions
2. active absorption bia the transcellular pathway
- amino acids
- driving force: sodium gradient created by Na+/K+ ATPase (on basolateral membrane)
transported across the cell membranes
- apical membrane: secondary active transport via sodium-amino acid cotransporter
- basolateral membrane: facilitated diffusion via amino acid carrier/transporter
describe the mechanisms of absorption for proteins: di- and tri- peptides
all absorbed in the small intestine
only ACTIVE absorption the transcellular pathway
- di- and tri- peptides
- driving force: negative membrane potential generated by Na+/K+ ATPase (on basolateral membrane) creates electrochemical gradient
transported across the cell membranes
- apical membrane: secondary active transport via H+-peptide cotransporter, inside the cytoplasm peptidases digest small peptides to amino acids
- basolateral membrane: facilitated diffusion via amino acid carrier/transporter
describe the mechanisms of absorption for lipids/fats: free fatty acids and monoglycerides
lumen and apical membrane:
- FFAs and monoglycerides move by simple diffusion from micelle into cell
- micelle not absorbed
- bile salts absorbed into ileum
intracellular:
- transported to the endoplasmic reticulum and resynthesized to triglycerides
- packaged as chylomicrons
baslateral membrane:
- TAGs exocytosed in chylomicrons
- enter lymph via lacteals of villi
describe the mechanisms of absorption for water
drink apron 1.5L a day to replace water lost, plus water secreted into the GIT from the organs is approx 8: in total per day.
- in total 9-10L of water arrive in the small intestine each day
- almost all of it needs to be reabsorbed/absorbed so that the body stays in water balance
Passive movement via osmosis:
driving force: osmotic gradient (driving force) generated by the absorption of nutrients and ions
Small intestine:
- 90% if water re/absorbed
- leaky epithelia with ‘leaky’ tight junctions
- mostly re/absorbed via the paracellular pathway
- some re/absorbed via the transcellular pathway (via. aquaporins)
Large intesine:
- 9% of water re/absorbed
- tight epithelia with ‘tight’ tight junctions
- all water re/absorbed via the transcellular pathway (via aquaporins)
**aquaporins - water channels in the cell membrane)
describe the mechanisms of absorption for sodium (Na+) in the small intestine
small intestine (90% of sodium absorption)
1. passive absorption via the paracellular pathway
- driving force: sodium ions diffuse down their concentration gradient (driving force) across the ‘leaky’ tight junctions
2. active absorption bia the transcellular pathway
- driving force: sodium gradient generated by Na+/K+ ATPase
transported across the cell membranes
- apical membrane: secondary active transport couples with monosaccharides, secondary active transport coupled with amino acids
- basolateral membrane: pumped out by Na+/K+ ATPase
describe the mechanisms of absorption for sodium (Na+) in the large intestine
variable amounts of sodium absorption depending on how much sodium the body needs
only ACTIVE absorption bia the transcelular pathway
- driving force: sodium gradient generated by Na+/K+ ATPase
transported across the cell membranes:
- apical membrane: sodium channels: sodium diffuses down its concentration gradient
- basolateral membrane: pumped out by Na+/K+ ATPase
describe the mechanisms of absorption for fat soluble vitamins
fat soluble vitamins: A,D,E,K
passive absorption via the transcellular pathway
- driving force: chemical (concentration) gradient
dissolved in micelles with fat
- apical membrane: lipid soluble, diffuses through the cell membrane
- basolateral membrane: packaged into chylomicrons with fat
