Transport 2 Flashcards
Absorption of glucose from intestine
What are epithelial cells joined by
Tight junctions
Absorption of glucose from intestine
Steps of glucose absorption
- Na+ in the ICF is kept low by the Na+ pump on the basolateral membrane
- Low intracellular Na+ creates a gradient for Na+ to enter from lumen, but there are no suitable Na+ channels on the apical membrane
- There are Na/Glu symporters on apical membrane
- As Na+ enters via this symporter down its gradient (high to low conc), the energy released is captured by the symporter to move glucose into the cells against its gradient (low to high conc)
- A glucose uniporter provides a mechanism for glucose to leave basolateral membrane and enter interstitial space
Absorption of glucose from intestine
Where must the sodium pump be
On the basolateral membrane
Absorption of glucose from intestine
Where must the Na/Glu symporter be
On the apical membrane
Absorption of water in gut
Steps of absorption of water in the gut
- H2O, Na+ and Cl- molecules in both the lumen & the interstitium
- Na+ in lumen moves causing an electrical imbalance
- This causes Cl- to follow to maintain electroneutrality
- This creates an osmotic gradient which allows H2O to follow by osmosis
Absorption of glucose from intestine
Where must the glucose uniporter be
On the basolateral membrane
Absorption of water in the gut
How do the cells move the Na+ in the first place????
- An Na+ pump forces Na+ out via basolateral membrane
- This lowers the conc of Na+ and creates a concentration gradient across the apical membrane
- The Na+ must cross the apical membrane but there is no Na+ channel
- Na+ uses the Na+/Glu symporter to enter cells down the concentration gradient
- Glucose pumped out of the cell by the Glucose uniporter in the basolateral membrane
- Then more Na+ and glucose enters the cell via symporter on apical membrane & are transported to interstitial space via Na+ pump and Glucose uniporter in basolateral membrane
- The net effect of this is a large osmotic gradient across the cell
- Which allows H2O into interstitial space by paracellular flow via “leaky” tight junctions
Where does the bulk absorption of water occur
- Gut
- Kidney tubules
Regulated absorption of water
What happens in the kidney collecting duct when you are well hydrated
- ADH levels low or absent
- APQ2 is retained in cytoplasm lining cells of collecting duct
- Water cannot be reabsorbed in collecting duct
- Urine is dilute & large volume H2O lost
Regulated absorption of water
What happens in the kidney collecting duct when you are dehydrated
- ADH levels increase
- AQP2 gets signal to move from cytoplasm to apical membrane
- Pores form
- Water can then be reabsorbed into collecting duct
- Urine is concentrated & water is retained
What do we need in addition to H2O and Na+ to rehydrate a severely dehydrated person
Glucose !
Exocytosis
Molecules carried and secreted out of the cell by secretory vesicles
Examples of Exocytosis
- Secretion of proteins
- Insertion of membrane proteins
- Release of neurotransmitters
Endocytosis
Molecules come into contact with cell membrane. The membrane surrounds them and eventually becomes a vesicle to transport them into the cell.
Examples of endocytosis
- Uptake of bacteria
- Uptake of LDL cholesterol in liver
