Lecture 9--absorption of water minerals & vitamins Flashcards
H2O transport is a ______ process
Passive process
Aquaporins
Specialised H2O channels, determine TRANSCELLULAR H2O permeability
________ gradients drive H2O transport
OSMOTIC gradients drive H2O transport
Describe TRANSCELLULAR absorption
Transcellular absorption = movement ACROSS cells (between the APICAL and BASAL membrane)
Involves passive AND active transport …
…active transport required on AT LEAST 1 surface (i.e. passive diffusion INTO (out of) the cell and active transport OUT OF (in to) the cell)
Is transcellular absoprtion an active or passive process?
BOTH
At least 1 surface (apical or basal) must have active transport, while the other surface can have passive diffusion
Describe PARACELLULAR absorption?
PASSIVE movement BETWEEN ADJACENT CELLS through TIGHT JUNCTIONS
What regulates paracellular absorption?
Permeability is regulated by structure of TIGHT JUNCTIONS (‘tightness’ v ‘leakiness’)
Is paracellular absorption passive or active?
paracellular absorption = PASSIVE PROCESS
Is electrolyte transport passive or active process?
Can be EITHER (passive or active)
Give a brief overview of the active transport mechanism(s) for ELECTROLYTE TRANSPORT
electrolytes can be transported actively or passively.
Active transport: derives energy from the hydrolysis of ATP. Moves solute against electrochemical gradient
Can be DIRECT ACTIVE TRANSPORT (e.g. N-ATPase pump)
or
INDIRECT ACTIVE TRANSPORT: in which active transport on one surface drives passive transport on other surface by establishing electrochemical gradient.
What is meant by INDIRECT ACTIVE TRANSPORT?
in which active transport on one surface drives passive transport on other surface by establishing electrochemical gradient.
Briefly outline PASSIVE TRANSPORT + the sub-classifications
Passive transport DOES NOT REQUIRE ATP
Moves solutes WITH electrochemical gradient
- FACILITATED DIFFUSION (carrier protein, specific to a certain molecule/class of molecules)
- PASSIVE DIFFUSION (pore, nonspecific)
What is FACILITATED DIFFUSION
Involves CARRIER PROTEINS, specific to a certain molecule or class of molecules
What is PASSIVE DIFFUSION
Involves a PORE that is non-specific
Outline the role of TIGHT JUNCTIONS in regulating water absorption (examples of this regulation?)
TJs OCCLUDE PARACELLULAR PASSAGE OF H2O & SOLUTES (the ‘tightness’ of TJs can vary)
The COLON has TIGHT TJs
The SMALL INTESTINE has LEAKY TJs (which allows things to pass in)
Outline the role of AQUAPORINS in regulating H2O absorption?
Aquaporins= specialised H2O channels (charge specific, size specific, tetrameric protein)
–UP-REGULATING EXPRESSION of APs on membranes = INCREASES PERMEABILITY
What is the importance of electrolytes in water absorption?
Electrolytes are moved across the membrane to establish osmotic gradient down which H2O will travel
What process powers Na+ transport into cell?
Electrochemical gradient established by Na/K pump
What are the (4) ways of SODIUM entering cells?
(1) PASSIVE DIFFUSION (Na+ channels)
(2) CO-TRANSPORT (Na/AA or Na/Glucose co-transporters)
(3) EXCHANGER (Na+/H+ exchanger…Na+ IN, Proton(H+) OUT)
(4) PARALLEL Na/H & Cl/HCO3 EXCHANGE
Outline how CO-TRANSPORTERS operate in Na+ absorption by cells
2 types of transporter: Na+/Glucose & Na+/amino acid
Location: the JEJUNUM (but also ileum)
Requirements:
=> Low intracellular [Na+] (=Na/K transporter)
=> Low intracellular [Glucose] (=glucose channels on basal membrane)
=> Low intracellular [Amino Acids] (=AA channels on basal membrane)
Outline how EXCHANGERS operate in Na+ absorption
Na+/PROTON (Na/H+) exchanger = brings 1 Na+ in, x1 H+ out)
Location: DUODENUM & JEJUNUM
Requirements:
(1) Low intracellular [Na+] (=Na/K transporter)
(2) Low intracellular [H+] (=Na+/H+ exchanger on basal surface removes 1H+ and brings in 1 Na+)
Outline how PARALLEL EXCHANGE operates in Na+ (and Cl-) absorption
Where does parallel exchange take place?
Involving products of reaction of: CO2 + H2O + Carbonic anhydrase ==releases==> H+ ion (used in Na/H exchanger on apical surface) and a HCO3- (hydrogen carbonate) (used in CL-/HCO3- exchanger on apical surface)
Takes place in the PROXIMAL COLON and the ILEUM
What energetically drives Parallel exchange of Na/H & Cl/HCO3-?
HYDROLYSIS OF H2O
What are the requirements for Parallel Na/H & Cl/HCO3- exchange
(1) Low intracellular [Na+]
(2) Low intracellular [Cl-]
(3) Hydrolysis of H2O
Outline the different mechanisms for Na+ absorption in the different regions of the GIT
- Jejunum
- Ileum
- Duodenum
- Proximal Colon
- Distal colon
- Jejunum: (1) Na/Gluc & Na/AA COTRANSPORT (2) Na/H EXCHANGER
- Ileum: (1) Na/Gluc & Na/AA COTRANSPORT (a little) (2) PARALLEL Na/H & Cl/HCO3- EXCHANGE
- Duodenum: (1) Na/H EXCHANGER
- Proximal Colon: (1) PARALLEL Na/H & Cl/HCO3- EXCHANGE
- Distal colon: PASSIVE DIFFUSION by Na+ channels
What are the (3) Mechanisms for CHLORIDE absorption?
(1) PASSIVE DIFFUSION
- Transcellular routes (Cl- channels, down ELECTROCHEMICAL gradient)
- Paracellular route (SOLVENT DRAG sucks Cl- through pores in tight junctions)
(Ileum=low absorb….Jejunum/Distal colon = high absorp)
(2) Cl-HCO3- exchange (ileum/proximal colon=moderate absorp)
(3) Parallel Na/H & Cl/HCO3- exchange(ileum/distal colon=moderate absorp….proximal colon = high absorb)
How is Cl- secreted in the GIT?
by Cl- CHANNELS
= GATED
—>Ca2+ gated
—>Volume gated (stretch; expand cell = let Cl- out to reduce volume)
—>Ligand-gated anion
—>CFTR (cystic fibrosis transmembrane conductance regulator)
=PASSIVE (mostly closed)
What mutation causes Cystic fibrosis? (and how)
Mutation in the CFTR Cl- channel (CYSTIC FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR)
- a transcellulra signalling cascade involving cAMP triggers opening of CFTR channel
- cAMP –> activate protein kinase A –> phosphorylates R-domain of CFTR –> conformational change = open
- Anything that up-regulates cAMP will cause CFTR to be open –> Cl- secreted –> creates SOLVENT DRAG –> H2O leaves cell –> DEHYDRATION
What is a SECRETAGOGUE?
anything that perturbs cAMP signalling is a SECRETAGOGUE for Cl- ions (.: anything that increases cAMP)
Where does Cl- secretion take place in the GIT?
Moderate secretion….
(1) Duodenum
(2) Jejunum
(3) Ileum
(4) Proximal colon
(5) Distal colon
What are the processes for K+ absorption?
What is special about K+ absorption?
(1) PASSIVE DIFFUSION (jejunum & ileum)
(2) ACTIVE K+/H+ EXCHANGER (distal colon)
- -NOTE in this case K+ absorption is an active process on BOTH the apical and basal surface (Na/K pump on basal). BECAUSE on both sides K+ is being pumped IN .: no electrochemical gradient for K+ movement into the cell is established.
Account for the ACTIVE absorption of K+ on the apical surface of cells in the distal colon?
K/H+ exchange requiring ATP…in this case K+ absorption is an active process on BOTH the apical and basal surface (Na/K pump on basal). BECAUSE on both sides K+ is being pumped IN .: no electrochemical gradient for K+ movement into the cell is established.
What method of K+ absorption occurs in different regions of the GIT?
(1) JEJUNUM: Passive diffusion
(2) ILEUM: Passive diffusion
(3) DISTAL COLON: ACTIVE Na/H exchange
What methods are used for K+ SECRETION by cells in the GIT
(1) Passive secretion
(2) Apical K+-channel powered by basal Na/K pump
check this answer with lecture
What factors influence BIOAVAILABILITY (Absorption) of minerals?
(1) COMPLEXES can either ENHANCE SOLUBILITY (aid absorption) or DIMINISH SOLUBILITY (hinder absorption)
(2) pH
(3) Redox state of metals (e.g. Fe3+ cannot be absorbed but Fe2+ can)
What complexes aid/hinder absorption of minerals (and how)
Complexes that AID absorption:
- -ASCORBATE
- -LACTATE
Complexes that HINDER absorption:
- -PHYTATES (in cereals)
- -TANNATES (in tea)
- -COMPETITION with other minerals (e.g. Ca2+ blocks Pb2+ absorption)
Ca2+ is usually bound to ______ and cannot be absorbed
Ca2+ is usually bound to OXALATE and cannot be absorbed
How much of the Ca2+ that we ingest, do we actually absorb?
1/2
What are the mechanisms for Ca2+ absorption in the GIT?
(1) ACTIVE TRANSCELLULAR ROUTE:
–> Ca2+ enters cell by the CAT 1 CHANNEL
–> within cell Ca2+ is bound to CALBINDIN (to stop it from being active in the cell)
–> Shuttled to apical surface, exported from cell by ACTIVE TRANSPORT
=> Ca2+ ATPase or;
=> Ca2+ ATPase antiporter (exchanges Ca2+ for Mg2+ or Na+)
(2) PARACELLULAR DIFFUSION (in the small intestine)
Explain active transcellular transport of Ca2+
ACTIVE TRANSCELLULAR ROUTE:
–> Ca2+ enters cell by the CAT 1 CHANNEL
–> within cell Ca2+ is bound to CALBINDIN (to stop it from being active in the cell)
–> Shuttled to apical surface, exported from cell by ACTIVE TRANSPORT
=> Ca2+ ATPase or;
=> Ca2+ ATPase antiporter (exchanges Ca2+ for Mg2+ or Na+)
Vitamin ____ activates absorption of Ca2+ by _____
Vitamin D activates absorption of Ca2+ by ;
TRANSCRIPTIONAL CASCADE IN CELL THAT UP-REGULATES:
>CAT1 channels
>CALBINDIN MOLECULE
Mg2+ absorption
–Apical surface:
–Basal surface:
–Apical surface: TRPM6 (transient receptor protein) (passive transport)
–Basal surface: Mg-ATPase (active transport)
Ca2+ absorption
- -Apical surface:
- -within cell:
- -Basal surface:
–Apical surface: CAT 1
– Within cell: CALBINDIN
–Basal surface:
==> ATPase
==> ATPase ANTIPORTER (exchange Ca2+ for Mg2+/Na+)
The amount of Mg2+ we absorb depends on ___________
the amount we have in our diet
Water soluble vitamin absorption can be dependent on what?
(1) Na+ DEPENDENT (vit B3/5/7, vitC)
(2) Na+ INDEPENDENT (vit B1/2/6)
(3) pH DEPENDENT (vit B1/3/6)
How are lipid soluble vitamins absorbed?
in the same way as fats –by BILE SALT MICELLES
What are the 2 categories of vitamins?
LIPID SOLUBLE
WATER SOLUBLE
What could cause deficiencies in WATER SOLUBLE VITAMINS?
pH changes
Na+ deficiencies
What could cause deficiencies in LIPID SOLUBLE vitamins?
Blocked bile ducts (=unable to emulsify fats = unable to absorb fat soluble vitamins)
What are the water soluble vitamins?
B vitamins
Vitamin C
What are the lipid soluble vitamins?
A,D,E,K
How is vitamin B12 absorbed?
We don’t have a channel for vitB12
(1) needs to be COMPLEXED WITH INTRINSIC FACTOR (in the DUODENUM)
(2) Take up complex by RECEPTOR MEDIATED ENDOCYTOSIS (in the ILEUM)
(3) CLEAVE B12 from intrinsic factor = FREE B12
(4) Bind free B12 –> TRANS-COBALAMIN 2 (TCII)
(5) EXOCYTOSIS to blood stream
Predominant site for H2O absorption in the GIT?
The SMALL INTESTINE
