Membrane Transport Flashcards
Definition of selective permeability
Only specific molecules can pass freely through the membrane
Definition of passive diffusion
Down conc grad, across membrane, passive until equilibrium reached
Definition of the partition coefficient
The rate at which the solute equilibrates
The more hydrophobic, greater the PC, the greater the rate of equilibration
Definition of facilitated diffusion
Down conc grad, through membrane channel proteins
Passive until equilibrium reached
Definition of gated ion channels
Ion channels allow facilitated diffusion selective for different ions
Definition of active transport
Solutes move against conc grad w membrane protein, ATP hydrolysis
Definition of primary active transport
ATP hydrolysis directly causes solute movement
Definition of uniport
Transport of 1 solute
Definition of symport
Transport of 2 solutes, same direction
Definition of antiport
Transport of 2 solutes, different directions
Definition of secondary active transport
ATP hydrolysis does not directly cause solute movement
What is permeable through the plasma membrane
Hydrophobic molecules
Small uncharged polar
Transport mechanisms
Simple diffusion Facilitated diffusion Gated ion channels Primary active transport Secondary active transport
Simple diffusion
Down conc grad until equilibrium reached
Rate depends on partition coefficient of solute
More hydrophobic, greater PC, equilibrate more quickly
Facilitated diffusion
Down conc grad until equilibrium reached
Requires channel protein
Kinetics of transport with simple and facilitated diffusion
As [S] increases, J increases in simple diffusion
As [S] increases, J increases more steeply initially, plates out in facilitated diffusion
1/2Jmax = Km
Tranporter affinity in diffusion
Low Km = High affinity
Types of glucose transporter
GLUT 1 GLUT 2 GLUT 3 GLUT 4 SGLUT
Location of GLUT 1
Everywhere, abundant in RBC
Low in skeletal muscle
Function of GLUT 1
Low Km, mediates constitutive glucose uptake in tissue
Location of GLUT 2
Liver, pancreas b cells
Function of GLUT 2
High Km, high Jmax
Transports glucose => hepatocytes and beta cells when [blood glucose] increases
Location of GLUT 3
Neurones
Function of GLUT 3
Low Km
Location of GLUT 4
Muscle, adipocytes
Function of GLUT 4
Km=5mM, similar to fed state [blood glucose]
Insulin regulated, can change glucose uptake ability
Insulin stimulated uptake of glucose
Stimulates glucose uptake in muscle, adipose, increase GLUT 4 in membrane
GLUT 4 on membranous vesicles in cytoplasm
Insulin binds to receptors, triggers movement of vesicles to membrane, merge, increase no on surface
Increase in GLUT4 in membrane, increase uptake of glucose into cell
Gated ion channels
Allow facilitated diffusion selective for different ions
Open/close in response to stimuli
Types of gated ion channels
Ligand gated
Voltage gated
Active transport
Up conc grad, w membrane protein
ATP hydrolysis
Conc of Na+ in and out of cell
12mM
145mM
Conc of K+ in and out of cell
140mM
4mM
Na+/K+ pump
Phosphorylation at cytoplasmic side with ATP = conformational change
Cytoplasmic end closes, external end opens
Pump binds 2K+ and releases 3Na+
Hydrolysis of Pi closes external end, opens cytoplasmic end
Release 2K+ into cell
Structure of the Na+/K+ pump
a2b2
Cotransoprt systems
Preestablished gradient used to drive transport of solute across membrane against gradient
ATP hydrolysis used to establish primary gradient
SGLUT cotrasnporter
Glucose absorption from intestine against gradient
Symport
Na+ gradient established by Na+/K+ pump, ATP hydrolysis drives glucose uptake
Secondary AT
Na+/Ca2+ cotransporter
Ca2+ export from muscle cells against gradient
Na+/Ca2+ cotransporter=antiport
Na+ gradient established by Na+/K+ pump and ATP hydrolysis used to drive CA2+ export from cells
Secondary AT
Secretion of insulin by B cells
Glucose =FD=> GLUT2 (b cells)
Glucose =metabolised=> ATP
Increase in ATP/ADP ration ==> closure of cell surface K ATP channels ==> cell membrane depolarisation
Voltage gated Ca2+ channels open, increase IC Ca2+
Increase Ca2+ ==>< exocytosis of insulin in vesicles
