Download 01.22.10 Lecture 5: Membrane transport

01.22.10
Lecture 5: Membrane transport
Ion concentrations within the cell are different
from those outside
Few molecules cross the membrane by
passive diffusion
Each cell membrane transports specific
molecules
Solutes cross membranes by passive or
active transport
•
Passive transport is driven by concentration gradients &
electrical forces
•
Active transport is requires energy
An electrochemical gradient is driven by 2 forces
•
Concentration gradient - ions move across a membrane from
high to low concentrations
•
•
Voltage across the membrane
High for sodium, low for potassium
There are 3 main classes of membrane
transport proteins
Passive transport by glucose carrier protein
(GLUT2)
protein randomly switches between two states
• Carrier
Glucose
moves down it’s concentration gradient
•
Active transport is mainly driven in 3 ways
•
•
•
1. Coupled transporters couple uphill transport of one solute to
the downhill transport of another
2. ATP-driven pumps use hydrolysis of ATP to uphill transport
3. Light driven pumps couple transport to light absorbtion
Example: the Na+-K+ pump
•
Uses ATP hydrolysis to pump sodium out, potassium in
•
Helps to maintain a negative electric potential inside the cell
Example: the Na+-K+ pump
Sodium gradients do work: glucose transport
•
•
Glucose-Na+ symport protein
Electrochemical Na+ gradient drives import of glucose
Two types of glucose carriers enable
epithelial cells to transport glucose in the gut
Ion channels are selective pores in the
membrane
•
Ion channels have ion
selectivity - they only
allow passage of specific
molecules
•
Ion channels are not
open continuously,
conformational changes
open and close
Gated ion channels respond to different kinds
of stimuli
The membrane potential is produced by the
distribution of ions on either side of the
bilayer
K+ leak channels establish the membrane
potential across the plasma membrane
The action potential provides rapid, longdistance communication
•
Action potential (nerve impulse): a wave of electrical activity
propagated along the length of a neuron
•
Very fast (~100 m/sec), dose not weaken over distance
Action potentials are propagated along an
axon
Voltage-gated Na+ channels mediate action
potentials
•
Exist in 3 states: closed opened, and inactivated
Action potentials are propagated along an
axon
Conversion of an electrical signal to chemical
signal
Conversion of biochemical signal back into
electrical