1. 01.22.10 Lecture 5: Membrane transport

2. Ion concentrations within the cell are different from those outside

3. Few molecules cross the membrane by passive diffusion

4. Each cell membrane transports specific molecules

5. Solutes cross membranes by passive or active transport
Passive transport is driven by concentration gradients & electrical forces
Active transport is requires energy

6. 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

7. There are 3 main classes of membrane transport proteins

8. Passive transport by glucose carrier protein (GLUT2)
Carrier protein randomly switches between two states
Glucose moves down it’s concentration gradient

9. 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

10. Example: the Na+-K+ pump
Uses ATP hydrolysis to pump sodium out, potassium in
Helps to maintain a negative electric potential inside the cell

11. Example: the Na+-K+ pump

12. Sodium gradients do work: glucose transport
Glucose-Na+ symport protein
Electrochemical Na+ gradient drives import of glucose

13. Two types of glucose carriers enable epithelial cells to transport glucose in the gut

14. 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

15. Gated ion channels respond to different kinds of stimuli

16. The membrane potential is produced by the distribution of ions on either side of the bilayer

17. K+ leak channels establish the membrane potential across the plasma membrane

18. The action potential provides rapid, long- distance 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

19. Action potentials are propagated along an axon

20. Voltage-gated Na+ channels mediate action potentials
Exist in 3 states: closed opened, and inactivated

21. Action potentials are propagated along an axon

23. Conversion of an electrical signal to chemical signal

24. Conversion of biochemical signal back into electrical