1. Stephen Fish, Ph.D.
Marshall University J. C. E. School of Medicine

2. Note to instructors:
I use these PowerPoint slides in cell biology lectures that I give to first year medical students. Copy the slides, or just the illustrations into your own teaching media. We all know that teaching science often requires compromises and simplification for specific student populations, or the requirements of a specific course. Please feel free to offer suggestions for improvements, corrections, or additional illustrations. I would be pleased to hear from anyone who finds my work useful, and am always willing to make it better. Also, the images have been compressed to screen resolution to keep PowerPoint file size down, and I can provide them at any resolution.
Stephen E. Fish, Ph.D.

3. Membrane Transport: Carriers & Channels

4. The membrane lipid barrier: Passive diffusion through the lipid bilayer
Concentration gradient up, diffusion up
Molecule lipid solubility up, diffusion up
Molecular size up, diffusion down
Molecule electrically charged, diffusion blocked

5. Specialized membrane proteins transport molecules across membranes
Simple diffusion
Species of molecule limited by membrane physics
Rate is slow and linearly related to concentration gradient
Membrane transport
Overall not limited by size, charge, or hydrophilia
Is highly selective for specific needed molecules
Rate is fast and not linear

6. Membrane protein transporter types
Channels facilitate diffusion
through an aqueous pore
when a conformational
change opens a gate
Some carrier types facilitate
diffusion, others use energy
to pump molecules against
Their gradient. They must
bind the solute to initiate
a conformational change

7. Carrier types Uniporter- transports only one molecule species
Symporter- coupled transport of 2 different molecular species in the same direction
Antiporter- coupled transport of 2 different molecular species in the opposite direction
Symporters & antiporters are usually pumps
Some types transport more than one molecule of a species/cycle

8. The glucose uniporter transports glucose across membranes
Ligand (glucose) binding flips the transporter to a different conformation (changes shape)
The new conformation releases glucose on the other side of the membrane
Release allows it to flip back to repeat the cycle

9. How carrier proteins change conformation
The ligand binding
site is exposed on
the upper membrane
surface

10. The folding pattern flips to a different position
The ligand binding
site is now exposed on
the lower membrane
surface

11. Without the ligand bound, conformation returns to the first state
The carrier is
now ready to
transport another
molecule

12. Band 3 facilitated diffusion anion antiporter in red blood cells
Multipass protein that binds to spectrin
Exchanges Cl- for HCO3-
Important for transporting CO2 to the lungs

13. Band 3 facilitated diffusion anion antiporter in red blood cells
When the bicarbonate diffusion gradient is reversed, the process reverses

14. Band 3 function in RBCs
Why HCO3- for CO2?
Why antiport Cl-?

15. Primary active transport example: The Na+- K+ antiporter pump
Pumps 3 Na+ ions out of cell & 2 K+ ions in
Maintains Na+ & K+ cell membrane gradients
Each cycle uses one ATP, 100 cycles/sec
Uses ¼ energy of most cells, ¾ for neurons

16. The Na+ - K+ pump cycle

17. The Na+- K+ ATPase pump is responsible for maintaining cellular osmotic balance
Charged intracellular
molecules attract ions
& increase internal tonicity
The pump’s net
effect is to
remove + ions

18. If the pump is blocked by ouabain
More water
enters

19. Secondary active transport example: The sodium-glucose symporter pump
Gradients from primary pumps power secondary active transport
Different types, can be antiporters or symporters
Pictured, the Na+ gradient powers conformational change
Glucose is pumped in against its gradient

20. Retrieval of GI tract glucose by enterocytes

21. Channels are selective for ion species
Some are very specific, others less
Specificity based on
Size
Charge
Special problem for K+ channels
Na+ is smaller & same charge
Requires a special filter

22. I K+ channel blocks Na+

23. II K+ channel blocks Na+

24. Most channel transporters are gated
Opening & closing of the gate mechanism
Ligand gated
Voltage gated
Mechanically gated
Other types later in the course
A few are not gated = leak channels

25. Leak channels Open all the time Best known type are K+ channels
K+ going down concentration gradient out of the cell
Increases inside negativity of the cell
Gradient created by the Na+-K+ pump

26. Ligand gated channels
Binding of ligand changes conformation of the channel
Gate opens to allow an ion (+ or -) to enter or exit the cell

27. The K+ leak channel charges up the membrane
The K+- Na+ pump charges up concentration gradients
Excess + ions out accounts for only a small portion of the -60mv membrane potential
The leak channel lets more + ions out
The electrical potential rises until it equals & balances the K+ concentration gradient = no more leak

28. Hormones can trigger secretion
Example- Pancreatic cells secrete digestive enzymes into the small intestine
The cell is charged up by the leak channel
Ligand opens gate on Ca++ channel
Membrane potential & Ca++ gradient sum
Ca++ entering triggers fusion of vesicles with membrane

29. Voltage gated channels
Are sensitive to voltage across the cell membrane
When the voltage changes to a trigger level, it opens
The gate will close again when the voltage returns to the trigger level
What is the problem with this picture?

30. Many channels are inactivated by a separate mechanism than the gate
The voltage gated Na+ channel serves as a good example
Opening the channel depolarizes the cell & if it stayed open the gate would never close
The inactivating mechanism provides for a short positive pulse of current into the cell

31. Mechanically gated channels: hair cells in the ear

32. Actually, I like to eat them proteins
Sherman says
Actually, I like to eat them proteins