1. Diffusion, channels and transporters
Cellular Processes
Diffusion, channels and transporters
This is a sample first topic page.

2. Cellular Membranes
Two main roles
Allow cells to isolate themselves from the environment, giving them control of intracellular conditions
Help cells organize intracellular pathways into discrete subcellular compartment, including organelles

3. Membrane Structure
Lipid bi-layer: phospholipids, primarily phosphoglycerides
Other lipids
Sphingolipids: alter electrical properties
Glycolipids: communication between cells
Cholesterol: increase fluidity while decreasing permeability

4. Membrane Proteins Can be more than half of the membrane mass
Two main types
Integral membrane proteins – tightly bound to the membrane, either embedded in the bilayer or spanning the entire membrane
Peripheral proteins – weaker association with the lipid bilayer
We will discuss membrane proteins that allow for flow of ions or for transport of molecules

5. Membrane permeability
Lipid bilayer

6. Membrane proteins we will discuss

7. Membrane Transport
Three main types
Passive diffusion
Facilitated diffusion
Active transport

8. Passive Diffusion Lipid-soluble molecules (alcohol, CO2)
No specific transporters are needed
No energy is needed
Depends on concentration gradient
High  low
Steeper gradient results in higher rates
Gradients can be chemical, electrical or both depending on the nature of the molecule
e.g., Membrane potential – electrical gradient across a cell membrane

9. Facilitated Diffusion
Hydrophilic molecules
Protein transporter is needed - Uniporter
No energy is needed
Depends on concentration gradient
Examples: amino acids, nucleosides, sugars (glucose)

10. Facilitated Diffusion, Cont.
Three main types of proteins
1. Ion channels – form pores, channel has to be open
Open/close in response to a membrane potential
b) Open via specific regulatory molecules
c) Regulated through interactions with subcellular proteins

11. Facilitated Diffusion, Cont.
2. Porins – like ion channels, but for larger molecules
Cool stuff: aquaporin allows water to cross the plasma membrane – 13 billion H2O molecules per second! But, as pointed out by T. Todd Jones that is only ml of water.
3. Permeases – function more like an enzyme. Binds the substrate and then undergoes a conformation change which causes the carrier to release the substrate to the other side. Ex. Glucose permeases

12. Facilitated Diffusion - Uniporter
GLUT1 – mammalian glucose transporter
Uses concentration gradient of glucose to drive transport
Can work in reverse
Used by most mammals

13. Electrical Gradients
All transport processes affect chemical gradients
Some transport processes affect the electrical gradient
Electroneutral carriers: transport uncharged molecules or exchange an equal number of charged particles
Electrogenic carriers: transfer a charge, e.g., Na+/K+ ATPase  exchanges 3Na+ for 2K+

14. Active transporters establish this gradient
Membrane Potential
Difference in charge inside and outside the cell ↔ electrochemical gradient
Active transporters establish this gradient
Two main functions
Provide cell with energy for membrane transport
Allow for changes in membrane potential used by cells in cell-to-cell signaling
Can be determined by Nernst equation and Goldman equation

15. Nernst equation
Used to calculate the electrical potential at equilibrium
Recall: ΔG = RTln([Xi]/[Xo]) + zFEm
Chemical component + electrical component
At equilibrium: zFEm = RTln([Xo]/[Xi])
Equilibrium potential is:
Ex = (RT/zF) ln [Xo]/[Xi]
where R – gas constant, T = absolute temperature (Kelvin),
z = valence of ion, F – Faradays constant
Example: K+ out: 0.01 M; K+ in: 0.1 M; T = 22oC
So, EK+ = (1.9872*295)/(1*23062) ln (0.01/0.1)
= -58 mV at 22oC

16. Nernst equation
Each ion has a different potential given the difference in concentration gradients.
Must have pores or channels to create potential!

17. Nernst equation and ion concentrations
Differences in Nernst potential reflect differences in chemical gradients!
We will discuss the protein pumps that are necessary to maintain these gradients.

18. Active Transport
Protein transporter is needed
Energy is required
Molecules can move from low to high concentration

19. Two main types: distinguished by the source of energy
Active Transport, Cont.
Two main types: distinguished by the source of energy
Primary active transport – uses an exergonic reaction ie ATP
Secondary active transport – couples the movement of one molecule to the movement of a second molecule

20. Primary Active Transport
Hydrolysis of ATP provides energy
Three types
P-type: pump specific ions, e.g., Na+, K+, Ca2+
F- and V-type: pump H+
ABC type: carry large organic molecules, e.g., toxins

21. P-class pumps: Na+/K+ ATPase pump
pumps 2 K+ in and 3 Na+ out
uses ATP as energy source
Blocked with poisons like ouabain or digitalis
Potential built up in the Na+ ions will be used by many different
processes i.e. cotransporters, neuronal signaling etc.

22. P-class pumps: Na+/K+ ATPase pump
Na+ binding sites switch from high affinity on inside to low affinity
on outside to allow for binding of Na+ on inside and release of Na+ ions
on outside.
K+ binding sites with from high affinity on outside to low affinity on inside

23. P-class pumps: Ca 2+ ATPase pump
pumps 2 Ca2+ ions out for every 1 ATP molecule used
Uses ATP to drive Ca 2+ out against a very large concentration gradient
Internal Ca 2+ binding sites have a very high affinity
Energy transfer from ATP to the aspartate of the Ca2+ ATPase causes
a protein conformational change and Ca2+ transported across membrane
Ca2+ binding sites on outside are low affinity and Ca2+ is released
The transfer of energy from the ATP to the pump triggers a conformational
change that moves the protein and allows the translocation of
Ca 2+ across the membrane
At the same time the Ca2+ binding sites change from high to low affinity.

24. P-class pumps: Ca 2+ ATPase pump cont.
In muscle cells the Ca2+ ATPase is the major protein found in
the membrane of the sacrcoplasmic reticulum (SR)
80% of the protein in the SR is the Ca2+ ATPase
SR is a storage site for Ca2+ that is release to drive
muscle contraction
Ca2+ ATPase will remove excess Ca2+ from the cytoplasm
and pump it into the lumen of the SR

25. V-class pumps: proton pumps
Transport H+ only
Found in lysosomes, endosomes and plant vacuoles
Transport H+ ions to make the lumen or inside of the lysosome acidic
(pH )
Many of these pumps are paired with Cl- channels to offset the electrical
gradient that is produced by pumping H+ across the membrane.

26. V-class pumps: proton pumps
H+ is transported into the lysosome
Cl- flows in to keep a balance. Why?

27. Secondary Active Transport
Use energy held in the electrochemical gradient of one molecule to drive another molecules against its gradient
Antiport or exchanger carrier: molecules move in opposite directions
Symport or cotransporter carrier: molecules move in the same direction

28. Secondary Active Transport
Uniporter: One molecule. Amino acids, nucleosides,sugars
Symporter/cotransporter: movement in the same directions.
Na+/glucose cotransporter in the intestine
Antiporter/Exchanger: Cl-/HCO3- exchanger in the red blood cell
Example of a Cotransporter

29. Membrane Potential and Na+
Animal cells are more negative on the inside than on the outside
(~ -80 to -70 mV)
Mostly due to K+ ions (inside > outside) created via Na+ /K+ pump, K+ leak channels and anions inside the cell (proteins etc)
Remember K+ will move down its concentration gradient
Nernst potential for K+ is - 80 to -70 mV.
Why is this important???
Transport of Na+ down the chemical gradient and the electrical gradient. Makes Na+ a powerful co-transporter!
Favorable conc. Gradient and electrical gradient
Favors movement of Na+ into the cell

30. Membrane Potential and Co-transporters
Na+/glucose co-transporter
Used by cells in the intestine to transport glucose against
a large concentration gradient
This is a symporter: both in the same direction
ΔG for 2 Na+ is -6 kcal/mol
Favorable conc. Gradient and electrical gradient

31. Membrane Potential and Co-transporters
3 Na+/Ca2+ antiporter
Important in muscle cells
Maintains the low intracellular concentration of Ca2+
Plays a role in cardiac muscle [Ca2+]i = mM
and [Ca2+]o = 2 mM
ΔG = RTln (2/0.0002) = 5.5 kcal.mol
ΔG = zFEm = 2(23062)(0.070Volts) = 3.3 kcal/mol
Total = 8.8 kcal/mol
►So must transport 3 Na+ in for 1 Ca2+ out
Favorable conc. Gradient and electrical gradient

32. Co-transporters HCO3-/Cl- antiporter Regulate pH
Carbon dioxide from respiration:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- in the presence of
carbonic anhydrase (enzyme)
Note: ~80% of the CO2 in blood is transported as HCO3-.
This is generated by red blood cells (RBC)
RBC have a protein (AE1) and this is the HCO3-/Cl-
antiporter
Pumps 1 X 109 HCO3- every 10 msec.
Clears the CO2 and Cl- transport ensures that there isn't a
build up of electrical potential
Favorable conc. Gradient and electrical gradient

33. Membrane Potential and Co-transporters
HCO3-/Cl- antiporter
Favorable conc. Gradient and electrical gradient

34. Co-transporters Other transporters that regulate pH Na+/H+ antiporter
Remove excess H+ when cells become acidic
Na+HCO3-/Cl- co-transporter
HCO3- is brought into the cell to neutralize H+ in the
cytosol: HCO3- + H+ ↔ H2O + CO2 in the presence of
carbonic anhydrase
Driven by Na+: Couples the influx of HCO3- and Na+ to
an efflux of Cl-
Favorable conc. Gradient and electrical gradient

35. Co-transporters Exchangers are regulated by internal pH and increase
their activity as the pH in the cytosol falls
Favorable conc. Gradient and electrical gradient