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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
Membrane Structure
Figure 3.20
Lipid bi-layer: phospholipids, primarily phosphoglycerides
Other lipids
Sphingolipids: alter electrical properties
Glycolipids: communication between cells
Cholesterol: increase fluidity while decreasing permeability
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
Passive Diffusion
Lipid-soluble molecules (alcohol, CO2)
No specific transporters are neededNo energy is neededDepends on concentration gradient
High lowSteeper 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
Facilitated Diffusion
Hydrophilic moleculesProtein transporter is needed - UniporterNo energy is neededDepends on concentration gradientExamples: amino acids, nucleosides,
sugars (glucose)
Facilitated Diffusion, Cont.
Three main types of proteins1. Ion channels – form pores, channel has to be open
a) Open/close in response to a membrane potential
b) Open via specific regulatory molecules
c) Regulated through interactions with subcellular proteins
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 0.000000000000018 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
Facilitated Diffusion, Cont.
Facilitated Diffusion - Uniporter
GLUT1 – mammalian glucose transporterUses concentration gradient of glucose to drive transportCan work in reverseUsed by most mammals
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+
Membrane Potential
Difference in charge inside and outside the cell ↔ electrochemical gradient
Active transporters establish this gradientTwo main functions
Provide cell with energy for membrane transportAllow for changes in membrane potential used by
cells in cell-to-cell signaling
Can be determined by Nernst equation and Goldman equation
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
Nernst equation
Each ion has a different potential given the difference in concentration gradients.
Must have pores or channels to create potential!
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.
Active Transport
Protein transporter is neededEnergy is requiredMolecules can move from low to high
concentration
Active Transport, Cont.
Two main types: distinguished by the source of energyPrimary active transport – uses an
exergonic reaction ie ATPSecondary active transport – couples the
movement of one molecule to the movement of a second molecule
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
P-class pumps: Na+/K+ ATPase pump
pumps 2 K+ in and 3 Na+ out important for many cellular functions (osmotic balance of cells) uses ATP as energy source can be blocked with poisons like ouabain or digitalis the potential built up in the Na+ ions will be used by many different
processes i.e. cotransporters, neuronal signaling etc.
P-class pumps: Na+/K+ ATPase pumpBinding of phosphate from ATP drives conformation change that allows ions to be
transported to appropriate sides: → an asparate residue becomes phosphorylated and the energy transfer changes the proteins conformational shape
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 for the same reason
P-class pumps: Ca 2+ ATPase pump
pumps 2 Ca2+ ions out for every 1 ATP molecule usedUses ATP to drive Ca 2+ out against a very large concentration gradientInternal Ca 2+ binding sites have a very high affinity
(in order to overcome extremely low Ca2+ concentrations inside cell)Energy transfer from ATP to the aspartate of the Ca2+ ATPase causes
a protein conformational change and Ca2+ transported across membraneCa2+ binding sites on outside are low affinity and Ca2+ is releasedThe 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.
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 contractionCa2+ ATPase will remove excess Ca2+ from the cytoplasm and pump it into
the lumen of the SR
V-class pumps: proton pumps
These 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 4.5 - 5.0) Many of these pumps are paired with Cl- channels to offset the electrical
gradient that is produced by pumping H+ across the membrane.
V-class pumps: proton pumps
H+ is transported into the lysosome Cl- flows in to keep a balance If Cl- doesn't flow in then there is rapid build up of potential (charge) across
the membrane which would block the further transport of H+. This would occur long before the lumen becomes acidic because
not that many ions need to be transported to produce the voltage potential
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
Secondary Active Transport
Uniporter: One molecule. Amino acids, nucleosides,sugarsSymporter/cotransporter: movement in the same directions. Na+/glucose cotransporter in the intestineAntiporter/Exchanger: Cl-/HCO3
- exchanger in the red blood cell
Example of a Cotransporter
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!
Favors movement of Na+ into the cell
Membrane Potential and Co-transportersNa+/glucose co-transporter
Used by cells in the intestine to transport glucose againsta large concentration gradient
This is a symporter: both in the same directionΔG for 2 Na+ is -6 kcal/mol
Membrane Potential and Co-transporters3 Na+/Ca2+ antiporter
Important in muscle cellsMaintains the low intracellular concentration of Ca2+
Plays a role in cardiac muscle[Ca2+]i = 0.0002 mM and [Ca2+]o = 2 mMSo ΔG = RTln (2/0.0002) = 5.5 kcal.mol
ΔG = zFEm = 2(23062)(0.070Volts) = 3.3 kcal/molTotal = 8.8 kcal/mol
►So must transport 3 Na+ in for 1 Ca2+ out
Co-transporters
HCO3-/Cl- antiporter
Regulate pHCarbon dioxide from respiration:CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3
- in the presence ofcarbonic 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
Co-transporters
Other transporters that regulate pH
Na+/H+ antiporter: Remove excess H+ when cells becomeacidic
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+: Couplesthe influx of HCO3
- and Na+ to an efflux of Cl-