Movement Across Cell Membrane Flashcards
Cell membrane
Allows for separation of an intracellular and extracellular environment
Cell membrane function determined by molecules in the membrane
Cell membrane excludes water soluble, charged molecules
Cell permeability
Membrane is a lipid barrier
Fluid in nature
Molecules within membrane can serve as transmembrane carriers
Lipophilic molecules cross membrane because traverse the lipophilic center
If carry a charge don’t cross membrane readily
Diffusion barrier for selective movement
Permeability
Ability of a molecule to cross the membrane
Phospholipid
Hydrophilic head and hydrophobic tail
Unsaturated fatty acid
Double bond
Energetically unfavorable
Planar phospholipid bilayer with edges exposed to water
Favorable when sealed compartment formed by phospholipid bilayer
Molecules that are permeable
Hydrophobic molecules, small uncharged polar molecules, large uncharged polar molecules, ions
Cells control internal environment
Control transport of water soluble molecules in the external environment
Cells in aqueous environment those in contact with membrane will not cross hydrophobic lipid center
Lipid soluble cross hydrophobic lipid center
Transmembrane movement of molecules is either by diffusion or protein mediated transport
Regulation of diffusion and protein transport, cell regulates internal environment
Transport by diffusion
Brownian motion
Molecules in constant motion, molecules move back and forth until equilibrium where still move but is equal
Net movement from high to low
Rate determined by Ficks law
Fick’s Law
J= DA(C1-C2)/ X
Rate of diffusion per unit time
Constant is P=D/X
D
Diffusion coefficient or diffusion constant
Diffusion, the concentration gradient is the driving force providing energy for net movement of molecules from 1 solution to another
Rates can vary
J, rate of diffusion
If form high concentration J is negative because concentration is decreasing
Ion diffusion
If molecule has electrical charge, it is an ion
Net flux is function of concentration and molecular potential difference if crosses
2 driving forces- concentration gradient and electrical potential gradient
Electrical potential gradients
Dependent on attraction of opposite charge or repulsion from like charge
If membrane impermeable to one type of ion
There are equal concentrations of ions on both sides of the membrane, then there will be zero potential difference and zero chemical difference
If conc. Of non permeable increases then electrical potential and chemical gradients exist, electrochemical gradient
Positive ions
If can cross the membrane but it is still impermeable to negative ions, then + diffuse towards - potential down their electrical gradient
Creates chemical gradient for the positive ions in opposite direction of electrical gradient and positive diffuses until these are equal
Electrical and chemical driving forces
Produce counter fluxes of ion with the net movement in the direction of the strongest driving force
When these two gradients are equal for the + ion there will be zero net flux across the membrane, electrochemical equilibrium
Electrochemical equilibrium
Net flux of ions is zero but there remains an electrical potential difference and a conc. Difference
Equation shows related to concentration and electrical gradients
Nernst equation
Ex= -(60/z)logXa/Xb
Positive value when for mM concentrations
For cell concentrations refer to inside or outside the cell Xo/Xi
Ion diffusion
Na+ with chemical and electrical into cell
Cl- chemical into cell
K+ in and out, chemical out and electrical in
Factors that influence movement of ions across a membrane
Need driving force
Concentration gradient, electrical gradient and permeability
Movement of through membrane
Permeability to different ions varies, membrane has low permeability for all ions
Controlled by ion specific channels and depends on whether open or closed which is regulated by membrane potential or other molecules
Distribution of small ions
K, Cl, and Na are influenced predominantly by large intracellular, membrane impermeable, negative charged ions like nucleotides and proteins
Active pumping of Na out
Out of cytoplasm decreases the intracellular osmotic pressure and is important for cell water and volume regulation, metabolic inhibitors cause cells to swell by stopping the Na/K pump
Resting membrane potential
With a semi permeable membrane and a concentration gradient for ions, there will be separation of charge if the membrane is impermeable to one ion
Ex: KCl
If membrane impermeable to both will diffuse down concentration gradient so have charge imbalance with inside of cell negative
Major ions
Na, Cl and K
Permeability for each is very different
Na+
Much less permeable than K+
K+ and Cl-
Nearly equal
Also large amount of non diffusible anion inside the cell
Nernst potential for ions
K+= -105 Cl- = -96 Na+ = 67
Electrode to muscle is about -90 inside the cell
Cl close to the electrochemical equilibrium
Electrochemical equilibrium definition
The concentration gradient equals the electrical gradient
Requires ion selective semipermeable membrane
Net flux by diffusion is zero
Na and K+
Not at electrochemical equilibrium
Large gradients for this to enter and exit cell
Net balance of the gradient results in negative inside
Resting membrane potential
Intracellular is negative
Value varies with different types of cells
Results in alignment of ions along the surface of membrane with positive ions outside the cell and negative inside
3 major influences on ion movement
Concentration gradient
Voltage gradient
Membrane permeability
Changes in RMP
- Depolarization, when RMP Becomes less negative
- Hyperpolarization: when RMP is becoming more negative
- Permeability changes for an ion, if this increases then RMP moves towards equilibrium
K hyperpolarization and Na depolarization
Ion permeability
Intrinsic proteins make channels through membrane for ions
Ion channels are specific for the type of the ion
Ion channels CNS be controlled by gates on the inside and outside of the channel
Channel types
Voltage gated channels
Ligand gated channels- AA, Amines, Peptides
Mechanically gated channels- pressure
Membrane permeability
Can add together permeability values for each ion
Passive membrane properties
Membrane capacitance
Membrane resistor
Time constant (R x C)
Space Constant (Sq. Rt. Diameter)
RC
Resistance x capacitance
Space constant
Passive membrane properties
Distance that signal can travel before decreases to 37% of initial value
Amplitude of potential change as a function of distance and is proportional to the square root of axon diameter
Larger diameter is lower resistance
Action potential
RMP to threshold then rapid depolarization Na channel activation and when reaches peak then Na channel deactivation which leads to depolarization below RMP and then repolarizes
Threshold
Membrane voltage at which the action potential is initiated
Rapid depolarization
Explosive depolarizing change in membrane potential
Overshoot
Magnitude of the positive, above 0, Change in membrane potential
Return to RMP
Repolarization of the membrane and termination of the action potential
Hyperpolarizing after potential
When the membrane potential reporarizes, the potential becomes more negative than the normal RMP for a brief time
All or none response
When the membrane potential reaches threshold, a stereotypic action potential occurs, if the threshold is not reached, no action potential occurs
Absolute refractory period
Period of time when a second action potential cannot be produced at the membrane site
Relative refractory period
Period of time when it is more difficult but not impossible to produce a second action potential at the membrane site
K+ activation
Near overshoot
