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Flashcards in Movement Across Cell Membrane Deck (50):

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



Ability of a molecule to cross the membrane



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



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



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

1. Depolarization, when RMP Becomes less negative

2. Hyperpolarization: when RMP is becoming more negative

3. 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)



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



Membrane voltage at which the action potential is initiated


Rapid depolarization

Explosive depolarizing change in membrane potential



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