APK 2105C Lecture Notes - Lecture 15: Electrochemical Gradient, Passive Transport, Membrane Potential
23 May 2018
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Chapter 4, Lecture 1
Cell Membrane Transport
• Direction of transport
o Transport of molecules across a membrane follows the same principles as
chemical rxns
▪ Spontaneous (passive) movement occurs from high energy to low energy
states (high conc to low conc)
• Requires no energy to happen
• Energy prefers to be spread out—helps them do this by going
from high to low
▪ Movement in the opposite direction requires ATP
o Energy of a soln depends on solute conc and ion charge
o Any difference in energy across a membrane acts as a driving force
▪ Chemical driving force
• Looks at how concentration gradient will drive an ion/solute across
a membrane
• Rate of transport depends on size of concentration gradient—how
much of a difference there is
• As soon as solute has opportunity to cross membrane, it’s going
to—will go out until there is no more concentration gradient
▪ Electrical driving force
• Affects charged particles (ions) only
• Does not have anything to do with concentration gradient
• Cells have an overall negative charge—positive charges want to
go towards neg charge
o Opposites attract, likes repel
o More negative charges will want to go towards positive
charges instead of staying with other negative charges
• Large the magnitude of charges = greater attraction or repulsion
▪ Electrochemical driving force
• Combined efforts of electrical and chemical driving force to move
a solute across a membrane
o Problems when driving forces want to go in opposite
directions
▪ One with greater magnitude will win out and cause
the ion to go in that direction
• Equilibrium potential (Ex) = membrane potential at which the
electrical driving force exactly opposes (balances) the chemical
driving force
o Equal magnitude of chemical and electrical driving forces
pushing in either directions
o Electrochemical gradient = 0
o Equilibrium = no net movement of the ions
o Each ion has its own Ex
▪ Ex of Na = +60 mV
▪ Ex of K = -94 mV
• High conc inside cell compared to ECF
o Chemical driving force is outward
• Positive ion but cell is still neg
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