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Lecture 4

BIOL 2160 Lecture Notes - Lecture 4: Resting Potential, Fluid Compartments, Extracellular Fluid

Biological Sciences
Course Code
BIOL 2160
Cross- Eyed

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CHAPTER 4: Cell Membrane Transport
Cell Membrane Structure
Phospholipid bilayer: proteins inserted in and associated with both sides.
o Hydrophilic heads on the outside and hydrophobic tails on the inside.
o Majority of membrane is nonpolar
o Semi-permeable
o Small and nonpolar can cross easily.
Anything that’s polar or charged, isn’t going to cross membrane on own.
(also large)
Categories of Membrane Transport
Passive: transport across the membrane does not require energy.
o Molecules move across membranes from high energy to low energy.
o Energy comes from amount of solute in solution, the more molecules bumping
into each other, the more energy you have.
Move to low concentration…. Always move toward area of lower energy
(happens naturally through diffusion)
o Solutes you need to know: Potassium (K+) and Sodium (Na+)
Create membrane potential which is the ratio of charge on the inside of the
cell compared to the outside of the cell.
You add up all the positive and negative ions inside and outside the
cell… if inside and outside isn’t the same membrane potential.
At rest, for most cells, resting membrane potential is -70 mV.
Tells you that the inside is negative compared to the outside.
More K+ Intracellular fluid than extracellular fluid. Reverse for
o Chemical Force
When Na+ is high outsidemoves inn
When K+ is high insidemoves out
o Electrical Force
Membrane potential
Affects charged molecules (ions)
Amount of electrical driving force depends on membrane potential
Membrane potential is a difference in electrical potential or voltage
across the plasma membrane.
Intracellular fluid contains more anions than extracellular fluid.
Most, if not all, healthy cells have a negative membrane potential.
With negative mem potential (SIMPLE):
o Electrical driving force on cations is inward.
o Electrical driving force on anions is outward.
o Amount of driving force depends on the specific value of
the membrane potential and the valence of the permeant
Mem. Potential is either going to be positive or negative.
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o Positive: positive inside compared to outside; a lot of
positive charge inside the cell.
Na+ goes out. K+ goes out
o Negative: inside of cell is negative compared to outside of
Na+ goes in. K+ goes in.
o Electrochemical Force
Determined by the combination of the chemical driving force and the
electrical driving force
The electrochemical driving force on an ion is set by the difference
between the ion’s equilibrium potential and the membrane potential
Equilibrium potential of an ion is the membrane potential at which the
electrical driving force on the ion is equal and opposite to the chemical
driving force
The Nernst Equation: Eion = 61mV log/z * [ion]out /[ion]in
ENa+ = +55 mV
EK+ = -94 mV
o These two are needed to counteract chemical force
o Numbers just effect specific ion, not each other.
Repel Na+ out with +55 mV
o Above this Na+ leaves… anything more negative then +55
Na+ goes in.
Equilibrium potential is where current changes
Bring K+ in with -94 mV
o Above -94 goes out, more negative then -94 it goes in.
o At -94: NO NET MOVT
The mem. Potential at ANY time, will always be approaching the
equilibrium potential of the MOST permeable ion.
Understanding this key concept will tie a lot of concepts together when
dealing with ion movement across a membrane.
Whichever ions is crossing membrane most is going to control
membrane potential
Still bring equilibrium toward membrane potential
When sodium leaves cell, makes it more negative
K+ concentrations on the inside of the cell are higher than the outside. At
rest, the inside of the cell is negative, relative to the outside.
So, the chemical gradient at rest tends to push K+ outside the cell. But the
electrical gradient tends to bring K+ into the cell.
At rest, the chemical gradient for K+ is stronger than the electrical
gradient. At equilibrium potential for K+ (-94mV), this is no longer the
Na+ concentrations are much higher on the outside of the cell than the
inside. At rest, the inside of the cell is negative, relative to the outside.
So, at rest, both the chemical and electrical gradients tend to move Na+
into the cell.
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