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BIOL 2021 Lecture Notes - Electrochemical Gradient, Membrane Transport, Lipid Bilayer

Course Code
BIOL 2021
Julie Clark

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Biol 2021- March 17 2009- lecture Chapter 11
CHAPTER 11- Membrane transport of small molecules and electrical properties
of membranes
The lipid bilayer of cell membranes prevents the passage of most polar molecules
This allows cell to maintain concentrations of solutes in its cytosol that differ from those in the
extracellular fluid and in each of the inter cellular membrane enclosed compartments
Cells use specialized transmembrane proteins to transport inorganic ions and small water
soluble organic molecules across the bilayer. Transmembrane transport proteins make up
usually 15-30% of all membrane proteins in a cell
Cell membrane= permeability barrier
Small hydrophobic molecules and small uncharged polar molecules (slow rate) are able to
diffuse through the membrane
Large uncharged polar molecules and ions cant diffuse through the membrane and require
*membrane is highly impermeable to charged molecules (ions) no matter how small they are
* In general the smaller the molecule and the more hydrophobic or non polar it is, the more
rapidly it will diffuse across the membrane bilayer.
Size of cargo Transport Mechanism
- Small molecules: - Transport proteins
Ions, sugar
- Marcomolecules: - Protein translocaters and
Proteins, RNA nuclear pores (chap 12)
- Large particules - Endocytosis or phagocytosis
Other cells
Cell needs transport to:
i. Ingest nutrients (glucose)
ii. Excrete waste (urea)
iii. Ion regulation
- Plasma membrane (PM), internal membrane (IM)

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Membrane transport proteins: transfer such solutes across cell membranes.
> Each transport protein only transports specific type of molecule
Two major classes of transport proteins:
1) Transporters: bind the specific solute to be transported and undergo a series of
conformational changes to transfer the bound solute across the membrane.
2) Channels: interact with the solute to be transported much more weakly. They form
aqueous pores that extend across the lipid bilayer; when open, these pores allow
specific solutes (usually inorganic ions of appropriate size and charge) to pass
through them and thereby cross the membrane. Faster.
- Free energy: the total change in free energy during a set of reactions determines whether or not
the entire reaction sequence can occur.
Enclosed system: collection of molecules that does not exchange matter with the rest of the
universe. Any such system will contain molecules with a total energy E.
First law of thermodynamics: in any process, the total energy of the universe remains constant.
Second law of thermodynamics: the universe constantly changes so as to become more
-h= ∆H= ∆E
Entropy (∆S): randomness of molecules
S= h/T
Gibbs Free Energy (G): Free energy change is a direct measure of the entropy change of the
- Free energy of universe= free energy of cell + free energy of environment
- G= H-TS
- At constant temperature :
-G= -∆H + T∆S
At equilibrium ∆G= 0 (only dead cells)
We are burning up energy and changing environment or universe by being alive
- Table 11-1 is important
- Ion gradients across membrane creates membrane potential
Voltage across membrane (-20mV to -150 mV)
- means negative inside cell

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Electrochemical gradient
- Electrical= charge difference
- Chemical = concentration difference
Electrical (how much energy does it take to push ion across membrane)
G= Zfv z= charge on ion
F= faradays constant
V= voltage ~ membrane potential
G= -RTln (concentration outside/concentration inside)
R: gas constant
T: temp in cells is constant
Electrical + chemical = amount of energy required for ion to go across membrane
Transporters that allow solutes to cross the membrane only passively
For a neutral molecule, passive transport occurs down a concentration gradient (high
concentration to low concentration)- the concentration gradient will determine the direction of
For a charged molecules, electrochemical gradient and concentration gradient influence
transport (electrochemical gradient- combination of both gradients)
Inside of cell is usually negative
Mediated channels are always passive
Proteins actively pump certain solutes across the membrane against their
electrochemical gradients; is mediated by transporters, which are called pumps.
Coupled with an energy source ie. ATP, light, ion gradient
Coupling to an energy source creates a gradient
- Transport proteins do not modify the solute it transports
- Transport proteins have multiple binding sites for its solute
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