Download 4 ways to penetrate the Cell Membrane

Membrane Transport
A Thermodynamic Perspective
4 ways to penetrate the Cell
Membrane
• Simple Diffusion
• Passive transport (facilitated
diffusion)
• Active transport (energy-dependent)
• Receptor-mediated endocytosis
Simple Diffusion
Initial
Final
High
Low
GA = the chemical potential of A
(also called the partial molar free energy)
GAo' = chemical potential of standard state
GA – GAo = RTln[A] (free energy varies with conc. A)
GA = GA(in) - GA(out) (final - initial)
[A]in
GA = GA(in) - GA(out) = RTln
[A]out
Thus:
If [A]out is > [A] in, GA is negative exergonic
If [A]out is < [A] in, GA is positive endergonic
If [A]out
= [A]in, GA is zero
Thermodynamics (ENERGY) of Transport
Diffusion
Rule:
A(out)
A(in)
Free energy is released when a solute moves
from an area of high concentration
to low concentration
Spontaneous
(out)
High
(in)
GA(in) < GA(out)
Low
Final state – Initial state =G = negative
(in)
(out)
Balanced
Rule:
Free energy change is zero when the
concentration of A on both sides
is the same
GA(in) = GA(out)
Final state – Initial state =G = 0
(out)
(in)
GA(in) > GA(out)
Final state – Initial state =G= positive
Rule: When chemical potential of A(in) is greater than
A(out), energy must be provided to drive A across
the membrane, i.e., make free energy change negative
Energy = ATP or a proton gradient
Rule:
The movement of ions presents a separate challenge
because not only must the mass difference
(chemical potential) be taken into account, but also the
charge differential (electrochemical potential)
electrochemical potential refers to the state of
(+) (-) charges on both sides of the membrane
The electrochemical potential is referred to as the
membrane potential when dealing with cells
Chemical potential
GA = GA(in) - GA(out) = RTln
and
[A]in
[A]out
Electrochemical potential
GA = GA(in) - GA(out) = ZAF
Membrane potential
Total Energy
GA = RTln
[A]in
[A]out
+ ZAF
Text
p398
Na
Na
+
+
+ Na +
_
_
_
+
Na
+
+
+ Na
+
Na Na
+
+ +
+
Na Na
Na
Na
+
Na
Na
+
15:1
+Na
150 mM
GNa+ = RTln
(- 60 mV)
[A]in
[A]out
= 8.314 x (310 K) x ln
= – 12.8 kJ/mole
+
10 mM
+ ZAF
[0.010]
[0.150]
+ (1) 96,500 x -0.06 volts
Na
Na
Na
+
+
Out
+ Na +
+
+ Na
+
Na Na
+
+
Na Na + +Na
Na
In
_
_
_
+
+
+
150 mM
(+ 60 mV)
GNa+ = RTln
= 8.314 x (310 K) x ln
= + 12.8 kJ/mole
[A]out
[A]in
[0.150]
[0.010]
+
+
Na
+
Na
+
Na
Na
10 mM
+ ZAF
+ (1) 96,500 x +0.06 volts
–
Out
Cl
–
Cl
–
Cl
–
+
Cl
–
Cl
+
–
– Cl
–
+
Cl
Cl
–
Cl
150 mM
In
_
_
_
(+ 60 mV)
GCl- = RTln
[A]in
[A]out
Cl
–
Cl
Cl
–
–
10 mM
+ ZAF
[0.010]
= 8.314 x (310 K) x ln
+ (1) 96,500 x +0.06 volts
[0.150]
= – 6.85 kJ/mole + 5.79 kJ/mol
= – 1.06 kJ/mol
–
Out
Cl
–
Cl
–
Cl
–
+
Cl
–
Cl
+
–
– Cl
–
+
Cl
Cl
–
Cl
150 mM
In
_
_
_
(– 60 mV)
GCl- = RTln
[A]out
[A]in
Cl
–
Cl
Cl
–
–
10 mM
+ ZAF
[0.150]
= 8.314 x (310 K) x ln
+ (1) 96,500 x – 0.06 volts
[0.010]
= + 6.85 kJ/mole + (– 5.79 kJ/mol)
= 1.19 kJ/mol
Facilitated Diffusion (Mediated Transport)
Modes of Transport
ATP-Driven (Active) Transport [Ca2+-ATPase]
Vesicle Trafficking
• The secretory pathway
• The trans-Golgi network
• The signal hypothesis
• Protein targeting
Rule: Proteins destined for secretion from a cell or for
relocation to a membrane or a specific organelle are
synthesized on the rough endoplasmic reticulum (RER)
Definition: The RER consists of ribosomes bound to
membranes enclosing an internal hollow space or cisternae
Selection: Proteins possess a signal sequence that is
recognized by a receptor on the membrane
Action: Proteins pass into the space and transit to the Golgi
while entrapped in vesicles
Protein inserted
in plasma membrane
Trans
Secretory
granule
Golgi
Cis Golgi
RER
Pre-lysosome
Signal Hypothesis
Proteins destined for secretion or transit to membranes and
organelles, have a signal peptide that allows them to enter the
RER cisternae
The signal peptide is recognized by a receptor called the
“signal recognition particle” (SRP) on the RER membrane
Signal sequences on the N-terminal represent a string of
leucine-rich hydrophobic amino acids that allow the
peptide to dock with the receptor
The signal peptide is removed after the peptide has
penetrated the membrane
Signal Hypothesis
Docking
SRP
SRP receptor
Signal Peptide
cleaved
Lumen of cisternae
+NH
3
+NH
Rough Endoplasmic Reticulum
3
Carbohydrate