Section
II: Properties of living cells and techniques to study them
Lecture 10: membranes and potential differences
Origins
of Life
The
self-organisation of lipid bilayers suggests possible start
to living cells.
There
is some evidence for organic molecules on pre-biotic Earth.
Radio-frequency spectroscopy shows many carbon compounds
in interstellar space. Meteorites (fallen on Earth's surface)
also contain hydrocarbons and amino acids. In some experiments,
spark produced proteins and amino acids. First cells are
thought to be vesicles from bilayers, which provided separation
of outside/inside as well as asymmetry arising from curvature,
asymmetric proteins spanning membranes and different media
trapped inside.
H+
export developed early: ATPase is conservative in structure:
bacterial, mitochondrial, chloroplast similar.
Membranes
All
living cells are enveloped by membranes. Inner compartments
and organelles are also membrane bound. Membranes provide:
·
diffusion barrier
·
energy-driven transporters (concentration
differences)
·
biochemical processes
·
signalling (electrical or chemical)
What
is the membrane like?
Very
thin: ~6 nm (two orders of magnitude too thin for optical
microscopes). Membranes are basically only two molecules
thick.
Some
history
End
of last century: Bernstein suggested living cells contain
electrolyte surrounded by thin layer not very permeable
to ions.
1899
- 1902 Overton compared rate of penetration of many substances
through cell membranes and lipid films.
1925
Gorter and Grendel: lipid extracted from red blood cells
occupied ~2x membrane area when spread as a monolayer: bilayer
arrangement?
1931
- 2 Harvey and Cole find surface tension of cell membranes
lower than expected for lipid/water interface
1935
Davson and Danielli propose protein coated lipid bilayer.
1959
Robertson proposes that all cell membranes conform to "unit
membrane": lipid bilayer
1962
Mueller et al. measure conductance of ions of pure lipid
membranes: much less than cell membranes!
1972
Thermodynamic considerations led Singer and Nicolson to
propose the fluid mosaic model: protein molecules imbedded
in the lipid bilayer.
Membrane
properties
The
lipid bilayers are resistive to electron current flow and
impermeable to ions. The conduction of specific ions happens
through channels and pumps (protein molecules that span
the lipid bilayer). Water, however, can penetrate membranes
rapidly both through the lipid bilayer and through specific
channels. This property of the membrane leads to large pressures
(up to 10 atm) in plant cells. Many membranes have the property
of building an electric potential difference between the
two sides and maintaining that difference. The potential
differences are used by plant cells to accumulate nutrients.
Both plant and animal cells need potential differences to
propagate signals.
Most
cells are too small to perform electrical measurements directly,
although technology provides us with increasingly thinner
electrodes and better electronics. So, great bulk of electrophysiology
in last century was performed on giant cells: squid axon
and charophytes.
Membrane
potential in Chara:
The
electrochemical potential is in J.mole-1. Substitute
the above values and divide by F to get the answer in V:
Taking
the membrane thickness dx = 6 x 10-9 m, then
the force on Cl ions:
Very
large! Repeat the procedure for K+:
Much
closer to equilibrium.
Active
and passive transport
In
general, some ions experience greater electrochemical gradients
than others. If the membrane is permeable to the ion, it
will flow down the gradient. However, some ions, such as
Cl- are not very permeable in biological membranes.
Low permeability delays the redistribution of uneven concentrations,
but this will eventually be reached, unless the concentration
differences are maintained by using free energy and performing
work by pumping ions "uphill". (As done by proton
pump, the uphill flow being coupled to chemical reaction).
Electroneutrality
and diffusion potential
While
bulk solutions contain equal numbers of positive and negative
ions, the different rate of permeation can cause local negative
potential difference (p.d.) inside the cell and local positive
p.d. outside. This p.d. is measurable across the membrane
and is called diffusion potential.
In
equilibrium, there is no net flow and the electrochemical
potentials of the inside and outside phases are equal. For
a single permeant ion:
(10.1)
where
z is the valence (+1 for K+ or Na+,
-1 for Cl-, +2 for Ca2+)
This
is Nernst potential, which shows if the ion is at or near
equilibrium (the concentration gradient balancing the electrical
gradient).
For
several permeable ions:
(10.2)
P's
are the permeabilities for each type of ion.
This
is the Goldman equation, which describes the relationship
between the diffusion potential and prevailing ion gradients
across the membrane. Goldman derived it from electrochemical
potentials by assuming that grady = constant in the membrane.
(Figures
from “Plant Physiology” by Taiz and Zeiger)
