• Bring voltmeter, AA batteries, 9V battery
• Show experiment with static electricity –
balloon,
• Show iPhone capacitance
Syllabus
• First Lab is next week
• Cells are enveloped in a
membrane dotted by channels,
pumps, receptors, and many
other transmembrane proteins
(machines)
• All of these machines operate in
water
• Humans are 60% water
Water is very different from air
1. Water is very concentrated: 55.5 M =
55.5 x 6 x 10^23 water molecules in 1 liter
Compare to air: 0.04M (water is >1,000 times
denser)
2. Water is polar
-
+
3. Various molecules interact differently with
water. Charged and polar molecules love
water  hydrophilic
Charged molecules love water  hydrophilic
• From Greek
philos=
“loving”
• Examples of
charged
molecules?
• all ions: K+,
Na+, Cl-, Ca++
Hydrophilic (charged and
polar) molecules LOVE
water molecules.
They come very close.
• An animation from a simulation of water molecules (red and white) around a
chloride ion (green)
• Notice that positive hydrogen of water molecules is attracted to the negative
chloride ion - the white dashed lines. These electrostatic interactions are called
hydrogen-bonds.
Urea is polar  is it attracted to polar
water molecules?
-
-
+
+
-
• Yes. Therefore
urea is hydrophilic
-
Glucose is polar  is it attracted to polar
water molecules?
+
-
• color indicates negative
charge distribution
Fats, oil are uncharged nonpolar
• Fats have no extra charge on
the surface 
• they cannot electrostatically
interact with polar water
molecules 
• Fats are hydrophobic
• from phobia == fear
Oil and water don’t mix
Hydrophobic
Hydrophilic (charged and
polar) molecules LOVE
water molecules.
They come very close.
Hydrophobic
molecules are
afraid of water.
They push
water
molecules away.
Charged molecules
Uncharged
extra electrons or lack of electrons
(=positive charges) that can interact
with water molecules
Polar
Nonpolar
electrons are
distributed unevenly
creating spikes of
charges that can
interact with water
molecules
Ions: K+, Na+, Cl-,
Ca++
H2O, Urea,
O2, CO2, N2,
glucose,
fats, oil,
proteins petrol/gasoline
Hydrophilic
Hydrophobic
Over 3.4 billion years ago… cells surrounded
themselves with a border (the membrane)
• Benefit of the
border?
• Control internal
environment (like
a political
country)
• Before we study
the border, let’s
take a quick look
at the evolution
1
4.5 BILLION
years ago
The Earth forms
2
1.5
3
Vertebrates
originated about 525
million YA
(Cambrian explosion:
Twice complete
genome duplication)
4
Dinosaurs originated
~230 million years ago
6
Primates originated
~70 million years ago
5
7
Human line split from
chimpanzee line
Modern humans:
~0.1 million YA
Bacteria
3.4 billion YA
Eukaryotes (mitochondria)
1.5 billion YA
Vertebrates (Cambrian explosion)
525 million YA
Dinosaurs
230 million YA
Mammals
200 million YA
Primates
70 million YA
Great apes ( orangutan, Gorilla, chimpanzee, bonobo & humans)
20 million YA
Human line split from chimpanzee line
6 million YA
First Homo (Homo habilis) makes stone tools
2.4 million YA
Significant brain growth in our ancestor (Homo erectus)
1.5 million YA
Last common ancestor of humans and Neanderthals
600 thousand YA
First anatomically modern humans
200 thousand YA
First behaviorally modern humans
100 thousand YA
Explosion of art, tools, religious beliefs, humans in Australia
50 thousand YA
Domestication of dogs
25 thousand YA
First agriculture
10 thousand YA
First Egyptian pyramids
5 thousand YA
Jesus Christ
2 thousand YA
Over 3.4 billion years ago… cells surrounded
themselves with a border
The border (membrane) consist of
phospholipids
• It is a phospholipid bilayer (or lipid bilayer)
A single phospholipid molecule:
-
-
The border (membrane) consist of
phospholipids
+
+
-
+
-
• It is a phospholipid bilayer
(or lipid bilayer)
-
• Variations of
phospholipid
molecules
• Always: hydrophilic
head and
hydrophobic tail
Can molecules and ions (O2, CO2, N2, K+,
Na+, Cl-, Ca++ , H2O, Urea, glucose, proteins)
penetrate through a cell membrane?
Molecules and ions size
1mm (millimeter)
1µm (micrometer)
• How thick is human nail?
• How thick is human hair?
Definition: A molecule is
a group of several atoms
held together by covalent
bonds.
1nm (nanometer)
Can molecules and ions (O2, CO2, N2, K+,
Na+, Cl-, Ca++ , H2O, Urea, glucose, proteins)
penetrate through a cell membrane?
0.1nm
(nanometer)
10µm
0.006µm
= 6nm
Appreciate cell membrane
thickness: if you scale a cell
to the size of a house, then
membrane thickness will
be similar to the house
wall thickness
• Appreciate membrane thickness (6nm): if you
scale a cell to the size of a house, then membrane
thickness will be similar to house wall thickness
Drop O2
molecules
O2
Measure
molecules
concentration
Drop
Biological membranes contain
channels, pumps, and other proteins
that can move molecules across a
membrane 
Let us first study an artificial
membrane, void of all proteins
Experiment:
• Take an artificial synthetic lipid bilayer (not a real biological
membrane)
• Drop molecules on one side
• Measure molecule concentration on the other side
• Some molecules easily diffuse through the membrane
• Some do not diffuse at all
• Oxygen easily diffuses through lipid bilayer
O2
K+
6nm
• Physics of molecule’s permeability through a lipid bilayer
• O2 is hydrophobic and small (0.1nm in diameter)  easily
soluble in lipids  O2 dissolves in lipid bilayer  diffuses
from high concentration to low concentration
• Potassium is hydrophilic  surrounded by several layers of
polar water molecules  does not solve in lipid bilayer 
repelled from lipid bilayer
Relative permeability through
an artificial membrane:
1,000,000,000
Like birds that fly
across any political
border
1,000,000
Like small animals
that can fit through
the fence
1,000
Like larger animals –
cats, dogs - that can
only rarely fit
through the fence
Like humans who
cannot fit through
the fence
<1
Artificial nonbiological lipid
membrane
Charged: extra
electrons or lack of
electrons
Uncharged
Polar
Ions: K+, Na+, Cl-,
Ca++
Nonpolar
H2O, Urea,
O2, CO2, N2,
glucose,
fats, oil,
proteins petrol/gasoline
Hydrophilic
Membrane-impermeable
Hydrophobic
Membrane-permeable
Relative permeability through
an artificial membrane:
1,000,000,000
Like birds that fly
across any political
border
1,000,000
Like small animals
that can fit through
the fence
1,000
Like larger animals –
cats, dogs - that can
only rarely fit
through the fence
Like humans who
cannot fit through
the fence
<1
Artificial nonbiological lipid
membrane
• Biological membranes have all kinds of proteins inserted:
channels, pumps, transporters, receptors – up to 100 million
of transmembrane proteins
• Water channels (aquaporins) are found in most cells
• Biological membrane is a dynamic structure
• Diffusion of phospholipids and some
transmembrane proteins is possible inside the 2dimentional surface of a membrane
How much water is in a raw steak?
• About 60%
• Total body water is about 60%
• What about fluid ionic composition?
Is it homogeneous?
What about fluid ionic composition?
Is ionic concentration identical inside and outside of a cell?
+
[K ]
?
inside
+
[K ]
Outside
Ringer experiments with frog’s heart
(published 1881 to 1887)
• Solution perfusing a
frog’s heart must
contain salts of sodium,
potassium and calcium
mixed in a definite
proportion if the frog’s
heart is to continue
beating for a long time
• RINGER SOLUTION
Ringer
solution
• Cations and
anions are well
mixed
• Take a smallest
drop of fluid  it
is electrically
neutral
• Most cell membranes have water channels (aquaporins) 
water molecules can easily pass through the membrane
• What happens if water concentration is different between the
inside of a cell and outside?  Osmosis
6 free water molecules
on each side
Add 2 NaCl molecules  2 Na
and 2 Cl ions “bind” 4 water
molecules, leaving only 2 free
water molecules in the right
compartment
6 free water molecules in the
left compartment and 2 free
water molecules in the right
compartment equilibrate  4
free water molecules on each
side
Water molecules
can pass through
• DEFINITION: Osmosis is the process of movement of water
molecules from a solution in which water concentration is
higher to one in which water concentration is lower
• DEFINITION: Osmosis is the process of movement of water
molecules from a solution in which water concentration is
higher to one in which water concentration is lower
• DEFINITION: The total solute concentration of a
solution is called osmolarity
• Solution of 1M Glucose has osmolarity of 1 Osm
• Solution of 1M NaCl has osmolarity of 2 Osm
• Solution of 1M MgCl2 has osmolarity of 3 Osm
• By comparing osmolarity
between two compartments,
we can tell the direction of
water flow
• Most body fluids (blood, intracellular fluid,
interstitial fluid) have osmolarity = 300 mOsm
• EXAMPLE: If you ware soft contact lenses and run
out of cleaning solution … how can you wash your
contact lenses?
• Soft contact lenses are manufactures to fit your eye
just right at osmolarity
of 300 mOsm
How to prepare cleaning solution
3. Use too much salt
1. Normal contact lenses cleaning
solution (300mOsm)
2. Tap water
To prepare contact lenses cleaning
solution:
3. Use too much salt
Use a tea spoon of
table salt dissolved in
a glass of water to
obtain 300mOsm
2. Tap water
Examples of importance of osmolarity
• Fresh water aquarium fish when placed in salt
water will die
• Salt water aquarium fish when placed in fresh
water will die
Electricity
• Humans have been harvesting electricity for
less than 200 years
• Biological cells have been using electricity for
as long as cells exist.
• This lecture: we will study how cells generate
electricity
• Next lecture: we will study how cells use
electricity to communicate (or disorient or kill
a pray as is the case with electric Ray)
Ringer
solution
K+
+
+
K+
-
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=4mM
• Potassium concentration is greater inside the cell  net flow
is from inside to outside
• How many potassium ion will leave the cell before the
equilibrium is reached?
• The correct answer is counter-intuitive
• The correct answer: 2 million out of 0.5 trillion potassium ions
will leave the cell = 0.0005%
Microscopic approach
+
+
+
+
-
+
+
-
-
+
-
- +
+
++
+
+
+
+
+ +- +
+
+
+
+ +- +
-
+
-
+
+
+
+
-
+
+
-
-
+ +
- +
+
- +
+
+
+
++
- + - +
+ -+
-+
+ - +
- +
+
+
+
• One potassium ion
leaves the cell
• That creates an
excess of one
negative ion inside
-
+
+
-
-
+
+
-
- +
+
++
+
+
+
+
+ +- +
+
+
+
+ +- +
+
-
+
+
-
-
+
+
+
+
+
-
-+
- +
- -
+
+
-
+
+
-
+
-
+ +
- +
+
+
+
++
+ - +
-+
+
-
+
• Two potassium
ions leave the cell
• That creates an
excess of two
negative ion inside
+
+
-
+
+
-
•
-
-
+
+
-
- +
+
++
+
+
-+
+
+ +- +
+
+
+
+
+ +- +
+
+
-
+
-
-
+
+
-
+
+
+
-
-
+
+
+
+
+
-
-+
- +
- -
+
+
-
+
+
-
+
-
+ + +
- +
+
+
+
++ +
+ - +
-+
+
+
+
-
+
+
As more potassium ions
leave the cell, there is a
build up of positive ions
on the outside of the
cell and build up of
negative ions on the
inside of the cell
• Why do these excess charges stick to the opposite sides of the cell membrane?
 Capacitance. The cell membrane is narrow enough for the charges to experience electric
attraction between positive ions on the outside and negative ions inside
+
With this build up of
positive ions on the
outside of the cell and
build up of negative ions
on the inside of the cell,
consider a potassium ion
flowing inside a channel:
–
+
–
+
–
+
–
+
–
–
+
–
–
–
+
–
+
+
+
–
+
+
+
+
–
–
+
–
+
–
+
–
+
–
–
+
• Every time another potassium ion leaves the cell, it adds to
the force that pushes potassium ions back into the cell.
• At some point (and quite fast) an equilibrium is reached
• It is reached after just 2 million out of 0.5 trillion potassium
ions leave the cell = 0.0005%
• At equilibrium, the net flow of potassium ions = 0
+
+
-96mV
?
–
+
–
+
–
+
–
+
–
–
+
–
–
–
+
–
+
+
+
–
+
+
+
+
–
–
+
–
+
–
+
–
+
–
+
• Insert an electrode into a cell that is at equilibrium
and measure voltage across cell membrane = -96mV
• Potassium equilibrium potential is the voltage
across cell membrane at which the net flow of
potassium across cell membrane is zero.
–
+
Microscopic approach
+
+
-
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
[K ]=150mM
+
-
[K+]=4mM
• Can we prevent potassium outflow with externally
applied electric field?
Macroscopic
approach
+
-96mV
+
+
-
+
+
-
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
[K ]=150mM
+
-
[K+]=4mM
• Can we prevent potassium outflow with externally
applied electric field?
• Yes. By applying -96mVolts across the cell membrane
• Why this electrical potential of -96mVolts across cell
membrane is able to hold the flow of potassium ions?
+
-96mV
-
+ + +
+
+
-
-
+
+
-
-
- +
+
+
+
- + +
• this electrical potential of -96mVolts across cell membrane
redistributes electrical charges over cell membrane (Note: there is no
flow of charges == a capacitor)
• This new charge distribution across cell membrane creates an
electrical force that pushes potassium ions back into the cell
• (positive charges outside repel the positive potassium ion; negative
charges inside attract the positive potassium ion)
+
K+
+
+
K+
-
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=4mM
• In reality, of course, there is no external
electrical potential applied and once the
potassium channels are open, potassium
freely leaks out of a cell following its
concentration gradient.
+
-0.1mV
-
+
-0.2mV
-
-96mV
+
+
+
+
+
-
+
+
-
-
+
-
- +
+
++
+
+
+
+
+ +- +
+
+
+
+ +- +
-
+
-
+
+
+
+
-
+
+
-
-
+ +
- +
+
- +
+
+
+
++
- + - +
+ -+
-+
+ - +
- +
+
+
+
• One potassium ion
leaves the cell
• That creates an
excess of one
negative ion inside
and membrane
voltage -0.1mV
-
+
+
-
-
+
+
-
- +
+
++
+
+
+
+
+ +- +
+
+
+
+ +- +
+
-
+
+
-
-
+
+
+
+
+
-
-+
- +
- -
+
+
-
+
+
-
+
-
+ +
- +
+
+
+
++
+ - +
-+
+
-
+
• Two potassium ions
leave the cell
• That creates an
excess of two
negative ion inside
and membrane
voltage -0.2mV
+
+
-
+
+
-
-
-
+
+
-
- +
+
++
+
+
-+
+
+ +- +
+
+
+
+
+ +- +
+
+
-
+
-
-
+
+
-
+
+
+
-
-
+
+
+
+
+
-
-+
- +
- -
+
+
-
+
+
-
+
-
+ + +
- +
+
+
+
++ +
+ - +
-+
+
+
+
-
+
+
• As more potassium
ions leave the cell,
there is a build up of
positive ions on the
outside of the cell and
build up of negative
ions on the inside of
the cell; membrane
voltage is decreasing
+
-96mV
-
K+
+
+
K+
-
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=4mM
• At equilibrium, voltage across the membrane = -96mV
• Potassium leakage channels are present in all cells
• Potassium leakage channels are always open, not
regulated by voltage across cell membrane
+
-96mV
+
+
K+
-
+
+
-
+
K+
+
-
+
-
+
+
-
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
K+
-
K+
K+
[K+]=4mM
K+
+
+
-
[K ]=150mM
K+
-
K+
+
+
?0 mV
mV
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=150mM
• Jack Kevorkian: championing a patient's right to die via physicianassisted suicide: “dying is not a crime”
• He claimed to have assisted to at least 130 patients
• Arrested in 1999, put in jail for 10-25 years
• Lethal injection of anesthetic, followed by 150mM KCl
• Resting membrane potential is primarily defined by K+ leaking from high
concentration inside cell to low concentration outside cell 
no K+ leaking  no resting membrane potential (Em=0mV)
+
-96mV
-
K+
+
+
K+
-
+
+
-
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=4mM
• One electrode inside a cell, another electrode
outside the cell measure difference in electrical
potentials between the inside and outside
• Both electrodes are inside a cell.
• What voltage will we measure?
• There is no potential difference inside any
conductive medium.
?mV
K+
+
+
K+
-
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=4mM
• Both electrodes are outside a cell.
• What voltage will we measure?
• There is no potential difference inside any conductive
medium.
?mV
K+
+
+
K+
-
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=4mM
+
-96mV
-
K+
+
+
K+
-
+
+
-
+
+
-
+
+
- +
+
+
++
+
- +
++ - - +
+
+
+ - +- + -+
- +
- + - +
+
- +
+
+ +
+- +
+
-
+
+
+
-
-
- +
+
+
+
++
+ - +
-+
+
-
+
[K ]=150mM
K+
K+
-
K+
K+
[K+]=4mM
• Let’s take a closer look at Potassium channels
• Potassium channels are transmembrane proteins
• There are many other types of channels
Channels = pores
• The cytoplasmic membranes of all organisms have
selective passive transport devices = channels
• Passive = downhill flux of ions (from high
concentration to low concentration)
• High rate (>million ions per second)
• Can be very specific - analogous to border
crossings: present a passport (size/electrical
charge combination)  allowed passage or
returned to ECF
• Can be regulated
Types of channels
1.
2.
3.
4.
Voltage-gated
Ligand-gated
Mechanically gated
Light-gated
1. Voltage-gated channels
outside
inside
Voltage-gated channels
outside
inside
Voltage-gated channels
outside
inside
Resting membrane potential:
inside is negative
Cell is depolarized: inside is positive
2. Ligand-gated channels
• DEFINITION: Agonist
= a chemical that
binds to a receptor
and activates the
receptor to produce
a biological
response (nicotine)
• DEFINITION:
Antagonist = a
chemical that blocks
the action of the
normal ligand
(naloxone)
3. Mechanically-gated channels
Mechanically-gated channels (cochlea)
4. Light-gated (Channelrhodopsin)
• serve as sensory
photoreceptors in
unicellular green
algae, controlling
movement in
response to light
• natural
channelrhodopsins
are nonspecific
cation channels,
conducting H+, Na+,
K+, and Ca2+
• Channels are passive transport devices that
allow downhill fluxes of ions
• Pumps (transporters) move ions uphill, against
their concentration gradient
Na+ – K+ pump (Na+ – K+ ATPase)
outside
nside
Na+ – K+ pump (Na+ – K+ ATPase)
• Na+ – K+ pump is ubiquitous = found in all cells
• It is electrogenic (one net positive charge taken
out of the cell for every cycle), but it is not why
cells are negative inside (limited to -11mV)
• Blocked by Ouabain that is extracted from seeds
of African shrub – powerful toxin used to tip
hunting arrows (Somali waba yo = arrow poison)
• Dependence: Na+ required, K+ is not.
• Complete cycle takes 10 ms.
Inside glucose is quickly metabolized to glucose-6-phosphate
 [glucose] inside is very low