LIFE ON THE EDGE
INTRO TO THE PLASMA MEMBRANE
MEMBRANE MODELS
(from past to present)
• 1915: membranes isolated from RBC’s were
analyzed & found to be made of lipids &
proteins
• 1925: scientists (Gorter & Grendel) suggested
membrane was made of phospholipid bilayer.
(hydrophobic parts are sheltered by the
hydrophillic parts).
Where are the
proteins?
• 1935: scientists Davidson & Danielli suggested
the membrane was like a sandwich.
– Phospholipid bilayer b/w layers of proteins.
• 1950’s: came the invention of the electron microscope.
– Pictures seemed to support the sandwich model
• 1960’s: scientists had problems with model
– Cells with different functions differed in structure and chemical
make-up.
– Proteins in membranes were not very soluble (they were
amphipathic-have a hydrophobic/non-polar & hydrophilic/polar end). If
these proteins were the “bread” part of the sandwich the
hydrophobic parts would be in an aqueous environment.
•1972: Singer and Nicolson proposed the membrane
proteins were dispersed & individually inserted into
the phospholipid bilayer.
•Only the hydrophillic portions are protruding
Fluid Mosaic Model
Freeze fracture (method of preparing cells
for the electron microscope)- confirmed
Singer and Nicolson’s model.
Fluid mosaic model
• Not static or rigid
• Proteins & lipids drift back & forth
(laterally)
• Movement is rapid
– Proteins move more slowly than lipids
• Some move as directed (motor proteins
attached)
• Some don’t move (anchored)
The fluidity of a lipid bilayer depends on both its composition
and its temperature
• The temperature at which
the membrane solidifies is
dependent on the types of
lipids in the membrane
• Unsaturated lipids create a
kink, preventing the fatty
acids from packing together
as tightly, thus decreasing
the melting temperature
(increasing the fluidity) of
the membrane.
Unsaturated vs Saturated Fatty Acids in
the Membrane
• Plants, animals, & bacteria adapt to decreasing temperatures
by increasing the proportion of unsaturated fatty acids in the
membrane.
• They decrease the proportion of unsaturated fatty acids in the
membrane & increase the saturated fatty acid content in the
membrane when temps are high.
The ability of some organisms to
regulate the fluidity of their cell
membranes by altering lipid
composition is called
homeoviscous adaptation.
NOW LET’S TALK
CHOLESTEROL
-Is amphipathic.
-The hydrophobic steroid ring
which is near the hydrocarbon tails
of the phospholipids tend to
immobilize the tails.
-Restricts random movement so
membrane doesn’t turn to mush.
-Keeps lipids more separated.
• This helps slightly immobilize the outer
surface of the membrane and make it less
soluble to very small water-soluble molecules
that could otherwise pass through more easily
• Without cholesterol, cell membranes would
be too fluid, not firm enough, and too
permeable to some molecules. In other
words, it keeps the membrane from turning to
mush.
How would you expect the saturation levels of
membrane fatty acids to differ in plants adapted
to cold environments & plants adapted to hot
environments?
• Plants in cold environments would probably have
more unsaturated fatty acids in their membrane,
since those remain fluid at lower temps.
• Plants in hot environments would probably have
more saturated fatty acids to so they would be closer
to each other, causing the membrane to be less fluid.
Being less fluid would help them stay intact at higher
temperatures.
It’s the “mosiac” turn 
• Proteins - Over 50 different types
• Different cells = different proteins
• Proteins determine the function of the
membrane therefore the function of the cell.
2 Major Types of Proteins
• Integral:
-many
are transmembranes – amphimathic
-exposed to both aqueous solutions (inside &
outside of the cell)
• Peripheral
-not
embedded in the lipid bilayer
-they are appendages to integral proteins & are
attached to the cytoskeleton
Integral
Peripheral
ENZYAMTATIC
Catalyze reactions
inside the cell
JOINING
Bind INTRACELLULAR
cells together to make
tissues
ATTACHMENT
TO CYTOSKELETON
Allows
cells to bind
to each other or a&
THE ECM
substrate
Moves molecules in & out of the cell
TRANSPORT
using passive or active passageways.
Convey signals from the outside of the cell
TRANSDUCTION
to theSIGNAL
inside; functions
like a light switch.
Allow immune system to recognize
RECOGNITION
what is self
& not self
CARBOHYDRATES OF THE MEMBRANE
• Cell to Cell Recognition:
– This comes in handy when in the embryonic stage
– Basis for rejection of organ transplants by immune system.
• 2 types
– Glycolipids- carbohydrate bonded to a lipid
– Glycoprotein- carbohydrate bonded to a protein
• Function as markers- distinguish one cell from the
other.
– Ex: human blood types: each have variation in their
carbohydrate markers.
How does the
endoplasmic
reticulum make
the plasma
membrane?
WHAT IS THE PLAMA MEMBRANE
MADE OF?
• LIPIDS
– PHOSPHOLIPIDS (amphipathic)
– CHOLESTEROL
• PROTEINS
– INTEGRAL PROTEINS (amphipathic)
– PERIPHERAL PROTEINS
• CARBOHYDRATES
– GLYCOLIPIDS (carbohydrate and lipid)
– GLYCOPROTEINS (carbohydrate and protein)
DRAWING A MODEL OF THE CELL MEMBRANE
•
•
•
Diagram a model of the cell membrane.
Make sure your model includes the following components:
a. Phospholipid bilayer (heads & tails!)
b. Integral proteins
c. Peripheral proteins
d. Glycolipids
e. Carbohydrates
f. Glycoproteins
g. Cholesterol
Make a key to show what each item represents
How do molecules pass in and out of
the plasma membrane?
• If you (the molecule) are small and non-polar
OR small & uncharged you can pass through
the lipid bilayer.
– N2 ,O2, CO2
• The hydrophobic tails of the lipid bilayer
impedes ions and polar molecules.
– This includes glucose and water!!
Transport Proteins
• Hydrophylic substances (some ions and polar
molecules) pass through the membrane by
way of the channel proteins
– aquaporins facilitate the passage of water (and
several diseases, such as congenital cataracts and nephrogenic diabetes
insipidus, are connected to the impaired function of these channels)
Transport Proteins
• Carrier proteins: these proteins change shape
as they pass substances through. They are
highly selective.
What mechanisms drive molecules
across the membrane?
• Passive Transport
– Diffusion
– Osmosis
– Facilitated diffusion
• Active Transport
– Sodium Potassium Pump/Electrogenic pump
– Cotransport
– Exocytosis
– Endocytosis
Diffusion
• A substance will travel from where it is more
concentrated to where it is less concentrated.
– Substances travel down its concentration gradient.
• No work needs to be done
– No need for energy
• It is a spontaneous process
• Most travel of substances across is by diffusion
– EX: oxygen transfer; CO2 transfer
Osmosis
• diffusion of water molecules
• across a selectively permeable membrane
• Movement is from areas of high potential
(high free water concentration) and low
solute concentration to areas of low
potential (low free water concentration) &
high solute concentration.
WATER MOLECULES
CLUSTERED AROUND
HYDROPHILIC MOLECULES
ARE UNAVAILABLE TO TRAVEL
THROUGH THE MEMBRANE.
Solutions of Osmosis
•HYPERTONIC:
•Has a higher solute concentration and a lower water
potential compared to the solution on the other side
of the membrane.
•HYPOTONIC:
•Has a lower solute concentration and a higher water
potential than the solution on the other side of the
membrane
•ISOTONIC:
•Have equal water potentials
Cells without cell walls
• In a hypotonic environment, water enters cell
and it swells and may burst
• In a hypertonic environment, water leaves the
cell and it shrinks
OSMOREGULATION:CONTROL OF WATER BALANCE
Turgor Pressure
• most plant cells live in hypotonic environment
• water moves into cells, pushing cell membrane
against cell wall
• cell wall is strong enough to resist pressure
• pressure from the water is called turgor pressure
Plasmolysis
•
•
•
•
plant cells in hypertonic environment
water leaves cells
cell membrane moves away from cell wall
loss of turgor pressure (wilting in plants)
• In plant cells, the presence of a cell wall prevents the cells from
bursting, but pressure does eventually build up inside the cell
and affects the process of osmosis.
• When the pressure inside the cell becomes large enough, no
additional water will accumulate in the cell even the though cell
still has a higher solute concentration than does pure water.
• So movement of water through the plant tissue cannot be
predicted simply through knowing the relative solute
concentrations on either side of the plant cell wall.
• Instead, the concept of water potential is used to predict the
direction in which water will diffuse through living plant tissues.
• In a general sense, the water potential is the tendency of water
to diffuse from one area to another under a given set of
parameters.
Osmosis and Water Potential
When water diffuses through a selectively permeable membrane from an
area of high water potential to an area of low water potential this is called
osmosis.
Water potential measures the tendency of water to diffuse from one
compartment to another compartment in plant cells based on solute
concentration and pressure.
ψ = ψP + ψS
Solute potential :AKA osmotic potential (ψs) is dependent on the solute concentration
Pressure potential (ψP) results from the exertion of pressure-either positive or negative
The water potential of pure water in an open beaker is zero because both
solute and pressure potential is zero.
Increase in positive pressure raises the pressure potential & the water potential.
Add solute; this lowers the solute potential thus decreasing the water potential
THE SOLUTE POTENTIAL (ΨS) = -iCRT
(osmotic potential)
i = ionization constant
(sucrose = 1; salt = 2)
C = molar concentration
R = pressure constant = 0.0831 liter bars/mole-K
T is temperature in °K = 273 + 0C
A 0.15M solution of sucrose at atmospheric pressure (ψP=0) and 250C has an
osmotic potential of -3.7bars and a water potential of -3.7bars.
A bar is measure of pressure & is the same as 1 atmosphere at sea level.
What if the solution was NaCl? What would the water potential be? -7.4bars
Calculating Water Potential
Example: You have allowed slices of cucumber to sit overnight in several different
sucrose solutions. You weighed the slices beforehand and find that the slice that
you put in the 0.5M sucrose solution has not had any change in its mass.
Using this data: 1) determine the molar concentration of solutes within the
cucumber cells, 2) calculate the solute potential (ψS) of the 0.5M sucrose solution,
and 3) calculate the water potential within the cucumber slice. Assume temperature
is 22 °C. 1bar = 0.1megapascal = 1kg/cm2
1) The concentration of solutes in the cucumber will be the same as the
concentration of sucrose at which the cucumber slice has not gained mass, 0.5M.
2) ΨS = -iCRT
T in °K = 273 + 22 = 295
ΨS = -(1.0)(0.5mol/liter)(0.0831liter bars/mole °K)(295 °K) = -12.26 bars
3) Ψ = ΨS + ΨP
Ψ = ΨS + 0
Ψ = -12.26
At equilibrium the water potential of the cucumber will be
equal to the water potential of the solution.
How can you measure water
potential in plant cells?
• You can do this by measuring/calculating
change in mass, change in length, or change in
volume over time in plant sections from
potatoes.
• DESIGN LAB:
– TO IDENTIFY THE CONCENTRATIONS OF THE
SUCROSE SOLUTIONS AND THEN USE THE
SOLUTIONS TO DETERMINE THE WATER
POTENTIAL OF THE PLANT TISSUES.
FACILITATED DIFFUSION
CHANNEL
PROTEIN
EX: aquaporins
MOVE CHARGED
POLAR MOLECULES
ACROSS
MEMBRANE
CARRIER
PROTEIN
EX: Cysteine transporter
Hydrophillic
passageway
ACTIVE TRANSPORT
• Where free energy (often provided by ATP) is
used by proteins embedded in the membrane
to “move” molecules &/or ions across the
membrane & to establish or maintain
concentration gradients.
• Membrane proteins are necessary
WHICH MEMBRANE PROTEINS
ARE USED?
CARRIER PROTEINS
AN EXAMPLE OF ACTIVE TRANSPORT
SODIUM-POTASSIUM PUMP
•Contributes to the membrane potential
•Pumps 3 Na+ out of cell for every 2 K+.
•Creates a positive charge from
cytoplasm to extracellular fluid.
•Stores energy in the form of voltage
•Major electrogenic pump of animals
•Proton pump for plants, fungi, &
bacteria.
What is a nerve impulse?
• Nerve impulse is misleading. We will call it an
action potential instead
• Can be measured in the same way as
electricity is measured
– Voltage
• Millivolts
• The conductor of a neuron is the axon
– Is covered by a myelin sheath
• Increases the rate at which an action potential passes
down an axon.
Resting potential
• Area of a neuron that is ready to send an action
potential but is not currently sending one.
• This area is considered polarized
– Characterized by the active transport of sodium ions
(Na+ ) out of the axon cell & potassium ions (K+) into
the cytoplasm.
– There are negatively charged ions permanently
located in the cytoplasm
– This collection of charged ions leads to a net positive
charge outside the axon membrane & negative charge
inside.
Action Potential
• Described as a self-propagating wave of ion movements in and
out of the neuron membrane
• This is the diffusion of the Na+ & the K+ .
– Sodium channels open & then potassium ones do to.
• This is the “impulse” or action potential
• It is a nearly instantaneous event occurring in one area of the
axon = depolarization
– This area initiates the next area on the axon to open up the channels.
• This action continues down the axon.
• Once an impulse is started at the dendrite end that action
potential will self-propagate itself to the far axon end of the
cell.
Action
Potential
Resting
Potential
Return to Resting Potential
• Remember that one neuron may send dozens of
action potentials in a very short period of time.
• Once an area of the axon sends an action
potential it cannot send another until the Na+ &
K+ have been restored to their positions at the
resting potential.
• Active transport is required to move the ions =
repolarization
– The time it takes for a neuron to send an action
potential & then repolarize is called: the refractory
period of that neuron.
Maintenance of Membrane Potential
by Ion Pumps
• Electrical potential across the membrane=
voltage (separation of opposite charges).
– Cytoplasm has a negative charge compared to the
extracellular fluid
• Voltage across membrane = membrane
potential
– Membrane potential favors passive transport of
cations (+) into cell & anions (-) out of the cell
So… what causes diffusion of ions?
• Electrochemical gradient
– Electrical force
– Concentration gradient
• EX: Na+ concentration inside a resting nerve is much
lower than the concentration outside it.
– When the cell is stimulated gated channels open & Na+
“fall” down their electrochemical gradient driven by the
concentration gradient of the Na+ & the attraction of the
cations to the negative side of the membrane.
COTRANSPORT
EXOCYTOSIS
PANCREAS SECRETING INSULIN TO THE BLOOD STREAM
CELLULAR
EATING
CELLULAR
DRINKING
ENDOCYTOSIS
What is the importance of active
transport in biological systems?
• It is responsible for enabling us to absorb more
food from our intestines.
• If there is no active transport, then most of the food
that we eat will be wasted.
• If absorption only depends on diffusion, then once
the concentration of food in and out of the intestinal
cells becomes the same, then absorption will stop.
• But as we have experienced, we can have 2nd
helpings or even 3rds of some of our favorite food.
So we have active transport to thank (blame?)for
that.
CHOLESTEROL
• Cholesterol is an important component in various
systems throughout the body.
• Cholesterol is necessary for the creation of certain bile
salts necessary for the digestion of saturated fats and
sugars.
• It is also used in the manufacturing of certain hormones,
like testosterone.
• Helps your body make vitamin D.
• Important in the creation of cells, because it contributes
to cell structure by strengthening cell walls.
High Density Lipoprotein
Average is around 40-60 mg/dL.
• HDL travels through the bloodstream after being
produced in the liver.
– As it travels it picks up excess cholesterol.
– It then carries this excess cholesterol back to the
liver where it is used as a building block for bile.
• By using the cholesterol to create bile there is
less in your arteries that can cause blockage.
• Therefore, HDL is considered good since it helps
to reduce cholesterol in the bloodstream.
Low Density Lipoproteins
Your goal is to have a number that is less than 100 mg/dL,
though 100-129 mg/dL is optimal.
• In general, LDL is not bad – it is having too much LDL
is when the problems begin.
• LDL carries cholesterol through the blood and
delivers it to cells that need it to build things such as
hormones or fat.
• The problem is that LDL is soft enough to get into the
walls of your arteries.
• This can lead to a process that builds up plaque in
the arteries, eventually leading to a heart attack.
ATHEROSCLEROSIS
Normally cholesterol (LDL’s) binds to a receptor site on
the cell membrane and then is brought into the cell by
way of endocytosis.
If LDL receptor
sites are defective
or just aren’t even
there…..
HOW ARE CARRIER PROTEINS
LIKE ENZYMES?
They have specificity, they have
conformational changes, they are
dependent on concentrations…