RHPT 243 Unit 2 Membrane Physiology
Dr. Moattar Raza Rizvi
PRESCRIBED LEARNING OUTCOMES
1. Apply knowledge of organic molecules to explain the structure and
function of the fluid-mosaic model
2. Explain why the cell membrane is described as “selectively
permeable”
3. Compare and contrast the following: diffusion, facilitated transport,
osmosis, and active transport.
4. Explain factors that affect the rate of diffusion across a cell
membrane.
5. Describe endocytosis, including phagocytosis and pinocytosis, and
contrast it with exocytosis.
6. Predict the effects of hypertonic, isotonic, and hypotonic
environments on animal cells
7. Demonstrate an understanding of the relationship and significance
of surface area to volume, with reference to cell size.
PLASMA MEMBRANE
SELECTIVELY PERMEABLE: Controls what comes in and out
of the cell. Does not let large, charged or polar things through
without help.
PLASMA MEMBRANE
• Fluid Mosaic Model
– Phospholipid bilayer with proteins and cholesterol embedded
within bilayer.
– Cholesterol makes bilayer stiffer or more viscous!!
– stops the membrane from becoming solid at room temperatures
– Membrane composition depends on function (ie. More lipid in
Schwann cells and more protein in mitochondria).
• Intrinsic/Integral vs. Extrinisic/Peripheral Proteins
– Intrinsic proteins span the entire membrane and contain
hydrophillic ends and a hydrophobic core, often serving as
transporters.
– Extrinsic proteins are present on one side of the bilayer or the
other and are anchored by electrostatic interactions.
– Glycolipids can be conjugated with either an intrinsic or extrinsic
protein and serve as a surface marker for the cell.
FLUID MOSAIC MODEL
The phospholipids move, thus allowing small non-polar
molecules to slip through.
FLUID MOSAIC MODEL
GLYCOLIPIDS and GLYCOPROTEINS: Act as
receptors – receive info. from body to tell cell
what to do.
FLUID MOSAIC MODEL
INTEGRAL PROTEINS: assists specific larger and charged
molecules to move in and out of the cell. Can act as ‘tunnels’
or will change shape.
FLUID MOSAIC MODEL
CHOLESTEROL: Reduces membrane fluidity by reducing
phospholipid movement. Also stops the membrane from
becoming solid at room temperatures.
FLUID MOSAIC MODEL
CYTOSKELETON: A cytoskeleton acts as a framework that gives the cell it's
shape. It also serves as a monorail to transport organelles around the cell.
TRANSPORT ACROSS THE MEMBRANE
Everything that is transported across the cell membrane
takes place by one of two fundamental processes:
1. Passive transport moves
molecules from a [high] to
[low] in order to establish
equilibrium.
The molecules may or may
not need to use a protein
channel or carrier.
TRANSPORT ACROSS THE MEMBRANE
TRANSPORT ACROSS THE MEMBRANE
2. Active transport moves molecules from [low] to [high],
AGAINST the concentration gradient and this process
requires energy in the form of ATP.
SIMPLE DIFFUSION
Simple Diffusion is a passive process ( no energy required).
Some substances will diffuse through
membranes as if the membranes
weren’t even there.
Molecules diffuse until they are
evenly distributed.
The molecules move from an area of [high] to [low].
EXAMPLES of molecules that easily cross cell membranes
by simple diffusion are: oxygen, carbon dioxide, alcohols,
fatty acids, glycerol, and urea.
Diffusion
• Diffusion is driven by concentration gradients.
C
• Fick’s 1st Law of Diffusion: J   DA
X
– Use to calculate Rate of Diffusion
– Note: ∆C = C1-C2 where C1 = target compartment
• Stokes-Einstein Equation:
kT
D
6r
– Use to calculate Diffusion Coefficient
• Partition Coefficient ()
Lipid Solubility
– Expresses relative
– 0 (lipid insoluble)  1 (completely lipid soluble)
SIMPLE DIFFUSION
The rate of diffusion will be increased when there is :
1. Concentration ∆C : The greater the difference in
concentration, the faster the diffusion.
2. Molecular size: smaller substances diffuse more quickly.
Large molecules (such as starches and proteins) simply
cannot diffuse through.
3. Shape of Ion/Molecule: a substance’s shape may prevent
it from diffusing rapidly, where others may have a shape
that aids their diffusion.
SIMPLE DIFFUSION
4. Viscosity of the Medium: the lower the viscosity, the more
slowly molecules can move through it.
5. Movement of the Medium: currents will aid diffusion. Like
the wind in air, cytoplasmic steaming (constant movement
of the cytoplasm) will aid diffusion in the cell.
6. Solubility: lipid - soluble molecules will dissolve through
the phospholipid bilayer easily, as will gases like CO2 and
O2.
7. Polarity: water will diffuse, but because of its polarity, it will
not pass through the non-polar phospholipids. Instead,
water passes though specialized protein ion channels.
where is diffusion important?
TRANSPORT ACROSS THE MEMBRANE
OSMOSIS
OSMOSIS
Osmosis is the diffusion of water across a selectively
permeable membrane driven by a difference in the
concentration of solutes on the two sides of the
membrane.
A selectively permeable
membrane is one that
allows unrestricted
passage of water, but not
solute molecules or ions.
So it is the WATER THAT
MOVES to create
equilibrium!!!
OSMOSIS
•
Osmosis requires NO ENERGY.
•
Osmosis is the net movement of
WATER molecules from the area
of [high] of water to the area of
[low] of water until it is equally
distributed.
•
Because membranes often
restrict or prevent the movement
of some molecules, particularly
large ones, the water (solvent)
must be the one to move.
OSMOSIS
OSMOSIS
•To cross the membrane,
water must move
through a protein ion
channel.
•In certain cellular
conditions, these protein
channels can be opened
or closed (ie: in the
kidneys, large
intestines) depending on
how much water is
needed by the body.
OSMOSIS
• Van’t Hoff’s Law: π=RT(iC)
o Use to calculate osmotic pressure
o π = pressure required to oppose the movement
of water from an area of high [H2O] (low
osmolarity) to an area of low [H2O] (high
osmolarity).
• Osmotic Flow Rate
o Vw=L∆π
o Use to calculate the osmotic flow rate of water
when the membrane is permeable to both water
and solute.
o  = reflection coefficient (0-1) - a high
reflection coefficient reflects a solute that does
NOT permeate the membrane well.
OSMOSIS
TONICITY OF A SOLUTION
The tonicity of a solution will affect the size & shape of cells:
ISOTONIC SOLUTION:
1. the solution concentration is
equal on both sides of the
membrane .
2. There is no net concentration
difference across the cell
membrane
3. Water moves back and forth,
but there is no net gain or
loss of water.
TONICITY OF A SOLUTION
TONICITY OF A SOLUTION
HYPERTONIC SOLUTION:
1. The solution outside the
cell is more concentrated
than inside.
2. There is more water
inside the cell and the
water will move out of the
cell.
3. This causes the cell to
shrink
4. *Memory Trick... Hyper
people get skinny!
TONICITY OF A SOLUTION
TONICITY OF A SOLUTION
HYPOTONIC SOLUTION:
1. The concentration inside the
cell is more concentrated
than outside.
2. Therefore there is more
water outside of the cell,
and water will move into the
cell.
3. This will cause the cell to
swell.
4. *Memory Trick... Hippos are
FAT!
TONICITY OF A SOLUTION
hyper, hypo, and isotonic
Red blood cells placed in Ringer's lactate solution will exhibit no change in
cell volume since the solution is isotonic to the cells
a 0.2% NaCl solution will exhibit hemolysis as this solution is hypotonic
Red blood cells placed in a 0.3 m urea solution (urea is permeable) will exhibit
hemolysis as will diffuse into the cell causing the cell to become hypertonic to
the solution
0.9% NaCl solutions
is isotonic relative
to blood plasma
hyper, hypo, and isotonic
Osmosis
In Biology we usually talk about the SOLUTION’S tonicity,
NOT the cells!
Hyperosmotic
*MEMORY TRICK: If you eat a lot of sugar (ie: solute)
you get HYPER. The solution with a lot of solute is
called HYPEROSMOTIC.
Hyperosmotic
hyper, hypo, and isotonic
where is osmosis important?
FACILITATED DIFFUSION
Facilitated Transport: Some molecules are not
normally able to pass through the lipid membrane,
and need channel or carrier proteins to help
them move across.
This does not require energy when moving
from [H] to [L] (with the concentration gradient).
Molecules that need help to move through the
plasma membrane are either charged, polar,
or too large.
FACILITATED DIFFUSION
If molecules are POLAR, CHARGED, or TOO LARGE they
need a protein the help them across the membrane
EXAMPLES: sugars, amino acids, ions, nucleotides ….
FACILITATED DIFFUSION
Each protein
channel or protein
carrier will allow
only ONE TYPE
OF MOLECULE to
pass through it.
FACILITATED DIFFUSION
Many channels contain a “gate” which control the channel's
permeability.
When the gate is open, the channel transports, and when the
gate is closed, the channel is closed.
These gates are extremely important in the nerve cells.
where is facilitated transport important?
ACTIVE TRANSPORT
Active Transport: the movement of
polar, large, and charged molecules
moving against the [ ] gradient
(uphill).
EXAMPLES of molecules that
move this way are all of the things
that require protein carriers to move
across the plasma membrane.
ions (like Na+ and K+ in cells, and
iodine) and sugars, amino acids,
nucleotides...
ACTIVE TRANSPORT
ACTIVE TRANSPORT
ACTIVE TRANSPORT
Low to High
EXAMPLES OF ACTIVE TRANSPORT
Example 1: the thyroid gland accumulates
iodine as it is needed to manufacture the
hormone thyroxin.
The iodine concentration can be as much as 25
times more concentrated in the thyroid than in
blood.
EXAMPLES OF ACTIVE TRANSPORT
EXAMPLES OF ACTIVE TRANSPORT
Example 2: a Na/K pump (mostly in nerve membranes).
These function to restore electrical order in a nerve after an
impulse has traveled along it.
In each cycle the sodium-potassium pumps transfer three sodium
ions out and two potassium ions in
EXAMPLES OF ACTIVE TRANSPORT
Example 3: In order to make ATP in the mitochondria,
a proton pump (hydrogen ion) is required.
where is active transport important?
ENDOCYTOSIS & EXOCYTOSIS
Vesicular Transport
Exocytosis
• Moves materials out of the cell
• Material is carried in a membranous vesicle
• Vesicle migrates to plasma membrane
• Vesicle combines with plasma membrane
• Material is emptied to the outside
Examples
- Secretion of digestive enzymes by pancreas
- Secretion of mucous by salivary glands
- Secretion of milk by mammary glands
ENDOCYTOSIS & EXOCYTOSIS
ENDOCYTOSIS
Endocytosis: (“Endo” means “in”).
Endocytosis is the taking in of molecules or particles by
invagination of the cell membrane forming a vesicle.
This requires energy.
ENDOCYTOSIS
There are two types of endocytosis:
1. pinocytosis (cell drinking): small
molecules are ingested and a vesicle
is immediately formed. This is seen
in small intestine cells (villi)
2. phagocytosis (cell eating): large
particles, (visible with light
microscope) are invaginated into the
cell (ie: white blood cells ‘eat’
bacteria).
ENDOCYTOSIS
ENDOCYTOSIS
EXOCYTOSIS
Exocytosis: (“Exo” means “out”.)
•Exocytosis is the reverse of
endocytosis.
•This is where a cell releases the
contents of a vesicle outside of the cell.
•These contents may be wastes, proteins, hormones, or
some other product for secretion.
•This also requires energy.
•Example: vesicles from the Golgi fuse with the plasma
membrane and the proteins are released outside of the cell.
Membrane Potentials
• Results because of an unequal distribution
of charge across a membrane
• Two equations you need to know:
1) Nernst Equation
2) Goldman’s Equation
Nernst Equation
60mV  X A
E  
log

 z
 X 

B
(Don’t forget about “z”…valence of ion)
-
Use to calculate the membrane potential of an ion at
equilibrium
Represents the electrical potential necessary to
maintain a certain concentration gradient of a
permeable solute.
Nernst Equation
Nernst Equation:
The Nernst equation enables us to calculate the membrane
voltage that exactly balances the diffusion of a particular ion
down its concentration gradient. To use the equation,
however, we must know all of the following,
• valence of the ion (for example, +1 for potassium; +2 for
calcium)
• intracellular concentration of the ion in mEq/L
• extracellular concentration of the ion in mEq/L
The sodium equilibrium potential, ENa, is about +60mV
Goldman’s Equation
P K   P Na  P Cl 

E  (60mv)log
P K   P Na  P Cl 

– Used to calculate overall membrane potential when
k
o
Na
o
k
i
Na
i
Cl
i
m
multiple ions are involved.
– Incorporates permeability of each ion.
– Permeability of K+ > Na+ > Cl- … thus..
K+ drives Resting
Membrane Potential
Cl
o
1. Write the difference between active and passive transport
with one example?
2. Describe simple diffusion?
3. Describe facilitated diffusion?
4. Describe osmosis?
5. Write the factors contributing to the rate of diffusion?
6. Explain Nerst Equation with formula?
7. Explain any 4 factors that affect the rate of diffusion across
a cell membrane?
8. Explain the 2 types of endocytosis?
9. Write any 3 examples of Active transport?
10. Define isotonic, hypertonic and hypotonic solution?
11. Explain the mechanism of exocytosis?
12. Expalin the mechanism of endocytosis?
13. Explain the importance of sodium potassium pump?
14. What is the difference between pinocytosis and phagocytosis?