Membrane Physiology
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
the relationship and significance
Membrane of
Physiology
2
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.
Membrane Physiology
3
PLASMA MEMBRANE
• Fluid Mosaic Model
– Phospholipid bilayer with proteins and cholesterol embedded
within bilayer.
– Cholesterol makes bilayer stiffer or more viscous!!
– 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
Membrane Physiology
4
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.
Membrane Physiology
5
FLUID MOSAIC MODEL
GLYCOLIPIDS and GLYCOPROTEINS: Act as
receptors – receive info. from body to tell cell
what to do.
Membrane Physiology
6
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.
Membrane Physiology
7
FLUID MOSAIC MODEL
CHOLESTEROL: Reduces membrane fluidity by reducing
phospholipid movement. Also stops the membrane from
becoming solid at room temperatures.
Membrane Physiology
8
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.
Membrane Physiology
9
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.
Membrane Physiology
10
TRANSPORT ACROSS THE MEMBRANE
Membrane Physiology
11
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.
Membrane Physiology
12
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, andMembrane
urea.Physiology
13
Membrane Physiology
15
Membrane Physiology
16
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
Membrane Physiology
– 0 (lipid insoluble)  1 (completely lipid soluble)
18
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.
Membrane Physiology
19
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 passesMembrane
though
specialized protein ion20
Physiology
channels.
where is diffusion important?
Membrane Physiology
21
TRANSPORT ACROSS THE MEMBRANE
Membrane Physiology
22
OSMOSIS
Membrane Physiology
23
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!!!
Membrane Physiology
24
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.
Membrane Physiology
25
OSMOSIS
Membrane Physiology
26
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.
Membrane Physiology
27
OSMOSIS
Membrane Physiology
28
• 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
Membrane
Physiology
29
NOT permeate the
membrane
well.
OSMOSIS
Membrane Physiology
30
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.
Membrane Physiology
31
TONICITY OF A SOLUTION
Membrane Physiology
32
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! Membrane Physiology
33
TONICITY OF A SOLUTION
Membrane Physiology
34
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!
Membrane Physiology
35
TONICITY OF A SOLUTION
Membrane Physiology
36
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
isMembrane
isotonicPhysiology
relative to
blood plasma
37
hyper, hypo, and isotonic
Membrane Physiology
38
Osmosis
In Biology we usually talk about the SOLUTION’S tonicity,
Membrane Physiology
39
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.
Membrane Physiology
40
Hyperosmotic
Membrane Physiology
41
hyper, hypo, and isotonic
Membrane Physiology
42
where is osmosis important?
Membrane Physiology
43
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.
Membrane Physiology
44
FACILITATED DIFFUSION
If molecules are POLAR, CHARGED, or TOO LARGE they
need a protein the help them across the membrane
Membrane Physiology
EXAMPLES: sugars, amino
acids, ions, nucleotides45 ….
FACILITATED DIFFUSION
Each protein
channel or protein
carrier will allow
only ONE TYPE OF
MOLECULE to
pass through it.
Membrane Physiology
46
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.
Membrane Physiology
47
where is facilitated transport important?
Membrane Physiology
48
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...
Membrane Physiology
49
ACTIVE TRANSPORT
Membrane Physiology
50
ACTIVE TRANSPORT
Membrane Physiology
51
ACTIVE TRANSPORT
Low to High
Membrane Physiology
52
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.
Membrane Physiology
53
EXAMPLES OF ACTIVE TRANSPORT
Membrane Physiology
54
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.
Membrane Physiology
55
EXAMPLES OF ACTIVE TRANSPORT
Example 3: In order to make ATP in the mitochondria, a
proton pump (hydrogen ion) is required.
Membrane Physiology
56
where is active transport important?
Membrane Physiology
57
ENDOCYTOSIS & EXOCYTOSIS
Membrane Physiology
58
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
Membrane Physiology
59
ENDOCYTOSIS & EXOCYTOSIS
Membrane Physiology
60
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.
Membrane Physiology
61
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).
Membrane Physiology
62
ENDOCYTOSIS
Membrane Physiology
63
ENDOCYTOSIS
Membrane Physiology
64
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 Physiology
65
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
Membrane Physiology
66
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.
Membrane Physiology
67
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
Membrane Physiology
68
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
Cl
o
multiple ions are involved.
– Incorporates permeability of each ion.
– Permeability of K+ > Na+ > Cl- … thus..
K+ drives Resting
Membrane Potential
Membrane Physiology
69