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Transcript 
Cells and Their
Housekeeping Functions –
Cell Membrane &
Membrane Potential
Shu-Ping Lin, Ph.D.
Institute of Biomedical Engineering
E-mail: [email protected]
Website: http://web.nchu.edu.tw/pweb/users/splin/

http://en.wikipedia.org/wiki/Cell_membrane#Lipid_bilayer
* Cell membrane, also
called plasma
membrane, separates
cell interior from
surroundings
Thin barrier = 7~10 nm
thick
 Controls traffic in & out of
the cell
 Selectively permeable




* Made of phospholipids,
proteins, carbohydrates
& other macromolecules
*Phospholipids arrange as
a bilayer:


Hydrophobic fatty acid tails
Hydrophilic phosphate
group head
Phospholipid Bilayer

Several important functions:
*A. Allow nutrients to enter cell, *B. Keep out unwanted molecules
and particles, *C. Transport waste out into extracellular fluid,
*D. Prevent needed metabolites and ions from leaving cell

Inherently amphipathic nature: possess both hydrophilic &
hydrophobic structures
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
More Than Lipids…

In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
It’s like a fluid…
It’s like a mosaic…
It’s the
Fluid Mosaic Model!
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion
.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9TuKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Cell membrane is composed of a double layer of phospholipid
molecules (called phospholipid or lipid bilayer), protein
molecules associated with lipid bilayer, and carbohydratecontaining cell coat called glycocalyx.
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
Cytoplasm
Transmembrane
proteins
Filaments of
cytoskeleton
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Membrane Fat Composition Varies

Fat composition affects flexibility

membrane must be fluid & flexible


about as fluid as thick salad oil
% unsaturated fatty acids in phospholipids


keep membrane less viscous
cold-adapted organisms, like winter wheat


increase % in autumn
cholesterol in membrane
http://www.ocvts.org/instructors/htm/asprague/Biot
ech/Ch06CellMembraneDiffusion.ppt+cell+membrane+
ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3sr9wakHHx6SCbBg9TuKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFaf
YX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp
-LymP07E_VLiGzjkC-wqjHTg
Membrane Proteins

Proteins determine membrane’s specific functions


Cell membrane & organelle membranes each have unique
collections of proteins
Membrane proteins:


Peripheral proteins

Loosely bound to surface of membrane

Cell surface identity marker (antigens)
Integral proteins

Penetrate lipid bilayer, usually across whole membrane
Transmembrane protein

Transport proteins


channels, pumps
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Why are
proteins the perfect
molecule to build structures
in the cell membrane?
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Classes of Amino Acids
What do these amino acids have in common?
nonpolar & hydrophobic
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Classes of Amino Acids
What do these amino acids have in common?
I like the polar
ones the best!
polar & hydrophilic
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9TuKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Proteins Domains Anchor Molecule

Within membrane

Nonpolar amino acids



Polar areas
of protein
Hydrophobic
Anchors protein
into membrane
On outer surfaces of
membrane

Polar amino acids


Hydrophilic
Extend into extracellular
fluid & into cytosol
Nonpolar areas of protein
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
identity marker
Cell adhesion
Cell surface
receptor
Attachment to the
cytoskeleton
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Membrane Carbohydrates


The carbohydrates are not inserted into the
membrane -- they are too hydrophilic for that. They
are attached to embedded proteins -- glycoproteins.
Play a key role in cell-cell recognition



Ability of a cell to distinguish one cell from another
 Antigens
Important in organ &
tissue development
Basis for rejection of
foreign cells by
immune system
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+p
pt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3sr9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Movement Across
the Cell Membrane
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Selectively Permeable










What molecules can
get through directly?
Small nonpolar molecules such
as carbon dioxide, nitrogen, and
oxygen diffuse freely across the
bilayer.
Lipid bilayer is also permeable to
small and uncharged polar
molecules such as urea and
ethanol.
Other small hydrophobic
molecules: fats and other lipids
NOT get through directly?
Ions: salts, ammonia (NH3)
Large uncharged or charged polar
molecules: starches, proteins
H2O molecule has 2 pathways:
Lipid pathway
Water channel-protein pathway
(polar molecules)
Diffusion Across Cell Membrane

Cell membrane is the boundary between inside
& outside…

Separates cell from its environment
Can it be an impenetrable boundary? NO!
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
OUT
IN
waste
ammonia
salts
CO2
H2 O
products
cell needs materials in & products or waste out
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Diffusion

2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion

movement from high  low concentration
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Diffusion

Move from HIGH to LOW concentration


“passive transport”
no energy needed

Osmotic pressure: the pressure
is required to stop the net flow of
water across a membrane
separating solutions of different
particulate concentration
diffusion
movement of water
osmosis
http://www.ocvts.org/instructors
/htm/asprague/Biotech/Ch06Cell
MembraneDiffusion.ppt+cell+mem
brane+ppt&hl=zhTW&gl=tw&pid=bl&srcid=ADGEES
hywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3
s-r9wakHHx6SCbBg9TuKL7xUvpYTtYHO3iB1SQtH0eo8bD
2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFR
csI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Zero flux
The Special Case of Water:
Movement of water
across the cell membrane
2007-2008
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Osmosis


Osmosis is diffusion of water
Diffusion of water from
high concentration of water to
low concentration of water

across a
semi-permeable
membrane
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt
+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9TuKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafYX0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg
Concentration of Water

Direction of osmosis is determined by
comparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Effect of Osmotic Pressure




Suppose an animal or a plant cell is placed in a solution of sugar or salt in
water.
If the medium is hypotonic — a dilute solution, with a higher water
concentration than the cell  The cell will gain water and wilt through
osmosis.
If the medium is isotonic — a solution with exactly the same water
concentration as the cell  There will be no net movement of water
across the cell membrane.
If the medium is hypertonic — a concentrated solution, with a lower water
concentration than the cell  The cell will lose water and shrink by osmosis.
http://en.wikipedia.org/wiki/Osmosis
Managing Water Balance

Cell survival depends on balancing water uptake & loss
freshwater

balanced
saltwater
Animal cells culture: must maintain in isotonic cell culture
medium (concentration of solutes is close to cell cytoplasm)
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Aquaporins


1991 | 2003
Cores of these channel proteins are hydrophilic
Water moves rapidly into & out of cells


Multiple water molecules pass through membrane at a rate
of 108 molecules/sec
Evidence that there were water channels
Peter Agre
John Hopkins
Roderick MacKinnon
Rockefeller
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Channels Through Cell Membrane

Membrane becomes semi-permeable
with protein channels

specific channels allow specific material
across cell membrane
inside cell
NH3
H 2O
salt
aa
sugar
outside cell
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Facilitated Diffusion

Diffusion through protein channels


Channels move specific molecules across
cell membrane
facilitated = with help
No energy needed
open channel = fast transport
high
low
“The Bouncer”
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Active Transport

Cells may need to move molecules against
concentration gradient



Shape change transports solute from
one side of membrane to other
Protein “pump”
“Costs” energy = ATP
conformational change
high
ATP
low
“The Doorman”
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Many Models & Mechanisms of
Active Transport

Ions and polar molecules across cell membranes include:




ATP-powered pumps
Transporter proteins
Ion channels
The arrows indicate the direction from high to low
concentration of the ion or polar molecule across
membrane
high
low
high
low
low
high
Getting through cell membrane

Passive Transport

Simple diffusion

diffusion of nonpolar, hydrophobic molecules



Facilitated transport


diffusion of polar, hydrophilic molecules
through a protein channel


lipids
high  low concentration gradient
high  low concentration gradient
Active transport
 diffusion against concentration gradient



low  high
uses a protein pump
requires ATP
ATP
Transport Summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQGCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRpLymP07E_VLiGzjkC-wqjHTg
Active Transport- Na+-K+ Pump

Sodium ion binds to transport protein in configuration 1  ATP
molecule associates with transport protein  After ATP
hydrolysis, phosphate group is transferred to transport protein
 Causing to switch of configuration 2  Sodium ion is
released to the outside of cell  Potassium ion is bound to
attachment site of transport protein  Binding of potassium
ion  Result in the release of phosphate group  Protein goes
back to configuration 1  Potassium ion is released into cell 
Cycle is completed with the attachment of sodium ion into
cavity of transport protein
Active Transport- Proton Pump
Proton pumps, in a lysosomal membrane, are used
by plants, bacteria, and fungi to create
electrochemical gradients (sodium-potassium pumps
are employed by animals for the same purpose)
www.mansfield.ohio-state.edu/~sabedon/lectures/.../campbl08.ppt
Examples
Retinal
chromophore
H+
NH2
Water channel
in bacteria
Porin monomer
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H+
Cytoplasm
proton pump channel
in photosynthetic bacteria
function through
conformational change =
shape change
Membrane Electrical Potential
Membrane potential: The electrical charge across a cell membrane; the
difference in electrical potential inside and outside the cell.
Axons have two basic electrical potentials:
1.
Resting membrane potential: The membrane potential of a neuron
when it is not being altered by excitatory or inhibitory postsynaptic
potentials.
2.
Action potential: The brief electrical impulse that provides the basis for
conduction of information along an axon.

www.psych.yorku.ca/desouza/PSYC3250/M/class2/Lecture2.../Lecture2.ppt
Electrochemical Gradient
• An Electrochemical Gradient is a Concentration Gradient with Ions:
- These ions want to move down their concentration gradient
- These ions (particularly) also want to move towards the opposite charge
found on the other side of the membrane
- This attraction for the other side of membranes (membrane potential) can
be harnessed to do work
- Electrochemical gradients essentially are batteries, i.e., means of
physically storing electrical energy
www.mansfield.ohio-state.edu/~sabedon/lectures/.../campbl08.ppt
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