Download Cell Membranes: Chapt. 6

Cell Membranes
The Cell
Membrane
Cell Membrane:
At Very High
Magnification
& in color
Membrane Structure
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html
Cell Membrane
Every cell is encircled by a membrane and most cells
contain an extensive intracellular membrane system.
Membranes fence off the cell's interior from its
surroundings. Membranes let in water, certain ions and
substrates and they excrete waste substances. They act
to protect the cell.
Without a membrane the cell contents would diffuse
into the surroundings, information containing molecules
would be lost and many metabolic pathways would
cease to work:
The cell would die!
www.biologie.uni-hamburg.de/b-online/e22/22.htm
Cell Membranes:
• Surround all cells
• Fluid-like composition…like soap bubbles
• Composed of:
– Lipids in a bilayer
– Proteins embedded in lipid layer (called
transmembrane proteins)
– And, Proteins floating within the lipid sea
(called integral proteins)
– And Proteins associated outside the lipid
bilayer (peripheral).
Membrane Lipids
• Composed largely of phospholipids
• Phospholipids composed of….glycerol and
two fatty acids + PO4 group
• P-Lipids are polar molecules…
P-Lipids are represented like this
Phospholipid Molecule
Model
phosphate (hydro philic)
polar
glycerol
fatty acids (hydro phobic)
nonpolar
Membrane Lipids
form a Bilayer
Outside layer
Inside Layer
Fluid Mosaic Membrane
Membrane Proteins
• Integral: embedded within bilayer
• Peripheral: reside outside hydrophobic
region of lipids
Membrane Proteins
Text pg 80
Integral membrane proteins
Peripheral membrane proteins
Membrane Models
Fluid Mosaic Model - lipids arranged in
bilayer with proteins embedded or
associated with the lipids.
Fluid Mosaic Membrane
Evidence for the Fluid Mosaic
Model (Cell Fusion)
Frey and Edidin
http://bio.winona.edu/berg/ANIMTNS/Frey.htm
Lateral Diffusion of Proteins
Membrane Functions
• allows for different conditions between
inside and outside of cell
• subdivides cell into compartments with
different internal conditions
• allows release of substances from cell via
vesicle fusion with outer membrane:
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/exocyt.gif
Membrane Permeability
• Biological membranes are physical
barriers..but which allow small uncharged
molecules to pass…
• And, lipid soluble molecules pass through
• Big molecules and charged ones do NOT
pass through
• Semi-permeable / selectively permeable
How to get other molecules
across membranes??
There are two ways that the molecules typically move through
the membrane:
passive transport and active transport
•Active transport requires that the cell use energy that it has
obtained from food to move the molecules (or larger particles)
through the cell membrane.
•Passive transport does not require such an energy
expenditure, and occurs spontaneously.
Membrane Transport Mechanisms
I. Passive Transport
• Diffusion- simple movement from regions
of high concentration to low concentration
• Osmosis- diffusion of water across a semipermeable membrane
• Facilitated diffusion- protein transporters
which assist in diffusion
Membrane Transport Mechanisms
II. Active Transport
• Active transport- proteins which transport
against concentration gradient.
• Requires energy input
– Endo cytosis
– Exo cytosis
Diffusion
Movement generated by random
motion of particles. Caused by
internal thermal energy.
Movement always from region of
high free energy(high
concentration) to regions of low
free energy (low conc.)
How Diffusion Works
Osmosis
Movement of water across
a semi-permeable
barrier.
Example: Salt in water,
cell membrane is barrier.
Salt will NOT move
across membrane, water
will.
How Osmosis Works
Osmosis in Hypertonic medium
cell
• Hypertonic solutions- shrink cells
Osmosis in Hypotonic medium
• Hypotonic solutions- swell cells
“Hypos make hippos”
For more animations view:
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm
For Osmosis in Action
View frozen frogs at:
http://www.pbs.org/wgbh/nova/sciencenow/
3209/05.html
How did the frog use the principles of
osmosis and diffusion to survive the
winter? Make sure you use the following
terms appropriately in your description:
hypertonic, hypotonic, solute, solvent,
diffusion, osmosis, cytolysis, crenation,
isotonic and semi-permeable membrane.
Endocytosis
• Transports macromolecules and large
particles into the cell.
• Part of the membrane engulfs the particle
and folds inward to “bud off.”
• Phagocytosis
• Pinocytosis
Endocytosis
•Receptor Mediated Endocytosis
Putting Out the Garbage
• Vesicles (lysosomes, other secretory vesicles) can
fuse with the membrane and open up the the
outside…
Exocytosis
(Cellular Secretion)
http://bio.winona.edu/berg/ANIMTNS/Secrtion.htm
Membrane Permeability
1) lipid soluble solutes go through faster
1) smaller molecules go faster
1) uncharged & weakly charged go faster
2) Channels or pores may also exist in
membrane to allow transport
1
2
Types of Protein Transporters:
Ion Channels
• work by facilitated diffusion No E!
• deal with small molecules... ions
• open pores are “gated”- Can change shape.
– How?
– How much gets in?
• important in cell communication
Receptors Linked to a Channel Protein
Ion Channels
• Work fast: No conform. changes needed
• Not simple pores in membrane:
– specific to different ions (Na, K, Ca...)
– gates control opening
– Toxins, drugs may affect channels
• saxitoxin, tetrodotoxin
• cystic fibrosis
Ion Channels
• Channel proteins or carrier
proteins allow the facilitated
diffusion of solutes down their
concentration gradients or
electrochemical gradients.
• Carrier proteins allow the
active transport of solutes up
their concentration gradients or
electrochemical gradients.
Cystic Fibrosis
• Fatal genetic disorder
• Mucus build-up results in lung and liver
failure
• Patients die between 4 and 30 yrs.
• Single gene defect
• 1 in 25 Caucasians carry 1 bad gene copy
• 1 in 2500 kids has it in Canada
• Testing
CF Cont…
• Proteins for diffusion of salt
into the airways don't work.
• Less salt in the airways
means less water in the
airways.
• Less water in the airways
means mucus layer is very
sticky (viscous).
• Sticky mucus cannot be
easily moved to clear
particles from the lungs.
• Sticky mucus traps bacteria
and causes more lung
infections.
http://www.the-aps.org/education/lot/cell/HotT.htm
Transport Proteins
Facilitated Diffusion
& Active Transport
• move solutes faster
across membrane
• highly specific to
specific solutes
• can be inhibited by
drugs
Types of Protein
Transporters
A. Facilitated Diffusion
Assist in diffusion
process.
Solutes go from
High conc
to
Low conc.
Examples: Glucose
transporters
http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm
Facilitated Diffusion
The Glucose Transporters
• Transport of glucose into cells mediated by proteins in the
GLUT (GLUcose Transport) family of transporters. There
are 7 different, but related, proteins. But, only four
(GLUT1-4) are known to be involved in glucose transport.
• All GLUT proteins share a set of similar structural
features and are all about 500 amino acids in length (giving
them a predicted molecular weight of about 55,000
Daltons)
• Glucose uptake shows saturation and glucose uptake can
be inhibited by drugs
A classic Membrane Transport protein
How Facilitated Diffusion Works
Glucose Transporter
Characteristics:
• integral protein: spans the membrane
• 12 alpha helices woven into membrane
• 55,000 mol. wt.
Glucose Transporter:
How it works..
• glucose binds to outside
of transporter (exterior
side with higher glucose
conc.)
• glucose binding causes a
conform. change in
protein
• glucose drops off inside
cell
• protein reassumes 1st
configuration
Types of Protein Transporters:
Active Transport
• carrier proteins
• go against the concentration gradients Low
to High
• require Energy to function (ATP, PEP, light
energy, electron transport)
Membrane Transport:
Active transport
• Movement from region of low free energy(low
concentration) to regions of high free energy (high
conc.)
• Requires energy input
Cotransport
Sodium-Potassium Exchange Pump
Endocytosis and Exocytosis
Proton Pump
Active Transport:
Sodium-Potassium Pump
Na+
high
Na+
low
K+
low
K+
high
Balance of the two ions goes hand-in-hand
ATP required for maintenance of the pump
The sodium/potassium pump
• All nerve and muscle cells have a high internal potassium ion
concentration and a low internal sodium ion concentration.
[Ki=166 mM; Ko=5 mM; Nai=18 mM; Nao=135 mM].
• Early on, it was thought that the nerve and muscle membranes
were relatively impermeable to these ions and that the difference
in ionic concentration was set up in early development of the
cells. The membrane then became impermeable.
• The later availability and use of radioactive Na and K ions
showed that this was not true and that there was a metabolic
pump that pumped Na out of the cell and K in; the ratio being 3
Na pumped out of the cell for every 2 K pumped into the cell.
How it Works
Is a Protein Involved ?
• Experiments showed a dependency of both
Na and K ions for pump to work
• Pump was inhibited by ouabain (a drug)
• 1957: an ATPase enzyme was found to be
associated with Na/K pumping
• Studies showed this ATPase capable of
pumping Na/K ions
• Text pg 90
Sodium/Potassium
ATPase Protein
• Made of 2 large and 2 small subunits
• 2 large units span membrane
– inside region: contains ATP binding site
– inside: binding sites for Na
– outside: binding site for K
• How does it work??
Sodium-Potassium Pump
http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/sppump.html
Na-K Pump Model: Part I
• 3 Na+ bind to inner region of protein
• Na+ binding triggers phosphorylation of
protein. ATP
ADP + Pi
• Phosphorylation causes conformation change
and Na+ binding site faces outside
• 3 Na+ released to outside
Na-K Active Pump: Part II
• 2 K+ ions on outside are able to bind
• K+ binding causes dephosphorylation and
new conformation change
• 2K+ ions exposed to inside and released
Cyclic process uses ATP energy to drive Na &
K ion transport against conc. Gradient
Na/Cl Pump
Cell Junctions
• Allow specific types of cells to stay together
to perform special jobs
• Layers of these types of cells…
– Line body cavities
– Cover body surfaces
4 Types of Cell Junctions
1.
Tight Junctions
2.
Desmosomes
3.
Gap Junctions
4.
Plasmodesmata
Tight Junctions
• intimate physical
connections linking cells
that line the inner or outer
surface of organs or body
cavities
• leakproof barriers that
prevent the movement of
molecules through the
spaces located b/t cells,
must diffuse to get by and
are therefore subject to the
precise control
mechanisms inherent to
tranpsort through cell mem
• e.g. bladder
Desmosomes
• junctions exhibiting
mechanical strength
• found in organs/tissues
exposed to mechanical
forces that subject
cells to much
stretching and
distortion
• maintains integrity of
cell
Gap Junctions
• permit small
molecules to move
b/t cells w/o passing
thru mem
• Communication
nerve & muscle
• six dumbell shaped
protein units in
mem, adjacent in the
cells
Put Them All Together…
Plasmodesmata
• Plasmodesmata
• similar to gap
juntions, but in
plant cells
• allow conitnous
flow of cytoplasm
through cells
Resources
• Directory of Animations:
http://bio.winona.edu/berg/ANIMTNS/Directry.htm
• http://www.aber.ac.uk/gwyddcym/cellbiol/transport/index.htm
• Anatomy & Physiology Chapter 3 Animations