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Cell Membranes
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The Cell Membrane
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Cell Membrane:
At Very High Magnification & in color
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Membrane Structure
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html
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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
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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).
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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
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Phospholipid Molecule Model
glycerol
fatty acids (hydro phobic) nonpolar
phosphate (hydro philic) polar
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Membrane Lipidsform a Bilayer
Outside layer
Inside Layer
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Fluid Mosaic Membrane
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Membrane Proteins
• Integral: embedded within
bilayer
• Peripheral: reside outside
hydrophobic region of lipids
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Membrane Proteins
Text pg 80
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Integral membrane proteins
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Peripheral membrane proteins
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Membrane Models Fluid Mosaic Model - lipids arranged in bilayer with proteins embedded or associated with the lipids.
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Fluid Mosaic Membrane
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Evidence for the Fluid Mosaic Model (Cell Fusion)
Frey and Edidin
http://bio.winona.edu/berg/ANIMTNS/Frey.htm
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Lateral Diffusion of Proteins
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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
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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
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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.
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Membrane Transport MechanismsI. Passive Transport
• Diffusion- simple movement from regions of high concentration to low concentration
• Osmosis- diffusion of water across a semi-permeable membrane
• Facilitated diffusion- protein transporters which assist in diffusion
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Membrane Transport MechanismsII. Active Transport
• Active transport- proteins which transport against concentration gradient.
• Requires energy input– Endo cytosis– Exo cytosis
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DiffusionMovement 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
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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
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Osmosis in Hypertonic medium
cell
• Hypertonic solutions- shrink cells
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Osmosis in Hypotonic medium
• Hypotonic solutions- swell cells
“Hypos make hippos”
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For more animations view: http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm
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For Osmosis in ActionView 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.
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Endocytosis
• Transports macromolecules and large particles into the cell.
• Part of the membrane engulfs the particle and folds inward to “bud off.”
• Phagocytosis
• Pinocytosis
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Endocytosis
•Receptor Mediated Endocytosis
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Putting Out the Garbage
• Vesicles (lysosomes, other secretory vesicles) can fuse with the membrane and open up the the outside…
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Exocytosis (Cellular Secretion)
http://bio.winona.edu/berg/ANIMTNS/Secrtion.htm
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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 transport1
2
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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
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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
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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.
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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
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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
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Transport ProteinsFacilitated Diffusion & Active Transport
• move solutes faster
across membrane
• highly specific to
specific solutes
• can be inhibited by
drugs
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Types of Protein Transporters
A. Facilitated DiffusionAssist in
diffusion process.Solutes go from
High conc to Low conc.
Examples: Glucose transporters
http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm
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Facilitated DiffusionThe 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
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Glucose Transporter Characteristics:
• integral protein: spans the membrane
• 12 alpha helices woven into membrane
• 55,000 mol. wt.
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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
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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)
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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 ExocytosisProton Pump
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Active Transport:Sodium-Potassium Pump
Na+high
K+low
Na+low
K+high
Balance of the two ions goes hand-in-hand
ATP required for maintenance of the pump
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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
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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
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Sodium/PotassiumATPase 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??
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Sodium-Potassium Pump
http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/sppump.html
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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
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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
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Na/Cl Pump
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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
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4 Types of Cell Junctions
1. Tight Junctions
2. Desmosomes
3. Gap Junctions
4. Plasmodesmata
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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
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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
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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
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Put Them All Together…
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Plasmodesmata
• Plasmodesmata
• similar to gap juntions, but in plant cells
• allow conitnous flow of cytoplasm through cells
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Resources
• Directory of Animations: http://bio.winona.edu/berg/ANIMTNS/Directry.htm
• http://www.aber.ac.uk/gwydd-cym/cellbiol/transport/index.htm
• Anatomy & Physiology Chapter 3 Animations