The Cell Membrane

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The Cell Membrane
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Cell Membranes

• 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 amphipathic molecules, with polar
  and nonpolar regions

phosphate (hydro philic)
polar glycerol fatty acids (hydro phobic)
nonpolar
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Membrane Proteins

• Integral embedded within bilayer
• Peripheral reside outside hydrophobic region of
  lipids

Text pg 80
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Fluid Mosaic Membrane
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Fluid Mosaic Membrane

• Fluid Mosaic Model - lipids arranged in bilayer
  with proteins embedded or associated with the
  lipids.

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Evidence for the Fluid Mosaic Model

• Frey and Edidin

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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
• 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 by random
  motion of particles caused by internal thermal
  energy.
• Osmosis- diffusion of water across a
  semi-permeable membrane
• Facilitated diffusion- protein transporters which
  assist in diffusion

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Diffusion Rates

• Factors affecting diffusion rate through a
  membrane
• temperature - ? temp., ? motion of particles
• molecular weight - larger molecules move slower
• steepness of concentrated gradient - ?difference,
  ? rate
• membrane surface area - ? area, ? rate
• membrane permeability - ? permeability, ? rate

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Concentration Gradient of Ions across a Membrane
and the Resulting Electrochemical Potential
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Tonicity

• Tonicity - ability of a solution to affect fluid
  volume and pressure within a cell
• depends on solute concentration and permeability
• Hypotonic solution
• low concentration of nonpermeating solutes (high
  water concentration)
• cells absorb water, swell and may burst (lyse)
• Hypertonic solution
• has high concentration of nonpermeating solutes
  (low water concentration)
• cells lose water shrivel (crenate)
• Isotonic solution normal saline

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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

• 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/w
estmin/science/sbi3a1/Cells/Osmosis.htm
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For Osmosis in Action

• View frozen frogs at
• http//www.pbs.org/wgbh/nova/sciencenow/3209/05.ht
  ml
• 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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Osmosis Food Preservation

• Food can be preserved by causing any
  microorganism that comes in contact with it to
  become plasmolysed and, therefore, shrivel and
  die. To do this food is placed in a high salt or
  sugar medium. The salt or sugar concentration is
  higher than the cytoplasm of bacteria or fungi.
  Bacteria or fungi, that contaminate the food,
  will lose water by osmosis and their metabolism
  will decline. Many will die but some bacteria may
  survive by forming dormant resistant endospores.
  Meat and fish are often preserved in salt. Fruit
  is commonly preserved in sugar as in jam or syrup.

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Membrane Transport Active Transport (Direct
Indirect)

• 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

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Endocytosis

• Part of the membrane engulfs the particle and
  folds inward to bud off.
• Phagocytosis
• Pinocytosis
• Receptor Mediated Endocytosis

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Exocytosis Cellular Secretion

• Vesicles (lysosomes, other secretory vesicles)
  can fuse with the membrane and open up the the
  outside

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Vesicular Transport(Active Transport)

• Endocytosis
• Phagocytosis, 2
• Pinocytosis
• Receptor Mediated Endocytosis
• Exocytosis

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Membrane Permeability

1. lipid soluble solutes go through faster
2. smaller molecules go faster
3. uncharged weakly charged go faster
4. Channels or pores may also exist in membrane to
   allow transport

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2
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Functional Roles of Membrane Protein Molecules
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Types of Protein Transporters Ion Channels

• Work fast No conformational 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
• work by facilitated diffusion No E!
• deal with small molecules... ions
• open pores are gated- Can change shape.
• important in cell communication
• Receptors Linked to a Channel Protein

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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

• 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.

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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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Facilitated Diffusion Glucose Transporters

• Transport of glucose into cells mediated by
  proteins in the GLUT (GLUcose Transport) family
  of transporters.
• 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
• These are uniporters, different from the
  Sodium-dependent glucose cotransporters (SGLT)

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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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Active TransportSodium-Potassium Pump
Balance of the two ions goes hand-in-hand ATP
required for maintenance of the pump
Na low
Na high
K low
K high
How it Works
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Sodium-Potassium Pump

• 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
• 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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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
• Tight Junctions
• Desmosomes
• Gap Junctions
• Plasmodesmata

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Intercellular Junctions
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Tight Junctions

• intimate physical connections linking cells that
  line the inner or outer surface of organs or body
  cavities
• Leak proof 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
  transport 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

Pemphigus is an autoimmune disease in which the
patient has developed antibodies against proteins
(cadherins) in desmosomes. The loosening of the
adhesion between adjacent epithelial cells causes
blistering. Carcinomas are cancers of epithelia.
However, the cells of carcinomas no longer have
desmosomes. This may partially account for their
ability to metastasize.
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Gap Junctions

• permit small molecules to move b/t cells w/o
  passing thru mem
• six dumbbell shaped protein units in mem,
  adjacent in the cells

The action potential in cardiac muscle provides
the rhythmic contraction of the heartbeat. At
some electrical synapses in the brain, gap
junctions permit the arrival of an action
potential at the synaptic terminals to be
transmitted across to the postsynaptic cell
without the delay needed for release of a
neurotransmitter. As the time of birth
approaches, gap junctions between the smooth
muscle cells of the uterus enable coordinated,
powerful contractions to begin.
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Plasmodesmata

• similar to gap junctions, but in plant cells
• allows continuous flow of cytoplasm through cells

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Resources

• Directory of Animations
• Anatomy Physiology Chapter 3 Animations
• Cell Membrane Just Passing Through
• Absorption in the Small Intestine
• GLUT4 Diabetes (Monogenetic disorder)