chapter 7 membrane structure and function artificial membranes phospholipids will self- assemble...
TRANSCRIPT
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Chapter 7Membrane Structure
and Function
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Artificial Membranes
Phospholipids will self-assemble into bi-layers
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Davson & Danelli 1935-1970
They didn’t know that back then.
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Current Fluid Mosaic Model
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Membrane FluidityCholesterol maintains fluidity of animal cell
membranes
Plant cell membranes have extra unsaturated fatty acids as
in winter wheat
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Membrane Proteins Drift About
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Membrane Components
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Integral (Trans-membrane) Protein
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Membranes Have Sides
• Cytoplasmic & extracellular sides differ
• Membrane is recycled• Loss & gain of plasma
membrane is equal• Carbs built by ER &
modified by Golgi
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Cell Membrane Proteins
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Membrane Carbohydrates
• Carbohydrates on extracellular surface made in ER and modified in Golgi
• Cell-to-Cell recognition
• Oligosaccharides on external side of plasma membrane
• Function as markers
• ABO blood group antigens
• Attached to proteins or lipids
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Blood Group Antigens
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Movement of Materials Across the Cell Membrane
• 1) Passive Transport - – The cell doesn’t need to expend energy to do
• 2) Active Transport – Cells need to expend energy to do it (ATP)
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Particles in Motion
• Particles of all states of matter (s,l,g) in constant motion = Brownian motion
• Particles will move so they are evenly spread out (dynamic equilibrium)
• Particles continue to move due to ambient heat
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Passive Transport
• 3 Types:• 1) Diffusion
• 2) Osmosis (Diffusion of Water)
• 3) Facilitated Diffusion
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Diffusion• The net movement of
materials from an area of high concentration to an area of low concentration. – Through pores in the
membrane– (Hi to Low) Down its
concentration gradient
Paul Lewis – Simple Diffusion Demonstration
http://hydrodictyon.eeb.uconn.edu/people/plewis/applets/Diffusion/diffusion.html
Click on the simulation to view it.
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Permeability of Lipid Bilayer
Hydrophobic molecules can dissolve in the lipid bilayer, and cross easily
Hydrophilic molecules, such as ions and polar molecules cannot easily cross
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Concentration Gradients
• The difference between concentrations on either side of a membrane
• If the particles are charged (+ or -), such as H+, Na+, K+, Cl-, the gradient is an electrochemical one
• Gradients have POTENTIAL ENERGY!
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Passive Transport is Diffusion Across a Membrane Down a
Concentration Gradient
The cell does not expend energy of its own
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Compare the following
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DIFFUSION
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Dynamic Equilibrium
• Diffusion will continues until particles become evenly spread out
• The concentration on both sides of the membrane become the same
• Some particles may not be able to reach equilibrium
Paul Lewis Simulation
http://hydrodictyon.eeb.uconn.edu/people/plewis/applets/Osmosis/osmosis.html
Click on the simulation to view it.
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Osmosis
• Diffusion of water across cell membrane– Water moves from where it is in higher
concentration to an area where it is in lower concentration
– *If you have a High conc. of dissolved substances, you will have a LOW conc. of water!
– “osmos” = Greek word for pushing
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OSMOSIS
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OSMOSIS
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Facilitated Diffusion
• Passive transport of substances across a membrane by means of channel and carrier transport proteins
• Hi to low concentration
• Takes place both directions (In or Out)
• No energy is expended
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Facilitated Diffusion
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Co-transport
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Factors Affecting Rate of Diffusion
• > Temperature = faster diffusion
• > Concentration = faster diffusion
• > Size of particles: smaller particles = faster diffusion
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Active Transport
• Requires ATP or another energy source such as an Na+ or H+ gradient
• Always used to concentrate materials against the normal direction of diffusion
• Ex. Roots collect minerals
• Cells build up gradients: Concentration and electrochemical
• Pump mechanisms: Proton, Na+/K+
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Active Transport (con’d)
• Uses energy to change shape of membrane proteins to allow substances to pass thru
• Moves materials from Low to High conc.
• One direction only - like turnstiles
• Exocytosis, endocytosis, phagocytosis
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Active Transport
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Active Transport
Click on picture to see video
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Cellular Transport
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Comparing Solutions
1. Isotonic
2. Hypotonic
3. Hypertonic
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Gotta love the Greeks!
ISOS=equalHYPO=below/underHYPER= above/over
Prefix refers to the amount of solute outside the cell!
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ISOTONIC SOLUTIONS
• Concentration of dissolved substances in solution is the same as concentration of dissolved substances inside the cell.
• No net water movement
• Dynamic Equilibrium
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Isotonic Importance
• Cells usually exist in isosmotic surroundings (increased salinity in lakes can kill the animals there!)
• Immunizations are isotonic solutions so they do not damage the cells by gain or loss of water.
• I.V. solutions must be isotonic too.
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HYPOTONIC SOLUTIONS• Concentration of dissolved substances
is lower in solution outside the cell than concentration inside the cell.
• There is more water outside the cell than inside.
• Water moves into the cell
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Cells immersed in hypotonic solutions
• EX: In animal cells, the pressure inside cell increases causing the cells to swell and sometimes burst!
• EX: In plant cells, the rigid cell wall prevents bursting, but the cells become more firm.
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Osmotic Pressure (Pressure Potential Ψp)
• Pressure created when water enters a cell pushing against the cell membrane and cell wall
• Animal cells can’t build up a Pressure Potential – They enlarge and burst
• Plant cell walls prevent plant cells from bursting = turgor pressure
• Water potential = 0 bars at equilibrium
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Osmotic Pressure
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HYPERTONIC SOLUTIONS
• Concentration of dissolved substances outside cell is higher than concentration inside cell.
• There is more water inside cell than outside.
• Water moves out of the cell
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Cells immersed in hypertonic solutions
• Ex: In plant cells, membrane and cytoplasm shrink away from cell wall and plant wilts.
• EX: In animal cells, the pressure decreases and the cells shrivel.
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What about the big boys?
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Endocytosis• The cell can engulf large particles that
are too large to fit through pores in the cell membrane
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Exocytosis• Large particles (waste, indigestable
material) are exported from cell
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Critical Thinking…..
• How does salting the roads in winter, sometimes kill plants next to the road?
• Why do marine fish constantly drink saltwater?
• Why do freshwater fish produce large quantities of dilute urine?
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Water PotentialΨ = Ψp + Ψs
• Ψ is 0 (bars or Mpa) for pure water open to the atmosphere
• Inversely proportional to solute content
• Directly proportional to pressure• Measures the relative tendency of
water to move from one location to another
• Systems move to lower free energy
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Ψs = Solute Potential
• Adding solute “decreases” Ψs • Proportional to solute concentration
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Ψp = Pressure Potential
• Physical pressure on a solution
• Pressure of the cell wall - Turgor
• Negative pressure is called Tension
• Ψp = 0 for water open to the atmosphere
• F.Y.I: Approximately 1 bar = 1 atm. = 101.3 kPa = .1 MPa
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At Dynamic Equilibrium
•Ψ = 0•No free energy•Ψp cancels Ψs
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