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Membrane Structure and Function. Chapter 8. Plasma membrane of cell selectively permeable ( allows some substances to cross more easily than others) Made mostly of proteins and lipids (phospholipids). Phospholipids and proteins create unique physical environment (fluid mosaic model).

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slide2
Plasma membrane of cell selectively permeable (allows some substances to cross more easily than others)
  • Made mostly of proteins and lipids (phospholipids).
  • Phospholipids and proteins create unique physical environment (fluid mosaic model)
slide4
Membrane - bilayer - hydrophilic (water loving) heads pointing outwards, hydrophobic (water fearing) tails pointing inwards.
  • Proteins help membrane to stick to water.
slide6
Fluid because lipids and proteins can move laterally.
  • As temperatures drop, liquid membrane can solidify.
  • Saturated fatty acid tails - more solid than unsaturated fatty acid tails.
slide8
Cholesterol found in membrane helps with fluidity of membrane.
  • Membranes need to be fluid to work properly - systems in place to help keep it fluid.
slide10
Two different types of proteins are found in membrane.
  • 1Peripheral proteins not in membrane, bound to surface of protein.
  • 2Integral proteins in membrane often spanning entire membrane.
slide11

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/MembraneProteins.gifhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/MembraneProteins.gif

slide12
Membrane helps keep cell’s shape.
  • Also aids in cell-to-cell recognition (ability of a cell to distinguish one type of neighboring cell from another)
slide14
Some substances move steadily across membrane (sugars, ions, and wastes like CO2)
  • Hydrophobic molecules (i.e. hydrocarbons, CO2, and O2) can dissolve in lipid bilayer and cross easily.
slide15
Charged particles and polar molecules have more difficulty passing.
  • Specific ions and polar molecules can cross lipid bilayer by passing through transport proteins that span membrane.
slide16
Diffusion - tendency for substance to spread out in open area.
  • Permeable membrane separating a solution with dye molecules from pure water, dye molecules will cross barrier randomly.
slide18
No force acting upon it - substance will tend to move down it’s concentration gradient from where it is more concentrated to less concentrated (passive transport).
slide20
Diffusion of molecules with limited permeability through lipid bilayer may be assisted by transport proteins.
slide21

http://library.thinkquest.org/27819/media/protein_channel.gifhttp://library.thinkquest.org/27819/media/protein_channel.gif

slide22
Difference in concentration - ions move from one area to other.
  • Solution with higher [ ] solutes - hypertonic.
  • Solution with lower [ ] solutes -hypotonic.
  • [ ] equal - isotonic.
slide25
Solution hypertonic - higher solute [ ] but lower H2O [ ].
  • H2O moves into solution and solute moves out.
slide26
Movement of H2O across selectively permeable membrane - osmosis.
  • 2 solutions isotonic, H2O molecules move at equal rates from one to the other, (no net osmosis)
slide27
Cell placed in hypertonic solution – H20 rushes out of cell (cell shrinks).
  • Cell placed in hypotonic solution – H2O rushes into cell (cell swells).
slide29
Organism does not have rigid walls must have ability to osmoregulate and maintain internal environment.
  • Plant cells expand when watered causing pressure to be exerted against cell wall.
  • Allows plant to stand up against gravity (turgid cell); not watered, plant will begin to wilt (flaccid cell).
slide33
Charged particles that cannot pass through membrane - proteins to pass through (facilitated diffusion - diffusion of substance down it’s [ ] gradient with help of transport protein)
  • Some channel proteins (gated channels) open/close depending on presence/absence of physical or chemical stimulus.
slide34

In this case, the protein actually rotates

to dump the materials to the inside

of the cell.

slide35
Sometimes materials need to be moved against [ ] gradient (active transport)
  • Active transport requires energy of cell to move substances from an area of low [ ] to an area of high [ ] (i.e. sodium-potassium pump in animal cells)
slide37
Sodium-potassium pump actively maintains gradient of sodium (Na+) and potassium ions (K+) across membrane.
  • Sodium-potassium pump uses energy of 1 ATP to pump 3 Na+ ions out and 2 K+ ions in.
slide39
Cells maintain voltage across plasma membranes.
  • Cytoplasm negative compared to opposite side of membrane (membrane potential - ranges from -50 to -200 millivolts)
slide41
Membrane potential favors passive transport of cations (positive ions) into cell and anions (negative ions) out of cell.
  • Creates an electrochemical gradient across membrane.
slide42
Some organisms have proton pumps that actively pump H+ out of cell (i.e. plants, bacteria, and fungi)
slide44
Materials leave membrane through lipid bilayer or through transport proteins.
  • Exocytosis - transport vesicle buds from Golgi apparatus - moved by cytoskeleton to plasma membrane.
  • When membranes meet - fuse - material is let out to outside of cell.
slide45
Endocytosis - cell brings in macromolecules and matter by forming new vesicles from plasma membrane.
  • Membrane is inwardly pinched off and vesicle carries material to inside of cell.
slide47
1Phagocytosis (“cell eating”) - cell engulfs particle by extending pseudopodia around it, packaging it in a large vacuole.
  • Contents of vacuole are digested when vacuole fuses with lysosome.
slide51
3Receptor-mediated endocytosis - specific in transported substances.
  • Extracellular materials bind ligands (receptors) - causes vesicle to form.
  • Allows materials to be engulfed in bulk (i.e. cholesterol in humans)