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)

Phospholipid

• Membrane - bilayer - hydrophilic (water loving) heads pointing outwards, hydrophobic (water fearing) tails pointing inwards.

• Proteins help membrane to stick to water.

• 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.

• 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.

• 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.

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

• 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)

• 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.

• 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.

• 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.

• 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).

• Diffusion of molecules with limited permeability through lipid bilayer may be assisted by transport proteins.

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

• Difference in concentration - ions move from one area to other.

• Solution with higher [ ] solutes - hypertonic.

• Solution with lower [ ] solutes -hypotonic.

• [ ] equal - isotonic.

http://www.biologycorner.com/resources/hypertonic.gif

http://www.biologycorner.com/resources/hypotonic.gif

• Solution hypertonic - higher solute [ ] but lower H2O [ ].

• H2O moves into solution and solute moves out.

• 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)

• Cell placed in hypertonic solution – H20 rushes out of cell (cell shrinks).

• Cell placed in hypotonic solution – H2O rushes into cell (cell swells).

• 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).

• Plant loses enough water, plasma membrane will pull away from cell (plasmolysis).

http://faculty.southwest.tn.edu/jiwilliams/plasmolysis.gif

• 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.

In this case, the protein actually rotates to dump the materials to the inside of the cell.

• 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)

http://www.sp.uconn.edu/~terry/images/anim/antiport.gif

• 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.

• Cells maintain voltage across plasma membranes.

• Cytoplasm negative compared to opposite side of membrane (membrane potential - ranges from -50 to -200 millivolts)

http://bioweb.wku.edu/courses/Biol131/images/neuronions.GIF

• Membrane potential favors passive transport of cations (positive ions) into cell and anions (negative ions) out of cell.

• Creates an electrochemical gradient across membrane.

• Some organisms have proton pumps that actively pump H+ out of cell (i.e. plants, bacteria, and fungi)

• 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.

• 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.

http://www.kscience.co.uk/as/module1/pictures/endoexo.jpg

• 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.

• 2Pinocytosis (cell drinking) - cell creates vesicle around droplet of extracellular fluid.

• 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)