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)
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.
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.
Difference in concentration - ions move from one area to other. • Solution with higher [ ] solutes - hypertonic. • Solution with lower [ ] solutes -hypotonic. • [ ] equal - isotonic.
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).
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)
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)
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.
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.