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Membrane Structure and Function. Chapter 7. 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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Plasma membrane of cell selectivelypermeable (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.
Cholesterol found in membrane helps with fluidity of membrane.
  • Membranes need to be fluid to work properly.
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.

Also aids in cell-to-cell recognition (ability of a cell to distinguish one type of neighboring cell from another)
pass through membrane easily
Pass through Membrane Easily
  • 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.
Diffusion - tendency for substance to spread out in open area.
  • Move from an area of Most concentrated to Least concentrated.
No force acting upon it - substance will tend to move down it’s concentration gradient (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. (water moves out)
  • Solution with lower [ ] solutes -hypotonic. (water moves in)
  • [ ] equal - isotonic.
Organism does not have rigid walls must have ability to osmoregulate and maintain internal environment.
  • Allows plant to stand up against gravity (turgid cell); not watered, plant will begin to wilt (flaccid cell).
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.

Active transport- materials need to be moved against [ ] gradient
  • 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.
Some organisms have proton pumps that actively pump H+ out of cell (i.e. plants, bacteria, and fungi)
more active transport
More Active Transport
  • Exocytosis - When membranes meet - fuse - material is let out to outside of cell.
  • Endocytosis - 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.
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)