cell membrane

• Fluid mosaic model


  Describes the cell membrane as fluid, with phospholipids and proteins free to move, composed of many different parts
• Phospholipid


  • Has a polar head and two nonpolar tails
  • Composed of a phosphate group, two fatty acid chains, and a glycerol molecule
  • Polar head is hydrophilic, nonpolar tails are hydrophobic
• Amphipathic


  Describes a molecule with both hydrophobic and hydrophilic parts
• Proteins in cell membrane

  • Globular proteins (peripheral)
  • Surface proteins (on exterior of membrane)
  • Integral proteins (embedded in membrane)
  • Transmembrane proteins (span entire membrane)
• Glycoprotein


  Protein with carbohydrate chain attached
• Glycolipid


  Lipid with carbohydrate unit attached
• Cell membrane


  • Semi-permeable
  • Allows small nonpolar molecules to diffuse in/out
  • Allows water to slowly diffuse in/out
  • Requires transport proteins for ions and large polar molecules
• Aquaporins


  Integral proteins that transport water across membrane
• Ion channels


  Transport proteins that allow ions to flow across membrane
• Carrier proteins


  Transport large polar molecules like glucose across membrane
• Cholesterol


  • Has polar and nonpolar regions
  • Maintains fluidity of membrane by preventing phospholipids from moving too far apart or too close together
• Increasing temperature increases fluidity of membrane
• Decreasing temperature decreases fluidity of membrane</b>
• Cholesterol acts to maintain membrane fluidity
• Phospholipids


  When the temperature goes down, they want to get closer, but cholesterol prevents them from getting too close
• Cholesterol


  • Maintains the fluidity of the cell membrane
  • Brings fluidity up when it goes down
  • Brings fluidity down when it goes up
• Cholesterol is an example of homeostasis or equilibrium in the cell membrane