3.2 Cell membranes

• Plasma membrane
    * also known as the cell membrane, lipid bilayer, fluid mosaic model
    * found in all cells
      * function
        * surround and protect the cell
        * control what enters and exits the cell
          * acts like a border guard
    * selectively permeable
      * only certain molecules can pass through freely
        * ^^small molecules^^ like water can pass through
        * ==large molecules== like carbohydrates, lipids and proteins CANNOT pass through
    * made up of phospholipids and proteins in an arrangement called a lipid bilayer
      * proteins are imbedded in the phospholipids
      * phospholipids are arranged with phosphate head toward the outside of the membrane and the lipid tails are orientated toward the interior of the membrane
        * function of phospholipids is to isolate cell from surroundings
    * phospholipids mostly hydrophobic

    inside of cell O~~O outside of cell
  * polar layer at the heads, hydrophobic tails
  * keep water out of the middle of the membrane
  * Fluid mosaic model
    * all cells live at least partly in touch with a water solution. in order to live, all cells must take in nutrients and eliminate wastes
      * the proteins create a pattern or “mosaic” on the membrane surface.
      * it is said to be “fluid” because the lipids are not rigid they move around one another
        * the proteins serve as identification badges or as channels to move substances in and out of the cell

• Going through the membrane
    * primary function of the cell membrane is to regulate what enters and exits the cell
      * types of transport across the membrane
        * passive transport
          * does not require energy (ATP)
          * diffusion, facilitated diffusion (proteins make it happen), and osmosis
        * active transport
          * requires energy (ATP)
          * molecular transport
          * endocytosis and exocytosis
    * environment of the cell is liquid on the inside and outside of the membrane with dissolved molecules on both sides of the membrane