c5 p1

• The plasma membrane forms a complete boundary around the cell
• Plasma membrane


  • Composed of phospholipids, proteins & cholesterol
  • The bi-molecular layer of phospholipid molecules forms the basic structure
  • Cholesterol molecules are inserted between the phospholipid molecules at regular intervals
  • Incorporates both INTEGRAL & PERIPHERAL proteins like "ICEBERGS floating in a sea of phospholipid"

  
• Lipid bilayer


  Serves as a highly impermeable barrier to most "charged (polar)" and "non-lipid soluble" substances
• Integral proteins


  Act as "pores, channels" or "carriers" to allow substances to cross the membrane

  
• Plasma membrane


  • Selectively permeable, some things can pass through & others cannot
  • Permeability is a function of solubility in lipids, driving forces, and molecular size

  
• Transport across the membrane

  1. Active transport

  2. Passive transport

  
• Passive transport


  • Diffusion through the lipid bilayer (lipid soluble substances)
  • Diffusion through ion channels (water soluble substances)
  • Facilitated diffusion using a carrier (water soluble substances)

  
• Active transport


  Requires cellular energy (ATP)
• Diffusion through the lipid bilayer

  1. Lipid-soluble substances (e.g. respiratory gases, lipids, small alcohols, urea) can diffuse across

  2. Concentration gradient is usually the driving force
• Diffusion across the lipid bilayer

  1. Water-soluble substances (e.g. ions, small sugars, amino acids, water) need integral membrane proteins to move across

  2. Small ions (channels)

  3. Water (channels)

  4. Sugars & amino acids (facilitated diffusion)

  5. Concentration or electrical gradient is often the driving force

  
• Facilitated diffusion


  1. Solute binds to a specific transporter on one side of the membrane and is released on the other side

  2. Rate of movement depends on steepness of concentration gradient and number of transporter proteins

  
• Ion channels


  • Some membrane proteins are ion channels
  • An electrochemical gradient is often the driving force
  • Ion channels are selective & specific
  • Some channels are continuously open, others only open transiently ("gated")
  • Transport much faster rate than facilitated diffusion
• Active transport


  1. Energy-requiring process that moves solutes against a concentration gradient

  2. Primary active transport derives energy directly from ATP

  3. Secondary active transport derives energy indirectly from ATP
• Primary active transport


  Energy is derived directly from ATP (metabolic/ATP hydrolysis)
• Secondary active transport


  Energy is derived indirectly from ATP (cotransport of Na+ or H+)
• Sodium-potassium ion pump


  • Most common primary active transport mechanism
  • Requires 40% of cellular ATP
  • All cells have thousands of them
  • Maintains low concentration of Na+ & high concentration of K+ in the cytosol
  • Operates continually
• Exocytosis


  1. Movement of large molecules OUT of the cell

  2. Occurs in secretory cells

  3. Secretions in vesicles (membrane packets) - vesicles fuse with cell membrane, e.g. neurotransmitter secretion at the synapse
• Endocytosis


  1. Movement of large molecules and particles INTO the cell

  2. Pinocytosis: engulfing small particles & fluids

  3. Phagocytosis: engulfing large particles

  4. Receptor-mediated endocytosis: movement of specific substances into the cell involving the caveolae regions of the cell membrane