AP Bio Cell Membrane

Membrane Structure

• Phospholipids
• Proteins
• Cholesterol

Phospholipids

• Have a polar “head”
    * Phosphate
• Have 2 nonpolar “tails”
    * Fatty Acids
• Polar side is attracted to water
• Nonpolar side is repelled to water
• Can have saturated hydrocarbon chains
    * Making the membrane viscous
• Can have unsaturated hydrocarbon chains
    * Making the membrane more fluid
• Move laterally, but rarely flip flop

Proteins

• Used for moving substances in and out of the cell
• Used for signal recognition
• Are embedded in the phospholipid bilayer based on hydrophobic interactions
• Can be integral
    * Through the cell membrane
• Glycoproteins are also common

Cholesterol

• Helps the membrane deal with temperature changes
• Keeps the membrane fluid when cooled
    * Keeps the phospholipids from packing tightly
• Keeps the membrane viscous when heated
    * Restrains the movement of molecules

Movement Across Membranes

Passive Transport

• Diffusion across the membrane
• No energy required
• Spontaneous
• Examples :
    * Diffusion
    * Osmosis
    * Facilitated Diffusion

Active Transport

• Often moves particles against the concentration gradient.
• Occasionally moves with the c.g., but at a faster rate than diffusion.
• Occurs when you need to accumulate particles
• Requires energy to move molecules
• Energy is required
• ATP used
• Examples :
    * The sodium-potassium pump
      * Involved with nerve cells
      * The transport protein has 2 conformations :
        * High affinity for Na+ with binding sites oriented toward the cytoplasm
        * High affinity for K+ with binding sites toward the exterior
      * ATP phosphorylates the transport protein and powers the conformational change from Na+ receptive to K+ receptive
      * 3 Na+ are moved out of the cell leaving room for 2 K+
      * This sets up an electrochemical gradient across the membrane
        * The difference in charge across a membrane is called the membrane potential
        * The combination of the membrane potential and the concentrations gradient is called the electrochemical gradient
      * With the correct stimulus, a gated channel opens
      * The electrochemical gradient is equalized
      * This is a nerve impulse
      * The nerve can’t work again until the gradient is set up

Diffusion

• Due to random movement of molecules
• Particles have a net movement from high concentration to low concentration
• Remember entropy
• Concentration gradient
    * Is the difference in concentration throughout space
    * Particles tend to move “with” or “down” their concentration gradient
    * From high concentration to low concentration
• Equilibrium
    * When the concentration is the same throughout space

Osmosis

• The diffusion of water across a membrane
• Moves down its concentration gradient
    * Toward higher concentration of particles
• Very important in cellular biology
• Water will move from a hypotonic solution to a hypertonic solution
• Hypotonic solution
    * Contains less solute (more water) than a hypertonic solution
• Hypertonic solution
    * Contains more solute (less water) than a hypotonic solution
• Water will move from a hypotonic solution to a hypertonic solution until :
    * Both solutions have equal concentrations (isotonic)
    * The pressure of the cell wall in plants stops the movement of water

Aquaporins

• Due the polarity of water, it has a difficult time moving directly through the membrane
• Water moves through protein channels called aquaporins

Water Control in Cells Without Cell Walls

• In isotonic environment, cells will stay the same (good)
    * There is no net movement of water
    * Cells become limp or flaccid.
    * Plant will wilt
• In hypertonic environment, cell will loose water and shrivel (crenate)
    * Cells will loose water
    * Plasmolysis may occur
      * When membrane pulls away from cell wall
    * Usually fatal to plant cells.
• In hypotonic environment, cell will gain water and swell
    * Water moves into the cell until the internal pressure of the cell wall equals the osmotic pressure

  * At this point, there is equal movement in and out of the cell.
  * Dynamic equilibrium
  * Ideal for most plants.
  * Turgor pressure builds (cells are turgid).

• If water uptake is excessive, the cell could burst (lyse)
• Organelles such as contractile vacuoles keep freshwater protists from bursting

Facilitated Diffusion

• Some molecules can’t diffuse freely across the membrane because they are too big or too charged
• They need the help of proteins.
• Facilitated diffusion
    * Is the diffusion of solute across a membrane with the help of transport proteins
• Does not require energy.
• Moves with the concentration gradient

Transport Proteins

• Solute specific
• Can be saturated
• Use various mechanisms for transport such as :
    * Conformational change
    * Selective channels
• Gated channels (only open with impulse)

Endocytosis

• Import particles into a cell by the formation of a vesicle

Three types are :

• Phagocytosis
    * “Cell eating”
    * Endocytosis of solid (large) particles
    * This is how amoebas eat
• Pinocytosis
    * “Cell Drinking”
    * Endocytosis of fluid droplets (small particles)
• Receptor mediated endocytosis
    * Happens when a specific molecule (called a ligand) binds to a receptor on the cell membrane

Exocytosis

• Exporting particles out of a cell by fusing a vesicle with the cell membrane

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