Movement of molecules through a membrane
       Cytoplasm = mixture of particles (molecules) in water
                  Particles = solute
                  Water = solvent
 
 
    Concentration = relative number of a particular molecule compared to total number of
                                    molecules present
                   10 % sucrose =10 % of molecules present are sucrose
                   90% of molecules present are not sucrose (are water molecules)
 
 
    Diffusion = passive movement of molecules from a region of high concentration to a
                            region of  lower concentration
              passive means no energy is spent on the process
                  molecules move along a concentration gradient — from high to low
         some molecules move through a membrane by simple diffusion
               occurs until concentration of molecules is the same on both sides of the membrane
               cell is now at equilibrium
                at equilibrium molecules still move through the membrane but rate of
                       movement is equal in both directions (into and out of the cytoplasm)
    Cell membrane does not allow all molecules to pass through it
           Cell membrane is semi-permeable — allows some molecules to move freely
                                                                    does not allow other molecules to move
 
 
    OSMOSIS = diffusion (passive) of water through a semi-permeable membrane from a
                        region of high water concentration to a region of low water concentration
           rate of water movement related to the difference in water concentration inside and
                        outside
               if water concentration equal inside and outside, there is no NET movement of water
               if 10% outside and 9% inside — water slowly enter
               if 15% outside and 9% inside — water enters more rapidly
               if 30% outside and 9% inside — water enters still more rapidly, etc.
              as water enters cell, it expands
              as water leaves a cell it shrinks
 
 
 
 
 
 
 
TONICITY  -  refers to difference in solute concentration on the two sides of the membrane
                      Isotonic = solute concentration are the same on both sides
                           Hypotonic = solute concentration is lower outside than inside
                                 (thus, solvent concentration is higher outside than inside) and water
                                 will enter the cell (move DOWN the concentration gradient)
                           Hypertonic = solute concentration is higher outside than inside
                                 (thus, solvent concentration is lower outside than inside) and water
                                  will leave the cell (move DOWN the concentration gradient)
 
                   Cell in a hypotonic solution will swell due to water entering the cell
                            To keep from expanding and bursting, living cells in hypotonic solutions
                                    May have contractile vacuoles to pump water out continuously
                   Cell in a hypertonic solution will shrivel due to water leaving the cell
                            To keep from losing water, living cells in hypertonic solutions (salt water)
                                    Increase solute concentration inside the cell
 
            Plant cell in hypotonic solution
                  plant cell wall keeps cell from expanding indefinitely
                         (like blowing up a balloon inside a coke bottle
                  Turgor pressure = pressure exerted INWARD by cell wall to counteract the
                          OUTWARD pressure of water trying to enter the cell by osmosis
 
 
Osmosis and excretion
      Too little water in body
          Osmoreceptors (cells) in brain shrink when body fluids are too concentrated (too little
                               water in body)
               Osmoreceptors signal pituitary gland to release antidiuretic hormone (ADH)
               ADH carried by blood stream to kidney tubule cells, causing them to alter their
                  permeability so they release water that goes back into blood capillaries
                  = body conserves water that would, without ADH be released in the urine

      Too much water in body —
           Osmoreceptors in brain expand when body fluids too dilute
               Inhibits release of ADH by pituitary
               Kidney tubule cells remain impermeable to water, ridding body of excess water
                      Aquaporins = proteins in kidneys control water reabsorption
 
 
 
 
 
 
Facilitated diffusion —
         polar or ionic molecules move through channels within carrier proteins in membrane
         passive process
         Carrier proteins have a channel allowing molecule to move through the hole to enter or
                         leave the cell
                carrier protein like a doughnut embedded in a cell membrane  -
                             doughnut = carrier protein
                             hole         = channel
         transports  glucose, amino acids, iron
 
 
 
Active transport —
         Active process (requires expenditure of energy, i.e., uses ATP)
         Moves molecules AGAINST a concentration gradient (from region of low concentration
                    to a region of high concentration)
         example = sodium-potassium pump
                      high concentration of potassium inside cell
                 1. cell carrier protein binds Na on inner surface of cell membrane
                 2. molecule of ATP is split, causing change in shape of carrier protein.
                      Causing Na to be released to outside of the cell, and phosphate to attach to
                                protein
                 3. this altered shape of carrier protein allows K to enter the cell as protein
                                resumes normal shape because of release of phosphate group.
                 4. carrier protein is now ready to start over again
 
 
 
Vesicle transport —
         Exocytosis      transports large particles out of cells
         Endocytosis   transports large particles into cells
         Receptor-mediated endocytosis  transports selected large particles into cells
  
   Exocytosis —
         Membrane-bound vesicle forms inside of cell containing large molecules (e.g., from ER,
                      or Golgi apparatus)
          Vesicle moves to the cell membrane and fuses with it (fluid mosaic membrane model)
          Releasing molecules to the outside of the cell
       Release of neurotransmitters from pre-synaptic neuron.
       Botulism and tetanus toxins block neuronal endo- and exocytosis resulting in fatal
                     paralysis
 
 
 
   Endocytosis —
         Membrane-bound vesicle forms by invagination of cell membrane around particles
                         being taken up
         Membrane-bound vesicle released from cell membrane and moves into cytoplasm
         After contents used, the membrane is recycled within the cytoplasm
   Receptor-mediated endocytosis —
         Receptor proteins on surface of cell membrane bind a specific protein (lock and key
                            recognition)
         Cell membrane invaginates to form a vesicle