HOMEOSTASIS AND TRANSPORT
5-1 PASSIVE TRANSPORT
 Cell membranes help maintain homeostasis by controlling what enters and leaves each cell
 Passive transport occurs spontaneously, requiring no energy (ATP) input by the cell
A. DIFFUSION
 Simplest type
 Molecules move from areas of high concentration to areas of low concentration
 Driven by the kinetic energy of molecules (they are in constant motion)
 Equilibrium will occur when the concentrations of molecules is the same throughout;
however, molecular motion still continues because of the random movement of molecules
 Regarding cells, membranes are semipermeable; so movement of molecules is dependent
upon concentration gradients, size type and chemical nature of the molecule
B. OSMOSIS
 Refers to the movement of water down its concentration gradient through a pore in the
semipermeable membrane – (aquaporin)
 When [solute] outside the cell is lower than inside the cell, HYPOTONIC; water moves into
the cell at a faster rate than it leaves cell; cell explodes or swells (cytolysis)
 When [solute] is higher outside the cell, HYPERTONIC; water moves out of cell at a faster
rate than it enters, cell shrinks or crenates
 When concentrations of dissolved particles equal outside and inside the cell, ISOTONIC;
water moves into and out of cell at same rate; cell does not change size
 How do animals and plants deal with life in hypotonic and hypertonic environments?
o Contractile vacuoles
o Turgor pressure
o Plasmolysis
o Cytolysis
C. FACILITATED DIFFUSION
 Process used for molecules that are insoluble in lipids or may be a bit too large to pass
through pores in the membrane
 This movement is assisted by carrier proteins (integral proteins)
 When molecule to be transported binds to a specific carrier, the protein changes shape and
allows molecules to pass through membrane
 Facilitated diffusion helps molecules move into or out of a cell , depending upon
concentration gradients
 CARRIER PROTEINS are SPECIFIC for one type of molecule (i.e., the carrier protein that
assists in transport of glucose will ONLY transport glucose)
D. DIFFUSION THROUGH ION CHANNELS
 Ions, insoluble in lipids, cannot cross cell membrane without help
 Carrier proteins do this; they are specific for their particular ion
 Some channels are always open; others have gates and open when electrically or
chemically stimulated
5-2 ACTIVE TRANSPORT
 MOVING MOLECULES AGAINST THEIR CONCENTRATION GRADIENT REQUIRES ATP
A. MEMBRANE PUMPS
o Done by carrier proteins
o One example is the Na+/K+ pump see figure 5.6
o This pump is important in the transmission of nerve signals
B. ENDOCYTOSIS AND EXOCYTOSIS
 Used to transport large quantities of molecules into or out of the cell at one time
 Requires energy to move the cytoskeleton
ENDOCYTOSIS
 Cells ingest extracellular fluid, macromolecules and large particles, including other cells
 Cell membrane encloses around “stuff” and pinches off inside; forms vesicle; vesicle
sometimes fuses with lysosome
 Pinocytosis = cell drinking
 Phagocytosis = cell eating
 Phagocytes
EXOCYTOSIS
 Reverse of endocytosis
 Cells use it to release large molecules (proteins, like neurotransmitters; mucus)