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)