UNIT 4: Homeostasis Chapter 9: Homeostasis: A Fine Balance pg. 426 - 9.4: Water Balance pg. 442 - 445 Osmosis An organism’s extracellular fluid must be maintained; a constant fluid volume, solute content, and temperature. The extracellular must continuously replenish and maintained. Aquatic organisms must maintain their internal environment, extracellular fluid, even with being surrounded by water. Solute concentrations vary between the internal and external environments. With a higher solute concentration internal compared to the external, water will move into the organism by osmosis. Water needs to be removed. The opposite may also occur. Figure 1: a) two solutions have different concentrations. b) Osmosis occurs when water moves across a selectively permeable membrane from an area of high concentration to an area of low concentration. Osmotic Pressure – is the pressure that results from a difference in solute concentration between the two sides of a selectively permeable membrane. Hyperosmotic – is the property of the solution on one side of a selectively permeable membrane that has the lower concentration of water. Hypoosmotic – is the property of the solution on one side of a selectively permeable membrane that has the higher concentration of water. Isoosmotic – is the property of two solutions that have equal water concentrations. For terrestrial organisms extracellular fluid must be maintained also. Water must be conserved and continuously replenished to maintain a homeostatic balance internally. In osmosis is the movement of water from an area of high water concentration to an area of lower water concentration. (solute concentration) Water moves through a semi-permeable membrane which selects against the movement of some solutes across the membrane. It is the solute concentration on either side of the membrane and the selectively permeable membrane that creates the osmotic pressure. Water moves across the membrane when two solutions separated by a membrane have different solute and water concentrations. Water will move across a membrane and continue to do so until the solutions have similar concentrations, Isoosmotic. Water pressure (hydrostatic) is also a factor that determines water movement. If the water pressure is equal to the osmotic pressure on the other side of the membrane, there will be no net water movement. The relationship between water pressure and osmotic pressure is important for the structure of plant cells. Turgor pressure created by water volume gives the plant its rigidity and allows the plant to stand upright. The plant will wilt if water moves out of the cell into the external environment. Animal cells do not have cell walls, if water moves in the volume will increase, the cell will eventually burst, if water moves out the cell will shrink. Osmoregulation and Excretion Osmoregulation – is the process of actively regulating the osmotic pressure of bodily fluids and cells. Osmosis is responsible for establishing and maintaining homeostasis. The extracellular fluid must be isoosmotic to the intercellular fluid, inside the cell. Osmoregulation is the process of actively maintaining the osmotic pressure of bodily fluids, water and solute concentrations are maintained by the continuous movement of water (osmosis) and solutes (diffusion) into and out of the cell. Vertebrates require a more complex control mechanism to maintain intercellular and extracellular fluids constant, but different from the external environment. Waste products from metabolism and cellular functions are continuously eliminated to maintain an aqueous homeostasis. Excretion The cell needs to regulate their ionic balance and pH balance, along with its osmotic concentration. Some ions and toxic (amino acids and nucleic acids) compounds must be eliminated form the cell. These waste products form a solution within the cell, and their elimination maintains osmotic pressure. Excretion is the elimination of waste products and foreign matter from the body. It maintains the ionic and osmotic equilibrium that necessary for cell functions. The kidneys and bladder are the organs of the excretory system which regulate the removal of wastes. Proteins that are broken down in the liver produce a number of nitrogen molecules that are waste products. Different organisms produce different nitrogen molecules for excretion. Bony fish – ammonia Mammals – urea Birds – uric acid During the break down of proteins, deamination occurs (removal of an amine group) the amine group is converted to ammonia. The ammonia molecule is highly toxic, a concentration greater then 0.005 mg/L can kill a human. The concentration needs to be diluted by large quantities of water. This is a problem for terrestrial animals but not to aquatic organisms, which have an abundant supply of water and secrete ammonia directly from the body. Mammals, some reptiles, and most amphibians, the ammonia is combined with carbonate ion (HCO3-) to create urea, a soluble substance with a 0.001% the toxicity of ammonia. 33 mg of urea can be dissolved in 100 mL of blood with no toxic effects. With this conversion less water is required to eliminate the waste. This allows terrestrial animals to maintain internal water balance.