Chapter 9: Homeostasis: A Fine Balance pg. 426 - 9.4: Water Balance

UNIT 4: Homeostasis
Chapter 9: Homeostasis: A Fine Balance
pg. 426 -
9.4: Water Balance
pg. 442 - 445
An organism’s extracellular fluid
must be maintained; a constant
fluid volume, solute content, and
temperature. The extracellular must
continuously replenish and
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
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
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.
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
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