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Biological Sciences
Course Code
Mary Olaveson

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LECTURE 8 (chapter 51)
Overview: A Balancing Act
oAll organisms need water, its the solvent for all of the bodies floating around
in different organisms
oOrganisms also evolved in salt water (ions)
oInternal fluids have very close approximation to sea water in terms of the
ionic concentrations
oHomeostasis in terms of ionic concentrations
oActive transport follow by osmosis
oPassive transport followed by the movement of water osmosis
Water Balance Crucial for Animal Cells
oHyper-osmotic: has more solute, more concentrated than the cytoplasm of the
cell so water will move from areas of higher water potential to lower water
potential, lower solute concentration to higher solute concentration
oAs the solute concentration increases, the solute potential becomes more
negative and thats all because pure water has a water potential of zero
because there are no solutes dissolved in the water. Water potential=solute
potential + pressure potential
oIf water potential is 0 then solute potential is also 0 since we ignore pressure
oAs soon as you start adding solutes to the water then you have less
opportunity that water isnt free to move or solutes are not as free to move
they’re dissolved and interacting with the water molecules
oSo consequently as soon as you add solutes, you increase the concentration in
the water, the solute potential will become negative so the highest water
potential you can get is 0
oIf solute potential becomes negative then so does water potential as more
material gets dissolved
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oIdeally for mammalian cells, (red blood cells) the osmotic potential or water
potential or solute potentials should be isotonic, should be similar on either
side of the membrane (iso-osmotic or isotonic solution)
oHyper-osmotic: fluid surrounding cell is more negative/more concentrated
solute levels and therefore more negative potential than inside the cell so
water goes from higher water potential to lower water potential and
consequently leaves the cell
oIn the hypo-osmotic, there is less negative water potential and less negative
water potential because its more dilute and the water will move into the
more concentrated regions inside the cell until the cell bursts
oIf you start at 0 then you can only go to negative #s when you keep adding
solutes and building up the concentration
What Roles Do Excretory Organs Play in Maintaining Homeostasis?
oWaters also moving passively in response to solute concentrations and
increasingly negative solute potentials
oWater moves through osmosis and thats passive
oSo what do the organs involved in the excretory system do in terms of
maintaining homeostasis while in large measure? one of the jobs is to
maintain the water balance and the ionic bonds so you dont have any of those
extremes and that the cells inside the animal can remain in a homeostatic
balance in terms of solute and water potentials
oIn so many ways the water and solute potentials are tied together
oIn terms of different types of habitats that organisms find their selves in and
in many of the marine systems you have high salt concentrations relatively
oSo for the most part, if you look at the average osmolarity inside most bodily
fluids of many animals, this is much more concentrated and therefore you got
the issues of the more concentrated external environment relative to the
internal environment so those organisms are dealing with issues of water loss
o Osmoregulation
Solutions to Osmotic Challenges
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