Organismal Physiology Lecture No. 17: Principles Of Water & Solute Balance
Thursday November 8 , 2012
-The capacity of air to hold water molecules increases exponentially as air temperature increases
linearly. Thus, the amount of water needed to saturate a volume of air at a cold temperature is less than
the amount of water required to saturate the same volume of air as it warms. As air warms over the day,
its relative humidity drops. So, leaves lose less water through transpiration (e.g. CAM photosynthesis)
than at noon.
Osmoregulation & Osmolarity/Osmolality:
-Osmoregulation is defined as the balancing of water and ions in the cells and the body in order to allow
physiological function. The osmolarity or osmolality is defined as the amount of solute present in a given
solution. 1 Mole of solutes is equivalent to 1 Osmole. The effect of osmolarity is cumulative (e.g. 0.2M of
5 things = 1 Osmole). Typically, osmolality involves the amount of solutes per kilogram of solvent, while
osmolarity involves the amount of solvent per litre of solvent.
Properties Of Water:
-Water is comprised of a highly polar bond between oxygen and hydrogen atoms. The consequences of
this include: Strong hydrogen bonding, a high heat of fusion and vaporisation, a large thermal capacity,
and strong cohesion (between molecules) as well as adhesion (to other substances). For life, water is
regarded as the universal solvent. Water also creates a “shell” of bound water around many
macromolecules like proteins (keeps the ions dissociated from each other).
Water In Cells:
-Water in cells is key for two reasons: it determines cell volume in animals (cell volume is often the
bottom level of regulation of water and ion balance) and it determines cell turgor pressure in plants (the
positive pressure or turgor pressure in the water potential equation).
Plant & Animal Cells At Low Concentrations:
-In hyperosmotic (concentration salt) solutions, animal cells shrivel up, while plant cells undergo
plasmolysis (cytoplasm pulls away from the cell wall). In hypoosmotic (distilled water) solutions, animal
cells burst, while plant cells don’t give way as easily (due to the rigidity of the cell wall). Isoosmotic
(normal salt concentration) solutions feature stability in the structure of both animal and plant cells.
Moving Water – Diffusion & Osmosis:
-In a solution of two different solutes (random molecular solutes), diffusion dictates that they move
from an area of high concentration to low concentration through a permeable membrane. As an
equilibrium is reached, there is no net effect on osmotic pressure. -As water moves by osmosis to areas of high solute concentration through a selectively-permable
membrane (only allows water through), there is an observed change in osmotic pressur