Properties of Water, Osmosis, Volume Regulation

February 3, 2010

pp. 25-30, 758-761

Properties of Water Figures

I. Introduction to Water Theme:

• water is important at all levels of biological investigation
• productivity of biological communities determined by water availability
•       (Sonoran Desert
, Mojave Desert, Antarctica, Peruvian Rain Forest and Stream)
•       limiting factor in arid environments (Utah)
• transport of minerals within multicellular organisms requires lots of water
• unicellular organisms must be in contact with water for exchange of materials with the external environment

II. Introduction to Water

a) earth is the only planet with significant quantities of water

b) oceans cover 71% of the earth to an average depth of 3.8 km

c) water is one of only three inorganic liquids that occur at normal temperatures and pressures found on earth's surface.

d) approximately 2/3 of the mass of most organisms is water.

III. Structure

a) water is the dihydride of oxygen

b) oxygen more electronegative, so although water molecule has no NET electrical charge, the shared electrons of the covalent bond are attracted more strongly to the oxygen atom, leaving the water molecule with a partial negative charge on the O end, and partial positive charges on the H end.

c) result is a polarity that attracts other polar molecules forming hydrogen bonds - weak and transient bonds.

IV. Properties of Water

a) high specific heat - amount of heat that needs to be added to or taken from 1g of water to affect a 1 degree temperature change. Consequences for thermal resistance, evaporative cooling.

b) crystal lattice-like arrangement of water molecules in ice decrease density of the solid. Ice floats!

c) cohesion, adhesion - polar water molecules attract each other (cohesion) and other polar molecules (adhesion). Capillary action, surface tension.

d) density - water is 775X as dense as air. Greatest density at ~4C.  Aquatic organisms can have reduced supporting or structural tissues (Nymphaea

, stems).

e) powerful solvent - dissociated ions attract polar water molecules that form a hydration shell preventing reassociation. Allows easy transport of dissolved materials.

f) diffusion 2 - water molecules in constant motion, and diffuse from areas of high concentration toward areas of lower concentration.

g) osmosis - water readily passes through selectively permeable biological membranes via aquaporins. If water potential is higher (solute concentration lower) on one side of a membrane than on the other, it is hypoosmotic, and water will migrate across the membrane to the side with lower water potential. Cells can lose water if external environment is hyperosmotic. Cells gain water if external environment is hypoosmotic.

Why don't cells burst? Bacteria, fungi, plants and many protist cells are surrounded by a strong cell wall that exerts a confining force equal to the osmotic pressure that builds within the cell.

Naked cells (animal and some protist cells) can expel water with a contractile vacuole (see link) or bathe cells in an isoosmotic solution that is carefully regulated (e.g. blood).

Study Questions:
1) What is a polar molecule?
2) What is specific heat?  What is the biological significance of the high specific heat of water?
3) How does the polarity of water contribute to its ability dissolve ions?
4) How does solute concentration affect osmotic potential?
5) If cell protoplasm is hypoosmotic compared to the external environment, which way will water flow?
6) What prevents plant cells from rupturing from taking up too much water?  Bacterial cells? Animal cells? Protist cells? Fungal cells?

Key Words: polar, hydrogen bond, specific heat, cohesion, adhesion, capillary action, hydration shell, diffusion, osmosis, hyperosmotic, hypoosmotic, isoosmotic, contractile vacuole