WATER TRANSPORT

A. Membrane Water Permeability: most (but not all) cell membranes are highly permeable to H2O molecules

B. Osmosis: the major driving force for water transport across cell membranes is the relative number of water molecules vs. the number of solute (dissolved) particles; water movement under this driving force is termed osmosis (similar to diffusion of dissolved particles)

C. Osmolarity (or Osmolality): the relative concentration of water can be derived from adding the molar concentration of all the solutes; this is termed the osmolarity of the solution

1. typical osmolarity for interstitial fluid or blood plasma in humans is about 300 mosmoles/liter (mosm/l) (range 285-310 mosm/l)

2. solutions are classified by their osmolarity relative to normal

isosmotic: same osmolarity as normal interstitial fluid or blood plasma
hyperosmotic: greater than normal osmolarity
hyposmotic: lower than normal osmolarity

D. Water moves from regions of low osmolarity (dilute) to regions of high osmolarity (concentrated)

Example: response of erythrocytes (red cells) placed in isosmotic saline (NaCl solution) vs. erythrocytes placed in distilled water

E. Solutes with the same intracellular and interstitial concentrations and solutes that equilibrate across the cell membrane do not contribute to net water movement
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Solutes with different intracellular-interstitial concentrations determine the tendency of cells to change size (swell or shrink); the sum of the concentrations of these substances is termed the tonicity of the solution (the term “tone” in this context refers to the tendency of cells to swell or shrink)

1. solutions can also be classified by their tonicity

isotonic: same tonicity as normal interstitial fluid
hypertonic: greater than normal tonicity (cells will shrink)
hypotonic: lower than normal tonicity (cells will swell)

Example: place erythrocytes in isosmotic urea (note: urea easily penetrates the membranes of most cells)