Biomedical Engineering Reference
In-Depth Information
The plasma membrane plays an important role in regulating cell volume by controlling the
internal osmolarity of the cell. Osmolarity is defined in terms of concentration of dissolved
substances. A 1 osmolar (1 Osm) solution contains 1 mole of dissolved particles per liter of
solution, while a 1 milliosmolar (1 mOsm) solution has 1 mole of dissolved particles per
1,000 liters of solution. Thus, solutions with high osmolarity have low concentrations of water
or other solvents. For biological purposes, solutions with 0.1 Osm glucose and 0.1 Osm urea
have essentially the same concentrations of water. It is important to note that a 0.1 M solution
of sodium chloride (NaCl) will form a 0.2 Osm solution, since NaCl dissociates into Na
þ
and
Cl
ions and thus has twice as many dissolved particles as a solution of a substance—for exam-
ple, glucose—that does not dissociate into smaller units. Two solutions are isotonic if they have
the same osmolarity. One solution is hypotonic to another if it has a lower osmolarity and
hypertonic to another if it has a higher osmolarity. It is important to note that tonicity (isotonic,
hypotonic, or hypertonic) is only determined by those molecules that cannot cross the plasma
membrane, since molecules that can freely cross will eventually reach equilibrium with the
same concentration inside and outside of the cell.
Consider a simple model cell that consists of a plasma membrane and cytoplasm. The
cytoplasm in this model cell contains proteins that cannot cross the plasma membrane
and water that can. At equilibrium, the total osmolarity inside the cell must equal the total
osmolarity outside the cell. If the osmolarity inside and the osmolarity outside of the cell
are out of balance, there will be a net movement of water from the side of the plasma
membrane where it is more highly concentrated to the other until equilibrium is achieved.
For example, assume that a model cell (Figure 3.6) contains 0.2 M protein and is placed in a
hypotonic solution that contains 0.1 M sucrose. The plasma membrane of this model cell
is impermeable to proteins and sucrose but freely permeable to water. The volume of the
PLASMA
MEMBRANE
WATER
WATER
0.2 M PROTEINS
0.1 M SUCROSE
FIGURE 3.6
A simple model cell that consists of cytoplasm, containing 0.2 M proteins, and a plasma membrane
is placed in a solution of 0.1 M sucrose. The plasma membrane is insoluble to proteins and sucrose but allows
water to pass freely in either direction. The full extent of the extracellular volume is not shown and is much larger
than the cell's volume of 1 nl.
