b. Isotonic Solutions. An isotonic solution is one in which the concentration of
the surrounding fluid, and the osmotic pressure outside a cell, is equal to the
concentration and the osmotic pressure inside the cell. The cell retains its original
shape. The cytoplasm remains unchanged and there is no osmosis.
c. Hypotonic Solutions. When the concentration and osmotic pressure are
lower in the surrounding fluid than in the cell, the cell begins to take up water, and the
surrounding solution is said to be hypotonic. The cell begins to take up water by
osmosis through a semipermeable membrane (the cell membrane) and greatly enlarges
by swelling. When the fluid inside the cell becomes diluted sufficiently to equal the
concentration outside the cell, further osmosis ceases (figure 16 A).
d. Hypertonic Solutions. When a cell is placed in a highly concentrated
solution that has a higher osmotic pressure than that of the inside of the cell, the cell
loses water from its cytoplasm and the surrounding solution is said to be hypertonic.
Until the two concentrations are equal, water passes from the cell to the surrounding
medium by osmosis (figure 16 B) and the cell shrinks.
Figure 15. Osmotic pressure.