Cells are bathed in a watery matrix and conduct most of their reactions in a similar watery fluid—a solution in which water is the solvent and the numerous molecules and ions dissolved in it are the solutes. The solutes include protons (H +), ions like sodium (Na +), potassium (K +), calcium (Ca 2+), organic molecules such as sucrose (C 12H 22O 11), polar and nonpolar molecules, and a host of other substances, the chemical nature of which determines the ease or difficulty with which they move across membranes.
All molecules possess kinetic energy and move in a random fashion. In solutions, solutes become distributed uniformly as they diffuse and occupy all available space. Diffusion is the net movement of a substance from a region of its higher concentration to a region of its lower as a result of the random movement of its individual molecules; or, in other terms, down a concentration gradient. The greater (steeper) the concentration gradient, the faster the movement. If nothing intervenes, the movement will continue until the concentration gradient is eliminated, i.e. until the substance is uniformly distributed. Most movement of materials in cells is by diffusion although it is neither the most efficient means nor can it be used for long distance moves.
Osmosis is a special kind of diffusion that pertains specifically to water: the movement of water across a selectively permeable membrane that permits the passage of water but inhibits the movement of the solute. The water moves down a concentration gradient from the region of its higher concentration of free water molecules (less solutes) to the region of its lower concentration of free water molecules (more solutes), or from high pressure to low pressure.
In comparing the relationship of the cell contents to those of the surroundings, three terms are used: 1.) isotonic: The two solutions have the same concentration of solutes, hence the same amount of water moves into the cell as moves out; 2.) hypotonic: The water outside the cell has less solute (hypo = less), and therefore more free water with the result that water moves into the cell at a greater rate than it moves out; 3.) hypertonic: The water outside the cell has more solute (hyper = more), and therefore less free water with the result that water moves out of the cell at a greater rate than it moves in.
In osmosis, water moves from a hypotonic solution to a hypertonic through a selectively permeable membrane. Water will diffuse across a selectively permeable membrane until the concentrations are the same on both sides (i.e. isotonic). If pressure is applied to the hypertonic side (the side into which the water is moving), it is possible to stop the inward flow of water. The amount of pressure needed to do so is called the osmotic pressure of the solution and is determined by the concentration of total solutes in the solution. Osmosis doesn't depend on the kinds of molecules or ions in solution, only on the amount of solutes.
Osmosis is vitally important for plants because it enables the plant to assimilate nutrients from the soil; the soil water is hypotonic to the root cells. Osmosis also makes the cells turgid (swollen) and gives rigidity to the plant. Water in the cell (mostly in the central vacuole) exerts a turgor pressure against the cell wall, which, in turn, exerts inwardly a mechanical wall pressure against the protoplast. The two equal and opposing pressures give strength to the cell and columns of water‐filled cells keep the plant erect. Forget to water a house plant and the cells lose water, the turgor and wall pressures lessen, the cells become flaccid (limp), and the whole plant wilts. Internally, as water leaves the cells the cytoplasm shrinks away from the wall and collapses into an interior clump; the cell is plasmolyzed, and the process is plasmolysis (an example of osmosis in action). The cells are not dead, but they stop active metabolizing. The wilted celery stalk retains more of its rigidity than the lettuce leaves because it has reinforcing strings of collenchyma cells among its thin‐walled parenchyma. Wash off the salt solution and immerse the salad greens in pure water and if the membranes were not broken, osmosis will rehydrate the cells to turgidity.