Molecular Movement

Molecules and ions (discussed in Chapter 2) are constantly in random motion. Visual evidence of this can be seen with an ordinary light microscope. If a drop of India ink is diluted with water and observed through a microscope under high power, the tiny carbon particles of the ink appear to be in constant motion. This motion, known as Brownian movement, is the result of the bombardment of the visible particles by invisible water molecules, which are in constant motion themselves.

is less than a millionth of a millimeter in diameter is going to take a long time to move just 1 millimeter, even though the amount of movement may be great in proportion to the size of the particle concerned. In gases, there is a great deal of space between the molecules and correspondingly less chance of the molecules bumping into each other and thus being slowed down. Accordingly, gas molecules occupy a space that becomes available to them relatively rapidly, while liquids do so more slowly, and solids are slower yet.

Large molecules move much more slowly than small molecules. If you added a tiny drop of a dye (which has relatively large molecules) to one end of a bathtub of water without disturbing the water in any way, it would take years for the dye molecules to diffuse throughout the tub and reach a state of equilibrium. In nature, however, wind and water currents distribute molecules much faster than they ever could be distributed by diffusion alone.


The differing intensity of smells discussed earlier involves molecules behaving somewhat like the tennis balls. Through their random motion, molecules tend to become distributed throughout the space available to them. Accordingly, if perfume molecules are kept in a bottle, they will become distributed throughout the bottle, but if the stopper is removed, they will eventually become dispersed throughout the room, even if there is no fan or other device to move the air.

This movement of molecules or ions from a region of higher concentration to a region of lower concentration is called diffusion (Fig. 9.2). Molecules that are moving from a region of higher concentration to a region of lower concentration are said to be moving along a diffusion gradient, while molecules going in the opposite direction are said to be going against a diffusion gradient. When the molecules, through their random movement, have become distributed throughout the space available, they are considered to be in a state of equilibrium. The rate of diffusion depends on several factors, including temperature and the density of the medium through which it is taking place.

Except within the area immediately surrounding the source, unaided diffusion requires a great deal of time because molecules and ions are infinitesimal. Something that


Despite the fact that the cytoplasm of living cells is bounded by membranes, it is now well known that water (a solvent) moves freely from cell to cell. This has led scientists to believe that plasma, vacuolar, and other membranes have tiny holes or spaces in them, even though such holes or spaces are invisible to the instruments presently available. It also has led to the construction of models of such membranes (see Fig. 3.11). Membranes through which different substances diffuse at different rates are described as differentially permeable, or semipermeable. All plant cell membranes appear to be differentially permeable.

In plant cells, osmosis is essentially the diffusion of water through a differentially permeable (semipermeable) membrane from a region where the water is more concentrated to a region where it is less concentrated. Osmosis ceases if the concentration of water on both sides of the membrane becomes equal.

A demonstration of osmosis can be made by tying a membrane over the mouth of a thistle tube that has been filled with a solution of 10% sugar in water (i.e., the solution consists of 10% sugar and 90% water). Fluid rises in the narrow part of the tube as osmosis occurs when the thistle tube is immersed in water (Fig. 9.3).

Osmosis Figure Mcgraw

Figure Q.2 Simple diffusion. A. A barrier separates two kinds of molecules. B. When the barrier is removed, random movement of individual molecules results in both kinds moving from a region of higher concentration to a region of lower concentration. C. Eventually, equilibrium (even distribution) is reached. The diffusion gradually slows down as equilibrium is approached.

Stern-Jansky-Bidlack: Introductory Plant Biology, Ninth Edition

9. Water in Plants


© The McGraw-Hill Companies, 2003

Water in Plants

Thistle Tube Osmometer

Figure 9.3 A simple osmometer, made by tying a membrane over the mouth of a thistle tube.


Figure 9.3 A simple osmometer, made by tying a membrane over the mouth of a thistle tube.


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