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Diffusion
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Diffusion is the natural tendency of molecules to flow from higher concentrations to lower concentrations. When the barrier between two substances is removed (as shown here), the molecules will diffuse throughout the entire container. While the number of molecules in the container is the same as it was before the barrier was removed, the substances are now at lower concentrations. The rate of diffusion depends on the weight of the molecules—heavy molecules diffuse more slowly than light molecules. |
Definition: Movement of particles of a region of higher concentration to that of lower concentration.
Diffusion of matter occurs most rapidly in gases, more slowly in liquids, and
most slowly in solids. The spreading of odoriferous molecules (a smell)
throughout a room is a common example of gaseous diffusion. A solid may dissolve
and diffuse through a liquid, as when a lump of sugar is placed in a cup of
water. This process is much slower than the diffusion of a gas; if the water is
not stirred, it may take weeks for the solution to become homogeneous. An
example of the slowest diffusion process, a solid diffusing into a solid, occurs
when gold is plated on copper. The gold will diffuse slowly into the surface of
the copper; however, diffusion of an appreciable amount of gold more than a
microscopic distance normally requires thousands of years.
As distinguished from stirring, which is a process of mixing masses of material,
diffusion is a molecular process, depending solely on the random motions of
individual molecules. The rate of diffusion of matter is therefore directly
proportional to the average velocity of the molecules. In the case of gases,
this average speed is greater for smaller molecules, in proportion to the square
root of the molecular weight, and is greatly increased by rise in temperature.
Metallic thorium, for example, diffuses rapidly through metallic
tungsten at temperatures around 2000° C (3632° F); the operation of
certain vacuum tubes is based on this diffusion.
Diffusion processes are of great biological importance. For example, digestion
is essentially a process of chemically changing food so that it will be able to
pass, by diffusion, through the intestinal wall into the bloodstream.
Shock, a condition that frequently follows surgery or injury, is a
state in which the blood fluids have diffused excessively through the
blood-vessel walls into the body tissues. Treatment of shock consists of
injecting chemicals, usually in the form of blood, plasma, or plasma expanders,
into the remaining blood fluid to compensate for the loss by diffusion and to
alter pressure in the blood vessels, thus obviating further loss .
Osmosis
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| The experiment shown above demonstrates the process of osmosis. Water flows through a semipermeable membrane into a sugar solution, diluting the solution. The sugar molecules cannot pass through the membrane, so the water outside remains pure. |
Osmosis, in botany and chemistry, the flow of one constituent of a solution through a membrane while the other constituents are blocked and unable to pass through the membrane. Experimentation is necessary to determine which membranes permit selective flow, or osmosis, because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow. In classic demonstration of osmosis, a vertical tube containing a solution of sugar, with its lower end closed off by a semipermeable membrane, is placed in a container of water. As the water passes through the membrane into the tube, the level of sugar solution in the tube rises visibly. A semipermeable membrane that may be used for such a demonstration is the membrane found just inside the shell of an egg, that is, the film that keeps the white of the egg from direct contact with the shell. In this demonstration, the water moves in both directions through the membrane; the flow is greater from the vessel of pure water, however, because the concentration of water is greater there, that is, fewer dissolved substances exist in this solution than in the sugar solution. The level of liquid in the tube of sugar solution will eventually rise until the flow of water from the tube of sugar solution, under the influence of hydrostatic pressure, just equals the flow of water into the tube. Thereafter, no further rise in level will occur. The hydrostatic pressure establishing this equality of flow is called osmotic pressure. A variety of physical and chemical principles is involved in the phenomenon of osmosis in animals and plants.
Active Transport
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| Region in intestine where active transport occurs. This process also occurs in plant roots to absorb mineral salts. |
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| Active transport across intestinal cells (villi). |
In order to move substances against a concentration gradient—that is, from the side of the plasma membrane where the concentration of a substance is lower to the side where it is already higher—a cell must expend energy in a process known as active transport. Active transport is achieved by membrane proteins called pumps, which have a docking site that is shaped to fit a specific substance. These pumps are open on either the inside or the outside of the cell. When the proper molecule or ion attaches to the docking site, the pump changes shape so that the docking site moves its opening to the other side of the plasma membrane, releasing the molecular cargo. Many pumps obtain the energy necessary to perform this work from adenosine triphosphate (ATP), a molecule that serves as the main energy currency of living cells.
