The most common mechanism by which substances move in biological systems is diffusion. Diffusion in the most general state refers to the random, thermal motion of particles. The first thing to remember about diffusion is that all movement is driven by heat energy. It is obvious that in our macroscopic world, items do not go moving around unless some force is pushing it. You would certainly be quite surprised if the pen on your desk should suddenly role off the table onto the floor. You would initially assume that something >caused= the pen to move as you know that pens do not just roll around on their own!. The same is true in the microscopic world of atoms and molecules. They do not move without some force pushing them (didn't Newton say that?). Obviously, it takes much less force to move a single molecule than to move an entire pen so only microscopic items are small enough to be pushed by the heat in the environment.. The heat in the air (especially in the Texas summer!) is energetic enough to push atoms and molecules but is not strong enough to move pens or other macroscopic (large) items. The force of the heat does not move the molecule in a particular direction but pushes it randomly. The atoms (and molecules and other microscopic particles) move every which way, constantly bumping into each other and changing direction. It may help to think of the atoms as balls rolling randomly around, always proceeding in a straight line until it hits another ball (or a wall) and bounces in another direction. The actual movement of the particle is random and we cannot predict where the heat energy will push any particular particle.
This is not very helpful in science where we hope to be able to predict the movement of particles. If diffusion is random, then what good is knowing about diffusion? To understand its value, we must examine diffusion a bit more closely.
Imagine that you have a container containing one liter of water (Solution A). Dissolved in the water is 1 mole of glucose (this would make Solution A a 1 M solution of glucose). Assuming that there is enough heat the system, both the water molecules and the glucose molecules will be in motion, moving randomly around the container, bouncing off each other and the walls of the container. Since the glucose is moving randomly (and there are so many glucose molecules - remember that 1 mole has 1 x 1023 molecules), the glucose molecules will be evenly distributed throughout Solution A. Now imagine that a second one liter of water containing no solutes (Solution B) is connected to the first by a membrane that is permeable to glucose. Since all of the glucose molecules are moving randomly in all directions (diffusing), there will always be some glucose molecules that are moving toward the membrane at any given moment. The number of these molecules that are moving to toward the membrane will be proportional to the number of glucose molecules present (i.e. the concentration). The greater the glucose concentration, the greater the probability that some glucose molecules will be moving toward Solution B. They will then meet the membrane. Since this membrane is permeable to glucose, these glucose molecules will move through the membrane and enter Solution B. Since initially, there are no glucose molecules in Solution B, no glucose molecules can move from Solution B to A. This means that there is a net movement of glucose from Solution A into Solution B. This movement is known as net diffusion. As the glucose moves from Solution A to Solution B , there will be an increase in the concentration of glucose in B and a decrease in the concentration of glucose in A. This means that there is a probability that the glucose in Solution B will move back into Solution A (and again, the number returning will be proportional to the concentration of glucose in B). Now there are competing reactions, glucose moving from Solution A to Solution B and glucose moving from Solution B to A. Since we have already concluded that the number of molecules moving out of the containers is proportional to the concentration of molecules in the container, more molecules will move from Solution A to Solution B as long as the concentration of glucose in Solution A is greater than that in Solution B and there will be a net movement of molecules from Solution A to B. (A similar argument can be made showing that if the concentration of glucose was greater in Solution B, then there would be a net movement of glucose into Solution A.) Enough glucose will eventually enter Solution B so that the glucose concentration of Solution B becomes equal to that in Solution A. At this point, the probability that glucose will move to Solution A from B is the same as the probability that glucose will move into Solution B from A. Therefore there will be no net movement of glucose. Glucose will still be moving, however, it is just that the same number of glucose molecules leaving Solution A for B will be entering A from Band thus the concentration of glucose in each compartment is constant. The solutions have reached a state of equilibrium which is defined as the state in which there is no net movement of solutes and thus the concentration of solutes remains constant.
Summary of Diffusion: (what you need to remember)
Diffusion is the random, thermal motion of particles.
Net diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.
Net diffusion can never move particles against a concentration gradient.
Therefore, substances will diffuse only from an area of higher concentration to an area of lower concentration.
Diffusion is considered a passive transport process as it does not utilize any cellular energy.
Equilibrium is defined as the state in which there is no net change in a system (solute concentration remains constant).
IF diffusion is the only force acting on a system, equilibrium will occur when the concentrations of solutes are equal. Please note that if there are other forces acting on the system, it is possible for equilibrium to occur when the concentrations of solutes are NOT equal.
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