Membrane Permeability

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Experiment 5

Introduction:

        All living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases (usually CO2 and O2), taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell. This membrane is a complex structure that is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell, and for interacting with the environment surrounding the cell.

        There are two ways that the molecules move through the membrane which are passive transport and active transport. Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. Passive transport does not require such energy expenditure, and occurs spontaneously.

        The principle means of passive transport is diffusion. Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout the system.

        An important concept in understanding diffusion is the concept of equilibrium. There are two types of equilibrium. Static equilibrium occurs when there is no action taking place. Dynamic equilibrium occurs when two opposing actions occur at the same rate. For example, consider a bucket full of water. It is in a state of static equilibrium because the water level stays the same. The water is not moving. If a hole was poked in the bottom of the bucket, water would leak out. This system would not be at equilibrium because there is action taking place where water is leaking out and the water level in the bucket would drop.

        However, if water being poured into the bucket at the same rate that it was leaking out, the water level in the bucket would stay the same because the rate at which the water is entering the bucket is equal to the rate at which it is leaking out. This is an example of dynamic equilibrium, and it applies to nearly everything that happens in the natural world.

        Diffusion occurs when a system is not at equilibrium. As an example, suppose one drop of ink was dropped into a glass of water. At first, all of the ink molecules are in a small space and they are moving around in a random way. They move in straight lines and change direction only when they collide with each other or the surrounding water molecules. Some of the ink molecules near the edge of the drop move away from the center of the drop. As a matter of fact, most of the molecules move away from the center of the drop.

        

        

        Most of the molecules continue to move away from the original center of the drop. They move in all different directions, and some may even move back toward the center. Still, more are moving away from the drop than toward it until they find the wall of the glass. Then they start moving back toward the center again. More and more molecules bounce off of the glass until they start moving toward the center, then they pass the center and move toward the other side. Eventually the number of molecules moving away from the center equals the number moving toward the center, and equilibrium is established. At this point the molecules are evenly spread throughout the water, and diffusion stops.

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        Several factors affect how fast a molecule will diffuse. The first of these is the kinetic energy of the molecule, which is most frequently measured as the temperature of the system. Molecules in a system at a higher temperature will have more energy and will move faster, and hence diffuse faster, than molecules of the same type in a low-temperature system. The size of the molecule also affects how rapidly it will diffuse. At the same temperature, smaller molecules will move more rapidly than larger molecules because it takes more energy to get ...

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