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Diffusion

Pearson
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In this lesson we will begin exploring how substances move through membranes. We will start by considering the process of diffusion. Specifically, we will answer the following questions: what is diffusion, how does diffusion occur, and what determines the rate of diffusion? Molecules and compounds in the body are constantly moving from one location to another. For example, air moves into and out of our lungs as we breathe. And the oxygen we breathe in is shifted around the body, carried in our blood, and pumped along by the action of the heart. But this is a very superficial view of how materials move in the body. How does the oxygen we bring into our lungs make its way into our blood, for example? And once that oxygen arrives at our tissues, how does it move from the blood into the cells that need it? In both cases, the answer is diffusion. A very simple way to understand diffusion is to think of it as molecules spreading out. For diffusion to occur, there must be a concentration gradient. That means that the concentration of the diffusing substance is higher in one area than in the other. Imagine baking cookies. As they start to bake, the molecules that make that yummy smell are highly concentrated in the oven, so there is a concentration gradient: a high concentration in the kitchen, but a low concentration elsewhere in the house. In time, though, the molecules will spread out, enter the other rooms, and eventually they'll be smelled throughout the house. This is diffusion. Diffusion is defined as the process by which molecules move from an area of higher concentration to an area of lower concentration. This directional movement is sometimes referred to as moving down, or moving along, the concentration gradient. Again, in other words, the molecules spread out. Diffusion happens spontaneously whenever there is a concentration gradient because molecules are always moving about. This general movement, known as Brownian motion, is random, jiggly, and nondirectional. But when the molecules bump into each other or into a barrier, they ricochet and then keep moving in that new path until they hit something else and ricochet again. Where the molecules are in high concentration, or located closely together, they bump into each other and bounce away more often. Think of being in an elevator. In a crowded elevator a small movement often means that you bump into someone and that person then usually moves away from you, perhaps bumping into someone else. But if there are only two of you in there, you can move more and not bump into each other as often. It's similar for molecules in motion. The more there are, the more they collide. Molecules tend to diffuse until they reach equilibrium. The term equilibrium usually refers to some sort of equality or balance. When we talk about molecules in motion, equilibrium means that they spread out so there is about equal space between them. In other words, equilibrium is reached when there is no longer a concentration gradient. Or if molecules are diffusing through a membrane, equilibrium is reached when the concentration is the same on each side of the membrane. Don't be fooled, though. Just because equilibrium is reached, the molecules don't stop moving. They keep moving about, but it is no longer a directional movement and they stay spread out. How quickly molecules diffuse depends on a few factors. In the strictest sense, diffusion is driven by thermal energy so temperature is the only factor to be considered. Heat makes molecules move faster, so the higher the temperature, the greater the rate of diffusion. In biology, though, we use a broader interpretation of diffusion so we also take into account the concentration gradient and the size of the molecules. The greater the concentration gradient, the more rapidly molecules tend to diffuse. In addition, smaller molecules move faster than larger molecules. Most diffusion is called simple diffusion or just diffusion. It occurs spontaneously without assistance. Recall, though, that cells are surrounded by a semipermeable cell membrane. Some molecules can diffuse directly through the membrane by simple diffusion but others cannot. When moving through a cell membrane, the nature of the molecules that are diffusing must also be considered. Recall that the cell membrane is largely nonpolar, so many nonpolar molecules, including gasses such as oxygen and carbon dioxide, easily diffuse directly through the cell membrane. The membrane contains a lot of lipids, so in general, lipid-soluble molecules can diffuse directly through it. Most polar molecules, which are instead water soluble, are allowed to pass through the cell membrane only through special doorways, the protein channels. These channels help, or facilitate, the diffusion of the molecules, so this process, diffusion with the aid of a protein channel, is referred to as facilitated diffusion. Some ions, such as sodium and potassium, and molecules such as glucose move into or out of the cells by facilitated diffusion. Either type of diffusion is a passive process, meaning it occurs spontaneously and without using any energy. Diffusion is just one way that materials move within the body and into and out of our cells. Back to our earlier example of breathing, by moving down its concentration gradient, the oxygen we breathe in diffuses through a membrane to move from our lungs and to our blood and circulates through our vessels to reach the tissues that are running low on oxygen. Once there the oxygen again diffuses across a membrane, now the wall of a capillary, to leave the blood and move into the tissues. From there the oxygen diffuses through the cell membrane to enter a needy cell. Without this spontaneous process none of the oxygen we need would ever reach its targets.
In this lesson we will begin exploring how substances move through membranes. We will start by considering the process of diffusion. Specifically, we will answer the following questions: what is diffusion, how does diffusion occur, and what determines the rate of diffusion? Molecules and compounds in the body are constantly moving from one location to another. For example, air moves into and out of our lungs as we breathe. And the oxygen we breathe in is shifted around the body, carried in our blood, and pumped along by the action of the heart. But this is a very superficial view of how materials move in the body. How does the oxygen we bring into our lungs make its way into our blood, for example? And once that oxygen arrives at our tissues, how does it move from the blood into the cells that need it? In both cases, the answer is diffusion. A very simple way to understand diffusion is to think of it as molecules spreading out. For diffusion to occur, there must be a concentration gradient. That means that the concentration of the diffusing substance is higher in one area than in the other. Imagine baking cookies. As they start to bake, the molecules that make that yummy smell are highly concentrated in the oven, so there is a concentration gradient: a high concentration in the kitchen, but a low concentration elsewhere in the house. In time, though, the molecules will spread out, enter the other rooms, and eventually they'll be smelled throughout the house. This is diffusion. Diffusion is defined as the process by which molecules move from an area of higher concentration to an area of lower concentration. This directional movement is sometimes referred to as moving down, or moving along, the concentration gradient. Again, in other words, the molecules spread out. Diffusion happens spontaneously whenever there is a concentration gradient because molecules are always moving about. This general movement, known as Brownian motion, is random, jiggly, and nondirectional. But when the molecules bump into each other or into a barrier, they ricochet and then keep moving in that new path until they hit something else and ricochet again. Where the molecules are in high concentration, or located closely together, they bump into each other and bounce away more often. Think of being in an elevator. In a crowded elevator a small movement often means that you bump into someone and that person then usually moves away from you, perhaps bumping into someone else. But if there are only two of you in there, you can move more and not bump into each other as often. It's similar for molecules in motion. The more there are, the more they collide. Molecules tend to diffuse until they reach equilibrium. The term equilibrium usually refers to some sort of equality or balance. When we talk about molecules in motion, equilibrium means that they spread out so there is about equal space between them. In other words, equilibrium is reached when there is no longer a concentration gradient. Or if molecules are diffusing through a membrane, equilibrium is reached when the concentration is the same on each side of the membrane. Don't be fooled, though. Just because equilibrium is reached, the molecules don't stop moving. They keep moving about, but it is no longer a directional movement and they stay spread out. How quickly molecules diffuse depends on a few factors. In the strictest sense, diffusion is driven by thermal energy so temperature is the only factor to be considered. Heat makes molecules move faster, so the higher the temperature, the greater the rate of diffusion. In biology, though, we use a broader interpretation of diffusion so we also take into account the concentration gradient and the size of the molecules. The greater the concentration gradient, the more rapidly molecules tend to diffuse. In addition, smaller molecules move faster than larger molecules. Most diffusion is called simple diffusion or just diffusion. It occurs spontaneously without assistance. Recall, though, that cells are surrounded by a semipermeable cell membrane. Some molecules can diffuse directly through the membrane by simple diffusion but others cannot. When moving through a cell membrane, the nature of the molecules that are diffusing must also be considered. Recall that the cell membrane is largely nonpolar, so many nonpolar molecules, including gasses such as oxygen and carbon dioxide, easily diffuse directly through the cell membrane. The membrane contains a lot of lipids, so in general, lipid-soluble molecules can diffuse directly through it. Most polar molecules, which are instead water soluble, are allowed to pass through the cell membrane only through special doorways, the protein channels. These channels help, or facilitate, the diffusion of the molecules, so this process, diffusion with the aid of a protein channel, is referred to as facilitated diffusion. Some ions, such as sodium and potassium, and molecules such as glucose move into or out of the cells by facilitated diffusion. Either type of diffusion is a passive process, meaning it occurs spontaneously and without using any energy. Diffusion is just one way that materials move within the body and into and out of our cells. Back to our earlier example of breathing, by moving down its concentration gradient, the oxygen we breathe in diffuses through a membrane to move from our lungs and to our blood and circulates through our vessels to reach the tissues that are running low on oxygen. Once there the oxygen again diffuses across a membrane, now the wall of a capillary, to leave the blood and move into the tissues. From there the oxygen diffuses through the cell membrane to enter a needy cell. Without this spontaneous process none of the oxygen we need would ever reach its targets.