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Osmosis

Pearson
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In this lesson we will explore a special type of diffusion know as osmosis. Specifically, we will answer the following questions: what is osmosis, under what conditions will osmosis occur, and why is osmosis important in the human body? In a previous lesson we discussed simple diffusion in which molecules move down, or along, their concentration gradient. That means they move from an area where they are more concentrated to an area where the are less concentrated. Another way to look at this is that molecules spread out to fill their space. Osmosis is a special type of diffusion. Specifically, it is the diffusion of water through a selectively permeable membrane. The movement of water molecules is changed from simple diffusion to osmosis simply by adding a selectively permeable membrane. In osmosis, water diffuses through a selectively permeable membrane. Let's say you want a cup of tea. You place a teabag into a cup of water. The bag acts as the selectively permeable membrane. Water can pass through, but the tea leaves are too large and cannot pass through. Water will enter the bag by diffusing through the selectively permeable membrane. This is osmosis. Next, some of the tea dissolves in the water and diffuses out of the bag into the cup. Although both the water and the tea are diffusing through the bag, remember that only water moves by osmosis. You may wonder why water entering through the membrane is osmosis, but tea coming out is diffusion. First, it has nothing to do with direction. Second, simple diffusion can occur with or without a selectively permeable membrane. Osmosis, though, has two specific requirements. Osmosis is specifically the diffusion of water through a selectively permeable membrane. No water, no osmosis. Similarly, no selectively permeable membrane, no osmosis. It really is that simple. What can make osmosis confusing is that water in the body contains other substances. Our body fluids are solutions in which one or more substances, called solutes, are dissolved in another substance, called the solvent. Here's a simple example. Salt, a solute, dissolved in water, a solvent, produces a solution, in this case saltwater. In the body, the solvent is always water, and solutes include things such as nutrients, waste, and electrolytes, for example, salts. When considering osmosis in solutions, we need to consider how much solute is present that cannot pass through the membrane. Notice I didn't ask what the solute is. For osmosis, that really doesn't matter. We care about the amount of solute present because solute molecules take up space. So wherever there is solute there can't also be water. That is important because, for osmosis, we need to know which side of the membrane has the most water because the water will move from there to the other side. Let's look at an example. These two compartments are separated by a selectively permeable membrane. The red balls represent the solute molecules and the rest of each solution, the blue part, is the solvent, water. You can see that solution A has more solute than solution B, but for osmosis you want to always focus on the water. So let's get rid of the solute. Solution B had less solute than solution A, so that means solution B has more solvent, or more water, than solution A. Recall, though, that by diffusion, molecules move from where they are in higher concentration to where they are in lower concentration. So when we talk about more or less water, we are really talking about the water concentration, not the number of molecules. Based on that, in this example, by osmosis, water will move from solution B, through the membrane, and into solution A. If the membrane does not allow the solute to pass through then the amount of solution A will increase because it gains water by osmosis. Likewise the amount of solution B would decrease, because it loses water. Three terms are used to compare solutions containing solutes that can't pass through the membrane. If two solutions have the same solute concentration, they are said to be isotonic. Iso means equal. Water will still move by osmosis but it will move about equally from each side, so there is no net, or overall, movement. If there is a difference in the solute concentrations, the solution with the most solute is said to be hypertonic to the other solution, which is said to be hypotonic to the first. Hyper means higher and hypo means lower. These terms are relative, though, so a solution may be hypertonic to one solution but hypotonic to a different one. In this example solution B is hypertonic to solution C but it's hypotonic to solution A. That's kind of like height. You may be taller than some of your friends but also shorter than some others. By osmosis, water always moves from a hypotonic solution to a hypertonic solution. Think about that for a moment. Tonicity reflect the amount of solute. Hypotonic means that there is less solute, and that means more water, or a higher water concentration, and water diffuses from where there's a higher water concentration to where there is a lower water concentration. Tonicity is very important to us. To maintain homeostasis our intracellular and extracellular fluids need to be isotonic. The water should be balanced between them. If you become very dehydrated, maybe from low fluid intake or from being ill with vomiting and diarrhea, for example, your extracellular fluid will become hypertonic to your intracellular fluid. This will draw water out of your cells and they will shrink and not be able to function well. How do you correct dehydration? Take in more water and the extracellular fluid's tonicity will be reduced, hopefully until the fluids are again isotonic. Water will enter the cells and the cells will expand hopefully back to normal. Here's a view of a red blood cell in an isotonic fluid. This is its normal shape and size. If you drink large amounts of water very quickly, your extracellular fluid would become hypotonic, and water would enter your cells, causing them to swell and perhaps even rupture. This condition is called water intoxication, which can be fatal and claims victims every year. For these reasons, IV fluids generally need to be isotonic to our body fluids, so our cells aren't adversely affected. Fortunately our bodies are well adapted to handle some fluctuations in the water balance between our body fluids and to adjust them as needed to keep them isotonic. In fact, maintaining this balance is one of the main jobs of our kidneys and the urinary system.
In this lesson we will explore a special type of diffusion know as osmosis. Specifically, we will answer the following questions: what is osmosis, under what conditions will osmosis occur, and why is osmosis important in the human body? In a previous lesson we discussed simple diffusion in which molecules move down, or along, their concentration gradient. That means they move from an area where they are more concentrated to an area where the are less concentrated. Another way to look at this is that molecules spread out to fill their space. Osmosis is a special type of diffusion. Specifically, it is the diffusion of water through a selectively permeable membrane. The movement of water molecules is changed from simple diffusion to osmosis simply by adding a selectively permeable membrane. In osmosis, water diffuses through a selectively permeable membrane. Let's say you want a cup of tea. You place a teabag into a cup of water. The bag acts as the selectively permeable membrane. Water can pass through, but the tea leaves are too large and cannot pass through. Water will enter the bag by diffusing through the selectively permeable membrane. This is osmosis. Next, some of the tea dissolves in the water and diffuses out of the bag into the cup. Although both the water and the tea are diffusing through the bag, remember that only water moves by osmosis. You may wonder why water entering through the membrane is osmosis, but tea coming out is diffusion. First, it has nothing to do with direction. Second, simple diffusion can occur with or without a selectively permeable membrane. Osmosis, though, has two specific requirements. Osmosis is specifically the diffusion of water through a selectively permeable membrane. No water, no osmosis. Similarly, no selectively permeable membrane, no osmosis. It really is that simple. What can make osmosis confusing is that water in the body contains other substances. Our body fluids are solutions in which one or more substances, called solutes, are dissolved in another substance, called the solvent. Here's a simple example. Salt, a solute, dissolved in water, a solvent, produces a solution, in this case saltwater. In the body, the solvent is always water, and solutes include things such as nutrients, waste, and electrolytes, for example, salts. When considering osmosis in solutions, we need to consider how much solute is present that cannot pass through the membrane. Notice I didn't ask what the solute is. For osmosis, that really doesn't matter. We care about the amount of solute present because solute molecules take up space. So wherever there is solute there can't also be water. That is important because, for osmosis, we need to know which side of the membrane has the most water because the water will move from there to the other side. Let's look at an example. These two compartments are separated by a selectively permeable membrane. The red balls represent the solute molecules and the rest of each solution, the blue part, is the solvent, water. You can see that solution A has more solute than solution B, but for osmosis you want to always focus on the water. So let's get rid of the solute. Solution B had less solute than solution A, so that means solution B has more solvent, or more water, than solution A. Recall, though, that by diffusion, molecules move from where they are in higher concentration to where they are in lower concentration. So when we talk about more or less water, we are really talking about the water concentration, not the number of molecules. Based on that, in this example, by osmosis, water will move from solution B, through the membrane, and into solution A. If the membrane does not allow the solute to pass through then the amount of solution A will increase because it gains water by osmosis. Likewise the amount of solution B would decrease, because it loses water. Three terms are used to compare solutions containing solutes that can't pass through the membrane. If two solutions have the same solute concentration, they are said to be isotonic. Iso means equal. Water will still move by osmosis but it will move about equally from each side, so there is no net, or overall, movement. If there is a difference in the solute concentrations, the solution with the most solute is said to be hypertonic to the other solution, which is said to be hypotonic to the first. Hyper means higher and hypo means lower. These terms are relative, though, so a solution may be hypertonic to one solution but hypotonic to a different one. In this example solution B is hypertonic to solution C but it's hypotonic to solution A. That's kind of like height. You may be taller than some of your friends but also shorter than some others. By osmosis, water always moves from a hypotonic solution to a hypertonic solution. Think about that for a moment. Tonicity reflect the amount of solute. Hypotonic means that there is less solute, and that means more water, or a higher water concentration, and water diffuses from where there's a higher water concentration to where there is a lower water concentration. Tonicity is very important to us. To maintain homeostasis our intracellular and extracellular fluids need to be isotonic. The water should be balanced between them. If you become very dehydrated, maybe from low fluid intake or from being ill with vomiting and diarrhea, for example, your extracellular fluid will become hypertonic to your intracellular fluid. This will draw water out of your cells and they will shrink and not be able to function well. How do you correct dehydration? Take in more water and the extracellular fluid's tonicity will be reduced, hopefully until the fluids are again isotonic. Water will enter the cells and the cells will expand hopefully back to normal. Here's a view of a red blood cell in an isotonic fluid. This is its normal shape and size. If you drink large amounts of water very quickly, your extracellular fluid would become hypotonic, and water would enter your cells, causing them to swell and perhaps even rupture. This condition is called water intoxication, which can be fatal and claims victims every year. For these reasons, IV fluids generally need to be isotonic to our body fluids, so our cells aren't adversely affected. Fortunately our bodies are well adapted to handle some fluctuations in the water balance between our body fluids and to adjust them as needed to keep them isotonic. In fact, maintaining this balance is one of the main jobs of our kidneys and the urinary system.