A water solution that contains nutrients, wastes, gases, salts and other substances surrounds cells. This is the external environment of a cell. The cell’s outer surface of the plasma membrane is in contact with this external environment, while the inner surface is in contact with the cytoplasm. Thus, the plasma membrane controls what enters and leaves the cell. Show The membrane permits the passage of some materials, but not all. The cell membrane is said to be selectively permeable. Small molecules, for example, may pass through the membrane. If no energy is required for substances to pass through the membrane, the process is called passive transport. We will discuss two examples of passive transport in this tutorial: diffusion and osmosis. Diffusion Diffusion is defined as the net movement of molecules from an area of greater concentration to an area of lesser concentration. The molecules in a gas, a liquid or a solid are in constant motion due to their kinetic energy. Molecules are in constant movement and collide with each other. These collisions cause the molecules to move in random directions. Over time, however, more molecules will be propelled into the less concentrated area. Thus, the net movement of molecules is always from more tightly packed areas to less tightly packed areas. Many things can diffuse. Odors diffuse through the air, salt diffuses through water and nutrients diffuse from the blood to the body tissues. This spread of particles through random motion from an area of high concentration to an area of lower concentration is known as diffusion. This unequal distribution of molecules is called a concentration gradient. Once the molecules become uniformly distributed, dynamic equilibrium exists. The equilibrium is said to be dynamic because molecules continue to move, but despite this change, there is no net change in concentration over time. Both living and nonliving systems experience the process of diffusion. In living systems, diffusion is responsible for the movement of a large number of substances, such as gases and small uncharged molecules, into and out of cells.  Figure \(\PageIndex{1}\). (CC BY-NC-SA) Osmosis Osmosis is a specific type of diffusion; it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Semi-permeable membranes are very thin layers of material which allow some things to pass through them, but prevent other things from passing through. Cell membranes are an example of semi-permeable membranes. Cell membranes allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but do not allow larger molecules like sucrose, proteins and starch to enter the cell directly. Figure \(\PageIndex{2}\). (CC BY-NC-SA) Example: If there was a semi-permeable membrane with more water molecules on one side as there were on the other, water molecules would flow from the side with a high concentration of water to the side with the lower concentration of water. This would continue until the concentration of water on both sides of the membrane were equal (dynamic equilibrium is established). Figure \(\PageIndex{3}\). (CC BY-NC-SA) Osmotic Pressure Figure \(\PageIndex{4}\). osmotic pressure (CC BY-NC-SA; LadyOfHats) If the two containers are connected, but separated by a semi-permeable membrane, water molecules would flow from the area of high water concentration (the solution that does not contain any sugar) to the area of lower water concentration (the solution that contains sugar). Figure \(\PageIndex{5}\). osmotic pressure (CC BY-NC-SA; LadyOfHats) This movement of water would continue until the water concentration on both sides of the membrane is equal, and will result in a change in volume of the two sides. The side that contains sugar will end up with a larger volume. Figure \(\PageIndex{6}\). osmotic pressure (CC BY-NC-SA; LadyOfHats) Water solutions are very important in biology. When water is mixed with other molecules this mixture is called a solution. Water is the solvent and the dissolved substance is the solute. A solution is characterized by the solute. For example, water and sugar would be characterized as a sugar solution. The classic example used to demonstrate osmosis and osmotic pressure is to immerse red blood cells into sugar solutions of various concentrations. There are three possible relationships that cells can encounter when placed into a sugar solution. 1. The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation the cell is in an isotonic solution (iso = equal or the same as normal). A red blood cell will retain its normal shape in this environment as the amount of water entering the cell is the same as the amount leaving the cell. 2. The concentration of solute in the solution can be greater than the concentration of solute in the cells. This cell is described as being in a hypertonic solution (hyper = greater than normal). In this situation, a red blood will appear to shrink as the water flows out of the cell and into the surrounding environment. 3. The concentration of solute in the solution can be less than the concentration of solute in the cells. This cell is in a hypotonic solution (hypo = less than normal). A red blood cell in this environment will become visibly swollen and potentially rupture as water rushes into the cell. Figure \(\PageIndex{4}\). (CC BY-NC-SA) Figure \(\PageIndex{4}\). (CC BY-NC-SA)
Osmosis n., plural: osmoses [ɒzˈməʊsɪs] Definition: net movement of water molecules across the membrane from areas of higher to lower water potential Osmosis is the net movement of solvent molecules through a semipermeable membrane. It is similar to diffusion as the movement is downhill, meaning from higher to lower concentration. In osmosis though, the movement has to occur across a semipermeable or selectively-permeable membrane. Without this element, it cannot be called osmosis. While diffusion is often depicted as the net movement of solutes between two solutions, osmosis is about the net movement of the solvent molecules (not the solute) (solvent such as water molecules). The differing concentration of water molecules between the two sides of the membrane is what drives the water to move so as to equalize the concentrations of the two areas. Osmosis DefinitionIn biology, osmosis is defined as the net movement of water molecules through a biological membrane (e.g. cell membrane) from an area of higher to an area of lower water potential. Other definitions of osmosis are as follows:
In chemistry, osmosis is defined similarly. It is the passage of a pure solvent from a solution of lesser to one of greater concentration of solutes when the two solutions are separated by a membrane that selectively prevents the passage of solute molecules while allowing the solvent molecules to pass through. Etymology: The term osmosis is a Latinized form of now obsolete osmose. A derived word is osmotic, which is defined as pertaining to or of the nature of osmosis. Thus, osmotic pressure, for instance, is a pressure that arises due to or is relevant to osmosis. Watch this vid about osmosis and water potential: How Osmosis WorksA schematic diagram showing how osmosis works Water molecules tend to move, and they move downhill, i.e. from an area of high water concentration (or fewer solutes) to an area of low water concentration (or greater solutes). If there is no net movement of water, it cannot be called osmosis. It should also incorporate a semipermeable membrane to move across. Without it, the process is only a case of diffusion and not osmosis. Since water molecules are polar molecules, they need channel proteins to move down their concentration gradient. These channel proteins are embedded in the cell membrane and provide a hydrophilic passageway through which water can move across. What drives the water molecules to move is the osmotic (pressure) gradient, i.e. differences in osmotic pressures between the two solutions. The measure of the relative tendency of water to move from one area to another is referred to as water potential. It is commonly represented by the Greek letter Ψ (Psi). Solutions that have different tonicities will cause a net flow of water across the cell membrane.
Note it! Question: How does osmosis occur?
Osmotic Types of Solutions
In this diagram, the cell is shown how osmosis and the tonicity of solutions affected the cell.
A solution is comprised essentially of the solute (a substance to be dissolved) and the solvent (the component that dissolves the solutes). The concentrations of the constituents of the two solutions shall determine if a solution is isotonic, hypotonic, or hypertonic as compared to another solution. An isotonic solution is a solution wherein the amount of solutes is basically the same as the number of solutes of another solution. For instance, a cell that is isotonic to the outside solution means that both the cell’s intracellular fluid and the surrounding fluid will have equal osmotic pressure and the same water potential. In this case, there will be no net movement of water molecules between the cell and the outside fluid. A hypotonic solution is a solution that has lower osmotic pressure (or has fewer solutes) than another solution to which it is compared. In this case, water moves toward the area with less water concentration or towards the more concentrated region so as to dilute the solution. For instance, when the fluid surrounding the cell is hypotonic, the water will move across the membrane towards the more concentrated solution, which is inside the cell. A hypertonic solution is a solution that appears to be the opposite of a hypotonic solution. A hypertonic solution will have more solutes and less water than the other solution. If a cell is immersed in hypertonic solution water will leave the cell to dilute the solution outside. Examples of OsmosisHere are examples of osmosis in animal and plant cells. In biological systems, osmosis is essential since many biological membranes are semipermeable, and it leads to different physiological effects. For example, when animal cells are exposed to a hypertonic surrounding (or lower water concentration) the water will leave the cells causing the cells to shrink. This condition is referred to as crenation. Conversely, when the animal cells are placed in a hypotonic surrounding (or higher water concentration), the water molecules will move into the cells causing them to swell. If osmosis continues and becomes excessive the cells will eventually burst. The cell bursting due to too much water influx does not happen in plant cells. Plants are able to counter excessive osmosis through their cell walls and vacuoles. The cell wall exerts osmotic pressure that stabilizes the plant cell. In fact, osmotic pressure is what makes plants stay upright. The large vacuole inside the plant cell also helps through osmoregulation, a regulatory process where water potential is regulated so that the osmotic pressure inside the cell is kept within the optimal range. The plant cells, though, are not protected by water efflux. When a plant cell is placed in a hypertonic surrounding, the cell wall cannot prevent the cell from losing water. This lead to cell shrinking or the cell becoming flaccid. Take the Osmosis Biology Quiz! Further Reading
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