Osmosis is a type of diffusion that, in biology, is usually related to cells. Diffusion is when molecules or atoms move from an area of high concentration to an area of low concentration. Osmosis is when a substance crosses a semipermeable membrane in order to balance the concentrations of another substance. In biology, this is usually when a solvent such as water flows into or out of a cell depending on the concentration of a solute such as salt. Osmosis happens spontaneously and without any energy on the part of the cell. Osmosis deals with chemical solutions. Solutions have two parts, a solvent and a solute. When solute dissolves in a solvent, the end product is called a solution. Salt water is an example of a solution; salt is the solute, and water is the solvent. In biology, there are three different types of solutions that cells can be in: isotonic, hypotonic, and hypertonic. Different types of solutions have different impacts on cells due to osmosis. An isotonic solution has the same concentration of solutes both inside and outside the cell. For example, a cell with the same concentration of salt inside it as in the surrounding water/fluid would be said to be in an isotonic solution. Under these conditions, there is no net movement of solvent; in this case, the amount of water entering and exiting the cell’s membrane is equal. In a hypotonic solution, there is a higher concentration of solutes inside the cell than outside the cell. When this occurs, more solvent will enter the cell than leave it to balance out the concentration of solute. A hypertonic solution is the opposite of a hypotonic solution; there is more solute outside the cell than inside it. In this type of solution, more solvent will exit the cell than enter it in order to lower the concentration of solute outside the cell. Osmosis affects plant and animal cells differently because plant and animal cells can tolerate different concentrations of water. In a hypotonic solution, an animal cell will fill with too much water and lyse, or burst open. However, plant cells need more water than animal cells, and will not burst in a hypotonic solution due to their thick cell walls; hypotonic solutions are ideal for plant cells. The optimal condition for an animal cell is to be in an isotonic solution, with an equal amount of water and solutes both inside and outside. When a plant cell is in an isotonic solution, its cells are no longer turgid and full of water, and the leaves of the plant will droop. In a hypertonic solution, water will rush out of both animal and plant cells, and the cells will shrivel (in plants, this is called plasmolyzation). This is why slugs and snails shrivel and die when salt is sprinkled onto them; water leaves their cells in order to balance the higher concentration of salt outside the cells. This figure shows the effects of osmosis on red blood cells:  Osmosis is how plants are able to absorb water from soil. The roots of the plant have a higher solute concentration than the surrounding soil, so water flows into the roots. In plants, guard cells are also affected by osmosis. These are cells on the underside of leaves that open and close to allow gas exchange. When the plant’s cells are full of water, the guard cells swell and open the stomata, small holes that allow the plant to take in carbon dioxide and release oxygen. Osmosis can have adverse effects on animals such as fish. If freshwater or saltwater fish are put into water that has a different salt concentration than they are used to, they will die from having too much water enter or leave their cells. Osmosis can affect humans as well; in a person infected with cholera, bacteria overpopulate the intestines, leaving the intestines unable to absorb water. The bacteria actually reverse the flow of absorption because osmosis causes water to flow out of the intestinal cells instead of in. This causes severe dehydration and sometimes death.
1. When a cell contains a lower concentration of solute than the solvent surrounding it, that cell is said to be in what kind of solution? A is correct. When a solution has a higher concentration of solute than a cell that it surrounds, the solution is hypertonic. When a cell is in a hypertonic solution, osmosis will cause water to flow out of the cell to balance the concentration of solute on either side of the semipermeable membrane. As too much water flows out of the cell, it will shrivel. 2. Isotonic conditions are ideal for which cells? B is correct. In isotonic solutions, the net movement of water into and out of the cell is the same, which keeps an animal cell balanced and functioning normally. Plant cells fare better in hypotonic solutions where they can be filled to the max with water. 3. What happens to an animal cell in a hypotonic solution? D is correct. When a cell is in a hypotonic solution, the concentration of solute in the cell is higher than the concentration of solute in the water surrounding it. Water will rush into the cell and can cause it to burst. A describes an isotonic solution, B describes a hypertonic solution, and C describes a plant cell in a hypotonic solution. The effects of isotonic, hypotonic, and hypertonic extracellular environments on plant and animal cells is the same. However, due to the cell walls of plants, the visible effects differ. Although some effects can be seen, the rigid cell wall can hide the magnitude of what is going on inside. Osmosis has different meanings in biology and chemistry. For biologists, it refers to the movement of water across a semipermeable membrane. Chemists use the term to describe the movement of water, other solvents, and gases across a semipermeable membrane. Both biologists and chemists define diffusion as the movement of solute particles (dissolved materials) from an area of higher concentration to lower concentration until equilibrium is reached. Osmosis is a passive transport system, meaning it requires no energy. It causes water to move in and out of cells depending on the solute concentration of the surrounding environment. This movement is caused by a concentration gradient created when there are different solute concentrations inside and outside the cell. It doesn’t matter what dissolved materials make up the solute, only the overall concentration. It is important to note that cells do not regulate the movement of water molecules in and out of their intracellular fluid. They rely on other systems in the body (such as the kidneys) to provide an isotonic external environment (see below). A cell in an isotonic solution is in equilibrium with its surroundings, meaning the solute concentrations inside and outside are the same (iso means equal in Latin). In this state there is no concentration gradient and therefore, no large movement of water in or out. Water molecules do freely move in and out of the cell, however, and the rate of movement is the same in both directions. A hypotonic solution has a lower solute concentration than inside the cell (the prefix hypo is Latin for under or below). The difference in concentration between the compartments causes water to enter the cell. Plant cells can tolerate this situation better than animal cells. In plants, the large central vacuole fills with water and water also flows into the intercellular space. The combination of these two effects causes turgor pressure which presses against the cell wall causing it to bulge out. The cell wall helps keep the cell from bursting. However, if left in a highly hypertonic solution, an animal cell will swell until it bursts and dies. In Latin, the prefix hyper means over or above. Hypertonic solutions have a higher solute concentration than inside the cell. This causes water to rush out making the cell wrinkle or shrivel. This is clearly seen in red blood cells undergoing a process called crenation. Plant cells in a hypertonic solution can look like a pincushion because of what’s going on inside. The cell membrane pulls away from the cell wall but remains attached at points called plasmodesmata. Plasmodesmata are tiny channels between plant cells that are used for transport and communication. When the inner membrane shrinks, it constricts the plasmodesmata resulting in a condition called plasmolysis.
The image above shows what happens to a cell in isotonic, hypertonic, and hypotonic solutions. References
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When the concentration inside a cell is the same as outside the cell is said to be in an?
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