Osmosis is the movement of fluid through a semipermeable membrane from low to high solute concentrations, resulting in an isotonic state. Plants and animals use osmosis to absorb nutrients and eliminate waste. Osmotic pressure can prevent isotonicity. Understanding osmosis is important in various fields, including medicine.
Osmosis is a process in which a fluid passes through a semipermeable membrane, moving from an area where a solute such as salt is present in low concentrations to an area where the solute is present in high concentrations. The end result of osmosis, excluding external factors, will be an equal amount of fluid on both sides of the barrier, creating a state known as “isotonic”. The most commonly used fluid in osmosis demonstrations is water, and osmosis with a wide variety of fluid solutions is the key to every living organism on Earth, from humans to plants.
There are a few key osmosis-related terms that may be helpful to know when thinking about how osmosis works. The fluid that passes through the membrane is known as a solvent, while the substance dissolved in the fluid is a solute. Together, the solvent and the dissolved solute form a solution. When a solution has low solute levels, it is considered hypotonic, while solutions with high solute levels are known as hypertonic.
In a classic example of osmosis, plants use osmosis to absorb water and nutrients from the soil. The solution in plant roots is hypertonic, absorbing water from the surrounding hypotonic soil. Roots are designed as selectively permeable membranes, admitting not only water, but some useful solutes, such as minerals that the plant needs to survive. Osmosis also plays a vital role in plant and animal cells, with fluids flowing in and out of the cell wall to introduce nutrients and eliminate waste.
The fluid passes both in and out of the semipermeable membrane under osmosis, but there is usually a net flow in one direction or the other, depending on which side of the membrane has a higher concentration of solutes. It is possible to alter the osmosis process by creating pressure in the hypertonic solution. When the pressure becomes so intense that the solvent of the hypotonic solution cannot pass through the membrane, it is called osmotic pressure and prevents the achievement of an isotonic state.
The principles that underlie osmosis are key to understanding a wide variety of concepts. For example, the sometimes fatal medical condition known as water intoxication occurs when people drink large amounts of water very quickly, causing the water that flows freely through their bodies to become diluted. This dilute solution is able to push through cell membranes, thanks to osmosis, and can cause cells to explode as they expand to accommodate water. Conversely, when people become dehydrated, cells shrivel up and die as the freely flowing water in the body becomes highly concentrated with solutes, causing water to flush out of cells in an effort to reach an isotonic state.
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