The difference between a hypertonic vs. hypotonic vs. isotonic solution is around concentration. A hypotonic solution is less concentrated than the cell, a hypertonic solution is more concentrated than the cell, and isotonic is balanced between the cell and outside solution.
Our cells are surrounded by a semi-permeable membrane that allows certain things to move in and out. This is a very selective barrier and plays an important role in osmosis and tonicity, which includes hypertonicity, hypotonicity, and isotonicity.
In biological systems, osmosis is the movement of water across the semi-permeable membrane of our cells. This movement occurs because the concentration of solutes inside of the cells wants to be the same concentration as the solutes outside of the cell. Since the cellular membrane is not permeable to many of these solutes, only small molecules like water are capable of passing through.
This means that water moves between the exterior and interior of the cells to balance the concentration of solutes instead of the solutes doing it. Osmosis is then related to tonicity, which measures the ability of the exterior solution to force water into or out of the cell so that a balanced level can be reached. Tonicity uses the concentration of molecules as a measurement.
This is an important function of our bodies because it is the main way that water is supplied to our cells and remain healthy and functional. It is also important for the structural integrity of the cells because regulated water levels support cellular stability and prevent it from bursting or collapsing.
Hypertonic
A hypertonic system occurs when the exterior solution contains a higher concentration of particles and the interior solution of the cell contains a lower concentration. This forces water to exit the cell into the exterior because it wants to dilute the exterior solution. This dilution will result in a lowered concentration on the outside, which would be closer to the concentration on the inside.
This movement of water from cells to the outside results in the cell shrinking. Red blood cells will shrink and become distorted if too much water is removed from it. This collapse of structure diminishes the red blood cell’s ability to function.
Without watering, plants will wilt and become distorted because water is moving out of the cells and causing a loss in turgor pressure, which is the plant’s internal pressure it uses to push against the cell wall and maintain its shape. When the get water again, they eventually restore that pressure and shape.
Species of saltwater fish all experience a hypertonic situation because the surrounding saltwater has a very concentrated solution relative to the interior of the fish and its cells. This means that the fish had to develop a means to deal with this problem. Their solution to simply drink a lot of saltwater and continuously remove the extra salt rather than the water.
Hypotonic
A hypotonic system is the exact opposite of a hypertonic system. In a hypotonic system, the concentration of solutes is higher on the inside of the cell relative to the exterior, which has a lower concentration. This forces water to enter the cell to dilute the interior. Again, this is meant to bring the concentration levels to a similar level and achieve balance.
As the water moves into the cells, it increases the internal pressures and causes the cell to swell up. If they become too swollen, the cells have the potential to burst. This can also be a good thing for some organisms.
As hypertonicity leads to lowered turgor pressure in plants, it is a bad thing. But, hypotonicity leads to higher turgor pressure and this is good for young plants. As plants grow, they promote processes that increase the turgor pressure because it pushes against the cell walls and facilitate the expansion of the plant cells so that they can continue to grow.
Water Overdose (Hyponatremia)
A rare, but occurring, consequence of hypotonicity is water overdose because there is such a thing as too much water. If we consume more water than what our cells can need and our kidneys can process, we risk creating an imbalance.
If we drink too much water, the exterior salt solution of our blood becomes so diluted that water rushes into the blood cells and causes them to swell up. When this swelling occurs in the brain, it requires immediate attention because it can be dangerous and fatal.
Symptoms are similar to exhaustion and heatstroke. They include feeling hot, headaches, malaise, and potentially vomiting. This is more prevalent in babies than adults because their small bodies cannot handle a lot of water as adults can. It is generally recommended that they stick to milk and formula rather than water to prevent this.
In adults, we generally must make sure that our pee is not too white, indicating too much water, or too dark, indicating too little water.
Isotonic
Osmosis and tonicity focus on bringing the cells to a balance of concentration between the interior and the exterior. When that balance occurs, the system is not isotonic. Water is moving into the cell at the same rate that it is moving out so there is a zero net movement of water.
This balance creates a stable shape for the cells and is generally the preferred state for most biological cells. Our red blood cells prefer this state over the other two to prevent loss of function.
Because these things are occurring in biological systems, they are relative terms that are constantly shifting and changing as we consume water or do not, as permeable solutes (like salts) move into and out of the cells, and as any other factor changes these concentrations.
Unlike our cells, plant cells like to be in a hypotonic state rather than an isotonic state because it increases the turgor pressure and keeps the cells in a more rigid and stronger shape.