payment page
In the past sections about cell membranes and membrane transport, we discussed how water can move passively across membranes when moving with the concentration gradient. In other words, water will move from an area of lower solute concentration to an area of higher solute concentration. Since water can move through the cell membrane, this means that the amount of water inside the cell is influenced by the solute concentration outside of the cell. Tonicity refers to the concentration of solutes outside of the cell relative to the concentration inside the cell. Osmoregulation refers to how the cell manages the tonicity to support proper cell function.
Tonicity
There are three main types of tonicity and each has a different effect on a cell.
-
An isotonic solution means that the solute concentration outside the cell is equal to the solute concentration inside the cell. Water molecules may move in or out of the cell at equal rates but overall cell volume stays the same.
-
A hypotonic solution means that the solute concentration outside the cell is lower than the solute concentration inside the cell. As a result, water will move into the cell causing the cell to swell or burst.
-
A hypertonic solution means that the solute concentration outside the cell is higher than the solute concentration inside the cell. As a result, water will move out of the cell causing the cell to shrink.
Water Potential
As we just described, water will move from areas of low solute concentration to areas of high solute concentration. This difference is also referred to as the difference in water potential. Areas with low solute concetration have high water potential and areas with hifh solute concentration hage low water potential. This is expressed by the equation:
Ψ = Ψp + Ψs
where Ψp = pressure potential and Ψs = solute potential
Osmoregulation
As you can imagine, is important for cells to maintain homeostasis by regulating the concentration of water and solutes within their membranes regardless of the external environment. Both hypotonic and hypertonic solutions can be detrimental to cell function if not managed on a cellular level. Cells can do this through a variety of methods in order to ensure proper cell function. These can include transport proteins or structural adaptations. Transport proteins are a very common way for osmoregulation to occur at a cellular level since they can move solutes across the cell membrane. Some organisms, often unicellular, contain contractile vacuoles that collect excess water inside the cell and release it to the outside when necessary.
Osmoregulation allows organisms to control their internal solute composition and water potential. The solute potential of a solution can be expressed by this equation:
Ψs = -iCRT
Where i = ionization constant
C = molar concentration
R = pressure constant = 0.0831((L*bars)/(mol*K))
T = temperature in Kelvin (°C + 273)
