In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the

water potential Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and mat ...

of two solutions separated by a partially-permeable cell

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

. Tonicity depends on the relative

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', '' number concentration'', ...

of selective membrane-impermeable

solutes In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...

across a cell membrane which determine the direction and extent of osmotic flux. It is commonly used when describing the swelling-versus-shrinking response of cells immersed in an external solution. Unlike osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an effective osmotic pressure. Solutes able to freely cross the membrane do not affect tonicity because they will always equilibrate with equal concentrations on both sides of the membrane without net solvent movement. It is also a factor affecting

imbibition Imbibition is a special type of diffusion that takes place when liquid is absorbed by solids-colloids causing an increase in volume. Water surface potential movement takes place along a concentration gradient; some dry materials absorb water. A ...

. There are three classifications of tonicity that one solution can have relative to another: ''hypertonic'', ''hypotonic'', and ''isotonic''.A hypotonic solution example is distilled water.

Hypertonic solution

A hypertonic is when the concentration inside the cell is high and the environment is dilute. In biology, the tonicity of a solution usually refers to its solute concentration relative to that of another solution on the opposite side of a

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

; a solution inside of a cell is called hypertonic if it has a greater concentration of solutes than the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

inside the cell and its environment is dilute. So when the cell is hypertonic(concentrated) the cell needs more water, so water is forced to enter inside the cell and the cell will swell up and eventually burst. When plant cells lose water they are hypotonic, because their inside is diluted(with more water) and the environment is concentrated so it needs water then the flexible cell membrane pulls away from the rigid cell wall, but remains joined to the cell wall at points called

plasmodesmata Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and spec ...

. The cells often take on the appearance of a

pincushion A pincushion (or pin cushion) is a small, stuffed cushion, typically across, which is used in sewing to store pins or needles with their heads protruding to take hold of them easily, collect them, and keep them organized. Pincushions are typ ...

. In plant cells the terms isotonic, hypotonic and hypertonic cannot strictly be used accurately because the pressure exerted by the cell wall significantly affects the osmotic equilibrium point.

Hypotonic solution

A hypotonic is when the concentration inside of the cell is lower than a solution outside of a cell is called hypotonic. If it has a lower concentration of solutes inside a cell then water is forced to move out. Due to this, water is removed out of the cell causing shrinking of the cell membrane. For cells without a cell wall such as animal cells, they lose water and shrivel. When plant cells are in a hypotonic solution, the central

vacuole A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...

loses water and the cell membrane sticks against the cell wall.

Isotonicity

A solution is isotonic when its effective

osmole Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L ...

concentration is the same as that of another solution. In biology, the solutions on either side of a cell membrane are isotonic if the concentration of solutes outside the cell is equal to the concentration of solutes inside the cell. In this case the cell neither swells nor shrinks because there is no concentration gradient to induce the diffusion of large amounts of water across the cell membrane. Water molecules freely diffuse through the plasma membrane in both directions, and as the rate of water diffusion is the same in each direction, the cell will neither gain nor lose water.

References

{{reflist Cell biology