Tonicity is a biological concept that describes the effective osmotic pressure gradient of a solution relative to another solution, typically the fluid inside a cell. It is a way to compare the concentration of solutes—dissolved substances like salts and sugars—on two sides of a cell membrane. Understanding this comparison determines how water moves in and out of cells, which governs cellular health and structural integrity in all living organisms. The terms isotonic, hypotonic, and hypertonic are the three comparative states used to categorize a solution’s tonicity.
The Mechanism of Osmosis
The differences in tonicity are driven by a passive process called osmosis, which is the movement of water across a semipermeable membrane. This membrane, such as the cell membrane, allows water molecules to pass freely but restricts the passage of most solutes. Water naturally moves to equalize the concentration of solutes on both sides of this membrane. Specifically, water will move from an area where its own concentration is high (low solute concentration) to an area where its concentration is low (high solute concentration). The net flow of water continues until the solute concentrations are balanced or until the pressure exerted by the water movement itself stops the flow.
Defining Isotonic, Hypotonic, and Hypertonic Solutions
A solution is defined as isotonic when the concentration of non-penetrating solutes is equal inside and outside the cell. The prefix “iso-” means equal. In this state, water molecules move in and out of the cell at the same rate, resulting in no net change in the cell’s volume.
A hypotonic solution has a lower concentration of solutes compared to the inside of the cell. The prefix “hypo-” means under or below. Because the water concentration is higher in the external solution, a net flow of water will move into the cell in an attempt to dilute the internal fluid.
Conversely, a hypertonic solution contains a higher concentration of solutes than the cell’s interior. The prefix “hyper-” means over or above. Consequently, this causes a net movement of water out of the cell and into the surrounding fluid.
Observable Effects on Animal and Plant Cells
The most direct way to determine a solution’s tonicity is by observing the resulting physical changes to cells under a microscope.
When a cell is placed in an isotonic solution, animal cells maintain their normal, characteristic shape because there is no net gain or loss of water. Plant cells in an isotonic environment are described as flaccid, meaning they are limp or soft because they lack the full internal water pressure that gives them structure.
In a hypotonic solution, the influx of water causes animal cells to swell. Because animal cells lack a rigid cell wall, they may expand until the cell membrane ruptures, a process known as lysis. Plant cells are protected by a strong cell wall, and the inward flow of water increases the internal pressure, called turgor pressure, which makes the cell firm and rigid (turgid).
When exposed to a hypertonic solution, both cell types lose water, but their reactions differ due to structure. Animal cells shrivel and shrink as water leaves the cell, a process specifically called crenation. Plant cells also lose water, causing the plasma membrane to pull away from the rigid cell wall, leading to the collapse of the cytoplasm, a condition called plasmolysis.