Final answer:
Tonicity describes the relative concentration of solutes in solutions and affects how cells respond to their environment. While related to osmolarity, which has measurable units, tonicity itself is a comparative term and therefore does not have units.
Step-by-step explanation:
The student's statement that tonicity does not have units is true. Tonicity is a term used in biology to describe the relative concentration of solutes in two solutions separated by a semipermeable membrane. Specifically, tonicity is related to the osmotic pressure gradient of two solutions, such as the internal environment of a cell versus the external environment.
Tonicity involves how an extracellular solution can change the volume of a cell by affecting osmosis. Descriptive terms such as hypotonic, isotonic, and hypertonic are used to describe the relative tonicity of solutions. In a hypotonic solution, the extracellular fluid has a lower concentration of solutes compared to the inside of the cell, resulting in water entering the cell. Meanwhile, an isotonic solution indicates equal solute concentrations inside and outside the cell, resulting in no net movement of water. A hypertonic solution has a higher concentration of solutes outside the cell, causing water to leave the cell. These conditions can affect cell survival because they influence cell volume and pressure.
Tonicity is indeed a concern for all living things, as it dictates how cells react to their surroundings. Organisms such as paramecia and amoebas, which lack cell walls, have developed structures like contractile vacuoles to help manage water balance, thereby preventing cell lysis in hypotonic environments. Tonicity is related to but different from osmolarity, which does have units (osmoles per liter). Osmolarity measures the total concentration of solute particles in a solution and is a physical property that can be quantified. While osmolarity can be measured, tonicity itself is a comparative concept that does not have units associated with it.