Tonicity & Osmoregulation (College Board AP® Biology): Study Guide

Tonicity & cells

• Compared to the cell contents, the external environment can be described as:

  • hypotonic

  • hypertonic

  • isotonic

• Tonicity is relative, so the tonicity of a cell's external environment is always described in relation to the cell contents, e.g. when a cell is placed in:

  • a hypotonic solution, the surrounding solution has a lower solute concentration than the cell cytoplasm

  • a hypertonic solution, the solution has a higher solute concentration than the cell cytoplasm

  • an isotonic solution, the solution and the cell contents have an equal solute concentration

• The external environment of a cell can also be described in terms of:

  • water potential: the tendency of water molecules to move from one place to another

  • osmolarity: the number of solute particles per liter of a solution

Tonicity & osmosis

• Osmosis can be defined as:

the movement of water molecules from areas of high water potential/low osmolarity/low solute concentration to areas of low water potential/high osmolarity/high solute concentration

• The tonicity of a cell's external environment will determine whether water moves into or out of the cell by osmosis:

  • Water will move into a cell placed in a hypotonic solution, as this solution has:

    • higher water potential

    • lower osmolarity

    • lower solute concentration

  • Water will move out of a cell placed in a hypertonic solution, as this solution has:

    • lower water potential

    • higher osmolarity

    • higher solute concentration

Determining the water potential of a solution

• The water potential (Ψ) of a solution is influenced by:

  • solute potential

  • pressure potential

• Solute potential (Ψ_s) is the effect that solutes in a solution have on water potential:

  • Pure water with no dissolved solutes has a solute potential of zero

  • As solutes are added to a solution its solute potential decreases and becomes more negative

  • Provided that pressure potential (see below) remains constant, a decrease in solute potential will cause a decrease in water potential

    • Solute molecules bind to water molecules via hydrogen bonds as they dissolve, reducing water potential

• Pressure potential (Ψ_p) is the hydrostatic pressure to which water is subjected

  • Most biological systems are at the same pressure as the surrounding atmosphere, and so will have a pressure potential of zero

    • In this situation the water potential will be determined by the solute potential alone

  • Pressure potential may deviate from zero in some situations, e.g.:

    • pressure potential inside plant cells is usually positive as the cytoplasm exerts pressure on the inside of the cell wall

    • negative pressure potential can occur in xylem vessels where water and dissolved minerals are transported under tension

• The relationship between water potential, solute potential and pressure potential is represented in the equation:

Ψ = Ψ_p + Ψ_s

Determining solute potential

• Solute potential can be calculated using the formula:

Ψ_s = -iCRT

• Solute potential depends on:

  • i = the number of molecules that a solute dissociates into when it dissolves in a solution, e.g.:

    • Sodium chloride, or NaCl, dissociates into Na⁺ and Cl^-, so has an i value of 2

    • Glucose does not separate in solution, so has an i value of 1

  • C = the molar concentration of the solute

  • R = pressure constant; this is 0.0831 liter-bars/mole-K

  • T = temperature in Kelvin (°C + 273)

Osmoregulation

• Osmoregulation is an example of homeostasis in living organisms; it maintains the internal water balance, e.g. between red blood cells and the blood plasma

• Failure to control internal osmolarity can result in harm to an organism:

  • Red blood cells may take on too much water and burst if the water potential of the blood plasma increases

  • Plant cells may lose water during a period of drought, causing a decrease in pressure potential; this can result in wilting, and may kill the plant over time

Example: contractile vacuoles in paramecium

• Paramecium are protists; single-celled eukaryotes

• Paramecium live in freshwater environments, meaning that their surroundings are hypotonic to their cytoplasm and they take in water by osmosis

• The water taken in by Paramecium is stored inside a specialized vacuole known as a contractile vacuole

• The contractile vacuole pumps excess water out of the cell

Example: central vacuoles in plant cells

• The central vacuole in plant cells plays a role in cell homeostasis, with functions including:

  • water storage; a full vacuole maintains pressure potential and provides structural support to the cell

  • regulation of ion concentration in the the cytoplasm; ion channels in the vacuole membrane can open and close to allow movement of ions into or out of the cytoplasm