Several terms are useful for discussing osmosis across the plasma membraneThe outer boundary of a cell that controls what enters and exits.. A solutionA homogeneous mixture of two or more substances. is composed of both solute and solvent. For example, sugar waterThe universal solvent essential for life. contains the solute sucrose and the solvent water. However, the concentration of sugar in the water could vary. The term tonicityThe ability of a solution to affect the water balance in a cell. refers to the amount of solute in a solution. This word is expressed in percentage.
The three terms of tonicity, hypertonicA solution with a higher solute concentration than the inside of a cell, causing water to leave th, hypotonic, and isotonicA solution with the same solute concentration as the inside of a cell, maintaining equilibrium. are relative terms and do not refer to absolute concentration. Therefore, a solution can only be called hypertonic, isotonic, or hypotonic when in reference to another solution. Ex: Donna is 28 years old is an absolute statement. Donna is older than Jane defines Donna’s age relative to Jane’s.
A solution is hypertonic to another solution if the concentration of solutes is higher.
Ex: Seltzer is hypertonic to pure water. Seltzer has carbon dioxide as a solute and water as the solvent. In this example, seltzer has a higher concentration of solute (carbon dioxide) and a lower concentration of solvent (water).
A solution is hypotonic if the concentration of solutes is lower.
Ex: Seltzer is hypotonic to club soda. Club soda has carbon dioxide and salt as the solutes and water as the solvent. In this example, seltzer has a lower concentration of solute and a higher concentration of solvent (water).
A solution is isotonic to another solution if the concentration of the solutes is the same.
A cell’s internal fluid, called the ICF (intracellular fluidThe fluid inside a cell, primarily composed of cytosol.) is said to be isotonic to the fluid of the surrounding environment, called the IF (interstitial fluidThe fluid surrounding cells within tissues.) when the concentration of dissolved moleculesGroups of atoms bonded together. is the same. The term ECF (extracellular fluid(ECF) Fluid outside cells, including plasma and interstitial fluid.) could also be used to refer to the cell’s external environment. However, this term, ECF, references all fluids outside cellsThe basic structural and functional units of life. in the body. Therefore, ECF would include the plasmaThe liquid component of blood. in the blood vessels as well as the fluid between cells (the interstitial fluid). In this lab, we will use ICF and IF. Be aware that other references may use ECF instead of IF.
Diffusion results in net movementA fundamental property of life involving motion of the body or its parts. of a substance from a region where the concentration of the substance is higher to a region where the concentration of the substance is lower. More specifically, osmosis refers to the movement of water only and the movement is across a selectively permeableA membrane that allows some substances to pass while blocking others. membrane. Osmosis results in net movement of water from an area of low solute concentration to an area with high solute concentration. In summary: Water always moves from hypotonic to hypertonic. Usually, when students see this statement, they are confused because they wrongly interpret the term hypotonic to refer to a low water concentration. However, these terms of tonicity refer to the percentage of solute, not solvent. Therefore, a hypertonic solution has less water in it.
When cells are submerged in interstitial fluid (IF) that is hypertonic to the cell’s contents (ICF), water will move through the cell membrane to the IF, where there is more solute. This will cause the cell to shrink, or create, as it loses water. This is also similar to the situation in your body when you become dehydrated. The salty IF sucks water from your cells.
When cells are submerged in the interstitial fluid that is hypotonic to the cell’s contents, water will move through the cell membrane to the ICF, where there is more solute. This is similar to the situation of hypotonic hydrationExcess water intake dilutes ECF, causing water to move into cells (ICF), leading to swelling. in which a diluted IF becomes hypotonic to the ICF. Because water will move from hypotonic to hypertonic, water enters the cells, inflating them, and increasing the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. of the contents, kind of like filling a water balloon. Eventually, the cells can burst. When red blood cells burst, it is called hemolysisThe rupture of red blood cells, releasing hemoglobin into the blood..
List of terms
- plasma membrane
- solution
- water
- tonicity
- hypertonic
- isotonic
- intracellular fluid
- interstitial fluid
- molecules
- extracellular fluid
- cells
- plasma
- movement
- selectively permeable
- hypotonic hydration
- pressure
- hemolysis