Estimating the Osmolarity in Tissues

As you are learning molecular biology, especially about passive transport, you will most likely encounter a lab where you should estimate the osmolarity – the concentration of a solution in terms of moles of solutes per liter of solution – in a tissue in a range of hypertonic (concentration of solutes is greater outside the cell than inside it) and hypotonic (concentration of solutes is greater inside the cell than outside it) solutions. So I thought it would be useful to give you a heads-up on what you should be expecting.

For instance, the osmolarity of a suitable plant tissue such as potatoes and carrots can be measured by putting them in varying concentrations of sodium chloride solutions. Like any other science lab, there are variables: independent variable (IV), dependent variable (DV), and control variables (CV). The IV is the concentration of NaCl, in which at minimum five different concentrations is recommended. The DV is the percent change in mass, which is calculated by subtracting the initial mass from the final mass and dividing that value multiplied by 100 by the initial mass. The mass of the plant tissue will be taken before and after it has been put in the NaCl solutions. Lastly, the control variables are the number of trials, temperature, age of the plant tissue, time in solution, and size of the plant tissue. All of the tissue pieces should be the same size and shape so that only the concentration of the IV affects the change in mass.

Effect of the Concentration of NaCl in Percent Change in Mass (Kognity)

The results of this investigation, when visualized into a graph, may look like the image above. In a hypotonic solution, the higher salt concentration in the potato cell will cause water to enter, increasing its mass. On the other hand, in a hypertonic solution, the higher concentration outside the potato cell will cause water to leave and a negative change in mass. The isotonic point is when the inside and outside of the potato cell have equal concentrations, so there is no net movement of water.

Works Cited

Allott, Andrew, and David Mindorff. IB Biology. Oxford University Press, 2014.