The Effect of Osmosis on Red Blood Cells

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Osmosis is defined as the movement of water molecules from a highly concentrated region to a lowly concentrated region across a semi-permeable membrane (Odom, Barrow, & Romine, 2017). It is also defined as the movement of any solvent molecules from a lowly concentrated region to a highly solute concentration region across any form of a semipermeable membrane. The movement of the molecules is defined by osmotic pressure which is the external force which pushes the movements of substances across the membrane. Osmotic pressure is dependent on a number of factors including the concentration of a specific solution. The movement of the molecules continues until an isotonic equilibrium is obtained. Osmosis remains a vital process in biological systems due to the presence of biological membrane which have the same properties as the semi-permeable membrane. Such membranes are normally impermeable to polar and large molecules including polysaccharides, proteins, and ions while at the same time being permeable to hydrophobic molecules like lipids and other small molecules like carbon dioxide, oxygen, and nitric oxide (Odom et al., 2017).  Osmosis has been indicated to play essential roles in both plants and animals. In plants, it has been implicated in regulating the opening and closing of the stomata, absorption of water molecules from the soil as well as providing the required support to plants. In animals, osmosis has been implicated in osmoregulation which determines the movement of fluids within internal and external tissues. The paper below investigates the effect of osmosis in red blood cells.


1. To investigate changes in red blood cells when placed in different solutions


Red blood cells do not undergo changes when placed in different solutions.

Materials and Methods

             Several materials were used in the above experiment. The materials included a microscope, pipette beaker, microscope cover glass, stopwatch, sheep’s blood, 10% sodium chloride, 0.85% sodium chloride, and distilled water.

A control experiment was initially set. Blood was smeared on the microscope glass slide and immediately viewed under the microscope within the addition of any other solution. The experimental set up involved smearing blood on a microscope and addition of approximately two drops of 10% sodium hydroxide, 0.85% sodium hydroxide and in distilled water. All experimental setups were left for four minutes before being observed in the microscope. The results were then recorded based on the shape of the red blood cells as viewed under the microscope.


             The shape of the red blood cells changed when placed in different solutions. In the hypertonic solution (10%) water moved out of the cell and as a result the cell shrink. When the red blood cell was placed in a hypotonic solution or a solution containing less concentration of the sodium chloride salt (0.85%) the cell grew in size to the point of bursting and releasing some of its components. In isotonic solutions, the shape of the red blood cells remained the same as highlighted in the images below.

Figure 1. Red blood cells placed in hypotonic, isotonic or hypertonic solutions. Changes in the shape of the red blood cells are noted in each of the beakers where the red blood cells had been placed. The biconcave shape is retained in the isotonic solution while the hypertonic solution contains red blood cells which have shrunk in size. The hypotonic solution shows red blood cells which are full and are close to bursting and releasing their content.

Discussion and Clinical Implications

            The results reject the hypothesis which indicated that no changes occur in red blood cells when placed in solutions of varying solute concentrations. When the red blood cell was placed in isotonic solutions the biconcave shape is retained was retained as an equilibrium had been established between the solute concentration inside and outside of the cell. The cell membrane of the red blood cell acted as the semipermeable membrane and facilitated the sustained movement of substance to achieve the equilibrium (Goodhead & MacMillan, 2017). However, when the red blood cell was either placed in the hypertonic or hypotonic solutions changes occur in the red blood cells.  Hypertonic solutions contain less water and as a result, the water moves from the low concentrated region within the cell to the high concentrated region which is directly outside of the cell. As a result, more water is lost from the cell, and the shrinking shape of the red blood cells takes effect. Within the hypertonic solution, a higher solute concentration exists within the cell while a lower solute concentration is present outside the cell. In such cases, water molecules move from the external environment which is more concentrated to within the cell through the semi permeable membrane (Goodhead & MacMillan, 2017). As a consequence, the cell enlarges until it burst into releasing some of its contents. Unlike plants cells which contain the cell wall which prevents bursting, animal cells like the red blood cells contain only the cell membrane as the only outer layer and as a result, their placement in a hypotonic solution normally increases the osmotic pressure which in turn causes the bulging of the cell and its subsequent bursting.  Some sources of errors could be bursting of red blood cells in isotonic solutions due to poor handling of the cell.

            Information collected from the above experiment could be useful when conducting any experimental procedures using red blood cells. Diluting the red blood cells with the appropriate isotonic solution would maintain the biconcave shape of the cell which will be essential in downstream processes to be carried out. The above information could also be useful in explaining some of the medical conditions within the body which occur as a result of dehydration. Pressure on red blood cells due to dehydration or in disease conditions could also be accounted for by osmosis.

            Future tests should be conducted to examine the effect of red blood cells and plant cells when placed in solutions of varying concentrations. Changes in the shape of red blood cells and plant cells will provide insights on the different functions of osmosis in plant and animal cells and how the presence of the cell wall and cell membrane acts to provide the two types of cells with different functions.

From the information provided in the above case, osmosis plays a major role in regulating the movement of solute and solvents across various cells through the semi-permeable membrane. The interaction between the cell membrane, the internal and external environment of the cells is essential in the regulation of critical functions of the cells. Cells to be used for any experimental procedure should always be kept at isotonic conditions to reduce some of the observed effects.


Goodhead, L. K., & MacMillan, F. M. (2017). Measuring osmosis and hemolysis of red blood cells. Advances in Physiology Education, 41(2), 298–305.

Odom, A. L., Barrow, L. H., & Romine, W. L. (2017). Teaching Osmosis to Biology Students. The American Biology Teacher, 79(6), 473–479.

August 04, 2023

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