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Diffusion and Osmosis: Twin Perils in the Life of the Cell


Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News & Views is delighted to present this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

Earlier we looked at what the human cell consists of and what it requires to live. Our cells need energy to perform their vital functions, including the ability to control their chemical content and volume. The cell faces a dilemma: it must let certain chemicals pass through its plasma membrane, while at the same time ridding itself of what is harmful. This dynamic exposes the cell to the laws of nature which if not resisted could drastically alter its chemical content and total volume, resulting in death. We turn now to the two main natural forces, diffusion and osmosis, that constantly threaten cell life. 

the-designed-body4.jpgDiffusion refers to the natural law that chemical particles in solution always remain in motion and spread out evenly in their medium. Therefore, when a solute (like salt) is dissolved in a solvent (like water) it forms a mixture that is homogeneous. This means that the salt particles in solution are equidistant from each other, and the chemical make-up of the salt water is the same everywhere. The salt water at the top of the container is chemically identical to the salt water in the middle and the salt water in the middle of the container is identical to the salt water at the bottom.

Moreover, when two solutions with different concentrations of salt are separated by a membrane that is permeable, meaning that it allows both the salt (solute) and the water (solvent) to pass through, diffusion naturally makes the salt from the solution with a higher concentration move into the one with a lower concentration.

This movement, called "diffusing down its concentration gradient," is like moving down the slope of a hill, from a higher to a lower elevation. Except in this case, the movement of salt from the solution with a higher concentration to a lower concentration is taking place by the power of diffusion rather than the force of gravity. The final result of this movement of salt between the two solutions is they end up having the same concentration, the actual numerical value being somewhere between the original two.

The biological significance to the cell is that the fluid inside of it has a high concentration of potassium and a low concentration of sodium while the fluid outside has a low concentration of potassium and a high concentration of sodium. The plasma membrane of the cell that separates these two fluids is permeable to potassium, sodium, and water. So, if left unchecked, by following the rules, the power of diffusion would make potassium move down its concentration gradient, from the fluid inside the cell to the outside, and sodium move down its concentration gradient, from the fluid outside the cell to the inside.

If there were no mechanism in place to resist this natural movement, by diffusion, of potassium out of the cell and sodium into the cell, then life as we know it would not exist. As noted already, one of the main things the cell has to do to survive is take control and maintain its chemical content. However, diffusion is not the only natural force the cell has to contend with to stay alive. The other one, which affects the cell’s ability to control its volume, is osmosis.

Osmosis takes place when two solutions of different concentration are separated by a semi-permeable membrane in which the solvent can pass through but not the solute. For salt water this would mean that the salt cannot pass through the membrane but water can. Osmosis would naturally make water move from the solution with less concentration of salt to the one with more. This is exactly the opposite of what happens in the diffusion of chemicals, like sodium and potassium, across a permeable membrane.

Since the salt cannot pass through the membrane, but water can, the water moves across in the opposite direction instead so the concentration on both sides will be the same, somewhere between the original two. However, since the semi-permeable membrane only lets water pass through, a change in volume also takes place on both sides. Due to the power of osmosis, the volume of the solution that had a higher concentration of salt, rises, while the volume of the solution that had a lower concentration of salt, falls.

The biological significance of osmosis to the cell is that the fluid inside the cell has a much higher concentration of protein than the fluid outside the cell. Although the plasma membrane is permeable to solutions of sodium and potassium, it is only semi-permeable to ones with protein, i.e., it lets water pass through but not protein. This takes place because sodium and potassium are very small ions that can slip through most biological membranes, but most proteins are very large molecules that can’t. This is important for survival. The cell makes many different proteins that perform vital functions, and if they were able to easily pass through the plasma membrane and leave the cell by diffusion, then the cell wouldn’t be able to work properly and would die.

However, the fact that protein can’t cross the membrane, but water can, makes the cell susceptible to the power of osmosis. As the potassium and sodium ions naturally move, by diffusion, in opposite directions across the plasma membrane, the much higher protein content inside the cell (which can’t leave it) follows the rules and makes water enter the cell by osmosis. If too much water enters the cell, causing its volume to rise and too much pressure to be applied against the plasma membrane, the cell can die by explosion, just like a balloon. As we once again see, one of the main things the cell needs to do to survive is take control and maintain its volume.

Cell death under these circumstances verifies that real numbers have real consequences. When the cell follows the rules, like diffusion and osmosis, it runs the risk of losing control and dying. So by what innovative mechanism do our cells combat the natural forces of diffusion and osmosis? That question must wait till next week.

Image by Adam Jones Adam63 (Own work) [CC BY-SA 3.0], via Wikimedia Commons.

Howard Glicksman

Dr. Howard Glicksman is a general practitioner with more than forty years of medical experience in office and hospital settings, who now serves as a hospice physician seeing terminally ill patients in their homes. He received his MD from the University of Toronto and is the author of “The Designed Body” series for Evolution News. Glicksman further develops the arguments from this series in a book co-authored with systems engineer Steve Laufmann, Your Designed Body (2022).



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