Osmosis (Chapter 6: Membrane Structure and Function)

Osmosis is one of the many biological processes that happen in our cells every day that people learn about in school and then forget. On top of that; osmosis is a vital process in our bodies that constantly takes in fresh water for the cells to use and takes out the excess water caused by respiration. So what exactly is osmosis? Well, the definition is the movement of water, through a selectively permeable membrane, to the higher concentration of solutes.

However that is a mouthful, and in able to actually understand the definition, we can break it down into three sections. The first and easiest section is “the movement of water”. Obviously that is the flow of water. However the term water is critical to the definition. If the water were to be replaced with grape soda or any other liquid, then that would not be osmosis. The second part is “through a selectively permeable membrane”. A selectively permeable (also known as semi-permeable) membrane is a special type of membrane that allows only small molecules, like water, to enter the cell on their own. Our cells’ plasma membranes are selectively permeable, and, not just our cells, but every cell that exists on Earth has a selectively permeable plasma membrane. The last part is “to the higher concentration of solutes”. Well, a solute is any substance that can dissolve in a certain liquid. That liquid is called a solvent. In the case of osmosis, the solvent is water. Two common solutes are sugar and salt. The water would flow through that selectively permeable membrane to the higher concentration, or where there are more of, solutes.

Now here is an example of osmosis; take a thistle tube (which looks like a fish bowl with a narrow, hollow tube sticking out of the bottom). The broad end of the tube is covered with a selectively permeable membrane, and (gently) dunked broad-end first in a bucket of water. This thistle tube that was just dunked had previously been filled with a small amount of water and solute. Over an extended period of time (most classroom experiments can have results seen overnight, however some other experiments may take longer), the water in the bucket will flow through the membrane and to where the solutes are at their highest concentration, which, since the bucket contains no solutes, is inside the thistle tube. At some point, the osmotic pressure (minimum amount of pressure to prevent water from moving through the membrane) will be reached, and the water levels will stop rising.

There are also real-life examples of osmosis as well. One example has to do with plant cells. Plant cells live in hypotonic conditions, or, conditions in which there are more solutes inside the cell than outside. So the water flows into the plant cells. In order to cope with this, there is a special organelle in plants called a vacuole that expands when there is a surplus of water caused by osmosis. As a result, the cell expands and the outer membrane pushes against the cell wall, which creates tugor pressure. This pressure keeps plants from wilting until the vacuole shrinks (along with the cell), and at that point in which the plant needs water.

Another example of osmosis can be seen in the ocean. Fish that live in the ocean live in a hypertonic environment. Simply the opposite of a hypotonic environment, a hypertonic environment is one where there is more solute outside the cell than inside. Ideally, the fish’s cells would simply shrink and dehydrate, but that doesn’t happen. In reality the fish’s cells stay at their normal size, due to the fish’s gills, which expel salt from the bloodstream, which keeps the fish’s cells in an isotonic environment. In an isotonic environment, the amount of solute inside and outside of the cell is equal, resulting in a constant flow of water through the cells. Fresh water keeps coming in, and excess water caused by respiration keeps coming out. All animal and human cells need to be in an isotonic environment to survive so there is just the right amount of water in the cell.

To demonstrate why our cells need to be in an isotonic environment, scientists have done experiments on red blood cells by placing some in three different solutions. One was hypotonic, another was hypertonic, and the last was isotonic. The cells in the hypotonic solution swelled up until the point of simply bursting, while the cells in the hypertonic solution shriveled up and dehydrated. The last cells were in an isotonic solution, and thus remained the same. This shows that one small, seemingly insignificant change can be catastrophic to life.

In conclusion, osmosis is the flow of water, through a selectively permeable membrane to the higher concentration of solutes. This definition was broken down into three sections, which were further explained in order to fully grasp the definition as something a bit more than a handful of words. After all, without knowing the definition of osmosis it would be difficult to understand the essay. This definition was followed by a laboratory example involving a thistle tube, which showed how the water flowed to the higher concentration of solutes until the osmotic pressure was reached, preventing any more water from coming in. Then came two real-life examples which introduced the terms hypotonic, hypertonic, and isotonic, which then led to showing why animals, and people, need their cells to live in an isotonic environment, or else the cells would die. If osmosis didn’t exist, life, as we know it, wouldn’t exist either.

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