In this article, you will learn what is osmosis and how it occurs in plant and animal cells through a semi-permeable membrane known as a cell membrane.

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Osmosis Defined

Osmosis is defined as:

"The process of net movement of water molecules from an area of higher water potential to an area of lower water potential, through a partially permeable membrane, i.e. a cell membrane, is referred to as an osmosis"

Osmosis is in fact a diffusion of water molecules from a region of a higher concentration, i.e. a dilute solution to a region of lower concentration, i.e. a concentrated solution through a cell membrane. We know that in a dilute solution, the concentration of water molecules is more and in a concentrated solution, the concentration of water molecules is less. Therefore, the water molecules move from a dilute to a concentrated solution.

Now, the question arises that what happens when the concentration of water molecules is the same on both sides. Well, the answer is simple. In this situation, the water molecules will move at the same rate in both directions and there will be no net movement of water molecules.

Osmosis In Living Cells

Cells have diluted solutions of sugars, amino acids, and ions. A dilute solution of ions, sugars, and amino acids are present inside the cells. The cell membrane that surrounds the cell is a partially permeable membrane or selectively permeable membrane. It means that it permits some materials to enter and leave the cell freely. Water moves in and out of the cell through a process known as osmosis.

In the next section of the article, we will discuss how osmosis occurs in plant and animal cells.

Osmosis in Plant Cells

When an individual plant cell is placed in water or any dilute solution, it will absorb water by osmosis. In plants, root hair cells take in the water from the moist or wet soil through this process. On the other hand, leaf cells of the plants on land are likely to lose water, unless it is raining or the weather is very humid.

We know that plant cells have a cell wall outside the cell membrane. Unlike a cell membrane which is a partially permeable membrane, the cell wall in the plant cells is a fully permeable membrane. It means that it allows all the molecules to enter the cell and supports the cell. It also prevents it from bursting when it takes in water through osmosis.

Certain experiments are done to fully understand the process of osmosis in plants. In these experiments, the plant cells are placed in different solutions of increasing solute concentration. Let us see what is concluded after these experiments.

Plant Cells Placed in Pure Water

When plant cells are placed in pure water, the structures of the cell like cytoplasm and vacuole push against the cell wall. As a result, the cell becomes turgid. The cells that are turgid fully provide support to the stems of non-woody plants.

Plant Cells Placed in a Concentrated Solution

When the plant cells are placed in a more concentrated solution, the structures of the cell lose water through osmosis. As a result, the cell shrinks and pulls away from the cell wall, and becomes flaccid instead of becoming turgid.

Do you know what happens when the plant cell becomes flaccid? To answer this question, let us take an example of a plant stem. When the stem of the plant becomes flaccid, then the turgor pressure inside the cell will decline and as a result, the stem will wither.

Plant Cells Placed in Highly-concentrated Solution

In a solution that is highly concentrated, the cell undergoes full plasmolysis because the cell loses a lot of water. If the water content in the soil is less, then the plants are exposed to a higher concentration of solutes. In this situation, the plants lose water through the process of osmosis. Usually, the soil becomes concentrated, i.e. it has less water when the plants are not watered enough or there is a drought condition.

For instance, when freshwater plants are placed in the sea or seaweed is placed in a rock pool, then the water evaporates in the sun and the plants lose water through osmosis.

In the next section, how animal cells undergo osmosis in different situations.

Osmosis in Animal cells

Like plant cells, animal cells also take in and lose water through osmosis. Unlike plant cells, the cell wall in absent in animal cells. Hence, they will alter their size and shape when they are placed in various solutions that have varying concentrations of water molecules.

For instance, when red blood cells are placed in a concentrated solution, they lose water and as a result shrink in size. Conversely, when the same cells are placed in a highly diluted solution, the cells absorb water and burst. In animals, the concentration of fluids in the body should be kept within stern limits because if the cells lose a lot of water through osmosis, then they do not function properly. Let us see what happens when the animal cells are placed in various solutions.

In a Pure Water or Dilute Solution

If the animal cell is put into pure water or a dilute solution, then the water goes into the cell through a semi-permeable membrane by osmosis because the pure water or dilute solution has a higher concentration of water molecules. In this situation, the cells will undergo osmosis to gain water and the cell membrane will be pushed too far that the cell will burst. This bursting of the cell is known as cytolysis.

Cytolysis does not occur in plant cells because they contain cell walls that prevent the cell from bursting when the pressure is increased. Cytolysis especially happens in the hypotonic environment. In a hypotonic environment, the solution outside the cell has a lower concentration of the solute than the solution inside the cell. Due to this reason, it is essential to maintain a constant water potential inside the bodies of animals.

In a Concentrated Solution

When the animal cells are placed in a solution having a lower water potential, i.e. a concentrated sucrose solution, then the water leaves the cell through the cell membrane by osmosis. As a result, the cell shrinks. This especially occurs in the hypertonic environment when the solution outside of the cell contains a higher concentration of the solute as compared to the solute inside the cell.

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Emma

Emma

I am passionate about travelling and currently live and work in Paris. I like to spend my time reading, gardening, running, learning languages and exploring new places.