What is Osmosis? 

In simple words, osmosis is the slow, gradual movement of a liquid through a membrane. But there are nuances to this process. For instance, there has to be an imbalance in the distribution of solute in the liquid on either side of the membrane, which has to be semipermeable. The liquid in the region with a lower solute concentration moves to the region with a higher concentration, thereby displacing the solution in the process. The objective is to equalize the concentration of molecules on both sides of the membrane; the displacement of liquid can be viewed as a byproduct of this equalization. Since there is no energy required to carry osmosis, it is considered a passive process.  

"Process of osmosis"

Are all Liquids Osmotic? 

Any solvent can undergo the process of osmosis. Osmosis can occur in gases and supercritical liquids too. There are, however, three types of osmotic solutions: isotonic, hypertonic, and hypotonic.  

A solution is called isotonic when it has the same concentration of solute on both sides of the membrane. In essence, this means that the levels of solute inside and outside the cell are the same. 

A hypertonic solution, on the other hand, has a higher solute concentration outside the cell as compared to the inside. Conversely, a hypotonic solution is the exact opposite as it has a higher solute concentration inside the cell. Simply put, as compared to the outside, a hypertonic solution has a low solute level inside the cell and a hypotonic solution has a high solute level inside the cell. 

Are there Different Types of Osmosis? 

There are essentially two types of osmosis: endosmosis and exosmosis.  

When a cell is introduced into a hypotonic solution, the solvent molecules move inside the cell. As a result, the cell becomes turgid, or distended and swollen due toexcessive water uptake and deplasmolysis. This process is called endosmosis. 

Similarly, when a cell is placed in a hypertonic solution, it becomes flaccid due to the outflow of water and plasmolysis. This process is called exosmosis. 

Why is Osmosis Important? 

Osmosis is extremely important for all living cells as it regulates the amount of water, nutrients, and waste within the cell. It is how plants’ roots absorb water and grow. It is also the reason why saltwater fish often die in freshwater reservoirs, and human fingers become wrinkledwhen placed in water. 

How osmosis impacts plant and animal cells are quite different. For example, an animal cell placed in a hypotonic solution will lyse, which means that the cell membrane will rupture due to excessive water intake. Plant cells, however, have thicker cell walls and need more water. Thus, plant cells won’t be affected as much as animal cells in the hypotonic solutions. A hypotonic solution is the most suited for plant cells.  

Animal cells can only thrive in isotonic conditions, which, interestingly, negatively impacts plant cells. It causes plant cells to lose turgidity, in turn, causing the leaves of the plants to droop. 

Can Osmosis be Stopped? 

The process of osmosis can be stopped and reversed as well with the help of external pressure. If the osmotic pressure is tweaked, the direction of osmosis can be changed, a process called reverse osmosis. 

"Process of reverse osmosis"

Osmotic pressure is the minimum pressure needed to keep the solvent transfer from occurring. Any changes to this pressure can make solvents flow inwards or outwards through the semipermeable membrane. For example, the loss of water due to changes in osmotic pressure causes the otherwise rigid plant cell wall to become less rigid, causing the cell membrane to shrink away from the cell wall.  

Does Osmosis have Applications in the Real World? 

For starters, be it a plant cell or an animal cell, osmosis helps to stabilize the internal environment by delicately balancing the water levels and intracellular fluid. The process keeps cells alive by regulating the distribution of nutrients and minerals that enter the cell by osmosis. But all the process of osmosis occurs at a cellular level, mostly unseen and undisturbed by human eyes.  

However, it will be wrong to say that osmosis doesn’t help humans on an everyday basis. For example, the process is used for the filtration of water and has been extremely successful in providing large populations access to clean water without expending additional energy or resources.  

Since osmosis plays a key role in providing nutrients to plants, it is central to agriculture. Using the understanding of osmosis, the amount of water needed to sustainplant life, germination, growth, leaf health can be determined.  

Another important application of osmosis is during dialysis of the kidneys. Patients suffering from kidney ailments often retain excess fluid in cells as they have difficulty passing urine. To get rid of this excess water, as it can be harmful to the cells, sugar is added to the dialysis fluid. The sugar molecules are transferred from the fluid into the body through the peritoneal membrane, while excess water from the body flows into the dialysis fluid. There is, therefore, an exchange that would have been impossible without osmosis. Without osmosis, patients with kidney issues would be unable to remove waste materials, toxins, excess salt, and fluids from their bodies. 

Quick Experiments to see Osmosis in Action 

While the process of osmosis cannot be seen without a microscope and other scientific equipment, its impact can be easily observed.  

  1. Eat something salty develops a feeling of thirst. 
  2. Soak some raisins, other dry fruits, or dried chickpeas in water. Within minutes or hours, depending on what has been soaked, the raisins or chickpeas will plump up. 
  3. Observe the fingers after a long shower. The skin will appear pruned up and wrinkly.  

Common Misconceptions 

  • Osmosis is not the same as diffusion! While diffusion is similar to osmosis, diffusion is the movement of a suspended substance from an area of higher concentration to an area of lower concentration whereas osmosis requires a semipermeable membrane. 
  • Osmosis is not limited to liquids. It occurs in gases and supercritical liquids too. 
  • Attractive forces between solute and solvent have no impact on osmosis. There is no pressure required, neither is any energy expended for osmosis.  
  • Osmosis happens both ways, from a lower concentration to higher, and from a higher concentration to lower (reverse osmosis). Reverse osmosis is a popular water filtration method across the world! 

Context and Applications  

This topic is significant in the professional exams for both Bachelors and Master courses related to biology. Some of the courses are listed below:  

  • Bachelors in Botany  
  • Bachelors in Biochemistry and Molecular Biology  
  • Bachelors in Zoology 
  • Masters in Biological Science  
  • Reverse osmosis 
  • Diffusion 
  • Osmotic pressure 
  • Cell structure 
  • Lysis 

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