What Is Osmosis

Defined as the spontaneous movement of solvent molecules from a region of low solute concentration to a region of high solute concentration through a semipermeable membrane in order to equalize solute concentrations on both sides of the membrane.


What Causes Osmosis and Why does it Occur

Occurs when any solvent moves across a semipermeable membrane, in order to attain the state of equilibrium for the system. The semipermeable membrane allows passage of solvent but not solute molecules separating two solutions of different concentrations. It is a passive process meaning that it occurs without any expenditure of energy and is sometimes called passive diffusion. Any solvent can undergo the process of osmosis, including gases and some special liquids called supercritical liquids.

Osmosis was first thoroughly studied in 1877 by a German plant physiologist, Wilhelm Pfeffer. The general term osmose (now osmosis) was introduced in 1854 by a British chemist, Thomas Graham.

Basic Characteristics of Osmosis

  • It is a particular form of diffusion and is the opposite of a simple diffusion process
  • The movement of the solvent is always through the semipermeable membrane
  • It is a slow and spontaneous process
  • This process undertakes in the liquid medium
  • Requires no energy expenditure
  • The rate depends upon the relative tendency of water to move from one area to another (water potential) of the solution. Water diffuses by osmosis from a region of high water potential to a region of low water potential
  • The process will continue until the concentration of the solution becomes even on both the sides of the semipermeable membrane
  • It depends on solubility, charge, and solute size of both the solutions present on opposite sides of the membrane

Examples of Osmosis

It is an important phenomenon occurring in both plants and animals. Some everyday-life examples of osmosis are:

In Plants

  • The roots of the plant have a higher solute concentration than the surrounding soil, so water flows into the roots which are absorbed by the plants through osmosis
  • The opening and closing of guard cells, the cells responsible for gas exchange in plants depends on the absorption of water by osmosis

In Animals

  • Human body infected with cholera causing bacteria, perform osmosis to reverse the flow of water absorption in the intestine allowing excess water to flow out of it thus causing severe dehydration and sometimes death
  • If freshwater or saltwater fishes are transferred in water that has a different salt concentration than they are normally used to, they will die of too much water entering or leaving the cells through osmosis
  • Cells take up nutrients and minerals into the cell and also get rid of their waste products and even toxic substances by osmosis

What Factors Affect Osmosis

i) Temperature

The rate of osmosis is directly proportional to temperature. Thus an increase in temperature increases the rate of diffusion.

ii) Area of Interaction

Osmosis is directly proportional to the area of interaction. Thus increasing the area of interaction of the molecules undergoing diffusion increases the rate of diffusion.

iii) Concentration Gradient or Osmotic Gradient

It is directly proportional to the extent of the concentration gradient. Thus higher the concentration gradient of the molecules between the two regions faster is its rate.

Osmosis will continue to happen until there is a difference in the concentration gradient on both sides of the membrane.

iv) Osmotic Pressure

The rate of osmosis is inversely proportional to the osmotic pressure of the solution. Thus increasing the osmotic pressure in the region of high solute concentration with respect to the low solute concentration region also decreases osmosis.

Osmotic pressure is the pressure that helps to prevent the passage of water through a selectively permeable membrane into a region of higher solute concentration.

What Are The Different Types Of Osmotic Solutions

The relative concentration of solutes dissolved in solution, which determines the direction and extent of diffusion is called the tonicity of the solution. In biology, there are three different types of solutions where the cells are found:

a) Isotonic Solutions

Solutions that have the same concentration of solutes both inside and outside the cell. For example, a cell with the same concentration of salt inside it as in the surrounding water/fluid is said to be present in an isotonic solution. Under these conditions, there is no net movement of a solvent, and the amount of water entering and exiting the cell’s membrane is equal.

b) Hypotonic Solutions

Solutions that have a higher concentration of solutes inside the cell than outside. When this occurs, more solvent will enter the cell than leave it to balance out the concentration of solute.

c) Hypertonic Solutions

Solutions that have a higher concentration of solutes outside the cell than inside it. In this type of solution, more solvent will exit the cell than enter it in order to lower the concentration of solute outside the cell.

What Are The Different Types Of Osmosis

Different types of solutions have different impacts on cells and thus determine the direction of water movement to and from cells. Accordingly, osmosis is classified into two main types:

i) Endosmosis

Water moves inside the cell when placed in a hypotonic solution causing them to swell up and make them rigid. Endosmosis occurs because the solute concentration of the surrounding solution is less than that inside the cytoplasm.

ii) Exosmosis

Water moves out the cell when placed in a hypertonic solution causing them to become flaccid.  Exosmosis occurs because the solute concentration of the surrounding solution is higher than that inside the cytoplasm. In some extreme cases, the cell loses excess water, and the cell membrane separates from the cell wall due to a lack of water pressure on it, a process known as plasmolysis.

Article was last reviewed on Friday, February 14, 2020

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