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Define osmosis

Osmosis is the movement of water molecules from a region of higher water potential to the region of lower water potential down a concentration gradient, across a partially permeable membrane. A high water potential solution is the one which has more amounts of water molecules, and less amount of the solute. Hence, it can also be called a low solute concentration solution. On the other hand, a low water potential is the one in which there is a low amount of water molecules, but more solutes, hence would be more concentrated. A solute can be salt or sugar that is present in a solution with water as the solvent. A semipermeable membrane only allows some molecules to pass through; for instance, only water molecules can pass through it and not the larger solute molecules like sucrose. “Down the concentration gradient” means how the water molecules move from the region where there are more water molecules to the region where there are less water molecules.

To understand the concept of osmosis, imagine we have three cells placed in three different types of solution. The different types of osmotic solutions are isotonic, hypertonic, and hypotonic. Isotonic solution has equal amount of solution concentration inside and outside the cell. Hypertonic solution has more solute concentration outside the cells as compared to the inside of cells. Hypotonic solution has less solute concentration outside the cells while more inside the cells. Hence, when a cell will be placed in these solutions, different types of osmosis will occur:

·    When a cell is placed in an isotonic solution, no net movement of water molecules happens by osmosis.

·      When a cell is placed in a hypertonic solution, then water leaves the cell by exosmosis and enters the solution, making the cell flaccid.

·     When a cell is placed in a hypotonic solution, then the water enters the cell by endosmosis, and becomes turgid.

There are many examples of osmosis which involve animal and plant cells.

Red Blood Cell (Animal Cell)

Plant cell

Red  blood cell swells when placed in an hypotonic solution, and eventually bursts

Plant cell becomes turgid when placed in a hypotonic solution. It exerts an osmotic pressure on the cell walls due to which plant cells stay upright

Red blood cell becomes shrink and crenation occurs

Plant cell becomes plasmolysed when placed in a hypertonic solution

Stay normal in isotonic solution

Stay normal in isotonic solution

Moreover, osmosis plays a significant role in various biological processes happening in the living organisms. In plants, osmosis is needed for absorption of water and then its transport through the xylem vessel, maintains the turgidity of cells like guard cells that can swell up to form stomatal pores. Also in humans the osmoregulation is important in the kidneys, and osmosis is important to maintain a balance between solute ions and water in different parts of the body.

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