what is Osmosis, hypertonic solution, Hypotonic solution and isotonic solution. class 9th biology

Osmosis is the movement of water across a semi-permeable membrane from a solution of lesser
solute concentration to a solution of higher solute concentration.
The rules of osmosis can be best understood through the concept of tonicity of solutions. The term
tonicity refers to the relative concentration of solutes in the solutions being compared.
• A Hypertonic solution has relatively more solute.
• A Hypotonic solution has relatively less solute.
• Isotonic solutions have equal concentrations of solutes.
When an animal cell, such as red blood cell, is placed in an isotonic solution, the cell volume
remains constant because the rate at which water is entering cell is equal to the rate at which it
is moving out. When a cell is placed in a hypotonic solution, water enters and cell swells and may
rupture like an over-filled balloon. Similarly, an animal cell placed in a hypertonic solution will lose
water and will shrink in size. So in hypotonic environments (e.g. freshwater) animal cells must
have ways to prevent excessive entry of water and in hypertonic environments (e.g. seawater) they
must have ways to prevent excessive loss of water.
Water balance problems are somewhat different for plant cells because of their rigid cell walls. Most
plant cells live in hypotonic environment i.e. there is low concentration of solutes in extracellular
fluids than in cells. As a result, water tends to move first inside cell and then inside vacuole. When
vacuole increases in size, cytoplasm presses firmly against the interior of cell wall, which expands a
little. Due to strong cell wall, plant cell does not rupture but instead becomes rigid. In this condition,
the outward pressure on cell wall exerted by internal water is known as turgor pressure and the
phenomenon is turgor. In isotonic environment, the net uptake of water is not enough to make
the cell turgid and it is flaccid (loose / not firm). In a hypertonic environment a plant cell loses water
and cytoplasm shrinks. The shrinking of cytoplasm is called plasmolysis.
#osmosis #hypertonicsolution