Biology PLASMOLYSIS AND IMBIBITION

KEY TOPICS

star Plasmolysis
star Imbibition

PLASMOLYSIS

● The color{violet}"behaviour" of the plant cells (or tissues) with regard to color{violet}"water movement" depends on the color{violet}"surrounding solution".

● If the color{violet}"external solution balances" the color{violet}"osmotic pressure" of the cytoplasm, it is said to be color{Brown}"isotonic". If the color{violet}"external solution" is more dilute than the color{violet}"cytoplasm", it is color{Brown}"hypotonic" and if the external solution is color{violet}"more concentrated", it is color{Brown}"hypertonic".

● color{violet}"Cells swell" in hypotonic solutions and color{violet}"shrink" in hypertonic ones.

● color{Brown}"Plasmolysis" occurs when color{violet}"water moves out" of the cell and the color{violet}"cell membrane" of a plant cell color{violet}"shrinks away" from its cell wall.

● This occurs when the color{violet}"cell (or tissue)" is placed in a solution that is color{violet}"hypertonic" (has more solutes) to the protoplasm.

● color{violet}"Water moves out"; it is first lost from the color{violet}"cytoplasm" and then from the color{violet}"vacuole".

● The color{violet}"water when drawn" out of the cell through color{violet}"diffusion" into the color{violet}"extracellular (outside cell)" fluid causes the color{violet}"protoplast" to shrink away from the walls.

● The cell is said to be color{violet}"plasmolysed".

● The color{violet}"movement of water" occurred across the membrane moving from an area of color{violet}"high water potential" (i.e., the cell) to an area of color{violet}"lower water potential" outside the cell.

● When the cell (or tissue) is placed in an color{violet}"isotonic solution", there is color{violet}"no net flow" of water towards the color{violet}"inside or outside".

● If the color{violet}"external solution balances" the osmotic pressure of the cytoplasm it is said to be color{violet}"isotonic".

● When color{violet}"water flows" into the cell and color{violet}"out of the cell" and are in color{violet}"equilibrium", the cells are said to be color{Brown}"flaccid".

● The process of color{violet}"plamolysis" is usually color{violet}"reversible".

● When the cells are placed in acolor{violet}"hypotonic solution" (higher water potential or dilute solution
as compared to the cytoplasm), color{violet}"water diffuses into" the cell causing the cytoplasm to build up a color{violet}"pressure against the wall", that is called color{Brown}"turgor pressure".

● The pressure color{violet}"exerted by the protoplasts" due to entry of water against the color{violet}"rigid walls" is called color{Brown}"pressure potential" p..

● Because of the color{violet}"rigidity of the cell wal"l, the cell does color{violet}"not rupture".

● This color{violet}"turgor pressure" is ultimately responsible for color{violet}"enlargement and extension" growth of cells.

IMBIBITION

● color{Brown}"Imbibition" is a special type of color{violet}"diffusion" when water is absorbed by solids – color{Brown}"colloids" – causing them to color{violet}"enormously increase" in volume.

● The color{Brown}"classical examples" of imbibition are absorption of water by color{violet}"seeds and dry wood".

● The pressure that is color{violet}"produced by the swelling" of wood had been used by prehistoric man to color{violet}"split rocks and boulders".

● If it were not for the color{violet}"pressure" due to imbibition, color{violet}"seedlings" would not have been able to color{violet}"emerge out" of the soil into the open; they probably would not have been able to color{violet}"establish"!

● Imbibition is also color{violet}"diffusion" since water movement is along a color{violet}"concentration gradient"; the seeds and other such materials have color{violet}"almost no water" hence they absorb water easily.

● color{violet}"Water potential gradient" between the color{Brown}"absorbent and the liquid" imbibed is essential for imbibition.

● In addition, for any substance to color{violet}"imbibe any liquid," color{Brown}"affinity" between the color{violet}"adsorbant and the liquid" is also a pre-requisite.