Archive for the ‘Diffusion and Osmosis’ Category
Imbibition and Matric Potential
Imbibition
Under certain circumstances water is taken up by the plant materials by a process called imbibition. In this process the water moves along a potential gradient from a region of higher potential to a region of lower potential as in diffusion. The substances that imbibe water or other solvents called imbibants. The imbibans are colloidal in nature. Water is held strongly with the imbibant by forces of attraction which may be electrostatic or chemical Forces.
Plasmolysis and Measurement of Osmotic Potential of Cell Sap
Plasmolysis
If a vacuolated plant cell is placed in hypertonic solution of the sucrose, with concentration greater than that of the cell sap, the water will diffuse out of the cell because the osmotic potential (Ψπ) of the solution is lower than that of the sap. The cell gradually decreases in volume until its cytoplasm is pulled away from the wall. The wall remains in position because it is rigid.
Osmotic Relations of a Plant Cell
The relations of water potential, osmotic potential and pressure potential in case of a vacuolated plant cell placed in pure water are similar to relations of these Osmotic quantities discussed in the case of an osmometer. Suppose that the cell is flaccid (not turgid) in the beginning and its sap has osmotic potential -12 bars. Being flaccid its pressure potential is zero. The water potential of the sap in this condition is equal to its osmotic potential, i.e., 12 bars. The water outside the cell has higher potential (zero, as it is pure water) than the water potential of the cell sap (12 bars). Therefore, water diffuses across the cell membranes, plasma lemma and tonoplast, by osmosis. By entry of water into the cell, the sap gets diluted and the volume of the cell increases to 1.5 times its volume in flaccid condition.
Osmosis
It is the diffusion of water through a differentially permeable membrane from a region of higher potential (pure water or weak solution) to a region of lower potential (more concentrated solution).
Diffusion of Water
The movement of water within a cell and from one cell to another is a peculiar case of diffusion. This diffusion occurs through differentially permeable membrane. A differentially permeable membrane is that through which some substances can penetrate, while others are either retained or pass through the membrane rather slowly. Plasma lemma, tonoplast and the membrane surrounding the cell organelles are differentially permeable membranes. Water can penetrate readily through these membranes, but the substances dissolved in water can either penetrate slowly or can not pass through them. Movement of water into or out of a cell membrane requires energy.
Diffusion
When a lump of sugar is dropped in a beaker of water, the molecules of sugar will begin lo move in all directions away from the lump. The direction of movement of sugar molecules is from the region of higher concentration of sugar, i.e., from the neighbourhood of the lump of the region of its lower concentration, i.e., away from the lump.