Water & Organisms
Plants
Osmosis and plant cells
Although the osmotic principles apply equally to plant and animal cells, a different set of terms is currently applied to the osmotic relationship of plant cells. Water potential is a measure of the tendency of water to leave a solution. Pure water is designated a water potential of zero. As the solute molecules in a solution tend to prevent the water molecules leaving it, the solution will have a lower water potential than pure water. Its value will therefore be less than zero, i.e. negative. The more concentrated the solution, the more negative is its water potential.
For practical purposes a plant cell can be considered as a solution of salts and sugars in the vacuole surrounded by a partially permeable membrane (tonoplast, cytoplasm and plasma membrane) and a slightly elastic but completely permeable cell wall. A plant cell therefore has a more negative water potential than pure water and will draw in water when surrounded by it. This entry of water forces the living part of the cell, known as the protoplast, against the cell wall. In effect, the water in the vacuole is being subjected to a pressure from the cell wall. This pressure is referred to as the pressure potential. In a turgid plant cell this is a positive value, although in the xylem of a transpiring plant it is negative. The water potential of a cell is changed by the presence of solute molecules. This change is referred to as the solute potential. As solute molecules invariably lower the water potential, its value is always negative. The relationship between these three terms is given as:
water potential = solute potential + pressure potential
Water for the support of plants
Water is almost impossible to compress (press into a smaller volume). This feature of water allows it to be used as a form of support by many smaller plants. Although such plants do not have a skeleton, they do have cells with strengthened walls the cell is filled with a gel-like...
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