It is often coluorless or greyish ,translucent and somewhat viscous semiliquid , that is , gelatinous in texture .Its viscosity varies from cell to cell in the same organism ,and from organism to organism .It is heavier than water .Some substances of the protoplasm such as sugars ,salts and simple acids and bases ,etc ., form crystalline or true solution but others exist in colloidal solution .Thus ,the protoplasm may be called a complex crystallo-colloid mixture .In colloidal system ,samll finely divided particales of matters [0.001u-0.1u] are suspended in a continuos medium .The colloids frequently posses glue-like consistency and are not crystalline in structure .A very viscous colloid is called a gel and more fluid colloid a sol . Sometimes sol and gel are converted into one another .Thus protoplasm is a reversible colloid , that is , it changes into sol or gwel when necessary .However it exists normally in a state between a sol and gel . View about physical structure .Under light microscope the protoplasm of the different cell shows different appearance or different structure , even the same cell shows different structures at different times .This has led the workers to think differently about the structure or appearance of the protoplasm. The view of the workers are in the form of the following theories : 1. Fibrillar theory .This theory , proposed by flemming , states that the suspended colloidal particles are in the form of fine fibres . 2. Alveolar theory .This theory ,proposed by Butschli [1892] ,holds that the protoplasm is an alveolar strucute .It appears to be a foaming mass of globules embedded in a viscid medium . 3. Granular theory .According to this theory , the protoplasm presents , a granular appearance ; many fine granules are evenly distributed in a liquid medium .This theory was proposed by altman [1893] . 4 Reticular theory . This theory states that the structure of the protoplasm is reticular , the suspended particles are in the form of a network of fibres.
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