A crawling-like type of movement in which the cell forms temporary cytoplasmic projections called pseudopodia (false feet) towards the front of the cell is called amoeboid movement. This type of movement is observed in amoebae, some protozoans, slime molds and vertebrate white blood cells. When an amoeba moves, it extends the gelatinous cytosol pseudopium, which then results in the more fluid cytosol flowing after the gelatinous portion where it congeals at the end of the pseudopium. Inside the amoeba, there are proteins that, can be activated to convert the gel into the more liquid state.
Locomotion of amoeba occurs due the sol-gel conversion of the cytoplasm within its cell. The central region of the cytoplasm is known as endoplasm which is fluid like sol and the region of the cytoplasm just lying down the plasma membrane is called ectoplasm, which is gel-like. The ectoplasm being called the plasma gel and the endoplasm the plasma sol. The conversion of the endoplasm to ecto and vice versa is called sol-gel conversion.
With the help of powerful microscope, the scientists observed that the various particles and membranes organelle embedded in the endoplasm are in continuous random motion. Ectoplasm contains a three-dimensional network of cross-linked actin fibres, and this gel region is responsible for the shape of the pseudopodium. It also transmits tension from the regions of cellular contraction to the sites of contact with the substratum. During the elongation of pseudopodium, the sol-like endoplasm streams into it, the region of the endoplasm near the tip of the pseudopodium apparently transforms into gel-like ectoplasm. Proteins such as actin, fimbrin and fodrin play a vital role in the sol-to-gel transition.
The regulation of sol-to-gel transition depends on both Ca2+ concentration and pH. In the low molar concentration of Ca2+ and 6.8 pH, the cytoplasm of amoeba sets as a gel. Conversely, if we raise pH or Ca2+ concentration, gel-to-sol transition takes place. Gelsolin or villin actively participate into gel to sol transition.