(hyaloplasm) The part of the *protoplasm
outside the nucleus in which the cisternal elements and membrane-bound *organelles
lie. The soluble phase of cytoplasm contains the principal components of the cell's biochemical pathways, i.e. ions, dissolved gases, and most of the enzymes and substrate molecules for metabolic processes. The cytoplasmic matrix also contains storage products, e.g. lipid droplets, aleurone grains, and starch grains. Complex networks and linear arrangements of fine microfilaments and *microtubules
are commonly present. The microfilaments. 0.5-7.0 nm in length, are composed of helical chains of F-actin, and when supplied with ATP they contract. Those at the periphery of the cell are involved in movements of the components of the plasma membrane, e.g. in *endocytosis
. Cytoplasm shows transitions between viscous (plasmagel) and fluid (plasmasol) phases. This is displayed in *cyclosis
and in amoeboid movement shown by some algal unicells. The F-actin molecules of the microfilaments, particularly at the periphery of cells, are unstable, readily breaking up into soluble components so that the cytoplasm becomes fluid.