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Figure 2–54 B. Pinocytosis in the cells of the root tip of Ricinus communis (After Frey-Wissling and Mühlethaler, 1965).
The notion of the cell envelope has been altered in the course of
studying the cell. As it was already mentioned, for a long time it has been
considered usual barrage wall of no special importance. Later on it became
clear that it is a multilayer formation performing various biological functions.
The chemical composition and structural organization of the cell
envelope have been an object of numerous investigations prompted by its
importance for cell formation and functioning as a biological system, as well
as by the historical fact that it is the first structure seen by R. Hooke in 16 65.
It is the cause for the discovery of cell, thus making a start with its
studying. The basic components of cell envelope are different
polysaccarides (mainly cellulose and starch), proteins, lipids, etc. The water
content is high in it — up to 95%. In the course of binding single
polysaccaride chains there are formed threads gathered together in
fascicles and shaped like sticks. It was established that they possess
dispersed and crossed parallel texture (Fig. 2–55).
There have been long arguments about the presence or absence of
outer cell wall. The reasons for that are mainly two: on one hand,
morphologically it has not been clearly observed under light microscope,
and on the other — in some cases it really is absent. Contemporary
biochemical and physical methods, especially using the electron
microscopy, made it possible to establish that normally the cells possess a
surface layer (cell wall) of high-molecular carbohydrates or in a complex
with proteins, that performs locomotory and protective functions and
internal plasmic membrane (plasmalemma) built mainly of lipid and protein
molecules.