Page 10 - 11-Bio-4 The Cell

4. The Cell eLearn.Punjab

solid particles) or pinocytosis (to take in liquid material). In neurons (nerve cells) the cell membrane
transmits nerve impulses from one part of the body to the other to keep coordination.

Cell Wall

The outer most boundary in most of the plant cells is cell wall. The cell wall of plant cell is different from
that of prokaryotes, both in structure and chemical composition. It is secreted by the protoplasm
of the cell. Its thickness varies in different cells of the plant. It is composed of three main layers:
primary wall, secondary wall and the middle lamella. The middle lamella is first to be formed in
between the primary walls of the neighbouring cells. The primary wall is composed of cellulose
and some deposition of pectin and hemicelluloses. Cellulose molecules are arranged in a criss
cross arrangement (Fig.4.6). The primary wall is a true wall and develops in newly growing cells. The
secondary wall is formed on its inner surface and is comparatively thick and rigid. Chemically it is
composed of inorganic salts, silica, waxes, cutin lignin etc. Prokaryotic cell wall lacks cellulose; its
strengthening material is peptidoglycan or murein. Fungal cell wall contains chitin.

Fig.4.6. Secret of the strength of plant structure is revealed by electron microscope photographs of the cell walls. The cellulose fibers
are arranged in layers, with the fibres of each layer at right angle to those of other layers.

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