Page 17 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

Microfibril
Matrix
2
Figure 2.8 Microfibrils arranged in an amorphals matrix

(iii) Each microfibril consists of about 100 chains of cellulose
molecule of 5-20 µm and a diameter of 1-2 µm.
(iv) The matrix is made up of complex polysaccharides such
as pectins and hemicellulose with long and branched
molecules.
(v) On the outer layer of the wall, there is a middle lamella
layer that consists of magnesium and calcium pectates for
cementing adjacent cells.
(vi) The primary cell wall is porous. It enables water to be
transported apoplastically along it.
(vii) It is elastic and strong. It enables parenchyma cells to
become turgid and support the whole plant especially in
herbaceous plants.
(viii) The wall is usually perforated with plasmodesmata for the
transport of substances between cells.
(b) Secondary cell wall
The secondary cell wall is a harder and usually thicker layer
of cell wall, formed between the plasma membrane and the
primary cell wall. It has the following characteristics:
(i) The wall is made up of regularly arranged microfibrils or
bigger macrofibrils. The fibrils are arranged in layers of
parallel rows, which are perpendicular to those of upper or
lower layers (Figure 2.9).

Microfibrils

Figure 2.9 Fibrils arranged in layers of parallel rows
(ii) The matrix in the secondary wall is impregnated with
lignin, forming a hard and impervious layer.
(iii) The deposition of lignin in xylem vessels is not uniform
but in patterns like rings, helices or networks.

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