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Biology for Advanced Level Secondary Schools
making the cell wall very hard and increases tensile strength. Suberin and cutin are
also found in the cell wall during secondary thickening. These are waxy materials
acting as a water proof coat which helps to prevent excessive water loss from the plant.
Middle lamella is a thin layer of pectin material (calcium and magnesium), which joins
neighbouring cell walls together.

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Close group of
plasmodesmata (primary
pit field) – primary cell
walls are thinner in this Adjacent primary cell walls of two cells
area as a result of slower
growth Middle lamela

TS before lignification

Plasmodesma

‘Pit cavity’
TS after lignification Secondary wall of lignin

Simple pit Primary cell walls

‘Pit membrane’ – middle
lamella plus two thin primary
walls
Figure 1.8: Structure of a cell wall
Functions of the cell wall therefore, they allow movements of
(a) It provides mechanical support and materials to long distances.
strength to cells and the plant as a (f) Cell walls possess small pores or
whole, due to presence of lignin. pits through which the materials can
(b) It maintains the shape of the cell, since pass from one cytoplasm to another
the cell wall is rigid. (symplastic movement).
(c) It prevents the osmotic bursting (g) Cutin and suberin deposits prevent
of the cell by inhibiting excessive loss of water from the cell surface
endosmosis due to its capability to through evaporation.
resist expansion. (h) The cell walls of root endodermal
(d) It protects the plant cells against cells are suberised to form casparian
pathogens and water loss due to the strips. These, among other functions,
presence of waxy cutin on the cell regulate the amount of water to be
wall. admitted into the plant.
(e) The walls of xylem vessels, tracheids (i) Cell walls of some cells store food.
and phloem sieve tubes are tubular;

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