Page 18 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

3. The functions of cell wall are as follows:
(a) It protects the cell from physical injuries and haemolysis.
(b) It supports the plant through cell turgidity or mechanical
strength for tall woody trees.
(c) It controls growth by limiting individual cell size and shape of
the cell through orientation of the fibrils in the wall.
(d) It forms a system of transport pathways for water and mineral 2
ions. Water can both be transported along the porous cell wall
in apoplast way and through the plasmodesmata of cells in
symplast way.
(e) It controls excessive loss of water from epidermal cells of the
leaves and stems by having a waxy cuticle on the surface of the cell
wall. Cork cells have suberised cell wall for the same purpose.
(f) Cell wall of the xylem vessels forms empty tubes for water
transport from the roots to the leaves. Tracheids form a water
transport system with a lot of pits so that water can be laterally
transported. Sieve tubes with a thinner wall but with sieve plates
(a sieve-like lignified cross wall) will give extra strength for
transport of organic compounds.
(g) It provides food storage in the form of hemicellulose in some
seeds.
(h) It provides large surface area to volume ratio in root hair cells
where absorption can take place.
(i) It controls passage of water and dissolved mineral ions into the
plant by having lignified Casparian strip or passage cells in the
endodermis of the root.

Cytoplasm
1. Cytoplasm is the protoplasmic part of the cell, which is outside the
nucleus and is surrounded by the plasma membrane. It is the aqueous
part of the cell, after all organelles are removed by centrifugation.
2. The cytoplasm of plant cell is usually referred to as protoplast,
excluding the sap vacuole.

3. The pH of the cytoplasm is 6.8 ± 0.2.
4. It has a considerable high density with a variety of solutes.
5. Cytoplasm can be divided into cytosol (ground substance) and
cytoskeleton (cell inclusion).

Cytosol (ground substance)
1. Cytosol or the ground substance is the soluble part of the cytoplasm.

2. The solutes can be divided into three groups.
(a) True solutes or crystalloids. These consist of:
(i) Micromolecules such as gases (O , CO and N ), mineral
2
2
2
2+
+
ions which include Na , K , Cl , Ca , Mg , Mn and Fe 2/3+ .
2+
–
+
2+
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