Page 40 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

roots. Its function is to produce secondary xylem (wood) and
phloem.
(c) Cork cambium (phellogen). It is found on the outer layer of
dicotyledonous woody stem and root. Its function is to produce
cork cells (phellem) on the outside and secondary cortex
(phelloderm) on the inside.
(d) Intercalary cambium. It is only found in monocotyledonous 2
plants, especially Gramineae. It is a thin layer of cells above the
node of the stem when it is young. Its function is to produce
more cells for the internode and later, it disappears.

Specialised Plant Cells

Parenchyma
1. Parenchyma is a tissue with the least differentiation, with a thin wall
and contains living protoplast and nucleus. 2017

2. It has the following structural features.
(a) The cells are spherical, isodiametric and may be oval in shape.
(b) The cells are the least differentiated i.e. with very few specialities.
(c) The cells are alive, with protoplast and nucleus. All the enzymes
within are active.
(d) Their cell walls are thin, which are made up of only primary cell
walls.
(e) The sap vacuole is big and centrally located. It stores water and
soluble mineral ions.
(f) Their protoplast may store starch and they have chloroplasts.
Some parenchyma are full of starch grains especially is storage
organs.
(g) The cells usually have large amounts of intercellular space. The air
spaces allow easy exchange of gases especially for photosynthesis.
(h) The cells can divide if stimulated by hormone such as auxin.

3. Its distributions are shown in all the eight basic organs (Figures 2.26
– 2.33).
(a) In the cortices (singular cortex, layer beneath epidermis) of
stems and roots.
(b) In the pith (centre of root or stem) of dicotyledenous stem and
monocotyledenous root.
(c) In the mesophyll of leaves.
(d) In the medullary rays of the secondary xylem and phloem.
(e) Around vascular bundles in stems and leaf stalk.
(f) In the epidermis with thickened wall and cuticle.

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