Page 45 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

(b) Parenchyma
(i) They differentiate to form vessels and tracheids.
(ii) They store food in the form of starch.
(c) Sclerenchyma
Their functions are mainly for support.

5. Xylem is found
2 (a) in the stem, towards the inside of the vascular bundles, as in
Figures 2.26, 2.27 and 2.28.
(b) in the roots, towards the inside of the vascular tissue, as in
Figures 2.29, 2.30 and 2.31.
(c) in the leaves, on the upper part of the vascular bundles, as in
Figures 2.32 and 2.33.

Phloem
1. Phloem is a complex vascular tissue that translocates organic food
especially sucrose and amino acid formed after photosynthesis.
2. Like xylem, phloem is divided into primary and secondary types.
3. Phloem consists of sieve elements, companion cells, parenchyma
and sclerenchyma.
(a) Sieve elements
(i) Sieve elements consist of sieve cells and sieve tubes.
(ii) Sieve cells are young cells with nuclei but with no defined
sieve plates.
(iii) Sieve tubes are mature cells, thin walled with no nucleus
but with protoplast and sap vacuoles.
(iv) Sieve tubes are long and slender cells connecting end to
end to form tubes for translocation.
(v) Their cross sections may be rectangular, round or polygonal.
(vi) The cross walls of sieve tubes form lignified sieve plates
with holes, allowing protoplast to flow from one tube to
another. These plates have supporting functions as they
prevent breakage of these thin-walled tubes under pressure.
(b) Companion cells
(i) They are small slender cells fitting neatly end to end, at
least one beside a sieve tube.
(ii) Their walls are thin and usually square in cross sections.
(iii) They have nuclei, compact protoplast with high organic
content, no sap vacuole but with a lot of mitochondria.
(iv) Their cytoplasm is connected by many plasmodesmata to
the sieve tubes. Sucrose and amino acid can diffuse to and
fro between companion cells and sieve tube.
(c) Parenchyma
(i) They exist as undifferentiated phloem or medullary rays
extended from xylem.
(ii) Their structures are the same as ordinary parenchyma but
smaller in size.

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