Page 23 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

(vii) The functions of the nuclear envelope are as follows:
• It protects the inner structure of the nucleus especially the
chromatin.
• It separates the nucleus from the cytoplasm so that reactions
occur in the nucleus are not affected by those of the cytoplasm.
• It controls the shape of the nucleus. This provides a three
2 dimensional space for processes such as the synthesis of DNA
and RNA.
• It controls the passage of substances like ribosomes and RNA
from the nucleus to the cytoplasm.
(b) Nucleoplasm (nuclear sap / karyoplasm)
(i) Nucleoplasm is the part of protoplasm inside the nucleus,
separated by the nuclear envelope.
(ii) Its composition is the same as that of the cytoplasm, consisting
mainly of water with crystalloids and dissolved colloids. It has
DNA, histone and pentoses that are not found in the cytoplasm.
(iii) The crystalloids are monosaccharides including glucose, ribose
and deoxyribose; amino acids, organic acids, nucleotides and
+
mineral ions like phosphates and K ions.
(iv) The colloids are DNA, RNA and proteins particularly histone
that mix with DNA forming chromatin.
(v) Chromatin is made of DNA and histone protein. Eight molecules
of histone, wound by DNA strand, form a nucleosome unit.
Chromatin is divided into two types, the euchromatin and the
heterochromatin.
(vi) Euchromatin composes of more DNA that is less wound on
histone protein. It is more lightly stained. It contains more
genes that are active and is found in the centre of the nucleus.
(vii) Heterochromatin found in the periphery of the nucleus
composes of DNA that is more wound with histone forming
more nucleosomes. The genes in it are not active.
(viii) The chromatin is easily stained with acidic eosin to form a purple
colour. This makes nucleoplasm different from cytoplasm.
(ix) It is common to find foreign structures that are not supposed
to be there. Such structures include mitochondria and parts of
the endoplasmic reticulum.
(x) Nucleoplasm performs various functions. It contains various
enzymes for metabolism including that for glycolysis, Krebs
cycle, phosphorylation and the synthesis of NAD, replication
and transcription of DNA.
(c) Nucleolus
(i) Nucleolus is a spherical structure that is the site for ribosome
synthesis in the interphase of nucleus.
(ii) Its location is not fixed, it is usually found in the centre or on
one side of the nucleus.

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