Page 22 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

6. Nucleus has a diameter of 10 – 20 μm and it occupies 75% of the cell
volume in the meristem.

7. The nucleus can be divided into
(a) Nuclear envelope
(b) Nucleoplasm
(c) Nucleolus
(d) Chromosome 2

(a) Nuclear envelope
(i) Nuclear envelope is the double lipoprotein membrane that
surrounds the nucleus.
(ii) The outer membrane is smooth, may have ribosomes attached
to it and is possibly continuous with the membrane of the
endoplasmic reticulum. Sometimes, it may be continuous right
to the plasma membrane.

Ribosomes
Smooth endoplasmic reticulum

Exocytosis

Nuclear pole
Golgi apparatus
Chromatin

Nucleolus Rough endoplasmic reticulum

Figure 2.12 Relationship of outer nuclear membrane with other membranes
(iii) The inner membrane is smooth; no ribosome is attached to it
and it is not folded.
(iv) This nuclear envelope disappears at prophase of cell division and
reappears at the end of telophase. Therefore, the components of
the membrane can be hydrated and reorganised into its original
state.
(v) There are nuclear pores in the envelope. The pores are relatively
big, 40 – 150 nm and cover a surface of 8% of the envelope.
However, the passage of substances is very well controlled.
The bigger pores are specifically for the transport of RNA and
ribosome subunits from the nucleus to the cytoplasm. Steroid
hormones may enter the nucleus through the phospholipid
layers by diffusion.
(vi) There is a perinuclear space of about 10 – 40 nm wide between
the outer and inner membrane of the envelope.

67

02[STPM Bio T1].indd 67 3/29/18 5:08 PM