Page 27 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

Exam Tips (xi) The chromosomes perform two major functions as follows:
• Chromosomes that contain genes, control the production of
Remember that organisation
of chromosomes includes RNA and proteins in cells. Through these proteins, especially the
not only structural enzymes, chromosomes control all the activities of the cell and
organisation but also that inheritable characters of an organism.
during interphase, their
organisation into pairs and • The compact chromosomes formed during metaphase enables
2 sex chromosomes. mitosis and meiosis to take place. These chromosomes can
move easily compared to the untidy long slender DNA. Hence,
chromosomes enable genes to be passed down from one mother cell
to daughter cells and thus, one generation to the next generation.

Endoplasmic reticulum (ER)
1. ER is a network of flattened sacs and tubules that interconnect to
form a complex structure in the cytoplasm for internal transport of
substances.
2. Each flattened sac or tubule is called cisterna. These interconnecting
cisternae form the basic units of function for ER.
3. The membrane of the ER is the typical lipoprotein type. The
membrane is not folded and the proteins on both sides are of
different types.
4. The content of the cisternae is a sol called matrix. The matrix varies
in content between different cells and contains a mixture of proteins.
5. The outside of the cisternae form a complex network of inter-
cisternal space. Its composition is the same as the cytoplasm but with
microfilaments attached on its outer membrane to maintain the ER’s
shape.
6. The membrane of ER may connect to the outer membrane of the
nucleus, which may continue to expand to form more ER membrane.
The ER itself will bud off to form the Golgi apparatus. Certain parts
of the ER may connect to the plasma membrane through the tubules.
7. The size of ER depends on the type of cell. In glandular cells and
liver cells, the ER is very big and complex.
8. ER can be divided into two types: the rough ER and the smooth ER.
The smooth ER is formed from the rough type.
(a) Rough ER
(i) The rough ER is the type with a lot of ribosomes attached
to its outer surface. It is found in glandular cells that
produce a lot of protein for secretion, such as the glandular
or goblet cells of the digestive system including pancreas,
stomach and small intestine.
(ii) These ribosomes produce proteins for export in the
cytoplasm attach themselves on the surface of the ER. Such
proteins have signal sequence to attach to the surface of
ER. The protein formed then enters the matrix of cisterna
through special pores. The protein is later moved to the
Golgi apparatus, packed into vesicles and exported through
exocytosis.
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