Page 26 - Biology_F5

Cytology

ribosomes bound to it at any one time reticulum once a specific protein-nucleic
are not a stable part of this organelle’s acid complex forms in the cytosol. This
structure, as they are constantly being special complex forms when a free
bound and released from the membrane. ribosome begins to translate the mRNA.
A ribosome only binds to the endoplasmic
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Ribosomes Cisternae

Cisternae

(a) (b)
Figure 1.10: The structure of endoplasmic reticulum (a) RER (b) SER

Functions of rough endoplasmic endoplasmic reticula are generally found in
reticulum adipose cells, interstitial cells, and glycogen
The rough endoplasmic reticulum has the storing cells of the liver. They also occur in
following functions: conduction fibres of the heart, spermatocytes
(a) They are sites for protein synthesis, due and leucocytes. The muscle cells are also
to the presence of ribosomes.
(b) They provide pathways for transportation rich in smooth endoplasmic reticulum known
as the sarcoplasmic reticulum.
of materials through the cell, such as
proteins synthesised on the ribosomes. Functions of smooth endoplasmic
(c) They provide a large surface area for reticulum
chemical reactions to take place. (a) It is essential for synthesis, secretion,
and storage of lipids, carbohydrate and
Smooth endoplasmic reticulum (SER) other non- protein products.
This type of endoplasmic reticulum possesses (b) It contains enzymes that breakdown
smooth walls, because the ribosomes are not chemical substances in liver cells.
attached to its membranes (Figure 1.10b). (c) It plays a fundamental role in the
The smooth endoplasmic reticulum occurs synthesis of steroids, which later form
hormones.
mostly in those cells which are involved in (d) It stores calcium ions in its lumen and
metabolism of glycogen and lipids, such as releases them into the cytoplasm when
phospholipids and cholesterol. The smooth needed.

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