5. Albert Claude discovered the endoplasmic reticulum
and published their research, in 1945. It was a very
important discovery, as this cell organelle plays a major
role in the functioning of every eukaryotic cell.
According to Dallmer (1966), Endoplasmic Reticulum is
originated from the plasma membrane by the
investigation. According to Derobertis (1970), the ER
originates from the nuclear envelope. The Endoplasmic
Reticulum (ER) is a complex structure within the cell
5
Discovery of ER
6. Endoplasmic Reticulum
The first of the two words - 'endoplasmic' (endo =
inside ) means that this organelle is contained within
the cytoplasm of a eukaryotic cell. The second word
'reticulum' means that this is a type of network that
is spread within the cell cytoplasm.
The whole endoplasmic reticulum is a continuously
connected network of tubules, cisterns, and vesicles
that is spread throughout the cytoplasm and is
covered with a special membrane.
6
8. Types
There are two types of endoplasmic reticulum. One
is smooth and the other is rough and both of these
types are interconvertible. The rough variety has
ribosomes attached to the surface, which are sites
for protein synthesis, while the smooth one is purely
a transport system for the ions and steroids in
particular.
8
9. Functions and Role
According to Sjostrand (1964),play a role in enzymic
reactions.
ER also plays important role in lipid metabolism and glycogen
synthesis.
Protein Synthesis and Folding occurs in the RER.
Lipid Synthesis The SER plays an important role in
cholesterol and phospholipid biosynthesis
The SER is an important site for the storage and release of
calcium in the cell
9
10. Ribosomes
Palade discovered in 1953 in
animals. Robinson and
Brown discovered ribosomes in
plant cells. Ribosomes are non-
membranous bounded
structures. Ribosomes are the
sites where the cell synthesizes
proteins according to genetic
instructions
A bacterial cell may have a few
thousand ribosomes. But a
human liver cell has millions
ribosomes.
10
ll
Fig 1
11. Types of Ribosomes
Ribosomes functions in two cytoplasmic locations.
Free Ribosomes: They are suspended in the
cytoplasm. Most proteins synthesized by the free
ribosomes functioned within the cytosol. The free
ribosomes are abundant in the cells cytoplasm
Bound Ribosomes: They are attached to the outside
of the endoplasmic reticulum. These proteins are
exported from the cell. Some cells specialize in the
protein secretion like pancreas and other glands. These
cells secrete digestive enzymes. They have a high
proportion of bound ribosomes.
11
12. Structure
Free ribosomes are structurally
identical and interchangeable. The
cells can adjust the numbers of
ribosomes.
Proteins 25-40 per
RNA 35-60 per
Two subunits
Large subunit
small subunit
3 sites
E P A
12
Fig 2
14. Functions of Ribosomes
The ribosome plays a key role in translating the genetic
message to form Proteins
The ribosomes of prokaryotes are smaller than
eukaryotes. They are different in their molecular
composition. This dissimilarity in a structure is
fundamental. It differentiates the prokaryotes from
eukaryotes. The difference in ribosomes is also
medically significant.
Certain drugs (antibiotics) can effect the prokaryotic
ribosomes. But they do not affect the eukaryotic
ribosomes.
14
15. ER
Rough ER
Smooth ER
The smooth endoplasmic
reticulum is 'smooth'
because it has no attached
ribosomes.
So technically, the RER and
Ribosomes both make
proteins.
Ribosome
Free ribosomes
Bounded ribosomes
the Ribosomes are
attached on the
surface of the RER,
making it appear
rough
15
