Ribosomes
Ribosomes -
The
electron
microscope
revealed
the
presence
of
dense
granules
150
to
200A
in
diameter
(Palade,
1955),
later
called
ribosomes.
These
granules
may
be
found
either
in
the
free state
(e.g.
in
the
cells
of
the
basal
epidermal
layer),
or
attached
to
the
outside
of
cytoplasmic
membranes
like
the
endoplasmic
reticulum.
In
the
latter
case
the
membranes
are
called
'rough
surfaced
membranes'
(Palade
and
Porter,
1954),
or
ergastoplasm
or
alpha
cytomembranes
(Sjostrand,
1956,
1962).
The
membranes
without
ribosomes
are
called
'smooth
profiles'.
Mature
mammalian
erythrocytes
have
no
ribosomes.
Ribosomes
are
of
two
basic
types,
70S
and
80S
ribosomes.
The
'S'
refers
to
Svedberg
units.
This
is
a
sedimentation
coefficient
which
shows
how
fast
a cell organelle
sediments
in
an
ultracentrifuge.
The
heavier
a
structure
the
more
is
its
sedimentation
coefficient.
Sedimentation
coefficients
are not additive.
Thus
the
30S
and
50S
subunits
of
a
ribosome
together
make
up
a
ribosome
with
a
sedimentation
coefficient
of
70S
and
not
80S.
80S
ribosomes
are
found
in
eukaryotes
(organisms
whose
cells
have
true
nuclei
bounded
by
nuclear
envelopes),
e.
g.
algae,
fungi,
higher
plants
and
animals.
The
80S
ribosome
of
animals
consists
of
a
large
60S
subunit
and
a
small
40S
subunit. 70S
ribosomes
are
relatively
smaller
and
are
found
in
prokaryotes
bacteria
e.g
.bacteria.
The
70S
ribosome
consists
of
a
large
50S
subunit
and
a
small
as
subunit.
Ribosomes
found
in
mitochondria
and
chloroplasts
of
eukaryotes
are
closer
to
prokaryote
ribosomes
rather
than
the
80S
eukaryote
ribosomes.
Vertebrate
mitochondria
for
example,
contain
55S
ribosomes,
each
with
a
large
40S
subunit
and
a
small
30S
unit.
The
sedimentation
coefficients
80S,70S
and
55D
are
rounded
off
values.
Actual
S
values
in
different
organisms
may
be
slightly
higher
or
lower.
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