storage of calcium within the cisternal space and regulated release into A typical cell contains millions of ribosomes. They may form groups
the cytoplasm. In general, RER is well developed in cells that produce (polyribosomes or polysomes) attached to messenger RNA (mRNA),
which they translate during protein synthesis for use outside the system
of membrane compartments, e.g. enzymes of the cytosol and cytoskel-
etal proteins. Some of the cytosolic products include proteins that can
be inserted directly into (or through) membranes of selected organelles,
such as mitochondria and peroxisomes. Ribosomes may be attached to
the membranes of the rough endoplasmic reticulum (see Fig. 1.5E).
In a mature polyribosome, all the attachment sites of the mRNA are
occupied as ribosomes move along it, synthesizing protein according
to its nucleotide sequence. Consequently, the number and spacing of
ribosomes in a polyribosome indicate the length of the mRNA mole-
cule and hence the size of the protein being made. The two subunits
have separate roles in protein synthesis. The 40S subunit is the site of