glycogen and pigments) and the cytoskeleton. The nuclear contents, Ribosomes, polyribosomes
the nucleoplasm, are separated from the cytoplasm by the nuclear
and protein synthesis
envelope.
Ribosomes are macromolecular machines that catalyse the synthesis of
Endoplasmic reticulum proteins from amino acids; synthesis and assembly into subunits takes
The endoplasmic reticulum is a system of interconnecting membrane- place in the nucleolus and includes the association of ribosomal RNA
lined channels within the cytoplasm (Fig. 1.4). These channels take (rRNA) with ribosomal proteins translocated from their site of synthesis
various forms, including cisternae (flattened sacs), tubules and vesicles. in the cytoplasm. The individual subunits are then transported into the
The membranes divide the cytoplasm into two major compartments. cytoplasm, where they remain separate from each other when not
The intramembranous compartment, or cisternal space, is where secre- actively synthesizing proteins. Ribosomes are granules approximately
tory products are stored or transported to the Golgi complex and cell 25 nm in diameter, composed of rRNA molecules and proteins assem-