Final answer:
Protein synthesis begins on ribosomes with the formation of an initiation complex, which starts the translation process. The synthesis of proteins destined for export continues directly into the rough endoplasmic reticulum (RER).
Step-by-step explanation:
The synthesis of most proteins starts at the ribosome, where the process of translation or protein synthesis takes place. It begins with the formation of an initiation complex that involves the small ribosomal subunit, the mRNA template, initiation factors, and a special initiator tRNA, specifically tRNAMet. The initiator tRNA carries the amino acid methionine, which is the first amino acid incorporated into the forming polypeptide chain, though it is often removed after translation is complete.
For proteins that are to be exported from the cell or directed to certain organelles such as lysosomes, their elongation continues into the cisternae of the rough endoplasmic reticulum (RER). This is due to the presence of a signal peptide sequence that directs the ribosome to dock on the RER, allowing the growing polypeptide to be co-translationally inserted into the lumen of the RER.
In summary, protein synthesis consumes significant cellular energy and involves complex molecular machinery. These proteins serve as vital components and perform numerous functions within living organisms, highlighting the intricacy and importance of this biological process.