During intracellular proteins synthesis, all proteins are made by free ribosomes in the cytoplasm and some, but not all ribosomes (those which make membrane or secretory proteins) move to the endoplasmic reticulum (ER) and attach to it during translation (called co-translational transport).
Why are proteins that will be exported to the outer of cell are not made in the ER directly?
What is the function of ER in intracellular protein synthesis?
What is the purpose of co-translational transport?
The secreted proteins are not made in the ER because ribosomes cannot get into the ER. Even individual ribosomal subunits are much larger (at least 10-fold greater in volume, if not hundreds of times greater) than a typical protein, so it is much easier to translocate the protein being produced by the ribosomes across the ER membrane than to translocate the ribosomes themselves. Even the protein being produced must be at least partially unfolded as it is being translocated.
That's not even taking into account that the mRNAs would need to be translocated as well if the ribosomes were to translate them while in the ER. The mRNA is again many times the mass of the protein (a nucleotide is ~3-4 amino acids "worth" of mass, and there are at least 3 nucleotides per amino acid encoded, more if you add the untranslated regions).
Why are proteins that will be exported to the outer of cell are not made in the ER directly? What is the function of ER in intracellular protein synthesis? What is the purpose of co-translational transport?
This is a eukaryotic cell that we are talking about.Once the mRNA is synthesized , it has to move out from the nucleus to the cytoplasm.During its exit, SRP (Signal Recognition Particle) recognizes specific signal sequences in the mRNA and forms a complex with that mRNA.then SRP complex targets ER membrane proteins for cotranslational-transport.
If the secretory proteins are not moved into the ER , disulfide bonds would not be formed from cysteine residues . Because the necessary Enzymes required for this process are found only and only in the ER (namely PDI and ERO1).
Plus, these secretory proteins often have to be glycosylated to perform their action correctly.The Enzymes are located in the ER.
Moreover I believe that these secretory proteins will aggregate and degraded later if they are not transported into the ER.
I do not know WHY this process exists this way. That is Evolution and it is just about adaptation to the environment and not necessarily based on pure logical reasons.