Year 12s: what will be the first way you research your future options? >>

if ribomes are attached to the outer surface of rough er, how it transfer protein int

..into the lumen of the rough ER?

Ribosomes can be associated with the ER or be free in the cytosol. Regardless, proteins destined for secretion will be fed into the lumen of the ER as they are synthesised.

Proteins to be secreted contain signal peptides. Signal peptides are a series of hydrophobic residues that are typically at the N-terminal end of the polypeptide and, hence, are the first part of the polypeptide exposed to the cytosol as the protein is being synthesised in the ribosome. When exposed, signal peptides are bound by signal recognition particles (SRPs). SRPs bring the ribosome-polypeptide complex close to the ER (or if its a ribosome embedded on the ER then it orientates the synthesis of the polypeptide to the ER) by binding to ER TRANSLOCONS, which are channels in the ER membrane through which the growing polypeptide can enter the ER lumen.

For example, during the insulin biosynthesis pathway, the SRP translocates the polypeptide-ribosome complex to the ER by binding to the sec61 translocon (famous example).

Once the ribosome is orientated with the translocon, translation resumes and the polypeptide grows to completion and enters the ER lumen for further processing. Typically (but not always) enzymes associated with the translocons cleave the signal peptides as the polypeptide enters the ER; as they are usually functionally redundant afterwards.