So I have read a few times that
b. SP probably forms loop not arrow. Loop enters channel (translocon) in membrane. SP loop is probably what opens (gates) the channel on the cytoplasmic side.
And I was wondering why this would be the case? It seems much more complicated to me for the protein to loop around than fior the signal peptide to just embed itself in the translocator pore and the rest of the polypeptide chain be synthesised in the cytoplasmic side.
Then if the protein was meant to pass through the membrane again, there wold be another start-transfer signal sequence (so the two first signal sequences would be two start-transfer sequences rather than a start transfer and a stop transfer).