The first part of the protein sequence has a single peptide which is specific for binding to the ER channel. A signal peptidase later removes the signal peptide. Transmembrane proteins destined to be incorporated into membranes are synthesized like proteins destined for the lumen of the ER except that the protein incorporates a hydrophobic sequence or sequences which anchor the protein in the membrane. Some proteins are moved between the cytosol and the ER, vesicles, and the Golgi apparatus by clathrin coated vesicles. Some proteins are covalently modified, often by addition of polysaccharide changes in the ER and in the Golgi.
Lysosomes are vesicles bound by a single membrane.
They appear in a tremendous range of sizes, ranging in diameter from 50 nm, the size of the smallest mycoplasmal cell to as much as 0.5 micrometers. Lysosomes are manufactured by the rough endoplasmic reticulum and then fuse with injested material or cell components which are to be broken down and recycled. The lysosome contains enzymes to break down just about everything. The necessity of having acid conditions protects the cell interior in case of a leak. These are endocytosed into cells lining the lungs and the vesicles fused with lysosomes.