how do proteins move across a cell membrane
Proteins are too large to move across the phospholipid bilayer of the cell membrane so therefore require help getting into (or out of) the cell. Proteins may be brought across the cell membrane by endocytosis - they may then be broken down into amino acid building blocks (by fusion of the endocytotic vesicles with the proteolytic enzyme containing lysosomes) which can then be used by the cell or moved out of the cell by transmembrane transport proteins.