for National Geographic News. A massive new study of human genetic diversity reveals surprising insights into our species' evolution and migrationsâincluding support for the theory that the first modern humans originated in Africaâscientists said today.or National Geographic News. A massive new study of human genetic diversity reveals surprising insights into our species' evolution and migrationsâincluding support for the theory that the first modern humans originated in Africaâscientists said today.
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.
Other ions or molecules are also carried across the cell membrane by carrier proteins. The ion or molecule binds to the active site of a carrier protein. The carrier protein changes shape, and releases the ion or molecule on the other side of the membrane. The carrier protein then returns to its original shape.acilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane. Facilitated diffusion is a type of passive transport. Even though facilitated diffusion involves transport proteins, it is still passive transport because the solute is moving down the concentration gradient.