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.
The two carrier proteins on the left are using ATP to move sodium out of the cell against the concentration gradient. The proteins on the right are using secondary active transport to move potassium into the cell. Active transport is the movement of a substance across a membrane against its concentration gradient. This is usually to accumulate high concentrations of molecules that a cell needs, such as glucose or amino acids.