Molecular model of the human nuclear pore complex (NPC), which contains at least 456 individual protein molecules and is composed of 34 distinct nucleoporin proteins. NPCs are complexes of proteins that are embedded in the nuclear envelope. All material moving between the nucleus and the cell cytoplasm passes through these channels. They allow passive transport (diffusion) of ions and small molecules and active transport (energy dependent) of proteins and RNAs (ribonucleic acids).