Nuclear pore complexes. Coloured atomic force micrograph (AFM) of the surface of an oocyte (egg cell) nucleus showing the nuclear pore complexes (NPCs). 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).