Nucleocytoplasmic transport. Illustration showing the movement of substances between the cell nucleus (upper centre) and the cytoplasm. Pores (ovals) in the nuclear membrane allow passive transport (diffusion) of ions and small molecules and active transport (energy dependent) of hormones, proteins and RNAs (ribonucleic acids). Ribosomes (blue ovals), which are assembled in the nucleolus inside the nucleus, are transported to the cytoplasm where they perform protein synthesis. Protein synthesis is the production of a protein (red string) from an mRNA (messenger RNA, green string) template.