The biological nitrogen fixation is carried out by some bacteria, cyanobacteria and symbiotic bacteria. In symbiotic association, the bacterium provides fixed nitrogen (NH3) to the host and derives carbohydrates and other nutrients from the latter.
Biological nitrogen fixation occurs in the presence of the enzyme nitrogenase which is found inside the nitrogen fixing prokaryote. In addition to this enzyme, a source of reducing equivalents (ferredoxin (Fd) or flavodoxin in vivo), ATP and protons are required.
Things enter the cell through the cell membrane through either active or passive transport which can tell you that the cell membrane controls what goes in and out.
