Which organelle forms a boundary around the nucleus and controls movement of substances in and out?

• A. Nuclear Membrane
• B. Chromosomes (DNA)
• C. Mitochondria
• D. Centrioles

Answer: (A)

The boundary around the nucleus and the control of what moves in and out is provided by the nuclear membrane, also called the nuclear envelope. This double membrane surrounds the nucleus and has nuclear pores that selectively regulate transport, letting RNA and ribosomal subunits exit to the cytoplasm while allowing needed proteins and signaling molecules to enter. This setup protects the genetic material inside the nucleus while keeping communication with the rest of the cell. Other organelles do not serve this boundary function: chromosomes are the genetic material inside the nucleus, mitochondria generate energy (ATP), and centrioles help organize spindle fibers during cell division.