Nuclear envelope and nuclear pore complexes
The nuclear envelope is a double membrane around the nucleus that contains nuclear pore complexes, channels built from many proteins that regulate what enters and exits.
Remember
Think of the lipid bilayers as the wall and the nuclear pore complexes as the gates: large molecules cannot diffuse across the wall, so proteins and RNA go in and out by the regulated route through the pores.
The nucleus is wrapped in the nuclear envelope, a double-layered membrane. Each membrane is a lipid bilayer, a fatty film made of two layers of phospholipid molecules, and a thin fluid space lies between the inner nuclear membrane and the outer nuclear membrane.
A lipid bilayer stops large molecules, so the envelope contains nuclear pore complexes. These are assemblies of many different proteins that span both membranes and form selective channels.
Small molecules can diffuse through passively, but macromolecules (very large molecules, such as proteins and RNA) are escorted through the channel by transport proteins, in a controlled way.
Traffic runs in both directions. Messenger RNA, the copy of a gene made from DNA in the nucleus, travels out to the cytoplasm, and proteins that act on DNA travel in. Ribosomes in the cytoplasm build proteins from that RNA, so the RNA has to leave through the pores.
The outer membrane of the envelope is continuous with the rough endoplasmic reticulum (the ribosome-carrying part of the network of membranes that extends through the cytoplasm), so ribosomes can sit on its outer face.
Key facts
- Envelope
- a double-layered membrane around the nucleus, made of an inner and an outer lipid bilayer with a thin fluid space between them.
- Pores
- nuclear pore complexes span both membranes and form selective channels, regulating the passage of proteins, RNA and other molecules.
- Small versus large
- small molecules can diffuse through, but larger molecules such as proteins and RNA are moved across with the help of transport proteins.
- Direction
- traffic runs in both directions, with messenger RNA going out and proteins that act on DNA coming in.
- Outer membrane
- continuous with the rough endoplasmic reticulum, so ribosomes can sit on its outer face.
Exam traps
- The outer nuclear membrane does not regulate macromolecular traffic, because it is a barrier and not a gated channel.
- The nuclear lamina is a meshwork of protein filaments under the inner membrane. It gives the nucleus mechanical strength but forms no gateway, so it does not carry proteins or RNA across.
- The perinuclear space is the narrow fluid-filled gap between the two membranes of the envelope, a space rather than a channel, so macromolecules do not travel through it to pass between the nucleus and the cytoplasm.
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