Nucleus: Nuclear Envelope, Nuclear Pores and Their Functions
Biology · Cell: The Unit of Life · NEET
The nuclear envelope is a double membrane (two parallel membranes) that wraps around the nucleus. Between the two membranes is a gap of 10 to 50 nm called the perinuclear space. The envelope has tiny holes called nuclear pores, and these pores let RNA and protein molecules move in BOTH directions between the nucleus and the cytoplasm. Memory hook: "Two walls, one gap, many gates" — two membranes, one perinuclear space, many pores (gates) that open both ways.
The nucleus is bounded by a nuclear envelope of two parallel membranes separated by the 10-50 nm perinuclear space. Nuclear pores (formed by fusion of the two membranes) let RNA and proteins pass both ways; the outer membrane is continuous with the ER and bears ribosomes.
Your doubts, answered
Is the nuclear envelope one membrane or a double membrane?
It is a DOUBLE membrane. NCERT says the nuclear envelope 'consists of two parallel membranes'. So the nucleus is a double-membrane-bound structure, like mitochondria and chloroplasts. This is why in a NEET question asking for a SINGLE-membrane organelle, the nucleus is NOT the answer (the answer there is the lysosome).
What exactly is the perinuclear space?
The perinuclear space is the gap BETWEEN the two membranes of the nuclear envelope. Its width is 10 to 50 nm. NCERT states this space, along with the envelope, 'forms a barrier between the materials present inside the nucleus and that of the cytoplasm.' So it keeps the nuclear contents separated from the cytoplasm.
Do nuclear pores allow movement in one direction or both directions?
BOTH directions. This is the most common trap. Nuclear pores are passages through which RNA and protein molecules move both INTO and OUT of the nucleus. So it is two-way (nucleus to cytoplasm AND cytoplasm to nucleus), not one-way.
What passes through nuclear pores — only RNA, or protein too?
Both RNA and protein molecules pass through nuclear pores. NCERT: 'These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions.' Do not pick 'only RNA' or 'only protein' — the correct answer always includes both.
How are the nuclear pores formed?
Nuclear pores are formed by the FUSION of the two membranes of the nuclear envelope. At many places the inner and outer membranes join together, creating a minute pore. So a pore is literally where the two membranes meet and open up.
Is the outer nuclear membrane connected to anything?
Yes. The OUTER membrane of the nuclear envelope usually stays continuous with the endoplasmic reticulum (ER) and also bears ribosomes on it. This link to the ER is a favourite NEET detail, so remember: outer nuclear membrane = continuous with ER + carries ribosomes.
Can a cell survive without a nucleus?
Some MATURE cells lack a nucleus. NCERT examples are erythrocytes (red blood cells) of many mammals and sieve tube cells of vascular plants. These are enucleate at maturity. That is why the statement 'mature sieve tube elements possess a conspicuous nucleus' is WRONG in NEET.
⚠️ The NEET trap ✗ Nuclear pores allow movement of only RNA, and only from the nucleus out to the cytoplasm (one direction). ✓ Nuclear pores allow BOTH RNA and protein molecules to move in BOTH directions between the nucleus and the cytoplasm. 🧠 NEET loves swapping 'both directions' for 'one direction' and dropping 'protein'. If an option limits the pore to one-way or to RNA only, it is the wrong statement.
Real NEET questions
NEET 2019 (Odisha) / NEET 2022
Which of the following is an incorrect statement?
A · The perinuclear space forms a barrier between the materials present inside the nucleus and that of the cytoplasm
B · Nuclear pores act as passages for proteins and RNA molecules in both directions between nucleus and cytoplasm
C · Mature sieve tube elements possess a conspicuous nucleus and usual cytoplasmic organelles ✓
D · Microbodies are present both in plant and animal cells
Solution: Statement C is INCORRECT. Mature sieve tube elements are enucleate at maturity — NCERT says some mature cells lack a nucleus, e.g. sieve tube cells of vascular plants (and mammalian RBCs). Their activity is controlled by the neighbouring companion cell. The other three are correct: the perinuclear space and envelope form a barrier (A), nuclear pores allow two-way movement of RNA and protein (B), and microbodies occur in both plant and animal cells (D). So the incorrect statement is C.
Solved Cell: The Unit of Life NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The nuclear envelope has two parallel membranes, so the nucleus is double-membrane-bound, along with mitochondria and chloroplasts.
What is the width of the perinuclear space?
The perinuclear space between the two nuclear membranes is about 10 to 50 nm wide (NCERT value).
What moves through nuclear pores?
RNA molecules and protein molecules move through nuclear pores, and they move in both directions between the nucleus and the cytoplasm.
Why does the outer nuclear membrane have ribosomes?
Because the outer membrane is continuous with the rough endoplasmic reticulum, which carries ribosomes; so the outer nuclear membrane also bears ribosomes.
Who first described the nucleus?
Robert Brown first described the nucleus in 1831. Later, Flemming named the stained material inside it 'chromatin'.
