Explain how the structure of the nuclear envelope enables selective communication between the nucleus and the cytoplasm.

IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The nuclear envelope is a double‑phospholipid bilayer that separates the nucleoplasm from the cytoplasm. Nuclear pore complexes (NPCs) span both the inner and outer membranes and act as selective channels, allowing regulated passage of molecules. Large molecules such as mRNA and ribosomal subunits exit the nucleus through the NPCs, while proteins such as transcription factors and RNA polymerase are imported into the nucleus through the same pores. The outer membrane is continuous with the rough endoplasmic reticulum, linking nuclear activity to protein synthesis and processing in the cytoplasm.

Examiner tips

  • Use the term ‘double‑phospholipid bilayer’ for the envelope; mention the inner and outer membranes. State that NPCs are selective channels and give examples of import and export. Note the rER continuity to show functional linkage. Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Forgetting to mention that the envelope is double‑membranous. Mixing up which molecules are imported versus exported. Omitting the link to the rough ER.

Mark scheme (4 marks)

  1. The nuclear envelope consists of two phospholipid bilayer membranes (inner and outer), which form a barrier separating the nucleoplasm from the cytoplasm.
  2. Nuclear pore complexes span both membranes and act as selective channels, allowing regulated passage of molecules between nucleus and cytoplasm.
  3. Large molecules such as mRNA and ribosomal subunits exit through nuclear pores, while proteins such as transcription factors and RNA polymerase are imported into the nucleus through the same pores.
  4. The outer membrane of the nuclear envelope is continuous with the rough endoplasmic reticulum (rER), linking nuclear activity to protein synthesis and processing in the cytoplasm.

Key terms in this question

nuclear envelope

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