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B2.2 Organelles and compartmentalization

Organelles and compartmentalization explain how eukaryotic cells divide work among specialized structures, controlled internal spaces, and directed vesicle transport routes.

Syllabus
First assessment 2025
Topic
B2.2
Level
SL

Learning objectives

B2.2.1Organelles as discrete subunits• Organelles are discrete cell subunits adapted for specific functions• Nuclei, chloroplasts, mitochondria, vesicles, ribosomes, and plasma membrane are organelles• Cell wall, cytoskeleton, and cytoplasm are not usually considered organellesB2.2.2Nucleus-cytoplasm separation• The nuclear envelope protects DNA and separates transcription from translation• Introns can be removed from pre-mRNA by RNA splicing before translation• Nuclear pores regulate mRNA export and protein import using selective transportB2.2.3Advantages of compartmentalization• Compartmentalization concentrates enzymes, substrates, and suitable pH for specific reactions• It separates incompatible biochemical processes so they can be controlled• Lysosomes and phagolysosomes contain acidic hydrolytic digestion away from cytoplasm

Compartments Turn One Cell into Many Workspaces

An organelle is a distinct cellular subunit adapted for a particular function. Membranes create local conditions, concentrate enzymes and substrates, and isolate incompatible reactions.

Cutaway diagram of a eukaryotic cell showing the nucleus, mitochondrion, rough endoplasmic reticulum, Golgi apparatus and lysosome as distinct compartments.

boundary → controlled composition → efficient reaction conditions; vesicles and transport proteins then connect the separate spaces into one coordinated cell

The Nucleus Separates RNA Processing from Translation

The nuclear envelope keeps DNA and transcription in the nucleus while translation occurs on cytoplasmic ribosomes. This separation permits pre-mRNA processing before mature mRNA reaches a ribosome.

The figure shows a polysome.

DNA transcription → 5′ cap, intron removal and poly-A tail → mature mRNA export through a nuclear pore → several ribosomes can translate it as a polysome

SL Summary: Explain the Advantage, Not Just the Organelle

  • organelle membrane → selected molecules and conditions → faster, regulated pathways
  • nucleus separated from cytoplasm → RNA processing before translation → controlled gene expression
  • distinct cytoplasmic spaces → incompatible reactions remain apart → simultaneous specialized work

For any compartment: name its boundary → state what differs across it → connect that difference to a function.

Organelles as discrete subunits

7 marks

Describe the organelles and other structures in animal cells that are visible in electron micrographs.

Advantages of compartmentalization

4 marks

Discuss the use of membranes for compartmentalization in eukaryotic cells.