Organelles and compartmentalization explain how eukaryotic cells divide work among specialized structures, controlled internal spaces, and directed vesicle transport routes.
An organelle is a discrete subunit of a cell adapted to perform a specific function. In the IB Biology scope, nuclei, chloroplasts, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, ribosomes and the plasma membrane are treated as organelles; the cell wall, cytoskeleton and cytoplasm are not. Compartmentalization allows enzymes and substrates to be concentrated, suitable conditions to be maintained and incompatible reactions to be separated. Cell fractionation using ultracentrifugation made it possible to study organelles individually.
This objective is assessed through structured response, commonly using Identify / State / Describe.
Identify / State / Describe
Build the answer around this relationship: Organelles are discrete cell subunits adapted for specific functions.
Giving an unqualified name such as Golgi when the evidence requires Golgi apparatus, complex, or body.
Representative question
Describe the organelles and other structures in animal cells that are visible in electron micrographs.
The nucleus keeps DNA and transcription separate from cytoplasmic translation, allowing RNA to be processed before it reaches ribosomes.
DNA remains protected while transcription occurs inside the nuclear compartment. Messenger RNA can then be modified and exported through pores, adding control between making RNA and making protein.
Trace the route: DNA transcription in nucleus; RNA processing and export; translation on cytoplasmic ribosomes.
A newly transcribed mRNA is processed in the nucleus before a ribosome translates it in the cytoplasm.
The nucleus does not make protein itself; it separates transcription from translation.
This objective is assessed through essay response, commonly using Discuss.
Discuss
Build the answer around this relationship: Internal membranes divide eukaryotic cells into functional compartments.
Representative question
Discuss the use of membranes for compartmentalization in eukaryotic cells.
A strong answer does not say only “organelles make cells efficient.” It gives a concrete reason: compartments concentrate enzymes and substrates, maintain suitable pH, separate incompatible reactions, and let the nucleus process RNA before translation.