Organelles are specialized cell subunits with recognizable structures and functions, allowing eukaryotic cells to organize metabolism, transport, and genetic control.
Coverage 2011–2025 · Updated 15 Jul 2026
An organelle is a discrete cell subunit adapted for a specific function. In this syllabus, nuclei, chloroplasts, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, ribosomes and the plasma membrane count as organelles; cell wall, cytoskeleton and cytoplasm do not.
Compartmentalization concentrates enzymes, substrates and metabolites, maintains conditions such as a suitable pH, and separates biochemical processes that would interfere with one another.
Lysosomes keep acidic hydrolytic enzymes separated from cytoplasm. A phagocytic vacuole can fuse with a lysosome so engulfed material is digested inside a controlled compartment rather than throughout the cell.
Cell fractionation followed by ultracentrifugation separates organelles by physical properties, allowing their structures and functions to be investigated independently.
A useful cell structure is not automatically an organelle under the IB convention. State the compartment, the concentrated or separated process, and the functional advantage.
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.
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.
