Organelles and compartmentalization — IB Diploma Biology SL
IB Biology B2.2: organelles, cell fractionation, and the advantages of compartmentalization in the nucleus and lysosomes.
Revision notes
1 short note, in the order of the specification. Each one in short:
Organelles are discrete subunits of cells that are adapted to perform specific functions, such as nuclei, ribosomes, vesicles and the plasma membrane. Compartmentalization brings advantages: the nucleus separates transcription from translation so mRNA can be modified first, and lysosomes concentrate digestive enzymes while separating digestion from the rest of the cytoplasm.
Exam questions
4 exam-style questions (12 marks), each with its mark scheme.
Answer the questionsFlashcards
5 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells (form and function) on one page, so you can see where this subtopic fits.
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Quick check questions
Name two structures that are not considered organelles.
Show answer
Any two of: the cell wall, the cytoskeleton, the cytoplasm.
In cell fractionation, which organelles form a pellet first, at the lowest speed?
Show answer
Nuclei.
Why must the hydrolytic enzymes of lysosomes be kept in a compartment?
Show answer
So they do not digest the cell's own molecules, and so they can be concentrated at their acid optimum pH.
Frequently asked questions
Is a ribosome an organelle?
Yes. In IB Biology an organelle is a discrete subunit of a cell adapted to perform a specific function, so ribosomes count even though they have no membrane. Nuclei, vesicles and the plasma membrane are organelles too, but the cell wall, cytoskeleton and cytoplasm are not considered organelles.
What are the advantages of compartmentalization in cells?
Compartments let a cell concentrate enzymes and their substrates in a small volume, so reactions run faster, and keep incompatible processes apart. For example, lysosomes hold hydrolytic enzymes that would digest the cell if free, and the nuclear membrane lets mRNA be modified before it meets ribosomes.
How does cell fractionation separate organelles?
Cell fractionation breaks cells open in a cold, isotonic, buffered solution and then spins the homogenate in a centrifuge at increasing speeds. Large, dense organelles such as nuclei form a pellet first; mitochondria pellet at higher speeds, and the smallest parts, such as ribosomes, need an ultracentrifuge. Each fraction can then be studied.
Written and checked against the IB Biology SL specification · Updated October 2026