What organelles are, and why cells wall off their reactions into separate compartments.
Theme B · Form & functionLevel 2 · CellsB2.2
This is the Standard Level lesson, covering the core statements B2.2.1–B2.2.3. Taking Biology at Higher Level? The HL version adds the detailed structural adaptations of the mitochondrion, chloroplast and nucleus, and the full secretory pathway.
Core content — SL & HL
👆 Click the organelles in the diagram
1. A cell is a set of compartments SL
An organelle is a discrete subunit of a cell, each adapted to perform a specific function (B2.2.1). Click each organelle in this eukaryotic (plant) cell to see what it does.
Interactive diagram · click each organelle
Click an organelleEach one is a compartment adapted to a specific job. Green outer layer = cell wall; the orange line inside it is the plasma membrane.
Compartmentalization gives a eukaryotic cell control that a single open bag of chemicals could never have (B2.2.3):
Separate incompatible reactionsReactions needing different conditions (e.g. digestion vs synthesis) run at the same time without interfering.
Concentrate enzymes & substratesKeeping reactants together in a small compartment raises local concentration, so reactions run faster and more efficiently.
Contain damaging substancesHydrolytic enzymes are held inside the lysosome, protecting the rest of the cell from being digested.
Optimum local conditionsEach compartment can hold its own pH or ion balance suited to the reactions happening there.
Separating the nucleus from the cytoplasm B2.2.2
Because eukaryotes keep DNA inside a nucleus, transcription and translation happen in different places — so mRNA can be modified before it is translated. In prokaryotes there is no nucleus, so the two processes are coupled. Toggle to compare:
EukaryoteTranscription happens inside the nucleus; the mRNA is processed, then exported to the cytoplasm where ribosomes translate it. The nuclear envelope keeps the two steps separate.
Going to HL? The HL lesson adds the specific adaptations of the mitochondrion (cristae), chloroplast (thylakoids & grana) and nucleus (double membrane), plus free ribosomes vs rough ER and the secretory pathway. → Open the HL version
Common mistakes examiners see
What is an organelle?✗ Any small molecule inside a cell.✓ A discrete subunit of a cell, each adapted to perform a specific function (e.g. nucleus, mitochondrion).
Why is compartmentalization useful?✗ It makes the cell bigger.✓ It lets incompatible reactions run at once, concentrates enzymes and substrates for efficiency, and contains harmful substances (e.g. lysosomal enzymes).
Why does separating the nucleus from the cytoplasm matter?✗ It stops the cell making protein.✓ Transcription (nucleus) is separated from translation (cytoplasm), so mRNA can be modified before it is translated. In prokaryotes the two are coupled.
Do all cells have a large central vacuole?✗ Yes, every eukaryotic cell has one.✓ A large permanent central vacuole is a feature of plant cells; animal cells have only small, temporary vacuoles if any.