It lets incompatible reactions run at once, concentrates enzymes and substrates for efficiency, and contains harmful substances (e.g. lysosomal enzymes). Learn it step by step →
This is the Higher Level lesson. The core statements B2.2.1–B2.2.3 (organelles, and the advantages of compartmentalization) are shared with SL; the HL-only statements B2.2.4–B2.2.9 — the detailed structural adaptations of organelles and the secretory pathway — carry a purple HL badge.
Core content — SL & HL
HL Additional Higher Level only
👆 Click the organelles in each 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.
3. How each organelle is built for its job HL
At HL you need the specific structural adaptations. Click the labelled features on the mitochondrion and chloroplast.
Mitochondrion · B2.2.4
Click: cristae · matrix
Chloroplast · B2.2.5
Click: thylakoids/grana · stroma
Click a feature on either organelleEach adaptation increases surface area or concentrates enzymes to make its reaction efficient.
Nuclear double membrane B2.2.6The nuclear envelope is two membranes with pores that control what enters/leaves; it can break down and reform during division and encloses the DNA.
Free ribosomes vs rough ER B2.2.7Free ribosomes make proteins used inside the cell (cytoplasm); ribosomes on the rough ER make proteins for secretion, membranes or lysosomes.
4. The secretory pathway HL
A protein destined for secretion is passed along a production line of organelles. Press play to follow one from ribosome to release. B2.2.7–B2.2.9
Ribosome on RER makes the protein → vesicle buds off → carries it to the Golgi → Golgi modifies & packages it → secretory vesicle → fuses with the plasma membrane (exocytosis). Vesicles form and fuse because the membrane is fluid.
L
Learn Organelles & Compartmentalization step by step
Free interactive mini-lessons: short screens, worked examples and quick checks. Pick your exam board to open its course map, then tap L on this topic:
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).
What do ribosomes on the rough ER make, versus free ribosomes?✗ They make exactly the same proteins.✓ RER-bound ribosomes make proteins for secretion, membranes or lysosomes; free ribosomes make proteins for use inside the cytoplasm. HL
Why do mitochondria have cristae?✗ To store DNA.✓ The folded inner membrane (cristae) increases surface area for the electron transport chain and ATP synthase, so more ATP is made. HL
Why can eukaryotes modify mRNA but prokaryotes struggle to?✗ Prokaryotes have no mRNA.✓ The nuclear envelope separates transcription (inside) from translation (cytoplasm), giving time and space to process mRNA before it is translated. In prokaryotes the two are coupled.
