IB Biology · Theme B: Form and function · SL and HL
B2.2 Organelles and compartmentalisation
A one-page summary of B2.2 Organelles and compartmentalisation, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How are organelles in cells adapted to their functions?
What are the advantages of compartmentalization in cells?
What B2.2 covers
Cells built from separate working parts
- B2.2.1A cell is built from discrete parts
- B2.2.2Separating the nucleus buys editing time
- B2.2.3Lysosomes keep dangerous enzymes contained
- B2.2.3Phagocytic vacuoles isolate what they engulf
Structure built for a single job
- B2.2.4 · HLThe mitochondrion is built for ATP
- B2.2.5 · HLThe chloroplast mirrors the same logic
- B2.2.6 · HLNuclear pores let the nucleus stay in touch
- B2.2.7 · HLFree ribosomes serve the cytosol
- B2.2.7 · HLMembrane-bound ribosomes serve export
- B2.2.8 · HLThe Golgi finishes what ribosomes started
- B2.2.9 · HLClathrin pinches off a vesicle
B2.2 Organelles and compartmentalisation: summary
What counts as an organelle
- Yes: nucleus, vesicles, ribosomes, plasma membrane.
- No: cell wall, cytoskeleton, cytoplasm.
Nucleus/cytoplasm split
- Separates transcription from translation.
- mRNA is processed before ribosomes see it.
Cytoplasmic compartments
- Lysosome: acidic enzymes, sealed away.
- Phagocytic vacuole: engulfs, then fuses to digest.
Powerhouses HL
- Mitochondrion: cristae, small intermembrane space.
- Chloroplast: grana, small thylakoid lumen.
Nucleus & ribosomes HL
- Nuclear pores: selective, not free diffusion.
- Free vs. bound ribosomes: signal sequence decides fate.
Golgi & vesicles HL
- Golgi: modifies/sorts/packages, cis → trans.
- Clathrin pinches off vesicles for transport.
Key terms
- Organelle
- A discrete, membrane-bound subunit within a cell, specialised for one function.
- Compartmentalisation
- Organising a cell into distinct regions, each with its own local environment and enzymes.
- Cristae HL
- Folds of the inner mitochondrial membrane that increase surface area for ATP production.
- Thylakoid HL
- Membrane-bound sac in a chloroplast; stacks form grana and house the light-dependent reactions.
- Stroma HL
- The fluid-filled space around the thylakoids, containing the Calvin cycle's enzymes.
- Clathrin HL
- A protein that coats a membrane and drives vesicle budding, as in endocytosis and Golgi export.
Sample exam questions
Three of the 28 multiple-choice questions for B2.2. Try each one before opening the answer.
Question 1. Which organelle is responsible for synthesising proteins destined for secretion or for incorporation into membranes?
- Golgi apparatus, which only modifies proteins after synthesis
- Lysosomes, which break down proteins rather than synthesising them
- Smooth endoplasmic reticulum (SER), which lacks ribosomes
- Rough endoplasmic reticulum (RER), studded with ribosomes on its cytoplasmic surface
Show the answer
Answer: D. The rough ER has ribosomes bound to its cytosolic surface. Proteins synthesised by these ribosomes enter the ER lumen for folding and modification. These are typically secretory proteins, membrane proteins, or lysosomal enzymes — all destined for the endomembrane system.
Question 2. What is the primary advantage of compartmentalisation in eukaryotic cells?
- It provides structural support to prevent the cell from collapsing under its own weight
- It allows incompatible chemical reactions to occur simultaneously in separate environments, each with optimal conditions
- It prevents any molecule from moving between different parts of the cell
- It enables the cell to store unlimited quantities of water
Show the answer
Answer: B. Compartmentalisation creates distinct membrane-bound environments (organelles) with unique conditions (pH, ion concentrations, enzymes) suited to specific processes. For example, lysosomes maintain pH ~5 for hydrolytic enzymes while the cytoplasm is ~pH 7.2 — these incompatible conditions can coexist because they are separated by membranes.
Question 3. The intermembrane space of a mitochondrion is kept small in volume. Why is this significant for ATP production?
- It concentrates the H⁺ ions pumped out of the matrix, creating a steep proton gradient that drives ATP synthase
- It reduces the total surface area available for the electron transport chain
- It prevents the Krebs cycle enzymes from functioning
- It increases the volume of the matrix available for glycolysis
Show the answer
Answer: A. Protons pumped across the inner mitochondrial membrane during the electron transport chain accumulate in the intermembrane space. Because this space has a small volume, the same number of H⁺ ions produces a higher concentration (steeper electrochemical gradient), maximising the proton-motive force available to drive ATP synthase.
Linking questions
Questions that connect B2.2 to other parts of the course, the kind that come up in Paper 2.
- Lysosomes contain enzymes that break down proteins into amino acids. How does the structure of these enzymes (B1.2) make them active only at the low pH maintained inside the lysosome? (see B1.2)
- The inner mitochondrial membrane is the site of oxidative phosphorylation. How does the membrane's phospholipid composition and embedded protein complexes (B2.1) contribute to chemiosmosis? (see B2.1, C1.2)
- Differentiated cells express different sets of proteins depending on their function. How does differential gene expression (B2.3) determine the organelle composition of a specialised cell? (see B2.3)
- During cell division (D2.1), organelles must be distributed between daughter cells. Explain how mitochondria and chloroplasts are inherited, and why they cannot be synthesised de novo by the cell. (see D2.1)
Practise B2.2
Study notes, every question and full markschemes for B2.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.