IB Biology · Theme B: Form and function · SL and HL
B2.2 Organelles and compartmentalization
A one-page summary of B2.2 Organelles and compartmentalization, 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
Organelles and compartments
- B2.2.1What is an organelle?
- B2.2.1 · Nature of scienceSeparating organelles: cell fractionation
- B2.2.2Why separate the nucleus?
- B2.2.3Compartments in the cytoplasm
- B2.2.3Lysosomes and phagocytic vacuoles
Organelles adapted to their functions
- B2.2.4 · HLThe mitochondrion
- B2.2.5 · HLThe chloroplast
- B2.2.6 · HLThe nuclear envelope
- B2.2.7 · HLFree ribosomes and rough ER
- B2.2.8 · HLThe Golgi apparatus
- B2.2.9 · HLVesicles and clathrin
- B2.2.7 – B2.2.9 · HLThe route of a secreted protein
B2.2 Organelles and compartmentalization: summary
Organelles
- Discrete subunits: nucleus, ribosomes, vesicles, plasma membrane…
- Not organelles: cell wall, cytoskeleton, cytoplasm. NOS: ultracentrifuges.
Nucleus and cytoplasm
- Transcription and translation separated.
- mRNA is modified before it meets ribosomes; not possible in prokaryotes.
Compartments
- Concentrate enzymes and metabolites; separate incompatible processes.
- Lysosomes (pH 5) and phagocytic vacuoles.
HL · Mitochondria and chloroplasts
- Mitochondrion: small intermembrane space, cristae, Krebs enzymes in matrix.
- Chloroplast: large thylakoid area, small thylakoid volume, Calvin enzymes in stroma.
HL · Nucleus and ribosomes
- Double envelope with pores; breaks into vesicles in mitosis and meiosis.
- Free ribosomes: proteins kept in the cell; rough ER: transport and secretion.
HL · Golgi and vesicles
- Golgi processes proteins for secretion.
- Vesicles move materials; clathrin coats help them form.
Key terms
- Organelle
- a discrete subunit of a cell adapted to a specific function.
- Compartmentalization
- division of a cell into separate membrane-bound spaces.
- Cell fractionation
- separating the organelles of broken cells by centrifugation.
- Post-transcriptional modification
- changes made to mRNA after transcription.
- Lysosome
- a vesicle of hydrolytic enzymes that works at about pH 5.
- Phagocytic vacuole
- a vacuole enclosing engulfed material, such as a bacterium.
- Cristae
- folds of the inner mitochondrial membrane.
- Matrix
- the inside of a mitochondrion, where the Krebs cycle happens.
- Thylakoid
- a flattened membrane sac in a chloroplast, with photosystems.
- Stroma
- the fluid of a chloroplast, where the Calvin cycle happens.
- Nuclear pore
- a channel through both membranes of the nuclear envelope.
- Rough ER
- ER with ribosomes that make proteins for transport or secretion.
- Golgi apparatus
- a stack of sacs that processes proteins for secretion.
- Clathrin
- a protein whose coat helps vesicles form.
Sample exam questions
Three of the 28 multiple-choice questions for B2.2. Try each one before opening the answer.
Question 1. What is an advantage of compartmentalization in eukaryotic cells?
- All reactions happen in one space.
- Enzymes and substrates are concentrated.
- Fewer membranes are needed in the cell.
- Every enzyme works at the same pH.
Show the answer
Answer: B. Enzymes and their substrates can be kept together at high concentrations, so reactions are faster.
Question 2. Which feature of a mitochondrion allows a steep proton gradient to build up quickly?
- The large volume of the matrix
- The circular DNA
- The outer membrane pores
- The small intermembrane space
Show the answer
Answer: D. Protons pumped into the small intermembrane space quickly build a steep gradient for chemiosmosis.
Question 3. Which structure synthesizes proteins that are to be secreted from the cell?
- Free ribosomes in the cytoplasm
- The Golgi apparatus
- Lysosomes
- Ribosomes on the rough ER
Show the answer
Answer: D. Membrane-bound ribosomes on the rough ER make proteins for secretion and for transport within the cell.
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), the nuclear envelope breaks into vesicles. Why must it break down, and how does it reform? (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.