Organelles and compartmentalization — IB Diploma Biology HL
IB Biology B2.2: organelles, cell fractionation, compartmentalization, and HL structure and function of mitochondria, chloroplasts, ER and Golgi.
Revision notes
4 short notes, 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.
The mitochondrion is adapted for aerobic respiration by a double membrane with a small intermembrane space, an inner membrane folded into cristae for a large surface area, and a matrix holding Krebs cycle enzymes and substrates. The chloroplast has a large area of thylakoid membranes with photosystems, small volumes inside thylakoids, and Calvin cycle enzymes in the stroma.
The nucleus is surrounded by a double membrane, the nuclear envelope, with pores that let mRNA out and proteins in, and it breaks into vesicles during mitosis and meiosis. Free ribosomes make proteins kept inside the cell, while ribosomes bound to the rough endoplasmic reticulum make proteins for transport within the cell or secretion.
The Golgi apparatus is a stack of flattened membrane sacs that processes proteins from the rough ER and packages them into vesicles for secretion. Vesicles are small membrane-bound sacs that carry materials around the cell. The protein clathrin forms a cage-like coat on the cytoplasmic side of a membrane, curving it into a bud that pinches off as a vesicle.
Exam questions
8 exam-style questions (25 marks), each with its mark scheme.
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15 cards: flip them, mark what you knew, and practise the rest.
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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.
HL only Which part of the mitochondrion contains the enzymes of the Krebs cycle?
Show answer
The matrix.
HL only What is the role of clathrin?
Show answer
It forms a coat that curves a membrane into a bud, forming a vesicle.
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.
How is the mitochondrion adapted for aerobic respiration?
HL only The mitochondrion has a double membrane. The inner membrane is folded into cristae, giving a large surface area for the electron transport chain and ATP synthase. The small intermembrane space lets a steep proton gradient build quickly, and the matrix keeps the enzymes and substrates of the Krebs cycle concentrated together.
What is the difference between free ribosomes and ribosomes on the rough ER?
HL only Both have the same structure, but they make proteins for different destinations. Free ribosomes in the cytosol make proteins that are retained and used inside the cell. Ribosomes bound to the rough endoplasmic reticulum make proteins that enter the ER lumen and are transported within the cell, for example to lysosomes, or secreted.
Written and checked against the IB Biology HL specification · Updated October 2026