IB Biology HLOrganelles & CompartmentsPaper 1 & 2~12 min read
Cell Organelles
A eukaryotic cell is not a bag of soup with a few things floating in it. It is a building with rooms, and each room is walled off by a membrane so that a different job can be done inside it. This page is the tour.
📚 What you need to know
Eukaryotic cells have a more complex ultrastructure than prokaryotic cells.
The cytoplasm is divided into membrane-bound compartments called organelles, bound by a single or a double membrane.
The cell wall, cytoskeleton and cytoplasm are not organelles, because they are not membrane-bound.
Compartmentalised structures include the nucleus, vesicles, ribosomes and the plasma membrane.
Compartmentalisation allows higher local concentrations of enzymes and substrates, keeps damaging substances separated, maintains optimal conditions, and lets the cell vary the number and position of organelles.
Organelles are specialised: the structure of each one is adapted to the function it carries out.
NOS: progress in science often follows the development of new techniques.
Cell fractionation — homogenisation, filtration and ultracentrifugation — made it possible to study organelles individually.
What counts as an organelle
The definition to hold on to is membrane-bound compartment. Some organelles are bound by a single membrane, such as lysosomes and the Golgi apparatus. Some are bound by a double membrane, such as the nucleus, mitochondria and chloroplasts.
That definition also tells you what is not an organelle, and this trips people up:
Structure
Organelle?
Why
Nucleus
Yes
Bound by a double membrane, the nuclear envelope
Mitochondrion
Yes
Bound by a double membrane
Lysosome
Yes
Bound by a single membrane
Cell wall
No
Not a membrane — it is a rigid layer of cellulose outside the membrane
Cytoskeleton
No
A network of protein filaments, with no membrane around it
Cytoplasm
No
It is the space organelles sit in, not a compartment in its own right
Ribosomes are the awkward one. They are not surrounded by a membrane, yet the syllabus lists them among the compartmentalised structures because so many of them are bound to the endoplasmic reticulum and therefore work inside a compartment. If you are asked directly whether a ribosome is membrane-bound, the answer is no.
A tour of an animal cell
Plant cells contain all of this too, plus a cellulose cell wall, chloroplasts, a large central vacuole and plasmodesmata linking neighbouring cells.
Structure follows function
Organelles look wildly different from one another, and that is the point. Each one is specialised, meaning its structure is adapted to the job it does. A few examples you can already justify:
A mitochondrion has a heavily folded inner membrane, because the reactions it hosts happen on that membrane and folding gives more room for them.
A lysosome has a single tough membrane, because its contents would digest the cell if they escaped.
The rough ER is studded with ribosomes, because proteins made there are fed straight into the compartment for processing.
The nuclear envelope is riddled with pores, because mRNA has to get out and enzymes have to get in.
Being separated by a membrane is what makes all of this possible. An organelle can run its own chemical reactions, at its own pH, without interference from the rest of the cell.
How we found out: cell fractionation
This is the nature of science point the syllabus wants: progress in science often follows the development of a new technique. Nobody could study what an individual organelle did until it was possible to get a tube containing nothing but that organelle. The invention of the ultracentrifuge made it possible, and organelle biology took off.
The process is called cell fractionation, and it has three stages.
The sample is kept cold, buffered and in an isotonic solution throughout — cold to slow enzymes, buffered to protect protein shape, isotonic to stop organelles bursting.
Spin
Speed
What settles into the pellet
First
Slowest
Nuclei — the largest and heaviest
Second
Faster
Chloroplasts and mitochondria
Third
Faster still
Endoplasmic reticulum and lysosomes
Fourth
Fastest
Ribosomes — the smallest and lightest
🧠
Remembering the order
Heaviest out first. Think of shaking a jar of gravel, sand and dust: the gravel drops immediately, the dust takes longest. Nuclei are the gravel, ribosomes are the dust.
Worked examples
WE 1
Applying the definition
A student lists the cell wall, the cytoskeleton and the Golgi apparatus as organelles. Identify which of these is correctly listed, and explain your answer. (3 marks)
Point 1: the correct one
Only the Golgi apparatus is an organelle.
Point 2: why
An organelle is a membrane-bound compartment, and the Golgi apparatus is made of membrane-bound sacs.
Point 3: the other two
The cell wall is a rigid cellulose layer and the cytoskeleton is a network of protein filaments. Neither is surrounded by a membrane.
membrane-bound is the test, and only the Golgi passes itstate the definition explicitly — that is usually where one of the marks sits
WE 2
Interpreting a fractionation result
A tissue sample is fractionated. The pellet from the second spin is found to contain high levels of ATP synthase. Suggest which organelle this pellet contains, and explain your reasoning. (3 marks)
Step 1: use the enzyme
ATP synthase is found on the inner membrane of mitochondria, where it is used in oxidative phosphorylation.
Step 2: use the order
The second spin is faster than the first, so the pellet contains organelles lighter than nuclei but still relatively large.
Step 3: combine them
Both lines of evidence point to mitochondria.
mitochondria — consistent with both the enzyme and the spin orderuse every clue in the stem; the marks are usually split between them
WE 3
The nature of science point
Explain how the invention of the ultracentrifuge advanced our understanding of cell biology. (3 marks)
Point 1: the problem it solved
Before it existed, organelles could be seen but not separated, so their individual functions could not be tested.
Point 2: what it allowed
Cell fractionation produces a pure sample of one type of organelle, which can then be studied on its own.
Point 3: the general point
This is an example of scientific progress following the development of a new technique rather than a new idea.
a new technique made a whole new kind of experiment possiblenature of science questions want the general principle stated, not just the example
💡 Exam tips
Define an organelle as a membrane-bound compartment. That single phrase answers a lot of questions.
Know which organelles have a double membrane: nucleus, mitochondrion, chloroplast.
Never call the cell wall, cytoskeleton or cytoplasm an organelle.
For fractionation, remember the three stages in order and be able to say why the solution is cold, buffered and isotonic.
Heaviest components pellet first, at the lowest speed.
When asked about an organelle’s structure, always finish the sentence with what that structure lets it do.
⚠ Common mistakes
Saying ribosomes are membrane-bound. They are not; some are attached to a membrane, which is different.
Getting the pellet order backwards. Ribosomes need the fastest spin, not the slowest.
Describing the homogenate as pure. It contains everything; the separation happens later.
Confusing the pellet with the supernatant. The pellet is what settles at the bottom; the supernatant is the liquid above it, and it goes on to the next spin.
Writing “the nucleus controls the cell” as an explanation. Say what it actually does: it houses the DNA and separates transcription from translation.
Treating plant and animal cells as completely different. They share almost all of these organelles.
Up next: Cell Compartmentalisation — we have said dividing the cell into rooms is an advantage. Now we work out exactly what that advantage buys the cell.
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