Key Idea: A eukaryotic cell is not one open bag of chemicals. It is split into tiny working parts called organelles, and most of them are wrapped in their own membrane — dividing the cell into separate compartments. This topic answers four linked questions. What is each organelle and what does it do? Why is it useful to divide the cell into compartments, and how does the amount of an organelle reveal the cell's job? Which organelles tell the cell types apart (prokaryote vs eukaryote, animal vs plant)? And where did mitochondria and chloroplasts come from in the first place? Two ideas run through all of it: structure reflects function (a cell that does a lot of a job carries a lot of the organelle that does it), and membranes create order (they keep reactions apart and gather the right molecules together). This topic is a regular on Paper 1A (identify a labelled organelle / pick the plant-only structure), Paper 2 (identify-and-state functions, distinguish cell types, explain endosymbiosis) and Paper 3 (suggest why a specialised cell has a lot of one organelle, or read evidence off a micrograph).
🔬 Identifying organelles & their functions
An organelle is a structure inside a cell that does a particular job — a 'little organ'. The core skill is simple but worth the most marks: recognise an organelle in a diagram or micrograph and state what it does. Almost every organelle is membrane-bound (wrapped in its own membrane); the ribosome is the one exception. That matters because a clearly membrane-bound structure can only belong to a eukaryotic cell — prokaryotes have no membrane-bound organelles, only ribosomes. A favourite exam link is the protein-export production line: ribosomes (on the rough ER) build a protein → the rough endoplasmic reticulum transports it → the Golgi apparatus modifies and packages it into a vesicle for export. So a structure that 'packages and modifies polypeptides into vesicles' is the Golgi apparatus.
Read the organelles off the eukaryotic cell on the right: the nucleus (holds the DNA), the mitochondrion (releases energy), the ribosomes (build proteins), the plasma membrane and the cytoplasm are all labelled. Most organelles are wrapped in their own membrane — the ribosome is the exception.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Organelle | Membrane-bound? | Its main function |
|---|---|---|
| Nucleus | yes (double membrane) | holds the DNA and controls the cell |
| Mitochondrion | yes | aerobic respiration — releases energy (ATP) |
| Ribosome | no | builds proteins (protein synthesis) |
| Rough endoplasmic reticulum | yes | makes and transports proteins |
| Golgi apparatus | yes | modifies & packages proteins into vesicles for export |
| Chloroplast (plant cells) | yes | photosynthesis — makes food using light |
An identify-and-state mark needs two things: the correct name AND a specific function. 'It helps the cell' scores nothing — say aerobic respiration / releases energy for the mitochondrion, or packages proteins into vesicles for the Golgi apparatus.
🏠 Compartmentalization & linking structure to function
Dividing the cell into membrane-bound spaces is called compartmentalization — think of the rooms of a house, each kept separate by walls so a different job can happen in each. Compartments give the cell several clear advantages: they separate incompatible reactions, concentrate enzymes and substrates together so reactions go faster, enclose harmful substances (e.g. digestive enzymes sealed inside lysosomes), provide extra membrane surface area (folded internal membranes), and let each compartment keep its own optimum conditions (such as pH). The part the exam loves most is structure reflects function: the amount of an organelle a cell contains reveals its job. A cell packed with mitochondria does a lot of respiration (it is very active, e.g. a muscle cell); a cell with extensive endoplasmic reticulum does a lot of making and processing molecules (e.g. a liver cell). The rule in one line: more of an organelle = more of its job.
| Advantage of compartments | Why it helps the cell |
|---|---|
| Separates incompatible reactions | reactions that would clash are kept in different spaces |
| Concentrates enzymes & substrates | molecules for one process are gathered in a small space → faster |
| Encloses harmful substances | e.g. lysosome enzymes can't digest the rest of the cell |
| Extra membrane surface area | folded internal membranes give more room for membrane reactions |
| Local optimum conditions | each compartment keeps its own pH / chemical mix |
For a 'Suggest why this cell has lots of organelle X' mark, link structure to function: say what X does, then say the cell does a lot of that process. Naming the organelle alone scores nothing. Many mitochondria → very active cell; extensive ER → lots of synthesis / processing / detoxification.
🧬 Comparing cell types by their organelles
We sort cells by which organelles they have and which they lack. Four structures are found in every cell — DNA, cytoplasm, a plasma membrane and ribosomes — so none of these can ever tell two cell types apart. The first split is prokaryotic vs eukaryotic, and it comes down to one structure: the nucleus. A prokaryote has no nucleus (its DNA is free in the cytoplasm) and no membrane-bound organelles, and is smaller (~1–5 µm). A eukaryote keeps its DNA in a nucleus, has membrane-bound organelles such as mitochondria, and is larger (~10–100 µm). Among eukaryotes, animal vs plant comes down to three structures that plant cells add: a cellulose cell wall, chloroplasts and a large central vacuole. So 'name an organelle in plant but not animal cells' is answered by chloroplast or large central vacuole (the safest answers — fungi and bacteria also have walls, just of different material).
Animal, plant and fungal cells are all eukaryotic and share a nucleus, mitochondria and ribosomes — but only the plant cell adds a cellulose cell wall, chloroplasts and a large central vacuole.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Structure | Prokaryote | Animal cell | Plant cell |
|---|---|---|---|
| Nucleus | ✗ | ✓ | ✓ |
| Membrane-bound organelles | ✗ | ✓ | ✓ |
| Chloroplast | ✗ | ✗ | ✓ |
| Large central vacuole | ✗ | ✗ | ✓ |
| Cellulose cell wall | ✗ | ✗ | ✓ |
| Ribosomes | ✓ | ✓ | ✓ |
No nucleus → prokaryote. Nucleus but no chloroplast → animal. Chloroplast present → plant. And a 'distinguish' answer must contrast both cell types in each point — 'eukaryotes have a nucleus' alone is only half the difference.
🦠 The endosymbiotic origin of organelles
Mitochondria and chloroplasts were not always part of eukaryotic cells. The endosymbiotic theory says they began as free-living prokaryotes that were engulfed by a larger host cell and survived inside it. The sequence is short: a host cell engulfs a bacterium by endocytosis → the bacterium survives (it is not digested) and reproduces → the relationship benefits both partners → so the bacterium is kept and becomes a permanent organelle. An aerobic bacterium became the mitochondrion (respiration), and a photosynthetic bacterium became the chloroplast (photosynthesis). The proof is still inside the organelle today: its own DNA, its own 70S ribosomes (the bacterial type — the host cytoplasm uses larger 80S ribosomes), a double membrane, and division by binary fission. This is why a leaf cell can contain two sizes of ribosome — the 70S ones are a leftover from the chloroplast's free-living past.
Endosymbiosis: an ancestral host cell engulfs a free-living aerobic bacterium by endocytosis; the bacterium survives inside, benefits both partners, and over many generations becomes a mitochondrion. The clues it left behind — its own DNA, its own 70S ribosomes, a double membrane and division by binary fission — are still inside the organelle today.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Feature | Mitochondrion | Chloroplast |
|---|---|---|
| Free-living ancestor | an aerobic bacterium | a photosynthetic bacterium |
| Job inside the host | aerobic respiration (energy) | photosynthesis (food) |
| Found in | almost all eukaryotes | plants and algae only |
| Shared evidence | own DNA, 70S ribosomes, double membrane, binary fission | own DNA, 70S ribosomes, double membrane, binary fission |
Say those four words in order and you can answer almost any 'explain the endosymbiotic theory' question. For a 3-mark Explain, name both organelles (mitochondrion AND chloroplast) and include the survival-inside-the-host step — one organelle alone caps you at 1 mark.
✍️ Worked examples
IB-style question — identify a labelled organelle
An electron micrograph of a salivary-gland cell shows an oval structure (labelled X) with a folded inner membrane. Identify organelle X and state its function. [2]
How to score both marks:
Identify it from the clue. An oval structure with a folded inner membrane is a mitochondrion. (Mark 1: mitochondrion.)
State its function. The mitochondrion is the site of aerobic respiration, releasing energy (ATP) for the cell. (Mark 2: aerobic respiration / releases energy.)
Organelle X is a mitochondrion; its function is aerobic respiration, releasing energy (ATP) for the cell.
IB-style question — suggest why a cell has lots of an organelle
A cell that secretes large amounts of protein contains extensive rough endoplasmic reticulum. Suggest why this cell contains so much rough ER. [1]
How to score the mark:
State what the organelle does. The rough endoplasmic reticulum makes and transports proteins (its surface is studded with ribosomes).
Link it to the cell's job. Because the cell secretes a lot of protein, it needs a large area of rough ER to make and process all of it. (The mark is for the LINK between the extensive ER and the large amount of protein made — not for naming the organelle.)
The rough ER makes and transports proteins, so a cell that secretes a lot of protein needs a large amount of it.
IB-style question — distinguish prokaryotic and eukaryotic cells
Distinguish between a prokaryotic cell and a eukaryotic cell. [3]
How to score all three marks:
Nucleus. A prokaryotic cell has no nucleus (its DNA is free in the cytoplasm), whereas a eukaryotic cell keeps its DNA inside a nucleus.
Membrane-bound organelles. A prokaryotic cell has no membrane-bound organelles, whereas a eukaryotic cell has them (for example mitochondria).
Size. A prokaryotic cell is smaller (~1–5 µm) than a eukaryotic cell (~10–100 µm). (1 mark per paired contrast, max 3 — each point must compare BOTH cell types.)
Prokaryote: no nucleus (DNA free in cytoplasm), no membrane-bound organelles, smaller. Eukaryote: DNA in a nucleus, has membrane-bound organelles, larger.
IB-style question — explain the origin of eukaryotic organelles
Explain the endosymbiotic theory for how eukaryotic cells and their organelles originated. [3]
How to score all three marks:
Start with the engulfing. A larger host cell engulfed a smaller free-living prokaryote (bacterium) by endocytosis, so it ended up inside the host.
Explain survival and benefit. The bacterium was not digested; it survived and reproduced inside the host, and the relationship was beneficial to both, so it was kept.
Name the organelles formed. An aerobic bacterium became the mitochondrion and a photosynthetic bacterium became the chloroplast. (1 mark engulfing; 1 mark survival/benefit inside the host; 1 mark BOTH organelles named.)
A host cell engulfed free-living bacteria that survived inside it; the relationship benefited both, so they were kept and became organelles — an aerobic bacterium became the mitochondrion and a photosynthetic bacterium became the chloroplast.
IB-style question — read the evidence off a micrograph [data]
The cytoplasm of a spinach leaf cell contains large 80S ribosomes, but the chloroplasts inside the same cell contain smaller 70S ribosomes. Suggest how this supports the endosymbiotic theory. [3]
How to score all three marks:
Identify the bacterial feature. The 70S ribosomes are the small, bacterial type (free-living prokaryotes use 70S; the host cytoplasm uses larger 80S).
Link it to a free-living ancestor. The chloroplast keeping its own 70S ribosomes is what you'd expect if it was once a free-living bacterium.
Draw the conclusion. So bacterial-type ribosomes inside the chloroplast are evidence that it descended from an engulfed photosynthetic bacterium, supporting the theory. (Mark 1: 70S is bacterial. Mark 2: organelle keeps the ribosomes of a free-living ancestor. Mark 3: this supports an endosymbiotic origin.)
70S ribosomes are the bacterial type; the chloroplast keeping its own 70S ribosomes is what you'd expect if it was once a free-living bacterium, so it supports the chloroplast having an endosymbiotic origin.
✅ Quick self-check
Tap each card to check yourself.
What is an organelle, and which one has no membrane? An organelle is a structure inside a cell that carries out a specific job. Most are membrane-bound; the ribosome is the only organelle that is NOT surrounded by a membrane.
Which three organelles form the protein-export production line? Ribosomes (on the rough ER) build the protein → the rough endoplasmic reticulum transports it → the Golgi apparatus modifies and packages it into a vesicle for export.
Give two advantages of dividing a cell into compartments. They separate incompatible reactions and concentrate the enzymes and substrates for a process in one small space. (Also: enclose harmful substances, add membrane surface area, keep local optimum conditions.)
How does the amount of an organelle reveal a cell's job? More of an organelle = more of its job. Many mitochondria → a very active cell that respires a lot; extensive ER → a cell that makes / processes a lot of molecules (e.g. a liver cell).
Name an organelle in plant cells but not animal cells, and a structure in every cell. Plant-only: a chloroplast or a large central vacuole (cellulose cell wall too, but fungi/bacteria have walls). In every cell: DNA, cytoplasm, a plasma membrane and ribosomes.
What does the endosymbiotic theory state, and what is its evidence? Mitochondria and chloroplasts began as free-living prokaryotes engulfed by a host cell that survived inside it. Evidence: each organelle has its own DNA, its own 70S ribosomes, a double membrane, and divides by binary fission.
Exam Tips
- Identify-and-state questions need TWO things per organelle: the correct name AND a specific function — 'it helps the cell' scores nothing.
- If a described job is 'packages / modifies polypeptides into vesicles', the organelle is the Golgi apparatus; if it 'releases energy', it is the mitochondrion.
- Almost every organelle is membrane-bound; the ribosome is the exception — so a membrane-bound structure can only belong to a eukaryotic cell.
- For a 'Suggest why this cell has lots of organelle X' mark, link structure to function — say what X does, then that the cell does a lot of it. Naming the organelle alone scores nothing.
- Advantages of compartments: separate incompatible reactions, concentrate enzymes/substrates, enclose harmful substances, add membrane surface, keep local optimum conditions.
- Found in every cell: DNA, cytoplasm, plasma membrane, ribosomes — never the answer to 'name something that tells two cell types apart'.
- Plant but not animal: chloroplast or large central vacuole are the safest answers (a cell wall alone is weak — fungi and bacteria have walls too).
- A 'distinguish' answer must contrast BOTH cell types in each point; in a tick table, read in order: no nucleus = prokaryote, nucleus + chloroplast = plant, nucleus no chloroplast = animal.
- For endosymbiosis: engulf → survive inside the host → benefit both → keep. A 3-mark Explain must name BOTH the mitochondrion AND the chloroplast.
- Evidence for endosymbiosis = the four bacterial features: own DNA, own 70S ribosomes, a double membrane, and division by binary fission (70S is bacterial; the cytoplasm uses 80S).
Key Idea: A eukaryotic cell is divided into organelles, most wrapped in their own membrane (the ribosome is the exception). The nucleus holds the DNA and controls the cell; the mitochondrion runs aerobic respiration (energy); ribosomes → rough ER → Golgi → vesicle make, package and export proteins. Membranes split the cell into compartments — separating incompatible reactions, concentrating enzymes and substrates, enclosing harmful substances, adding surface area and keeping local conditions — and the amount of an organelle reveals the cell's job (more of an organelle = more of its job). Every cell has DNA, cytoplasm, a plasma membrane and ribosomes; prokaryotes stop there (no nucleus, no membrane-bound organelles, smaller), eukaryotes add a nucleus and membrane-bound organelles, and plant cells add a cellulose cell wall, chloroplasts and a large central vacuole that animal cells lack. Finally, mitochondria and chloroplasts arose by endosymbiosis — free-living bacteria (aerobic → mitochondrion; photosynthetic → chloroplast) that were engulfed and survived inside a host — still betrayed today by their own DNA, own 70S ribosomes, a double membrane and binary fission.