The big idea: An organelle is a tiny structure inside a cell that does a particular job — a bit like an organ does a job inside a body.
Key points
- A eukaryotic cell is divided into many organelles.
- Most of them are wrapped in their own membrane.
- To answer this topic you need to recognise each organelle in a diagram or micrograph and be able to state what it does.
Read the organelles off the eukaryotic cell on the right: the nucleus (holds DNA), mitochondrion, ribosomes, plasma membrane and cytoplasm are all labelled.
Interactive diagram
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- Organelle
- A specialised structure inside a cell that carries out a specific function (for example the nucleus or a mitochondrion).
- Membrane-bound organelle
- An organelle surrounded by its own membrane, which keeps its contents separate from the rest of the cell (e.g. nucleus, mitochondrion, Golgi apparatus).
- Nucleus
- The large organelle that holds the cell's DNA and controls the cell's activities.
- Mitochondrion
- The organelle where aerobic respiration takes place, releasing energy for the cell (plural: mitochondria).
- Ribosome
- A tiny organelle that builds proteins; it is the one organelle that is NOT surrounded by a membrane.
- Cytoplasm
- The jelly-like fluid filling the cell, in which the organelles sit.
Organelle = a 'little organ': The word organelle means 'little organ'.
Just as your heart, lungs and stomach each have a job in your body, each organelle has a job in the cell — and the exam wants you to match the organelle to its job.
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The skill the exam tests most is simple: identify an organelle, then state its function.
Function follows structure: The function follows from the structure — so if you learn what each organelle does, you can usually work out the rest.
The organelles you must know
- Nucleus — holds the DNA and controls the cell.
- Mitochondrion — site of aerobic respiration; releases energy (makes ATP).
- Ribosome — builds proteins (protein synthesis); the only organelle with no membrane.
- Rough endoplasmic reticulum (rough ER) — makes and transports proteins.
- Golgi apparatus — modifies, packages and sorts proteins into vesicles for export.
- Vesicle — a small sac that transports materials around or out of the cell.
| Organelle | What it looks like | Its main function |
|---|---|---|
| Nucleus | Large, round, with a double membrane (envelope) | Holds the cell's DNA and controls the cell's activities |
| Mitochondrion | Oval, with a folded inner membrane | Site of aerobic respiration — releases energy (makes ATP) |
| Ribosome | Tiny dot, NOT membrane-bound | Builds proteins by joining amino acids (protein synthesis) |
| Rough endoplasmic reticulum | Folded membranes studded with ribosomes | Makes and transports proteins |
| Golgi apparatus | Stack of flattened membrane sacs | Modifies, packages and sorts proteins into vesicles for export |
| Vesicle | Small membrane-bound sac | Transports materials around or out of the cell |
| Chloroplast (plant cells) | Green oval containing stacks of membranes | Site of photosynthesis — captures light to make food |
| Vacuole (large, in plant cells) | Large fluid-filled sac | Stores water and keeps the cell firm (turgid) |
The protein-export production line
1
Ribosomes build it
Ribosomes (on the rough ER) build the protein.
2
Rough ER transports it
The rough ER transports the protein.
3
Golgi packages it
The Golgi apparatus modifies it and packages it into a vesicle.
4
Vesicle releases it
The vesicle carries it to the membrane to be released.
Three organelles work together to make and export proteins — a favourite exam link.
Spot the Golgi: When a question describes a structure that packages and modifies polypeptides into vesicles, the answer is the Golgi apparatus.
Membrane-bound or not?: Almost every organelle is membrane-bound — wrapped in its own membrane.
The big exception is the ribosome, which has no membrane.
This matters because prokaryotes have no membrane-bound organelles at all, but they do have ribosomes. So a structure that is clearly membrane-bound can only belong to a eukaryotic cell.
| Feature | Membrane-bound organelles | Ribosomes |
|---|---|---|
| Surrounded by a membrane? | Yes — wrapped in their own membrane | No — they are not membrane-bound |
| Examples | Nucleus, mitochondrion, Golgi, ER, chloroplast, vacuole | Ribosomes (the only non-membrane organelle) |
| Found in eukaryotes? | Yes | Yes |
| Found in prokaryotes? | No (prokaryotes have no membrane-bound organelles) | Yes — prokaryotes do have ribosomes |
| Visible as | A structure with its own outline / compartment | Tiny dots scattered in the cytoplasm |
Energy & control organelles
- Nucleus — holds DNA, controls the cell
- Mitochondrion — aerobic respiration → energy
- Chloroplast (plants) — photosynthesis → food
- All are membrane-bound
Protein-making organelles
- Ribosome — builds proteins (no membrane)
- Rough ER — makes & transports proteins
- Golgi apparatus — packages proteins into vesicles
- Vesicle — carries proteins out of the cell
An easy way to remember: Mighty-chondria make energy. The nucle-us is the boss (it holds the DNA). Ribo-somes build the bodies of proteins.
And: lots of an organelle = lots of that job. A cell packed with mitochondria needs lots of energy; a cell packed with ribosomes and Golgi makes lots of protein.
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The big idea: An organelle is a discrete subunit of a cell: a separate part with its own structure, adapted to one job.
The points to remember
- Organelles: the nucleus, mitochondria, chloroplasts, lysosomes, vesicles, ribosomes and the plasma membrane.
- Ribosomes have no membrane, but they are still organelles.
- Not organelles: the cell wall (outside the cell), the cytoskeleton and the cytoplasm.
- Organelles that hold base pairs: the nucleus, mitochondria and chloroplasts (DNA), and ribosomes (RNA).
- Each organelle's structure suits its job, so its shape helps you name it.
- Watch for microtubules in a list: they are part of the cytoskeleton, so that list is not all organelles.
Remember it as: A separate part with its own job = an organelle.
Real example: red blood cells: As a human red blood cell matures it loses its nucleus and mitochondria. The space is filled with haemoglobin, so it carries more oxygen.
The big idea: The job of each organelle could only be studied once scientists could separate one kind of organelle from all the others.
The points to remember
- Cell fractionation: cells are broken open, then spun in a centrifuge.
- Heavier organelles sink first: nuclei at a slow spin, then mitochondria, then ribosomes at a very fast spin in an ultracentrifuge.
- Each pellet holds one kind of organelle, so its function can be tested on its own.
- Progress in science often follows a new technique: here, the ultracentrifuge.
Remember it as: Spin faster, catch smaller.
Real example: finding the lysosome: In 1955 Christian de Duve spun liver cells in a centrifuge and found a new pellet of small sacs full of digestive enzymes. He named them lysosomes.
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How this is tested: Most questions give you a cell diagram or micrograph. You identify organelles and state what they do.
What you may be asked
- The classic format: a labelled cell diagram or electron micrograph. Identify organelles (often two or three labelled X, Y, I, II, III) and state a function for each — typically 1 mark for the name and 1 mark for the function.
- A 1-mark version may describe a job ('packages and modifies polypeptides into vesicles') and ask you to name the organelle — here, the Golgi apparatus.
- A 2-mark Deduce version shows an unfamiliar cell (e.g. a fungus). It asks what the labelled membrane-bound structures could be, with a reason — so you must justify your answer, not just name it.
Exam-style question — identify a labelled organelle
An electron micrograph of a pancreas 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 (in the form of ATP) for the cell to use. (Mark 2: aerobic respiration / releases energy / makes ATP.)
- Tie the structure to the function. The folded inner membrane gives a large surface area for the respiration reactions — a neat way to show you understand, though the two marks come from the name and the function.
Final answer
Organelle X is a mitochondrion; its function is aerobic respiration, releasing energy (ATP) for the cell.
Why this scores full marks: There are two separate scoring points: the name of the organelle and a correct function.
A common slip is naming the organelle but giving a vague function ('it helps the cell') — the function must be specific (aerobic respiration / releases energy).
| Organelle | What it looks like | Its main function |
|---|---|---|
| Nucleus | Large, round, with a double membrane (envelope) | Holds the cell's DNA and controls the cell's activities |
| Mitochondrion | Oval, with a folded inner membrane | Site of aerobic respiration — releases energy (makes ATP) |
| Ribosome | Tiny dot, NOT membrane-bound | Builds proteins by joining amino acids (protein synthesis) |
| Rough endoplasmic reticulum | Folded membranes studded with ribosomes | Makes and transports proteins |
| Golgi apparatus | Stack of flattened membrane sacs | Modifies, packages and sorts proteins into vesicles for export |
| Vesicle | Small membrane-bound sac | Transports materials around or out of the cell |
| Chloroplast (plant cells) | Green oval containing stacks of membranes | Site of photosynthesis — captures light to make food |
| Vacuole (large, in plant cells) | Large fluid-filled sac | Stores water and keeps the cell firm (turgid) |
A two-mark State on a labelled cell drawing:
IB-style questionState[2 marks]
A drawing of a cell from the pancreas shows structure X, flattened sacs covered in ribosomes, and structure Y, a curved stack of flattened sacs with small vesicles beside it. State a function for structures X and Y.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.