Identifying organelles and their functions

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.

A eukaryotic cell is divided into many organelles, and 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.

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.

The skill the exam tests most is simple: identify an organelle, then state its function.

The trick is that 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.

The protein-export production line: Three organelles work together to make and export proteins — a favourite exam link.

Ribosomes (on the rough ER) build the protein → the rough ER transports it → the Golgi apparatus modifies and packages it into a vesicle → the vesicle carries it to the membrane to be released.

So 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.

A memory hook: 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.

How this is tested: The classic format gives you a labelled cell diagram or electron micrograph and asks you to 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) and asks what the labelled membrane-bound structures could be, with a reason — so you must justify your answer, not just name it.

✓ 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).