Cell Organelles and Their Functions (With Exam Answers)

Quick answer: Cell organelles are the specialised structures inside a cell, each with one main job. The nucleus stores DNA and controls the cell, mitochondria release energy in aerobic respiration, ribosomes build proteins, rough endoplasmic reticulum folds them, the Golgi apparatus packages and exports them, lysosomes digest waste, and chloroplasts carry out photosynthesis in plant cells only.

I lost four marks on this topic in a mock last year and it still annoys me. I wrote "mitochondria give the cell energy". My teacher circled it and wrote "released, not given, and from what?". That is the whole lesson of cell biology: you probably know the organelle names already, but the marks live in exact wording and in the link between a structure and what it lets the cell do.

What each organelle does, in mark-scheme wording

Learn each function as a phrase you could write straight into an answer, not as a nickname.

• Cell membrane — partially permeable, controls what enters and leaves the cell.
• Cytoplasm — jelly-like, site of most chemical reactions in the cell.
• Nucleus — contains DNA in chromosomes, controls the cell's activities by coding for proteins.
• Mitochondria — site of aerobic respiration, releases energy from glucose as ATP.
• Ribosomes — site of protein synthesis, joins amino acids into a chain.
• Rough endoplasmic reticulum — has ribosomes on its surface, folds and transports proteins.
• Smooth endoplasmic reticulum — makes lipids and steroids.
• Golgi apparatus — modifies, packages and exports proteins in vesicles.
• Lysosomes — contain digestive enzymes that break down worn-out organelles and pathogens.
• Vacuole — in plants, a permanent sap-filled sac that keeps the cell turgid.
• Chloroplasts — contain chlorophyll, absorb light energy for photosynthesis.
• Cell wall — made of cellulose in plants, freely permeable, stops the cell bursting.

"Contains chlorophyll" scores better than "does photosynthesis", because it names the substance doing the work. If you change one thing after reading this, change your cards so the answer side includes the how, not just the label. I rebuilt mine as organelle flashcards in about twenty minutes and my recall score went from 11 out of 20 to 18 out of 20 in a week.

The structure-to-function link is where the marks are

Almost every longer question here is really asking the same thing: name a feature, say what it lets the cell do, then say why that matters. Three steps, usually three marks.

• Root hair cell — long thin extension, so a large surface area, so faster water uptake by osmosis.
• Red blood cell — no nucleus and biconcave shape, so more room for haemoglobin and a bigger surface area for oxygen diffusion.
• Sperm cell — many mitochondria in the mid-piece, so more ATP released, so the tail keeps moving.
• Palisade mesophyll cell — chloroplasts packed near the upper surface, so more light absorbed for photosynthesis.
• Pancreas cell — large amounts of rough ER and Golgi, because it exports digestive enzymes constantly.

Write those as chains with "so that" between each part. If your sentence contains no "so that", you have described rather than explained, and the second mark is gone.

Why cells stay small: surface area to volume ratio

This is the piece most students skip, and it is exactly what separates a strong AP Biology or Cambridge answer from an average one.

Take a cube-shaped cell of side 1 unit. Surface area = 6 x 1 x 1 = 6. Volume = 1 x 1 x 1 = 1. Ratio = 6:1.

Now double the side to 2 units. Surface area = 6 x 2 x 2 = 24. Volume = 2 x 2 x 2 = 8. Ratio = 24:8 = 3:1.

The cell got bigger, but its surface area per unit of volume halved. Less membrane per unit of contents means diffusion cannot supply the inside fast enough. That one fact explains microvilli in the small intestine, the folded cristae inside mitochondria, and the flattened red blood cell. Whenever a question asks why a membrane is folded, the answer is increased surface area for exchange.

Plant, animal and bacterial cells side by side

Plant cells have three things animal cells do not: a cellulose cell wall, chloroplasts, and one large permanent vacuole. Animal cells have centrioles and usually more lysosomes, and no wall at all.

Bacteria are what people get wrong. A bacterial cell is prokaryotic, which means:

• No nucleus. Its DNA is a single circular loop, free in the cytoplasm.
• Extra small rings of DNA called plasmids.
• No mitochondria, no chloroplasts, no ER, no Golgi. No membrane-bound organelles at all.
• Ribosomes are present, but smaller.
• A cell wall that is not made of cellulose.
• Much smaller: roughly 1 to 5 micrometres, against 10 to 100 micrometres for a typical eukaryotic cell.

Bacteria still respire. They just do it across the cell membrane instead of inside mitochondria. Writing "bacteria cannot respire because they have no mitochondria" is a guaranteed lost mark.

Worked example: finding the real size of a cell

This calculation turns up constantly, and it is free marks once you fix your units.

A drawing of a palisade cell is printed at a magnification of 1500 times. You measure the drawing with a ruler and get 60 mm. How long is the real cell?

1. Write the formula: magnification = image size / actual size.
2. Rearrange for what you want: actual size = image size / magnification.
3. Substitute: actual size = 60 / 1500 = 0.04 mm.
4. Convert, because cells are quoted in micrometres and 1 mm = 1000 micrometres: 0.04 x 1000 = 40 micrometres.

Now the reverse, which examiners like even more. The real cell is 40 micrometres and the drawing is 60 mm. Find the magnification. Convert first, so both are in the same unit: 60 mm = 60000 micrometres. Then 60000 / 40 = 1500. The magnification is 1500 times, and magnification never has units. If you wrote 1500 mm, you would lose the mark.

Two rules save you: convert before you divide, never after, and always measure the drawing in millimetres so your ruler and your working agree. If a step feels shaky, paste the question in and have it explained line by line.

Mistakes I actually made

• Calling mitochondria "the powerhouse". No mark scheme accepts it. Say "site of aerobic respiration".
• Mixing up cell wall and cell membrane. Every cell has a membrane. Only plants, fungi and bacteria have a wall.
• Saying chloroplasts and mitochondria do the same job. Chloroplasts store energy in glucose, mitochondria release it.
• Labelling diagrams with arrowheads and crossing lines. WAEC and Cambridge want straight ruled labels that touch the structure.
• Answering "explain" with a bare list. GCSE boards (AQA, Edexcel, OCR) and CBSE want reasoning sentences there.

Test yourself

1. A cell has ribosomes but no nucleus and no mitochondria. What type of cell is it, and how does it respire?
2. A drawing of a cell measures 45 mm at a magnification of 900 times. What is the actual length in micrometres?
3. Give one structural feature of a root hair cell and explain, using "so that", why it helps the plant.

FAQ

What is the easiest way to memorise all the organelles?

Stop memorising them as a list. Group them by pathway: the nucleus sends the instruction, the ribosome builds the protein, rough ER folds it, Golgi packages it, and a vesicle ships it out. Five organelles become one story, and stories survive exam stress far better than lists do. Then test it back with a quick generated quiz.

Do I need to know organelle sizes?

You need rough orders of magnitude, not exact figures. Know that a bacterial cell is about 1 to 5 micrometres, an animal cell roughly 10 to 30 micrometres, and that 1 mm equals 1000 micrometres. Those three facts cover almost every magnification question you will be set.

Which organelles appear in both plant and animal cells?

Nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, rough and smooth ER, and Golgi apparatus appear in both. The only differences are the cell wall, chloroplasts and the large permanent vacuole, which are plant-only, and centrioles, which are effectively animal-only.

Is cell biology worth revising in depth if it is a small topic?

Yes, because it feeds everything else. Respiration, photosynthesis, transport, enzymes and genetics all assume you can name the structure involved and say what it does. More routes through the subject are collected on the biology hub.

In short: learn each organelle as a function phrase rather than a nickname, link every structure to what it lets the cell do with a "so that", make your micrometre conversions automatic, and treat bacteria as a genuinely different kind of cell rather than a simpler animal one. Do those four things and this becomes some of the most reliable marks on the paper.