You will be handed a grey, unlabelled electron micrograph and asked what it is. There is a reliable order to work in, and once you know it these become some of the most predictable marks in the paper.
📚 What you need to know
Work in a fixed order: nucleus? then cell wall or large vacuole? then which organelles?
No nucleus → prokaryote. Nucleus → eukaryote.
Cell wall and a large vacuole → plant. Neither → animal.
You should be able to recognise: nucleoid, prokaryotic and plant cell walls, nucleus, mitochondria, chloroplast, sap vacuole, Golgi, rough and smooth ER, chromosomes, ribosomes, plasma membrane and microvilli.
Then deduce the function: whichever organelle is unusually common tells you what the cell spends its energy on.
The order to work in
Do not start by naming organelles. Sorting the cell type first stops you claiming a chloroplast in what is obviously an animal cell.
Recognising organelles by eye
Organelle
What to look for
Nucleus
Large, roughly round, with a clear membrane and a dark nucleolus inside
Mitochondrion
Roughly oval, double membrane, often with visible folds (cristae)
Chloroplast
Bigger than a mitochondrion, oval, with distinctive stacks of thylakoids
Vacuole
Very large, mostly empty-looking pale area — a strong sign of a plant cell
Cell wall
A straight, even line right around the outside of the cell
Rough ER
Layered membrane sheets with dark dots (ribosomes) along them
Golgi apparatus
A stack of curved flattened sacs with small vesicles nearby
Microvilli
Many finger-like projections along one edge — a brush border
Chloroplast or mitochondrion is the identification students get wrong most often. Chloroplasts are noticeably bigger and have stacks inside. Mitochondria are smaller and have folds, not stacks. And if there is no cell wall anywhere, it cannot be a chloroplast.
Deducing what a cell does
Once you know the type, look at what the cell has too much of. Cells invest in whatever they do most, so an unusual amount of an organelle is a direct clue.
🧩 Reading the clues
Lots of mitochondria → the cell uses a great deal of energy. Think muscle, or anything doing active transport.
Lots of rough ER and Golgi → the cell makes and exports proteins. Think a secretory or gland cell.
Lots of lysosomes → the cell digests things. Think a white blood cell such as a macrophage.
Microvilli → the cell absorbs. Think the lining of the small intestine.
Many chloroplasts → a photosynthesising plant cell, such as a palisade mesophyll cell in a leaf.
An unusual nucleus shape → the cell squeezes through gaps. Think a phagocyte moving out of a capillary.
Worked examples
WORKED EXAMPLE
An electron micrograph shows a cell with a nucleus, no cell wall, no vacuole, many mitochondria and many lysosomes. Identify the type of cell and suggest its function. [3]
Step 1: nucleus present
Therefore it is a eukaryotic cell.
Step 2: no cell wall or large vacuole
Therefore it is an animal cell.
Step 3: read the organelles
Many mitochondria means a high energy demand; many lysosomes means it digests material it takes in.
An animal cell that engulfs and digests, e.g. a macrophage“therefore” after each observation is what turns an identification into an explanation
WORKED EXAMPLE
A micrograph shows a cell with a nucleus, an even outer line around the whole cell, a very large pale central area, and several oval organelles containing stacks. Identify three structures and the cell type. [4]
Structure 1
The even outer line is the cell wall.
Structure 2
The large pale central area is the permanent vacuole.
Structure 3
The oval organelles with stacks inside are chloroplasts.
Conclusion
Cell wall + large vacuole + chloroplasts + nucleus.
A plant cell
WORKED EXAMPLE
A micrograph shows a cell with a brush border of tightly packed projections along one edge and a very large number of mitochondria. Suggest where in the body this cell is found, and explain. [3]
Step 1: identify the projections
They are microvilli, which greatly increase the surface area.
Step 2: interpret the mitochondria
Large numbers mean a lot of ATP is needed, which suggests active transport.
Step 3: put it together
A cell built for absorbing substances quickly and against a concentration gradient.
Epithelial lining of the small intestine
💡 Exam tip
Always rule out prokaryote first. It takes two seconds and prevents a whole wrong answer.
Write your reasoning as observation → therefore. Examiners award the link, not just the label.
If the question says “suggest”, you are not expected to be certain — but you must justify.
Size matters: the nucleus is the biggest organelle in most animal cells, and chloroplasts beat mitochondria.
A large pale space nearly always means a plant vacuole in these questions.
Count organelles roughly. “Unusually many” is the whole basis of the deduction.
⚠ Common mix-up
Calling a mitochondrion a chloroplast. Stacks mean chloroplast; folds mean mitochondrion.
Naming a chloroplast in an animal cell. Sort the cell type first and this cannot happen.
Confusing the plasma membrane with a cell wall. The wall is thicker, straighter and outside the membrane.
Identifying without explaining. “It is a plant cell” alone rarely gets full marks.
Assuming grey blobs are organelles. Some are artefacts from the preparation.
Forgetting scale. If the whole image is a few micrometres across, you are probably looking at a prokaryote.
Up next: Drawing Cells (Skills) — the drawing conventions examiners actually mark against, and the habits that quietly lose marks.
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