The big idea: A micrograph is just a photograph taken down a microscope. You are not recalling a memorised picture — you are looking for evidence.
Read the clues and decide two things
- What type of cell is this? (prokaryotic or eukaryotic; plant, animal or fungus)
- Which structures can I see and name?
- Micrograph
- A photograph of a specimen taken through a microscope.
- Electron micrograph
- A micrograph taken with an electron microscope — high magnification, so small organelles such as mitochondria are visible.
- Organelle
- A structure inside a cell that does a specific job (for example the nucleus or a mitochondrion).
- Prokaryotic cell
- A cell with no nucleus and no membrane-bound organelles (for example a bacterium).
- Eukaryotic cell
- A cell with a nucleus and membrane-bound organelles (for example an animal, plant or fungal cell).
Two questions, every time: Train yourself to ask, in order:
1. Is there a nucleus? (the biggest clue to prokaryote vs eukaryote) 2. What else can I name? (cell wall, chloroplast, mitochondrion, vacuole…)
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The single most useful clue is the nucleus. Prokaryotic cells also differ in size and organelles.
Eukaryotic cell
- Has a nucleus — a large, dark, membrane-bound structure
Prokaryotic cell
- Has no nucleus: its DNA sits free in the cytoplasm as a single loop
- Much smaller
- No membrane-bound organelles at all (no mitochondria, no endoplasmic reticulum)
| Clue in the micrograph | Prokaryotic | Eukaryotic |
|---|---|---|
| A nucleus (dark, membrane-bound) | absent | present |
| Membrane-bound organelles (mitochondria, ER) | absent | present |
| DNA | free in the cytoplasm (a single loop) | inside a nucleus |
| Typical size | small (about 1–5 μm) | larger (about 10–100 μm) |
| A cell wall | always present | plant/fungus yes, animal no |
Once you know it is eukaryotic, a few extra clues tell you which kind:
Looks like a PLANT cell
- Has a cell wall (a straight, rigid edge)
- Often has chloroplasts (where photosynthesis happens)
- Has one large central vacuole
Looks like an ANIMAL cell
- No cell wall (a soft, irregular edge)
- No chloroplasts
- Only small vacuoles, if any
A fungus is a third option: A fungal cell is eukaryotic and has a cell wall (like a plant) but has no chloroplasts (like an animal). Its wall is made of chitin, not cellulose.
So: cell wall + chloroplasts → plant; cell wall + no chloroplasts → fungus; no cell wall → animal.
Naming organelles by how they LOOK: The exam also asks you to Identify a single labelled organelle — for example 'Identify organelle X' — so you must recognise the common organelles by their appearance in an electron micrograph, not just decide the cell type.
Each organelle has a shape you can spot. Match the picture to the name using the table below.
| Organelle | How it looks in the micrograph | What it does |
|---|---|---|
| Nucleus | Large and round, with a double membrane pierced by pores; dark chromatin inside | Stores the DNA |
| Mitochondrion | Oval (sausage-shaped) with folded inner membranes called cristae | Releases energy (respiration) |
| Rough endoplasmic reticulum (rough ER) | Stacked membrane sheets studded with dotted ribosomes on the surface | Makes and transports proteins |
| Smooth endoplasmic reticulum (smooth ER) | A network of tubes with a smooth surface — no ribosomes (no dots) | Makes lipids; no protein synthesis |
| Golgi apparatus | A neat stack of flattened, curved sacs, often with small bubbles (vesicles) nearby | Modifies and packages proteins |
- Cristae
- The folds of the inner membrane of a mitochondrion — they give it its oval, ridged look.
- Ribosome
- A tiny dot-like structure where proteins are made; ribosomes stuck to membranes make the ER look 'rough'.
- Endoplasmic reticulum (ER)
- A system of membranes inside the cell. Rough ER carries ribosomes (dots); smooth ER does not.
- Golgi apparatus
- A stack of flattened membrane sacs that modifies and packages proteins ready to leave the cell.
ROUGH ER
- Membranes covered in dots (ribosomes)
- Often stacked, parallel sheets
- Job: makes and ships proteins
SMOOTH ER
- Membranes with a plain surface — no dots
- Looks like a network of curved tubes
- Job: makes lipids (not proteins)
The dots are the giveaway: The fastest way to separate the two kinds of ER is the ribosomes: dots on the membrane → rough ER; no dots → smooth ER.
And don't mix up the look-alikes — a mitochondrion is oval with internal cristae, whereas the Golgi apparatus is a stack of flat curved sacs.
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The big idea: In an electron micrograph every structure has a shape you can learn. Match the shape to the name.
The points to remember
- Nucleus: the largest round shape, a double membrane with pores. Chromosomes show as thick dark threads in a dividing cell.
- Mitochondrion: an oval with inner folds (cristae). Chloroplast: larger, with stacks of membranes.
- Rough ER: membranes dotted with ribosomes. Smooth ER: tubes with no dots. Golgi: a stack of curved sacs.
- Sap vacuole: a large pale space. Cell wall: a thick layer outside the plasma membrane, a thin dark line.
- Microvilli: finger-like folds of the plasma membrane on some animal cells.
- A prokaryote has a pale nucleoid region of DNA, no nucleus, small ribosomes and a cell wall.
Remember it as: Dots on a membrane mean ribosomes: that is rough ER.
A prokaryote has none of these membrane-bound organelles. Look for these instead:
Name it in full: Write 'plasma membrane', not 'membrane' alone, and 'rough ER', not just 'ER'. A part name is not enough.
Chromosomes only show when a cell is dividing; the rest of the time the DNA is spread out as chromatin inside the nucleus.
Real example: E. coli: E. coli, a bacterium that lives in your gut, is about 2 µm long. In an electron micrograph its DNA shows as a pale, tangled patch in the middle: the nucleoid region.
The big idea: Read a micrograph in two steps: decide the type of cell from what you can see, then name its structures.
The points to remember
- Prokaryote: no nucleus, a nucleoid, small (about 1–5 µm). Eukaryote: a nucleus and membrane-bound organelles.
- Plant cell: a cell wall, chloroplasts and a large sap vacuole. Animal cell: no wall; it may have microvilli.
- A light micrograph shows the cell wall, nucleus, chloroplasts and vacuole. Smaller organelles need an electron micrograph.
- Give a reason you can see: a nucleus, a vacuole, chloroplasts. Never one you cannot, such as 80S ribosomes.
- A cell wall alone does not prove a eukaryote: bacteria have walls too.
Real example: the small intestine: Each cell lining your small intestine is covered in microvilli. They give a large surface for absorbing digested food, so a micrograph with microvilli shows an animal cell.
'Organelles' is not evidence on its own: Name one you can see, such as the nucleus or a mitochondrion. 'It has organelles' does not earn the mark.
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How this is tested: You will be asked to read micrographs: deduce the cell type or identify a labelled structure.
Where it comes up
- Paper 2: you are often given a micrograph and asked to Deduce the cell type — and you must justify it with a feature you can see (for example 'no nucleus, so prokaryotic').
- Paper 1A: Identify a labelled organelle.
- Paper 1B: name a structure labelled X, or say from what you can see whether a cell is prokaryotic or eukaryotic.
Exam-style question — (a) deduce the cell type
An electron micrograph shows a small cell, about 3 μm across, with its DNA lying free in the cytoplasm and no nucleus or mitochondria visible.
Deduce whether this is a prokaryotic or a eukaryotic cell, giving a reason. [2]
How to score both marks
- Pick out the evidence. There is no nucleus and no membrane-bound organelles, and the DNA is free in the cytoplasm — these are all prokaryotic features.
- Answer the command term (Deduce). State the cell type and link it to the evidence: this is a prokaryotic cell because it has no nucleus (its DNA is free in the cytoplasm).
Final answer
Prokaryotic — because there is no nucleus (the DNA lies free in the cytoplasm) and no membrane-bound organelles.
IB-style questionDeduce[2 marks]
An electron micrograph shows a cell from the lining of a human airway, with a scale bar of 2 µm. A large round structure with a double membrane and several ovals with inner folds can be seen. Deduce, using evidence from the micrograph, whether the cell is prokaryotic or eukaryotic.
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