The big idea: A micrograph is just a photograph taken down a microscope.
Your job in the exam is to read the clues in it and decide two things:
- What type of cell is this? (prokaryotic or eukaryotic; plant, animal or fungus) - Which structures can I see and name?
You are not recalling a memorised picture — you are looking for evidence.
- 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. A eukaryotic cell has a nucleus — a large, dark, membrane-bound structure. A prokaryotic cell has no nucleus: its DNA sits free in the cytoplasm as a single loop.
Prokaryotic cells are also much smaller and have 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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How this is tested: On 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').
On Paper 1A you Identify a labelled organelle, and on Paper 1B a 3-mark Draw question can ask you to draw and label a structure as it appears under an electron microscope.
IB-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 question — (b) draw and label the nucleus
Using the same micrograph technique, draw and label a diagram of a eukaryotic cell nucleus as it appears under an electron microscope. [3]
How to score all three marks
- Draw the outline. Show a roughly round structure enclosed by a double membrane (the nuclear envelope) — draw it as two close lines, not one.
- Add the gaps. Mark a few small breaks in the membrane as the nuclear pores.
- Label the inside. Show and label the darker chromatin (DNA) and a dense round nucleolus inside. (Mark 1: double membrane with pores. Mark 2: chromatin shown. Mark 3: nucleolus labelled.)
Final answer
A round nucleus bounded by a double membrane (nuclear envelope) with nuclear pores, containing labelled chromatin and a denser nucleolus.
How to draw the nucleus for full marks: a DOUBLE-membrane nuclear envelope (two lines) pierced by nuclear pores, with the chromatin (DNA) and a dense round nucleolus inside.
Interactive diagram
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Check your drawing: A good electron-micrograph nucleus has four labelled features: the double membrane (nuclear envelope), the nuclear pores, the chromatin and the nucleolus. Drawing the envelope as a single line is the most common lost mark.