Cell Structure and Organisation: Question 1
Syllabus 2.1
A microbiologist takes a single bacterium from a laboratory culture and examines it under a powerful electron microscope. The image shows a cell surrounded by a cell wall and a cell membrane, with cytoplasm containing ribosomes, one large loop of circular DNA and several small plasmids.
Which structure, present in both plant and animal cells, is missing from this bacterial cell?
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Worked solution
Step 1: Recall which structures a bacterial cell has
A bacterial cell has a cell wall, a cell membrane, cytoplasm, ribosomes, one loop of circular DNA and small circles of extra DNA called plasmids. All of these are described in the stem, so all of them are present in this cell.
Step 2: Recall which structure a bacterial cell lacks
Unlike plant and animal cells, a bacterial cell does not have a nucleus. Its DNA is not enclosed in a nuclear membrane. Instead it lies free as a loop in the cytoplasm, exactly as described in the stem.
Step 3: Match this to the question
The question asks for a structure found in both plant and animal cells but missing from this bacterial cell. Plant and animal cells both have a nucleus, but this bacterial cell does not, so the nucleus is the missing structure.
Why the other options are wrong
- A (cell wall): bacterial cells do have a cell wall, so this structure is not missing.
- B (cytoplasm): the stem states cytoplasm is present, so this structure is not missing.
- D (ribosomes): bacterial cells do have ribosomes, so this structure is not missing.
Final answer
The missing structure is the nucleus, option C.