A student observes two types of cells under a microscope. One cell has a nucleus, mitochondria, and a cell wall made of cellulose. The other cell has no nucleus, no mitochondria, and a cell wall made of a different material. Explain how these observations allow the student to identify which cell is a plant cell and which is a bacterial cell, and describe one key difference in how genetic material is stored in each cell type.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student notes that one cell is approximately 100 µm in diameter and the other is approximately 2 µm in diameter.
Model answer (5 marks)
The cell that has a nucleus, mitochondria and a cellulose cell wall is a plant cell. The other cell, lacking a nucleus and mitochondria and having a different cell‑wall material, is a bacterial cell – a prokaryote.
In the plant cell the genetic material (DNA) is stored inside a nucleus, which is surrounded by a nuclear membrane. In the bacterial cell the DNA is not enclosed in a membrane; it is free in the cytoplasm (a nucleoid).
The size difference also helps: the plant cell is about 100 µm in diameter, whereas the bacterial cell is only about 2 µm, reflecting the general rule that prokaryotes are much smaller than eukaryotes.
In the plant cell the genetic material (DNA) is stored inside a nucleus, which is surrounded by a nuclear membrane. In the bacterial cell the DNA is not enclosed in a membrane; it is free in the cytoplasm (a nucleoid).
The size difference also helps: the plant cell is about 100 µm in diameter, whereas the bacterial cell is only about 2 µm, reflecting the general rule that prokaryotes are much smaller than eukaryotes.
Examiner tips
- Use the exact terms ‘nucleus’, ‘mitochondria’, ‘cellulose’, ‘prokaryote’, ‘nucleoid’
- Show the size comparison to justify the identification
- Explain the location of DNA in each cell type
Common mistakes
- Confusing the cell wall material – cellulose only in plants, not bacteria
- Saying the bacterial DNA is in a ‘nucleus’ or ‘chromosome’ inside a membrane
- Ignoring the size difference or giving wrong units
Mark scheme (5 marks)
- The cell with a nucleus, mitochondria, and cellulose cell wall is the plant cell
- The cell with no nucleus and no mitochondria is the bacterial cell, as it is a prokaryotic cell
- In the plant cell, the genetic material (DNA) is stored inside the nucleus, enclosed by a nuclear membrane
- In the bacterial cell, the genetic material is not enclosed in a nucleus but floats freely in the cytoplasm
- The size difference supports the identification, as bacterial cells are much smaller than plant cells (prokaryotic cells are smaller than eukaryotic cells)
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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