Compare prokaryotic and eukaryotic cells
Compare cell organisation, DNA, ribosomes, size and organelles using the evidence supplied for each cell.
AS Level · Cambridge International Biology 9700, 2028–2030. Interpret only the structures supported by the provided image or description. Written practice is self-checked; it does not demonstrate practical microscopy competence.
Before you begin
- Recognise that organisms are made of cells and that a light microscope produces a magnified image.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where is a typical bacterium’s circular DNA found?
What you will learn
- Compare a typical bacterial cell with typical plant and animal cells
- Use shared and distinguishing features in a structured comparison
Compare the same features in each cell
A typical bacterium is prokaryotic, unicellular and generally 1–5 µm in diameter. It has circular DNA not enclosed in a nucleus, 70S ribosomes, a peptidoglycan cell wall and no double-membraned organelles.
Typical plant and animal cells are eukaryotic and have a nucleus containing linear chromosomes. Their cytoplasm contains 80S ribosomes and membrane-bound organelles such as mitochondria. Typical plant cells also have a cellulose wall, chloroplasts in photosynthetic tissues and a large permanent vacuole.
State similarities as well as differences
All three cell types have a cell-surface membrane, cytoplasm, DNA and ribosomes. Use a named feature for each comparison rather than writing only “more complex”.
A comparison should specify both sides: for example, bacterial DNA is circular and not enclosed in a nucleus, whereas eukaryotic chromosomes are linear and enclosed within a nucleus. If a typical animal cell is compared with a typical plant cell, avoid presenting plant-specific structures as shared by both.
Pause and explain
Which feature is shared by bacterial, plant and animal cells?
Worked example
Compare a typical bacterium with a typical animal cell using DNA organisation, ribosomes and membrane-bound organelles.
Show the worked solution
- State both structures for each feature.
- Include a correct similarity or limitation if useful.
Answer A typical bacterium has circular DNA not enclosed in a nucleus, whereas a typical animal cell has linear chromosomes within a nucleus. Bacteria have 70S ribosomes, whereas eukaryotic cytoplasm has 80S ribosomes. A typical bacterium lacks double-membraned organelles such as mitochondria, while a typical animal cell contains mitochondria.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare a typical bacterium and an animal cell by three features.
Give me a hint
Use the same feature in each side of every comparison.
Compare my reasoning
- Bacteria are smaller and lack a nucleus.
- They differ in DNA organisation, ribosome type and double-membraned organelles.
A typical bacterium is generally 1–5 µm in diameter and an animal cell is typically larger. Bacterial circular DNA is not enclosed in a nucleus, whereas animal-cell linear chromosomes are nuclear. Bacteria have 70S ribosomes and lack double-membraned organelles; animal-cell cytoplasm has 80S ribosomes and contains mitochondria.
Look for these in your work
- Three paired comparisons
- Accurate size or organisation
State two similarities and two differences between a typical plant cell and a typical bacterium.
Give me a hint
Compare shared boundary/contents and wall or organelles.
Compare my reasoning
- Both have a cell-surface membrane and cytoplasm.
- Bacteria lack a nucleus and have peptidoglycan; plant cells have a nucleus and cellulose wall.
Both have a cell-surface membrane and cytoplasm. A typical bacterium has circular DNA not enclosed in a nucleus and a peptidoglycan wall; a typical plant cell has a nucleus with linear chromosomes and a cellulose wall, and may contain chloroplasts and a large permanent vacuole.
Look for these in your work
- Two similarities
- Two differences correctly paired
A comparison table says “both cells have 70S ribosomes.” Identify the error when one cell is a typical eukaryotic animal cell.
Give me a hint
Consider the ribosome location.
Compare my reasoning
- Bacterial cytoplasm has 70S ribosomes.
- Eukaryotic cytoplasm has 80S; 70S occurs in mitochondria and chloroplasts.
A typical bacterium has 70S ribosomes. A eukaryotic animal-cell cytoplasm has 80S ribosomes. Eukaryotic mitochondria have 70S ribosomes, but that does not make cytoplasmic ribosomes 70S.
Look for these in your work
- Bacterial type
- Eukaryotic location distinction
Common mistakes
- Using “prokaryotes have no DNA”
- Claiming eukaryotic cells lack 70S ribosomes in organelles
- Comparing two different features rather than the same feature
What can you explain now?
Where is a typical bacterium’s circular DNA found?
Compare with the explanation
Not enclosed in a nucleus
Prokaryotic DNA is not enclosed by a nuclear envelope.
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Make it stick
Which features are shared by all cells, and which distinguish prokaryotes from eukaryotes?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.