Describe a typical bacterial cell
A typical bacterium is a small unicellular prokaryote with circular DNA, 70S ribosomes and a peptidoglycan cell wall.
Section 1.2 · Cells as the basic units of living organisms
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
What material forms a typical bacterial cell wall?
What you will learn
- Outline the stated features of a typical bacterial cell
- Distinguish prokaryotic DNA and ribosomes from eukaryotic structures
A small unicellular prokaryote
A typical bacterium is unicellular and generally 1–5 µm in diameter. It has a cell-surface membrane, cytoplasm and a cell wall made of peptidoglycan.
Its DNA is circular and is not enclosed in a nucleus. A typical bacterium has 70S ribosomes and lacks organelles surrounded by double membranes, such as mitochondria and chloroplasts.
Use “typical” carefully
Bacteria vary in shape and in accessory structures. The syllabus comparison focuses on a typical bacterium and lists the main features above. Do not add a plasmid, capsule or flagellum unless the question or image supplies evidence.
The cell wall material is a useful distinction: peptidoglycan in a typical bacterial cell, compared with cellulose in plant cell walls. The cell-surface membrane is present in both prokaryotic and eukaryotic cells.
Pause and explain
What is the usual ribosome type in a typical bacterium?
Worked example
A cell is 2 µm across, has circular DNA outside a nucleus, 70S ribosomes and a peptidoglycan wall. Classify it.
Show the worked solution
- Use its size and DNA organisation.
- Compare the ribosome type and wall.
Answer The described cell is consistent with a typical bacterium: it is within the general 1–5 µm size range, has circular DNA not enclosed in a nucleus, 70S ribosomes and a peptidoglycan cell wall.
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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.
List four syllabus features of a typical bacterial cell.
Give me a hint
Include size, DNA, ribosomes or wall.
Compare my reasoning
- It is unicellular and generally 1–5 µm in diameter.
- It has circular DNA, 70S ribosomes and a peptidoglycan wall.
A typical bacterium is unicellular, generally 1–5 µm in diameter, has circular DNA, has 70S ribosomes and has a peptidoglycan cell wall. It lacks organelles surrounded by double membranes.
Look for these in your work
- At least four accurate features
- Prokaryotic distinction included
A diagram shows a bacterium with circular DNA in the cytoplasm and no mitochondrion. Explain why this is consistent with a prokaryote.
Give me a hint
Look for a nucleus and double-membraned organelles.
Compare my reasoning
- The DNA is not enclosed in a nucleus.
- Prokaryotes lack double-membraned organelles.
The circular DNA is not enclosed in a nucleus, and the cell lacks a double-membraned organelle such as a mitochondrion. These features are consistent with a prokaryotic cell.
Look for these in your work
- Nucleus distinction
- Organelle distinction
A student calls a bacterial wall “cellulose.” Correct the description and compare it with a plant wall.
Give me a hint
Name each wall material.
Compare my reasoning
- A typical bacterial wall contains peptidoglycan.
- A plant cell wall contains cellulose.
A typical bacterial cell wall is made of peptidoglycan. A plant cell wall is made mainly of cellulose in the cells considered here; the materials should not be confused.
Look for these in your work
- Bacterial material corrected
- Plant comparison accurate
Common mistakes
- Calling a bacterium’s circular DNA a chromosome inside a nucleus
- Confusing peptidoglycan with plant cellulose
- Saying all bacteria have every accessory structure
What can you explain now?
What material forms a typical bacterial cell wall?
Compare with the explanation
Peptidoglycan
The syllabus specifies peptidoglycan cell walls in typical bacteria.
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Make it stick
Which features distinguish a typical bacterium from a eukaryotic cell?
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