Lesson 4 of 6 / Cells and microscopy
Recognise a bacterial cell
How can a cell function without a nucleus?
I can describe the characteristic organisation of a typical bacterium.
I can describe typical bacterial cell structure.Syllabus
Syllabus 9477, Core 1(d). Describe typical bacterial cell structure.
About 5 min
Make a guess. You are not marked.
A bacterium has no mitochondria. Can it carry out aerobic respiration?
Show the answer
It can, using its cytoplasm and membrane.
Aerobic bacteria run respiratory reactions in the cytoplasm, with electron carriers in the plasma membrane.
The key idea
A bacterium has a membrane, cytoplasm, DNA and ribosomes, but no membrane-bound nucleus or other membrane-bound organelles.
Explore the idea
A cell without a nucleus
DNA occupies a nucleoid without a nuclear envelope. Circular describes a continuous DNA molecule, not a perfectly circular shape in the cell.
A typical bacterium is small and unicellular. Optional capsules, pili and flagella are omitted here; their absence from this schematic is not a claim that all bacteria lack them.
Original schematic. Features, cell sizes and distances are not to scale. Use several visible clues when interpreting an unfamiliar section.
Explanation
A typical bacterium is small and unicellular. Its plasma membrane encloses cytoplasm containing enzymes and 70S ribosomes. DNA is usually organised as a circular chromosome in a nucleoid region, which is not enclosed by a nuclear envelope. Plasmids may occur as additional small DNA molecules.
The typical bacterial wall contains peptidoglycan, unlike the cellulose wall of plants. Some bacteria also have a capsule, flagellum or pili, but these are not universal defining requirements. Describe the feature shown rather than inventing every optional structure.
Bacteria lack membrane-bound organelles such as mitochondria, chloroplasts and endoplasmic reticulum. This does not mean they lack metabolism: reactions occur in their cytoplasm and at the plasma membrane. In aerobic bacteria, the cell membrane can house components for respiratory energy transfer.
Eukaryotic cytosolic ribosomes are 80S, whereas typical bacterial ribosomes are 70S. These labels describe sedimentation behaviour, not ordinary additive sizes. Bacterial small size often gives a large surface-area-to-volume ratio, facilitating exchange, but does not make bacteria viruses.
Step by step
- 1
Find cellular organisation
Look for membrane-enclosed cytoplasm and ribosomes.
- 2
Locate the DNA
A nucleoid is not a nucleus.
- 3
Compare like features
Compare wall material, ribosomes and organelles with a eukaryote.
Worked example
Work through the evidence
A cell has a peptidoglycan wall, 70S ribosomes and DNA without an envelope. Classify it and name one absent structure.
One way to explain it
It is a bacterial cell. A membrane-bound nucleus, mitochondrion or endoplasmic reticulum would not be expected in the typical bacterial model.
Why this answer works
- Use several consistent features.
- Absence of a nucleus does not mean absence of DNA.
Watch out for this
A student says: "Without mitochondria, bacteria cannot respire." What is wrong with this?
Show the answer
Respiratory reactions can occur in the cytoplasm and at the bacterial cell membrane.
Check your understanding
Which comparison is correct?
Show the answer
A bacterial nucleoid is not enclosed by a nuclear envelope.
The DNA region is not a membrane-bound nucleus.
Exam-style questions
Check your understanding
Which statements describe a typical bacterial cell? 1: Its ribosomes are 70S. 2: Its DNA is enclosed in a nuclear envelope. 3: Its wall contains peptidoglycan. 4: It has no mitochondria.
Show the answer
1, 3 and 4 only
Bacteria have 70S ribosomes, a peptidoglycan wall and no membrane-bound organelles. Their DNA lies in a nucleoid with no envelope, so statement 2 is false.
Quick recall
How can a cell function without a nucleus?