Lesson 4 of 5 / Cells and microscopy
Recognise a bacterial cell
How can a cell function without a nucleus?
In this lesson: Describe the characteristic organisation of a typical bacterium.
About 5 min
The key ideaA 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 ER. 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 ER 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.
Is this true? "Without mitochondria, bacteria cannot respire."
Respiratory reactions can occur in the cytoplasm and at the bacterial cell membrane.