Bacteria are single-celled prokaryotes with structures that help them survive, reproduce, and interact with their environment. A bacterial cell is much smaller and simpler than a plant or animal cell, but it is highly organized. Understanding bacterial cell structure helps explain infection, antibiotic action, genetic exchange, and how bacteria live in extreme environments.
A typical bacterium has protective outer layers, a plasma membrane, cytoplasm, ribosomes, and DNA located in a nucleoid region. Some bacteria also have capsules, flagella, pili, and plasmids that give them special advantages. Unlike eukaryotic cells, bacteria do not have a nucleus or membrane-bound organelles, so many life processes happen directly in the cytoplasm or across the plasma membrane.
Key Facts
- Bacteria are prokaryotic cells, meaning they lack a nucleus and membrane-bound organelles.
- The nucleoid contains the main circular bacterial chromosome, which is not enclosed by a nuclear membrane.
- Plasmids are small circular DNA molecules that often carry extra genes, such as antibiotic resistance genes.
- The cell wall gives shape and protection to most bacteria, and in many species it contains peptidoglycan.
- The plasma membrane controls what enters and leaves the cell and is the site of many energy-related reactions.
- Binary fission growth can be modeled by N = N0 x 2^n, where N is final cell number and n is the number of divisions.
Vocabulary
- Capsule
- A sticky outer coating that helps some bacteria attach to surfaces and avoid being destroyed by host defenses.
- Cell wall
- A rigid outer layer that supports the bacterial cell, protects it from bursting, and helps determine its shape.
- Nucleoid
- The region of a bacterial cell where the main circular chromosome is located without a surrounding membrane.
- Plasmid
- A small circular piece of DNA that can carry useful genes and may be transferred between bacteria.
- Flagellum
- A long whip-like structure that rotates to help some bacteria move through liquids.
Common Mistakes to Avoid
- Calling the nucleoid a nucleus is wrong because bacteria do not have a membrane-bound nucleus.
- Assuming all bacteria have capsules is wrong because capsules are common in some species but absent in many others.
- Confusing pili with flagella is wrong because pili mainly help with attachment or DNA transfer, while flagella mainly provide movement.
- Thinking plasmids are required for survival is wrong because plasmids often provide extra advantages, but the main chromosome carries essential genes.
Practice Questions
- 1 A bacterium divides by binary fission every 20 minutes. Starting with 1 cell, how many cells will be present after 2 hours?
- 2 A sample starts with 500 bacteria. If the population doubles 6 times, what is the final number of bacteria?
- 3 Explain why an antibiotic that damages peptidoglycan in the cell wall can kill many bacteria without directly damaging human cells.