What you need to know
- Identify the main structures of a bacterial cell.
- Compare bacterial cells with animal and plant cells.
- Describe binary fission and factors affecting bacterial growth.
- Calculate population size from a mean division time.
- Recognise yeast as a single-celled fungus.
Structure of a bacterial cell
A bacterium is a prokaryotic cell. It has cytoplasm surrounded by a cell membrane and a cell wall. Its genetic material is not contained inside a nucleus.
| Structure | Function or importance |
|---|---|
| Cell membrane | Controls movement of substances into and out of the cell. |
| Cytoplasm | Contains enzymes and is where many reactions occur. |
| Ribosomes | Make proteins; bacterial ribosomes are smaller than eukaryotic ribosomes. |
| Cell wall | Supports and protects the cell. It is not made from cellulose. |
| Circular DNA | The main loop of genetic material, free in the cytoplasm. |
| Plasmids | Small rings of DNA carrying extra genes, such as antibiotic-resistance genes. |
| Slime capsule | May protect the bacterium and help it attach to surfaces. |
| Flagellum | A tail-like structure that may rotate to move the cell. |
Write circular DNA or a loop of chromosomal DNA. A bacterium does not have chromosomes inside a nucleus.
Bacterial cells compared with eukaryotic cells
Bacterial cells are usually much smaller than animal and plant cells. They have no nucleus, mitochondria or chloroplasts. They do have a cell membrane, cytoplasm and ribosomes.
Bacterial cell
- Prokaryotic
- No nucleus
- Circular DNA and possible plasmids
- No membrane-bound organelles
- Cell wall not made of cellulose
Animal or plant cell
- Eukaryotic
- Nucleus present
- Linear chromosomes in nucleus
- Mitochondria present
- Plant cell wall made of cellulose
Binary fission and population growth
When conditions are suitable, a bacterium grows, copies its circular DNA and divides into two genetically identical cells. This is binary fission. Each new cell can divide again, so the population doubles repeatedly.
Growth is fastest when bacteria have enough nutrients, a suitable temperature and appropriate moisture, oxygen and pH. If a bacterium divides every 20 minutes, one cell can produce eight cells after 60 minutes because three divisions have occurred.
A culture starts with 250 bacteria. The mean division time is 30 minutes. Calculate the population after 3 hours.
- Convert 3 hours to 180 minutes.
- Divisions = 180 ÷ 30 = 6.
- Population = 250 × 26 = 250 × 64 = 16,000 bacteria.
Repeated doubling produces exponential growth. Answers can become very large, so Higher-tier questions may require standard form.
Yeast: a microbial eukaryote
Yeast is a single-celled fungus. Unlike a bacterium, a yeast cell is eukaryotic and has a nucleus and mitochondria. It also has a cell wall, but this wall is not made from cellulose. Yeast can respire anaerobically, producing ethanol and carbon dioxide, which makes it useful in brewing and baking.
Misconception AlertEvery microorganism is a bacterium.Select to reveal the correctionSelect to hide the correction
Microorganisms include bacteria, microscopic fungi such as yeast, protists and microscopic algae.
Quick retrieval check
1. Where is the main DNA in a bacterial cell?
2. What is a plasmid?
3. Name three conditions that affect bacterial growth.
4. A bacterium divides every 15 minutes. How many divisions occur in two hours?
Exam connection
A culture starts with 40 bacteria. The population doubles every 20 minutes. Calculate the population after 2 hours.
Show the answer
2 hours = 120 minutes. Divisions = 120 ÷ 20 = 6. Population = 40 × 26 = 40 × 64 = 2,560 bacteria.
Always calculate the number of divisions before applying repeated doubling.