By the end of this lesson
- Describe the common structural features of bacteria.
- Distinguish the main bacterial chromosome from plasmids.
- Use nutrition and examples to show that bacteria are diverse.
BIG QUESTION
How can a cell function without a nucleus or mitochondria?
Bacteria are structurally simple, but their membrane, cytoplasm, ribosomes and DNA support independent life processes.
A complete living cell
A bacterium is a unicellular prokaryoteA single-celled organism with no nucleus or membrane-bound organelles, such as a bacterium.. It has a cell membrane, cytoplasm and ribosomes, usually surrounded by a cell wall. Its main chromosome is a loop of circular DNA lying free in the cytoplasm because there is no nucleus.
Bacteria do not have mitochondria, chloroplasts or other membrane-bound organelles. This does not prevent respiration or photosynthesis: the relevant reactions occur in the cytoplasm or on specialised cell membranes rather than inside eukaryotic organelles.
Chromosomal DNA and plasmids
The main circular chromosome carries genes needed for essential cell functions. Many bacteria also contain plasmidA small ring of DNA in a bacterial cell, separate from the main chromosome, used as a vector in genetic engineering., much smaller rings of DNA carrying additional genes.
A plasmid is not the bacterium’s only DNA, and not every bacterium contains every plasmid. Plasmids can be copied and transferred between bacteria, which matters when they carry genes such as antibiotic resistance.
Structures found in some bacteria
Some bacteria have a flagellumA long, rotating protein structure that moves some bacterial cells; a sperm cell also uses a flagellum (tail) for movement. that rotates to move the cell. Some produce a slime capsule that helps adhesion and can reduce water loss or recognition by host defences. Do not describe these optional structures as universal.
The bacterial cell wall supports the cell but is not made of cellulose or chitin. The key distinction is that a wall may be present while a nucleus and membrane-bound organelles are absent.
Different ways to obtain nutrients
Some bacteria are heterotrophs and absorb organic nutrients. Lactobacillus converts lactose to lactic acid and is used in yoghurt production. Other bacteria use light or chemical energy to manufacture organic molecules.
Some bacteria are pathogens, but most bacterial species do not cause human disease. Pneumococcus is a course example of a spherical pathogenic bacterium that can cause pneumonia. “Bacterium” describes a cell type; “pathogen” describes an effect on a host.
Reading a bacterial cell
Link each structure to evidence about how the cell survives.
| Structure | What it is | Why it matters |
|---|---|---|
| Cell membrane | Boundary around the cytoplasm. | Controls exchange and supports membrane-associated reactions. |
| Cytoplasm | Aqueous interior containing enzymes. | Site of many metabolic reactions. |
| Ribosomes | Small protein-making structures. | Build proteins from amino acids. |
| Circular chromosome | Main loop of DNA free in the cytoplasm. | Carries essential genetic information. |
| Plasmid | Small additional ring of DNA. | May carry transferable advantageous genes. |
| Cell wall | Rigid layer outside the membrane. | Supports the cell and helps prevent osmotic bursting. |
Language that earns precision
- prokaryote
- An organism whose cell lacks a membrane-bound nucleus and other membrane-bound organelles.
- plasmid
- A small circular DNA molecule separate from the main bacterial chromosome.
- chromosome
- The main DNA molecule carrying genes; bacterial chromosomes are usually circular.
- flagellum
- A rotating protein structure that moves some bacterial cells.
- pathogen
- A microorganism or infectious agent (including viruses) that causes disease.
Common misconceptions
Claim: Bacteria have no DNA because they have no nucleus.
Correction: Their circular chromosome and any plasmids lie free in the cytoplasm.
Claim: Bacteria cannot respire because they have no mitochondria.
Correction: They carry out respiratory reactions in the cytoplasm and across cell membranes.
Claim: Photosynthetic bacteria contain chloroplasts.
Correction: They use pigments and membrane systems but do not possess chloroplast organelles.
Claim: All bacteria are pathogens.
Correction: Only some cause disease; many are harmless or beneficial.
Match the answer to the command word
Name the stated structures accurately; do not add organelles that bacteria lack.
Link structure to function, such as ribosomes to protein synthesis.
Give a paired difference between the two things named.
Recall, apply, then justify
Recall, apply, then justify (continued)
State three structures found in a typical bacterial cell.
Hint
Choose common, not optional, structures.
Suggested marking points
- Any three from cell membrane, cytoplasm, ribosomes, cell wall, circular chromosome.
Explain why lacking mitochondria does not prevent a bacterium from respiring.
Hint
Name where bacterial respiratory reactions occur.
Suggested marking points
- Bacteria have enzymes needed for respiration.
- Reactions occur in the cytoplasm and/or across the cell membrane rather than in mitochondria.
Distinguish the main bacterial chromosome from a plasmid.
Hint
Compare size, role and whether it is additional DNA.
Suggested marking points
- The chromosome is the main large circular DNA molecule.
- It carries genes essential for normal cell function.
- A plasmid is a smaller additional ring that may carry accessory genes and can be transferred.
A cell has a wall, ribosomes and circular DNA but no nucleus. Identify it and justify your answer.
Hint
Name the group, then use two diagnostic features.
Suggested marking points
- It is a bacterium or prokaryote.
- Its circular DNA is not enclosed in a nucleus.
- Ribosomes plus a wall are consistent with a bacterial cell.
Explain how an antibiotic-resistance plasmid could increase in frequency after antibiotic treatment.
Planning hint
Use variation, selection, survival and reproduction.
What a strong design includes
- Some bacteria already carry the resistance plasmid.
- The antibiotic kills susceptible bacteria or prevents their reproduction.
- Resistant bacteria survive and reproduce.
- They pass or transfer the plasmid, increasing its frequency.