For three billion years, prokaryotes were the only life on Earth. Understanding their structure isn't just biology history — it's the reason antibiotics work at all.
Here's a statement that many students believe:
"Prokaryotic cells are just simpler, smaller versions of eukaryotic cells — they have all the same parts, just fewer of them."
Do you agree or disagree? Write your reasoning. You'll revisit this at the end of the lesson.
Come back to this at the end of the lesson.
Core Content
Life appeared on Earth approximately 3.8 billion years ago. For the first three billion of those years — roughly 80% of life's entire history — every living thing on the planet was prokaryotic. Eukaryotes, the complex cells that make up plants, animals, fungi and protists, are a relatively recent innovation.
Prokaryotes survived ice ages, meteorite impacts, mass extinctions, and the oxygenation of Earth's atmosphere. They colonised hydrothermal vents, salt lakes, Antarctic ice and human intestines. Their success is directly tied to their structure: simple, fast-replicating, and extraordinarily adaptable.
Every prokaryotic cell, without exception, contains these structures:
Phospholipid bilayer surrounding the cytoplasm. Controls what enters and exits the cell. Present in all living cells.
Gel-like fluid filling the cell. Site of most metabolic reactions. Contains the nucleoid, ribosomes, and dissolved molecules.
Area where the single circular chromosome of DNA is located. Not enclosed by a membrane — this is the defining prokaryotic feature.
Site of protein synthesis. Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S). This size difference is why many antibiotics work.
Rigid outer layer providing structural support and shape. Made of peptidoglycan in bacteria (different from plant cell walls which use cellulose). Absent in animal cells entirely.
These structures are not universal — they appear in some prokaryotes depending on species and environment:
| Structure | What it is | Function | Present in |
|---|---|---|---|
| Capsule | Thick outer layer of polysaccharides outside the cell wall | Protection from desiccation, immune evasion, attachment to surfaces | Many pathogenic bacteria (e.g. Streptococcus pneumoniae) |
| Flagella | Long whip-like protein appendages | Motility — allows bacteria to move toward nutrients or away from toxins | E. coli, Salmonella, many others |
| Pili (fimbriae) | Short hair-like protein projections | Attachment to host cells or surfaces; sex pili transfer DNA between cells (conjugation) | Many gram-negative bacteria |
| Plasmids | Small circular loops of DNA separate from the main chromosome | Carry accessory genes — often antibiotic resistance genes — that can be transferred between bacteria | Common in bacteria; not universal |
The HSC requires you to compare prokaryotic and eukaryotic cells. This table covers the features most commonly examined:
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | Absent — DNA in nucleoid region | Present — DNA enclosed in membrane-bound nucleus |
| DNA form | Single circular chromosome; may have plasmids | Multiple linear chromosomes enclosed in nucleus |
| Membrane-bound organelles | Absent (no mitochondria, ER, Golgi, etc.) | Present — mitochondria, ER, Golgi body, etc. |
| Ribosomes | 70S (smaller) | 80S (larger) |
| Cell wall | Present — peptidoglycan (bacteria) | Present in plants (cellulose), fungi (chitin); absent in animals |
| Size | Typically 1–10 µm | Typically 10–100 µm |
| Cell division | Binary fission | Mitosis / meiosis |
| Examples | Bacteria, Archaea | Animals, plants, fungi, protists |
Misconception: Prokaryotes have no DNA.
Prokaryotes have a single circular chromosome of DNA located in the nucleoid region. They may also carry additional DNA in plasmids. What they lack is a membrane-bound nucleus to contain it.
Misconception: The prokaryotic cell wall is the same as the plant cell wall.
Bacterial cell walls are made of peptidoglycan, a unique polymer of sugars and amino acids. Plant cell walls are made of cellulose. This difference is precisely why antibiotics like penicillin can target bacteria without harming plant or animal cells.
Misconception: Prokaryotes are less successful than eukaryotes because they're simpler.
Prokaryotes are the most numerically abundant, metabolically diverse, and ecologically widespread organisms on Earth. Simplicity enables rapid reproduction and fast evolution — major advantages in changing environments.
Activities
In your book, draw a labelled diagram of a prokaryotic cell. Your diagram must include:
Then, below your diagram, write two sentences explaining why the nucleoid is fundamentally different from a eukaryotic nucleus. Do not just say "there is no membrane" — explain what the functional consequence of this difference is.
Draft your two sentences here.
A microbiologist isolates a single-celled organism from a hot spring. It has the following characteristics:
Write your responses here or in your book.
Assessment
1. Which of the following structures is present in ALL prokaryotic cells?
2. Penicillin disrupts the synthesis of peptidoglycan in bacterial cell walls. Why does this antibiotic not damage human cells?
3. A scientist observes a cell with a nucleoid region, 70S ribosomes, and a cell wall. There are no membrane-bound organelles. Which domain does this organism most likely belong to?
4. Which statement correctly describes the difference between prokaryotic and eukaryotic ribosomes?
5. A bacterium acquires a plasmid carrying a gene encoding beta-lactamase — an enzyme that breaks down penicillin. The bacterium then uses pili to transfer this plasmid to a neighbouring cell. This process is best described as:
1. Draw and label a prokaryotic cell. Identify one structure present in some prokaryotes only and explain its function. (3 marks)
1 mark for accurate labelled diagram with ≥4 structures; 1 mark for correct optional structure named; 1 mark for function explained
2. Compare prokaryotic and eukaryotic cells. In your answer, refer to the nucleus, ribosomes, and cell wall. (3 marks)
1 mark per feature correctly compared
3. Explain how the structural features of prokaryotic cells make them both a target for antibiotics and capable of developing resistance to those antibiotics. (3 marks)
1 mark antibiotic target (cell wall or ribosome); 1 mark why human cells are unaffected; 1 mark resistance mechanism via plasmids/pili
Answers
SA1: A labelled diagram should include: cell membrane, cytoplasm, nucleoid (circular DNA), 70S ribosomes, cell wall (peptidoglycan). An optional structure example: Flagellum — a long whip-like protein appendage that rotates to propel the bacterium through its environment, enabling movement toward nutrients or away from toxins.
SA2: Nucleus: prokaryotes lack a membrane-bound nucleus; their DNA sits in an unenclosed nucleoid region. Eukaryotes have a true nucleus where DNA is enclosed within a double membrane. Ribosomes: prokaryotes have smaller 70S ribosomes; eukaryotes have larger 80S ribosomes — a difference exploited by antibiotics. Cell wall: prokaryotic bacteria have cell walls made of peptidoglycan; eukaryotic plant cells have cell walls made of cellulose; eukaryotic animal cells have no cell wall.
SA3: Antibiotics target structures unique to prokaryotes. Penicillin disrupts peptidoglycan synthesis in the bacterial cell wall — a structure not found in human (eukaryotic) cells, which have no cell wall. Streptomycin binds to prokaryotic 70S ribosomes, inhibiting protein synthesis — human 80S ribosomes are structurally different and unaffected. Resistance develops because bacteria can carry plasmids containing resistance genes (e.g. beta-lactamase, which breaks down penicillin). These plasmids can be transferred between bacteria via pili through conjugation, spreading resistance rapidly through a bacterial population without requiring cell division.
You were asked whether prokaryotes are just "simpler, smaller versions of eukaryotic cells with fewer parts." What's the verdict?
This is wrong in an important way. Prokaryotes aren't missing parts — they have a fundamentally different organisation. They lack the entire membrane-compartmentalisation system that defines eukaryotes. No nucleus, no ER, no mitochondria, no Golgi. Some structures (like the cell wall) are present in both but made of completely different materials with different properties.
The 70S vs 80S ribosome difference alone is enough to show this: prokaryotic and eukaryotic ribosomes are built differently at the molecular level — not just scaled down versions of each other.