Prokaryotic Cells and Viruses
Bacteria are prokaryotes: cells without a nucleus or any of the membrane-bound organelles described in Eukaryotic cells. Viruses are not cells at all. Knowing exactly how these differ from eukaryotic cells explains why antibiotics kill bacteria but not viruses, why cholera and TB are treated so differently from HIV, and why viruses are hard to classify as living. Examiners ask for the structural features of a typical bacterium, a comparison table with plant and animal cells, and the structure of a virus.
The prokaryotic cell
Prokaryote means "before nucleus". The DNA of a prokaryote lies free in the cytoplasm rather than inside a nuclear envelope. All bacteria are prokaryotes.
Key structural features of a typical bacterium (syllabus list):
- unicellular (each organism is a single cell, though cells may stick together in chains or clusters);
- generally 1–5 µm in diameter;
- peptidoglycan cell wall;
- circular DNA;
- 70S ribosomes;
- absence of organelles surrounded by double membranes (no nucleus, mitochondria or chloroplasts).
Each structure in more detail
Cell wall. Made of peptidoglycan (also called murein): long polysaccharide chains cross-linked by short peptide chains, forming a strong net around the cell. It prevents the cell from bursting by osmosis when water enters and maintains the cell's shape. Its chemistry is completely different from the cellulose wall of a plant cell, which is why penicillin, which stops cross-links forming in peptidoglycan, harms bacteria but not plants or humans (see Antibiotics).
Cell surface membrane. A phospholipid bilayer with proteins, as in eukaryotes. In bacteria, the membrane also carries the enzymes for respiration (and, in photosynthetic bacteria, the pigments for photosynthesis), because there are no mitochondria or chloroplasts.
Circular DNA. The main genetic material is a single, circular DNA molecule, sometimes called the bacterial chromosome. It lies in a region of the cytoplasm called the nucleoid, with no nuclear envelope around it. Unlike eukaryotic chromosomes, bacterial DNA is not associated with histone proteins in the same way, and it is not organised into a linear chromosome with a centromere or telomeres.
Plasmids. Many bacteria also contain one or more plasmids: small, circular loops of DNA separate from the main DNA, which replicate independently. Plasmids often carry genes that give an advantage, such as antibiotic resistance, and can be passed between bacteria. Plasmids are not on the syllabus list of "key features", but they are needed to explain how resistance spreads.
70S ribosomes. Smaller than the 80S ribosomes in eukaryotic cytoplasm, and the same size as those in mitochondria and chloroplasts. Many antibiotics (such as streptomycin and tetracycline) bind to 70S but not 80S ribosomes, stopping bacterial protein synthesis without stopping ours.
Absence of double-membrane organelles. There is no nucleus, no mitochondria and no chloroplasts. In fact bacteria have no membrane-bound organelles at all: no ER, Golgi body or lysosomes.
Other structures found in some bacteria.
- A capsule (or slime layer) outside the wall, made of polysaccharide, which protects against drying out and helps the bacterium resist being engulfed by phagocytes. Pathogenic bacteria often have capsules.
- One or more flagella (singular flagellum): long, corkscrew-shaped protein structures that rotate to move the cell. A bacterial flagellum is much thinner than a eukaryotic flagellum and does not contain the 9 + 2 arrangement of microtubules.
- Pili (singular pilus): short, hair-like protein projections used to attach to surfaces or other cells, and in some cases to transfer plasmids between bacteria.
Older textbooks show mesosomes, infoldings of the bacterial membrane. These are now known to be artefacts produced when bacteria are prepared for electron microscopy. Do not draw or describe them as real structures.
Comparing prokaryotic and eukaryotic cells
This comparison is one of the most frequently set tables in AS Biology. Learn it as pairs.
| Feature | Prokaryotic cell (typical bacterium) | Eukaryotic cell (plant or animal) |
|---|---|---|
| Size | generally 1–5 µm diameter | generally 10–100 µm |
| Nucleus | absent; DNA free in the cytoplasm (nucleoid) | present, surrounded by a nuclear envelope |
| DNA | circular, not associated with histones | linear, in chromosomes, associated with histones |
| Plasmids | often present | usually absent |
| Ribosomes | 70S only | 80S in cytoplasm (70S in mitochondria and chloroplasts) |
| Membrane-bound organelles | none (no mitochondria, chloroplasts, ER, Golgi body) | present |
| Site of respiration | cell surface membrane (and cytoplasm) | mitochondria (and cytoplasm) |
| Cell wall | always present; peptidoglycan | cellulose in plants; absent in animals |
| Flagella (if present) | thin, no microtubules, rotate | thicker, 9 + 2 microtubules, beat (not in flowering plants) |
| Capsule | sometimes present | absent |
| Cell division | binary fission (no mitosis, no spindle) | mitosis (or meiosis), with spindle |
| Organisation | always unicellular | unicellular or multicellular |
What they share: a cell surface membrane, cytoplasm, ribosomes, DNA as the genetic material, and the same genetic code. These shared features show that all cells have a common ancestry.
Compare the structure of a typical bacterium with that of a palisade mesophyll cell.
Solution
Award one mark for each valid comparison, including at least one similarity:
- Both have a cell wall, but the bacterial wall is made of peptidoglycan, while the plant cell wall is made of cellulose.
- Both have a cell surface membrane, cytoplasm and ribosomes.
- The bacterium has 70S ribosomes; the plant cell has 80S ribosomes in its cytoplasm (and 70S in its chloroplasts and mitochondria).
- The bacterium has circular DNA free in the cytoplasm; the plant cell has linear DNA in chromosomes inside a nucleus with a nuclear envelope.
- The bacterium has no chloroplasts or mitochondria (no double-membrane organelles); the palisade cell has both.
- The bacterium has no ER, Golgi body or large vacuole; the plant cell does.
- The bacterium is much smaller (1–5 µm) than the plant cell (about 40–100 µm).
- The bacterium may have plasmids, a capsule or flagella; the plant cell does not.
Viruses
Viruses are far smaller than bacteria, about 20–300 nm across, so they can only be seen with an electron microscope. They are not cells: they have no cytoplasm, no cell surface membrane of their own, no ribosomes and no metabolism. A virus can only reproduce by entering a living host cell and using the host's ribosomes, enzymes, nucleotides, amino acids and ATP to make copies of itself. For this reason viruses are described as obligate intracellular parasites.
All viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein. Some viruses have an outer envelope made of phospholipids.
Structure of a virus
- Nucleic acid core. Either DNA or RNA, never both. It may be single-stranded or double-stranded. It carries the genes needed to make new virus particles. HIV, influenza virus and the coronavirus that causes COVID-19 are RNA viruses; the viruses that cause smallpox and herpes are DNA viruses.
- Capsid. A protein coat made of many identical protein subunits called capsomeres. The capsid protects the nucleic acid and, in viruses without an envelope, its proteins attach the virus to receptors on the host cell.
- Envelope (some viruses only). An outer layer of phospholipids, taken from the host cell's membrane as new virus particles leave the cell. It contains glycoproteins coded for by the virus, which bind to receptors on the host cell. HIV and influenza virus are enveloped.
- Some viruses carry enzymes inside the capsid. HIV contains reverse transcriptase, which makes DNA from the viral RNA once inside the host cell.
A worked comparison: virus, bacterium, eukaryotic cell
| Feature | Virus | Bacterium | Eukaryotic cell |
|---|---|---|---|
| Cellular? | no | yes | yes |
| Size | 20–300 nm | 1–5 µm | 10–100 µm |
| Nucleic acid | DNA or RNA, not both | DNA (and RNA) | DNA (and RNA) |
| Ribosomes | none | 70S | 80S (70S in organelles) |
| Outer layer | protein capsid; some have phospholipid envelope | peptidoglycan wall, cell surface membrane | cell surface membrane; wall in plants |
| Metabolism | none outside a host cell | yes | yes |
| Reproduction | only inside a host cell | binary fission | mitosis and cytokinesis |
| Affected by antibiotics? | no | yes (many) | no |
Are viruses alive?
Living things show characteristics such as nutrition, respiration, growth, excretion, response to stimuli and reproduction, and are made of cells. Viruses only show one of these, reproduction, and only inside a host cell. They do contain genetic material and they evolve. Because of this, viruses are usually described as being on the boundary between living and non-living, and they are not placed in any of the kingdoms of living organisms. The syllabus wants you to recognise that viruses challenge the idea that the cell is the basic unit of life.
An electron micrograph of HIV particles has a magnification of . One particle is 22 mm across in the image. Calculate its actual diameter in nm and explain why HIV cannot be seen with a light microscope.
Solution
.
The resolution of the light microscope is about 200 nm, limited by the wavelength of light. The 110 nm particle is smaller than this, so it cannot be distinguished; an electron microscope (resolution about 0.5 nm) is needed.
Using your knowledge of cell structure, explain why antibiotics used to treat cholera would have no effect on HIV.
Solution
- Antibiotics act on structures or processes found in bacteria, such as the synthesis of the peptidoglycan cell wall or protein synthesis on 70S ribosomes.
- Viruses have no cell wall and no ribosomes (and no metabolism of their own).
- HIV reproduces inside host cells, using the host's 80S ribosomes and enzymes, which antibiotics do not affect.
- So there is no target for the antibiotic in HIV.
An organism is found to have: a cell wall that does not contain cellulose; circular DNA; 70S ribosomes; no nucleus; and a diameter of 2 µm. A second structure has a protein coat, a single strand of RNA, a diameter of 90 nm, and no ribosomes. Identify each, giving reasons, and state which could be grown on a nutrient agar plate.
Solution
The first is a prokaryote (bacterium): no nucleus, circular DNA, 70S ribosomes, a non-cellulose (peptidoglycan) wall, and a size in the typical 1–5 µm range.
The second is a virus: it has only a nucleic acid core (RNA) and a protein capsid, no ribosomes, and is in the virus size range (20–300 nm).
Only the bacterium could be grown on nutrient agar, because it is a cell with its own metabolism and ribosomes. The virus can only reproduce inside living host cells, so it would need a culture of living cells.
- Use exact syllabus phrasing: "peptidoglycan cell wall", "circular DNA", "70S ribosomes", "no membrane-bound organelles" (or "no organelles surrounded by double membranes").
- Say bacterial DNA is "not in a nucleus" or "free in the cytoplasm", not "bacteria have no DNA".
- For viruses, the four marking points are: non-cellular; nucleic acid core of either DNA or RNA; protein capsid; some have a phospholipid envelope.
- Do not call a virus a "cell" or a "microorganism with organelles", and do not say a virus "has a cell membrane". The envelope is made of phospholipid taken from the host.
- "Prokaryotes have no ribosomes" is a common wrong answer. They have ribosomes; they are 70S.
- Prokaryotes (bacteria) are unicellular, generally 1–5 µm, with a peptidoglycan wall, circular DNA, 70S ribosomes and no double-membrane organelles.
- Many bacteria also have plasmids, a capsule, flagella (without microtubules) and pili.
- Eukaryotic cells are larger, with a nucleus, linear DNA with histones, 80S cytoplasmic ribosomes and membrane-bound organelles.
- Viruses are non-cellular: a nucleic acid core of DNA or RNA, a protein capsid, and in some an outer phospholipid envelope.
- Viruses are 20–300 nm and can only reproduce inside host cells; they have no ribosomes or metabolism.
- Antibiotics target bacterial structures (peptidoglycan wall, 70S ribosomes), so they do not affect viruses.
Practice questions
- List the key structural features of a typical prokaryotic cell.
- State two structures found in both a bacterium and a eukaryotic cell, and two found only in the eukaryotic cell.
- Describe the structure of a virus. (3 marks)
- Explain why viruses are described as non-cellular.
- A bacterium is shown in a micrograph at . The image is 24 mm long. Calculate its actual length and state whether this is typical for a bacterium.
- Compare the DNA of a bacterium with the DNA in the nucleus of an animal cell. (3 marks)
- State the difference between the cell walls of bacteria and plant cells, and explain why this difference is useful in medicine.
- Explain why an enveloped virus such as HIV has phospholipids in its envelope, even though viruses cannot make phospholipids.
- Compare the structure of a bacterium, a virus and a liver cell. (6 marks)
- Some scientists argue that cells are the basic unit of life. Discuss how viruses challenge this view. (4 marks)
Answers
- Unicellular; generally 1–5 µm diameter; peptidoglycan cell wall; circular DNA; 70S ribosomes; no organelles surrounded by double membranes (no nucleus, mitochondria or chloroplasts). (Plasmids, capsule, flagella, pili in some.)
- Both: cell surface membrane, cytoplasm, ribosomes, DNA. Eukaryotic only: any two of nucleus (nuclear envelope), mitochondria, ER, Golgi body, 80S ribosomes, chloroplasts (plants), lysosomes.
- Non-cellular; nucleic acid core of DNA or RNA; protein capsid (made of capsomeres); some have a phospholipid envelope (with glycoproteins).
- They have no cytoplasm, no cell surface membrane, no organelles such as ribosomes, and no metabolism of their own; they consist only of nucleic acid and protein (and sometimes an envelope) and can only reproduce inside host cells.
- ; this is typical (within 1–5 µm).
- Bacterial DNA is circular, the animal cell's linear; bacterial DNA is free in the cytoplasm (nucleoid), the animal cell's is inside a nucleus with a nuclear envelope; bacterial DNA is not associated with histones, animal DNA is wound around histones in chromosomes; bacteria may also have plasmids; both are made of DNA with the same genetic code.
- Bacterial walls are made of peptidoglycan; plant walls of cellulose (human cells have no wall). Antibiotics such as penicillin can target peptidoglycan synthesis, killing bacteria without harming human cells.
- As new virus particles leave the host cell, they bud through the host's cell surface membrane, taking part of it with them as the envelope; the phospholipids come from the host cell.
- Bacterium: cellular, 1–5 µm, peptidoglycan wall, circular DNA, 70S ribosomes, no membrane-bound organelles. Virus: non-cellular, 20–300 nm, DNA or RNA core, protein capsid, possibly a phospholipid envelope, no ribosomes. Liver cell: cellular, about 20–30 µm, no cell wall, nucleus with linear DNA and histones, 80S ribosomes, mitochondria, ER, Golgi body, lysosomes. Credit similarities: bacterium and liver cell both have cell surface membrane, cytoplasm, ribosomes; all three contain nucleic acid.
- Viruses are not cells but contain genetic material, reproduce (inside host cells) and evolve by natural selection, which are properties of life; however, they cannot reproduce, respire or carry out any metabolism outside a host cell, so they depend on cells. They therefore support the view that cells are needed for life (viruses cannot function without them) while challenging the idea that only cellular things show features of life. A balanced conclusion: viruses sit on the boundary between living and non-living.