How do prokaryotic cells and viruses differ from eukaryotic cells?
The structure of prokaryotic cells, including the cell wall, cell-surface membrane, capsule, circular DNA, flagella and plasmids, and how prokaryotic cells differ from eukaryotic cells; the structure of viruses as acellular, non-living particles including the genetic material, capsid and attachment proteins.
A focused answer to the AQA 3.2 dot point on prokaryotic cells and viruses. Covers bacterial cell structure, the key differences from eukaryotic cells, and why viruses are classed as acellular non-living particles.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA wants you to describe the structure of a generalised prokaryotic (bacterial) cell, list the precise ways it differs from a eukaryotic cell, and explain the structure of a virus and why it is not classed as a cell or as living.
The answer
Prokaryotic cell structure
Prokaryotes (bacteria) are small (typically 0.5 to 5 micrometres) and have no membrane-bound organelles.
Features you must know:
- Cell wall. Made of murein (a glycoprotein, also called peptidoglycan). Provides support and prevents the cell bursting. Note it is not cellulose.
- Cell-surface membrane. A phospholipid bilayer controlling what enters and leaves.
- Capsule. A slimy protective layer outside the wall in some bacteria. Protects against drying out and against host immune cells.
- Circular DNA. A single loop of DNA free in the cytoplasm, not associated with histone proteins.
- Plasmids. Small rings of DNA separate from the main loop, often carrying antibiotic-resistance genes. They can be exchanged between bacteria.
- Flagella. One or more long tail-like structures that rotate to move the cell.
- Ribosomes. Smaller (70S) than eukaryotic (80S) ribosomes.
- Mesosomes and food granules may also be present.
How prokaryotes differ from eukaryotes
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No true nucleus; DNA free in cytoplasm | Membrane-bound nucleus |
| DNA | Circular, no histones | Linear, wound around histones |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, Golgi, etc.) |
| Ribosomes | Smaller, 70S | Larger, 80S |
| Cell wall | Murein | Cellulose (plants), chitin (fungi), or none (animals) |
| Extra DNA | Plasmids | No plasmids (with rare exceptions) |
| Size | Smaller (0.5 to 5 micrometres) | Larger (up to about 100 micrometres) |
Viruses
Viruses are acellular, non-living particles, much smaller than bacteria (about 20 to 300 nanometres).
Their structure is simple:
- Genetic material. A core of DNA or RNA carrying the genes to make new viruses.
- Capsid. A protein coat surrounding and protecting the genetic material.
- Attachment proteins. Proteins projecting from the surface that bind to specific receptor molecules on a host cell. This binding is highly specific and determines which cells a virus can infect.
Some viruses (for example HIV) also have a lipid envelope taken from the host membrane, and may carry enzymes such as reverse transcriptase.
Examples in context
Example 1. Antibiotic resistance via plasmids. Genes coding for enzymes that break down antibiotics (for example beta-lactamase) are often carried on plasmids. Because plasmids can be passed between bacteria by conjugation, resistance can spread rapidly through a bacterial population, which is why hospitals monitor resistant strains such as MRSA.
Example 2. The specificity of viral attachment proteins. SARS-CoV-2 attachment (spike) proteins bind to the ACE2 receptor found on human respiratory and gut cells. Because attachment is receptor-specific, the virus can only infect cells displaying that receptor, which explains its tissue tropism and why mutations in the spike protein change transmissibility.
Try this
Q1. Give three structural differences between a bacterial cell and a human cheek cell. [3 marks]
- Cue. Bacterium has circular DNA free in cytoplasm vs nucleus; no membrane-bound organelles vs present; murein wall vs no wall; 70S vs 80S ribosomes (any three).
Q2. Explain how plasmids contribute to the spread of antibiotic resistance. [2 marks]
- Cue. Plasmids carry resistance genes and can be transferred between bacteria, so resistance passes from one bacterium to another.
Q3. State two reasons a virus cannot be classed as a living cell. [2 marks]
- Cue. It is acellular (no membrane, cytoplasm or organelles); it cannot reproduce or metabolise independently and must use a host cell.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA3 marksExplain why a virus is described as acellular and non-living.Show worked answer →
A 3-mark answer needs the features a cell has that a virus lacks.
- A virus is acellular because it has no cell-surface membrane, no cytoplasm and no organelles; it is just genetic material inside a protein capsid.
- It is non-living because it cannot carry out metabolism or reproduce by itself.
- It can only replicate by infecting a host cell and using the host's ribosomes and enzymes to make new virus particles.
Markers reward the absence of cellular structures and the dependence on a host cell.
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