Structure of Prokaryotic Cells
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
Prokaryotic cells are much smaller than eukaryotic cells. They also differ from eukaryotic cells in having:
- cytoplasm that lacks membrane-bound organelles
- smaller ribosomes
- no nucleus; instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins
- a cell wall that contains murein, a glycoprotein.
In addition, many prokaryotic cells have:
- one or more plasmids
- a capsule surrounding the cell
- one or more flagella.
Details of these structural differences are not required.
Now that we’ve looked at eukaryotic cells, let’s compare them to prokaryotic cells. Prokaryotes are much simpler. Bacteria are an example. The key difference is that prokaryotes do not have membrane-bound organelles. No nucleus, no mitochondria, no ER. Just a cell with its DNA floating freely in the cytoplasm.
Structures Found in Prokaryotic Cells
- Circular DNA (nucleoid): A single, circular DNA molecule that is not associated with histone proteins. It is free in the cytoplasm (no nuclear envelope). This carries the main genetic information of the cell.
- Plasmids: Small, extra circles of DNA, separate from the main circular DNA. They often carry genes for antibiotic resistance. Plasmids can be transferred between bacteria and are used in genetic engineering as vectors.
- 70S Ribosomes: Smaller than eukaryotic ribosomes (70S vs 80S). Still the site of protein synthesis (translation). This size difference is why some antibiotics can target bacterial ribosomes without affecting human cells.
- Cell wall (murein): Contains murein (peptidoglycan), a glycoprotein. Provides structural support and prevents the cell from bursting. Different from plant cell walls (cellulose) and fungal cell walls (chitin).
- Capsule: A slimy layer outside the cell wall, found in some bacteria. Provides protection against the host’s immune system (e.g. prevents phagocytosis) and helps the bacterium adhere to surfaces.
- Flagella: Long, whip-like projections that rotate to move the bacterium. Not all prokaryotes have them. Some have one, others have several.
Compare prokaryotic and eukaryotic cells
Test yourself: click a button to reveal that column.
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Size | Smaller (1–5 μm) | Larger (10–100 μm) |
| Nucleus | No: DNA free in cytoplasm | Yes: DNA enclosed in nuclear envelope |
| DNA | Single, circular, not associated with proteins | Linear, associated with histone proteins |
| Ribosomes | 70S (smaller) | 80S (larger) |
| Membrane-bound organelles | None | Yes (mitochondria, ER, Golgi, etc.) |
| Cell wall | Yes: contains murein (a glycoprotein) | Plants/fungi have cell walls, but not made of murein |
Viruses
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
Viruses are acellular and non-living. The structure of virus particles to include genetic material, capsid and attachment protein.
Viruses are not cells. They are described as acellular (without cells) and are considered non-living. They cannot carry out metabolic processes on their own and can only replicate inside a host cell.
Why are viruses considered non-living?
Why are viruses considered non-living?
- They have no cell membrane, no cytoplasm, no ribosomes, so they cannot carry out metabolic reactions
- They cannot reproduce on their own, so they must hijack a host cell’s machinery to replicate
- They do not grow or respond to stimuli
- They are essentially a piece of genetic material in a protein coat
Structure of a Virus
- Genetic material: Can be DNA or RNA (not both). Can be single-stranded or double-stranded. This carries the instructions for making new virus particles. Different viruses have different types, e.g. HIV has RNA, while bacteriophages have DNA.
- Capsid: A protein coat that surrounds and protects the genetic material. Made of protein subunits called capsomeres. The shape of the capsid varies between viruses (e.g. icosahedral, helical).
- Attachment proteins: Proteins on the surface of the virus that bind to specific receptors on the host cell. This is what determines which cells a virus can infect. For example, HIV has attachment proteins that bind to receptors on T helper cells.
Exam tip
Viruses are much smaller than bacteria. Don’t confuse them. Bacteria are living prokaryotic cells; viruses are non-living acellular particles. Antibiotics work against bacteria but not against viruses (a very common exam question).
Want more on viruses?
The full structure and replication of HIV (including reverse transcriptase) is covered on the HIV and AIDS page.
How this topic is tested
This analysis is based on past paper data from 2017 to 2025. It is intended for interest only and is not predictive of what will appear in future papers.
- Tested in 3 of 9 years (2017–2025): 11 question parts worth 19 marks.
- 41st most-examined topic overall by marks, 6th in Unit 2.
Marks by year
Most-tested spec points
- Structure of Viruses: tested in 5 parts (9 marks)
- Prokaryotic Cell Structure: tested in 3 parts (4 marks)
Maths and practical skills
- Choosing and using a statistical test (MS 1.9): e.g. 2025 P1 Q8.3
- Plotting graphs (PS 3.1): e.g. 2017 P3 Q2.5
- Percentages, ratios and fractions (MS 0.3): e.g. 2025 P1 Q8.4
- Using equations (MS 2.3): e.g. 2025 P1 Q8.2
- Logarithms and log scales (MS 2.5): e.g. 2017 P3 Q2.4
- Evaluating experimental design (PS 2.1): e.g. 2017 P3 Q2.3
Tips from examiner reports
What students commonly get wrong
- Features of ALL prokaryotesWatch out: plasmids, a capsule and flagella are only found in some prokaryotes. Features of all prokaryotes are: no membrane-bound organelles, circular DNA not associated with proteins, and murein in the cell wall. Don't give 70S ribosomes as unique to prokaryotes: eukaryotic cells contain them too, in their mitochondria and chloroplasts. 2022 P1 Q3.1 · 2025 P1 Q4.1
- Capsule vs capsidWatch out: a capsule is a slimy layer around some bacteria. A capsid is the protein coat of a virus. Don't mix them up. 2021 P1 Q10.3 · 2025 P1 Q4.1
- What all viruses haveAll virus particles have genetic material (DNA or RNA, not always the same one), a capsid and attachment proteins, which bind to receptors on host cells. Don't include features only some viruses have (e.g. reverse transcriptase, a lipid envelope), or bacterial structures. 2023 P1 Q1.1
- What "acellular" meansWatch out: acellular means viruses aren't made of cells: they have no cytoplasm and no organelles. Don't confuse it with non-living: "they need a host cell to reproduce" explains why they're non-living, not why they're acellular. 2023 P1 Q1.2
Exam Question Practice
A few exam-style questions on this topic to try. Have a go at each one before opening the answer.
Prokaryotic and eukaryotic cells differ in their structure.
Give two features found in all prokaryotic cells but not in eukaryotic cells.
(1 marks)
Hint
The question says all prokaryotes. Which structures are only in some? Which features are also found in eukaryotic cells?
Mark Scheme
Any two of these (1 mark):
- No membrane-bound organelles
- Circular DNA
- DNA not associated with histones
- DNA free in the cytoplasm
- A cell wall containing murein (peptidoglycan)
Also accepted / rejected
Tips
- Give features found in all prokaryotes, e.g. no membrane-bound organelles, circular DNA not associated with histones, murein cell wall
- Plasmids, capsules and flagella are only in some prokaryotes
- “70S ribosomes” alone doesn’t work, as eukaryotic cells have them too (in mitochondria and chloroplasts)
Some structures are found in some prokaryotic cells but not in all of them.
Name three of these structures.
(2 marks)
Hint
Every prokaryotic cell has a cell-surface membrane, cell wall, 70S ribosomes and circular DNA. Which other structures are found in only some of them?
Mark Scheme
Capsule, plasmids and flagella (2 marks)
All three correct = 2 marks; two correct = 1 mark
Also accepted / rejected
Also accepted
- Pili, or photosynthetic membranes (lamellae)
Tips
- Don’t give viral structures. A capsid belongs to a virus; the prokaryotic structure is a capsule
- Don’t give 70S ribosomes, as they are found in all prokaryotic cells
- Don’t give organelles that are only found in eukaryotic cells, such as mitochondria or a nucleus
- Spell key terms correctly, e.g. pili
What earns marks
- Capsule, plasmids and flagella are the most common correct answers
Below are four statements about the structure of prokaryotic cells.
- All prokaryotic cells have a cell-surface membrane.
- All prokaryotic cells contain one or more plasmids.
- No prokaryotic cell has a nucleus.
- All prokaryotic cells contain ribosomes.
Which statements about the structure of prokaryotic cells are correct?
Tick (✓) one box.
(1 marks)
Hint
Check each statement: is it true for every prokaryotic cell?
Mark Scheme
Third box ticked (1 mark)
Answer key: C – statements 1, 3 and 4
Tips
- Statement 2 is wrong: not all prokaryotic cells have plasmids
- Tick only one box
Virus particles share some structural features.
Name three of these features, and describe the function of one of them.
(2 marks)
Hint
Which features do virus particles share? What contains the genetic material, what surrounds it, and what helps the virus attach to host cells?
Mark Scheme
- Genetic material (DNA or RNA), a capsid and attachment proteins (1 mark)
- One correct function (1 mark), e.g.
- the genetic material codes for viral proteins
- the capsid protects the genetic material
- attachment proteins bind to receptors on the host cell
Also accepted / rejected
Also accepted
- “Nucleic acid” or “genome” for genetic material (point 1)
- “Glycoprotein” for attachment protein
- A correct function of a wrong feature still earns point 2, e.g. reverse transcriptase makes DNA from RNA
Not accepted
- “DNA and RNA” (point 1: a virus has one or the other)
Tips
- Give the three shared features: genetic material (DNA or RNA), capsid and attachment proteins
- Some viruses have DNA and others RNA, so write “DNA or RNA”, not that all viruses have one of them
- Don’t give features only some viruses have (reverse transcriptase, lipid envelope) or bacterial structures (capsule, plasmid, flagella)
- For attachment proteins, say they bind to receptors on the host cell
Viruses are described as acellular and non-living.
Explain why.
(2 marks)
Hint
Acellular means ‘not made of cells’. What would a cell have that a virus lacks? Think about organelles and cytoplasm.
Mark Scheme
- Acellular: they are not made of cells; they have no cell-surface membrane (1 mark)
- Non-living: they have no metabolism; they can’t replicate on their own (only inside a host cell) (1 mark)
Also accepted / rejected
Also accepted
- No cytoplasm or no organelles (point 1)
- They can’t respire, move or excrete on their own (point 2)
Not accepted
- “No membrane-bound organelles” (point 1: prokaryotic cells don’t have these either)
Tips
- Acellular means not made of cells: no cell-surface membrane, cytoplasm or organelles
- Don’t define acellular using non-living features (e.g. “can’t reproduce on their own”), or as “smaller than a cell” or “unicellular”
- Non-living: they have no metabolism and can’t replicate or respire independently
- “No mitochondria” isn’t a reason: prokaryotic cells lack mitochondria but are alive
What earns marks
- Explaining non-living as being unable to replicate without a host cell
Also accepted
Not accepted