Prokaryotic Cell Features: Plasmids, Capsule, and Flagella Revision Notes | A Level AQA Biology

1. Why Prokaryotic Cell Features Matter

Bacteria are the most abundant organisms on Earth, and their success comes down to structure. The features covered in this lesson — plasmids, capsules, and flagella — are the very structures that allow bacteria to resist antibiotics, evade your immune system, and move towards nutrients.

You already know that prokaryotic cells lack a membrane-bound nucleus and have a cell wall, a cell surface membrane, ribosomes, and a single circular chromosome. This lesson builds on that foundation by examining the additional structures that many prokaryotic cells possess.

The AQA specification requires you to know that many prokaryotic cells also have one or more plasmids, a capsule surrounding the cell, and one or more flagella. Crucially, you do not need to know the detailed molecular differences between prokaryotic and eukaryotic versions of these structures — the spec explicitly states that details of structural differences are not required.

2. Plasmids: Extra DNA for Extra Survival

Picture the prokaryotic chromosome as the cell's main instruction manual. Now imagine a small, separate booklet tucked alongside it — that is a plasmid. A plasmid is a small, circular DNA molecule that exists independently of the main chromosomal DNA within the cytoplasm of a prokaryotic cell.

A prokaryotic cell can carry one or more plasmids simultaneously. Each plasmid typically carries only a small number of genes, but those genes can be critically important. Antibiotic resistance genes are a classic example — a bacterium carrying a plasmid with a resistance gene can survive exposure to an antibiotic that would otherwise kill it.

Plasmids can also be transferred between bacterial cells through a process called conjugation, which is one reason antibiotic resistance spreads so rapidly through bacterial populations. This has enormous clinical significance, though the mechanism of conjugation itself is beyond the AQA A Level specification.

Plasmids are also central to genetic engineering techniques — scientists use them as vectors to introduce foreign genes into bacteria. This application is covered elsewhere in the AQA specification, but it reinforces why understanding plasmid structure is so important at A Level.

3. The Capsule: A Protective Outer Layer

Moving outward from the chromosome, past the cytoplasm and cell wall, some prokaryotic cells are surrounded by an additional protective layer called the capsule. Think of it as a thick, sticky coat wrapped around the outside of the cell wall.

The capsule sits outside the cell wall, making it the outermost layer of the cell. Its polysaccharide composition makes it highly hydrophilic, helping the cell retain water and resist drying out. In pathogenic bacteria, the capsule is a major virulence factor — it physically prevents white blood cells from engulfing and destroying the bacterium through phagocytosis.

A well-known example is Streptococcus pneumoniae, which causes pneumonia. Encapsulated strains are far more dangerous than non-encapsulated strains precisely because the capsule shields the bacterium from the host's immune response. This was demonstrated in Griffith's famous transformation experiment, though the molecular details of that experiment are covered in the genetics section of the specification.

Not all prokaryotic cells possess a capsule — it is an additional feature present in many, but not all, species. When answering exam questions, use the phrase 'some prokaryotic cells' rather than stating all bacteria have a capsule, to reflect this accurately.

4. Flagella: Structures for Movement

Some prokaryotic cells need to move — towards nutrients, away from toxins, or towards light. The structure that makes this possible is the flagellum (plural: flagella). A flagellum is a long, whip-like appendage that extends from the cell surface and rotates like a propeller to drive the cell through its environment.

Prokaryotic flagella are composed of a protein called flagellin, which assembles into a hollow, helical filament. The filament is anchored to the cell by a basal body embedded in the cell wall and membrane. Rotation of the flagellum is driven by a proton gradient across the cell surface membrane — a molecular motor powered by the flow of hydrogen ions. You do not need to know the detailed molecular structure of the basal body for AQA A Level.

A prokaryotic cell may have one flagellum or several, and their arrangement varies between species — some have flagella at one end, others have them distributed all over the cell surface. The number and arrangement affect how efficiently the cell can move, but again, these finer structural details are beyond the specification.

It is worth noting that eukaryotic cells also have flagella and cilia, but their internal structure is fundamentally different from prokaryotic flagella — eukaryotic flagella contain microtubules arranged in a '9+2' pattern and are powered by ATP hydrolysis rather than a proton gradient. The AQA specification explicitly states that details of these structural differences are not required, so you do not need to compare them in depth.

5. Bringing It Together: The Full Prokaryotic Cell Picture

Now that each additional feature has been examined individually, it helps to see them as a coordinated set of adaptations. The table below summarises the three additional structures, their location within the cell, their composition, and their function — exactly the kind of comparison an examiner might ask you to produce.

Notice how each structure addresses a different survival challenge: plasmids deal with chemical threats (antibiotics), the capsule deals with biological threats (immune cells), and flagella deal with the challenge of finding resources. Together, they make prokaryotic cells remarkably resilient.

6. Applying Your Knowledge: Exam-Style Reasoning

AQA exam questions on prokaryotic cell features tend to test three things: accurate recall of structure and function, the ability to explain how a feature contributes to survival, and the ability to suggest how a feature might be relevant in a novel context. The following worked example models how to approach a 'suggest' question.

Worked example: Answering an Exam 'Suggest' Question on Prokaryotic Features

A strain of bacteria is found to be highly resistant to being destroyed by the immune system of a host organism. Suggest two features of prokaryotic cells that could explain this resistance. [2 marks]

1. Command word: 'Suggest' — apply biological knowledge to a new context. Two features are needed, each worth one mark. — For 'suggest' questions, there may be more than one valid answer. You must link a named feature to a specific mechanism that explains the observation.
2. Feature 1: Capsule — The capsule is the most directly relevant structure here because its primary function is protection against the host immune system.
3. The capsule surrounds the cell wall and prevents white blood cells from engulfing the bacterium by phagocytosis. — A mark-winning answer names the feature AND explains the mechanism. writing 'capsule' without explanation would not secure the mark in a 'suggest' question.
4. Feature 2: Plasmid — Plasmids can carry genes for antibiotic resistance, which is part of the immune/treatment response against bacteria.
5. Plasmids may carry genes that encode enzymes capable of breaking down antibiotics used in treatment, allowing the bacterium to survive drug therapy. — This links the structural feature (plasmid) to a functional outcome (antibiotic resistance) in the context of the question (resistance to destruction by the host).

Answer: 1. The capsule prevents phagocytosis by white blood cells, protecting the bacterium from the host immune response. 2. Plasmids may carry antibiotic resistance genes, allowing the bacterium to survive drug treatment.

Notice that each answer point follows the same pattern: name the structure, then explain the mechanism. This is the pattern that secures marks in AQA biology questions — a named feature alone is rarely sufficient for full credit.

7. Exam Technique and Common Mistakes

Questions on prokaryotic cell features appear regularly in AQA A Level Biology papers, often as part of a cell structure question or as a context-based 'suggest' or 'explain' question. The marks are accessible, but students consistently lose them through imprecise language and structural confusion.

To summarise the full picture: many prokaryotic cells possess plasmids (circular DNA in the cytoplasm carrying additional genes), a capsule (polysaccharide outer layer beyond the cell wall), and flagella (protein appendages that rotate to drive movement) — each of these structures extends the core prokaryotic cell plan and contributes directly to bacterial survival and pathogenicity, satisfying the AQA specification requirements for this topic.