In addition, many prokaryotic cells have: one or more plasmids a capsule…

The idea

These three structures are the optional extras of prokaryotic cells, and the examinable word is many, not all. A plasmid is a small circular loop of DNA separate from the main circular DNA molecule. It carries genes that are not essential for survival, most usefully antibiotic resistance genes, and it replicates independently of the main loop, so copy number varies. Plasmids can be passed between bacteria, which is why resistance spreads so quickly, and why plasmids become vectors in recombinant DNA technology. A capsule is a slime layer of polysaccharide outside the cell wall. It helps the cell stick to surfaces, retains moisture, and makes the bacterium harder for a phagocyte to engulf. A flagellum is a whip-like structure that rotates to propel the bacterium, allowing it to move towards nutrients.

What earns the marks

• Naming a plasmid as a small circular DNA molecule separate from the main circular DNA, replicated independently of it.
• Plasmids carrying additional genes such as those for antibiotic resistance, which can be transferred between bacteria.
• The capsule lying outside the cell wall and protecting the cell, for example against phagocytosis or drying out, or helping it adhere to surfaces.
• The flagellum producing movement of the whole cell.
• Recognising that these structures are present in many but not all prokaryotes.

Common mistakes and how to put them right

• Listing a plasmid, capsule or flagellum when the question asks for features of all prokaryotic cells.

• Confusing the plasmid with the single circular DNA molecule, or calling either a eukaryotic chromosome.

• Describing a bacterial flagellum as beating like a eukaryotic flagellum instead of rotating.

• Saying the capsule is the cell wall, or that it is made of murein.

• Claiming plasmids carry the genes the bacterium needs to survive.

Exam tips

• Read whether the stem says all or many; the same three structures are creditworthy in one wording and incorrect in the other.
• When asked for a function, give the benefit to the cell, not just the location: sticks to surfaces and resists phagocytosis, not simply surrounds the cell.
• Keep plasmid answers ready for the genetic technology topic; the same structure returns as a vector in 3.8.4.1.