Eukaryotes and prokaryotes: Bacterial cells (prokaryotic cells) are much smaller in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.

The idea

Prokaryotic cells, such as bacteria, are typically around 1–5 µm across, far smaller than most eukaryotic cells, which are often 10–100 µm. A bacterial cell still has cytoplasm where reactions occur and a cell membrane controlling what enters and leaves. Outside the membrane is a cell wall that supports and protects the cell. Unlike eukaryotes, the genetic material is not enclosed in a nucleus. Instead there is a single loop of DNA carrying the main genes, and there may also be one or more small rings of DNA called plasmids. Plasmids are additional, separate from the main loop, and can carry genes such as those for antibiotic resistance. A useful comparison is a human cheek cell (eukaryotic, with a nucleus) beside a bacterium such as Escherichia coli (prokaryotic, no nucleus, DNA loop plus possible plasmids).

What earns the marks

• States that prokaryotic cells are much smaller than eukaryotic cells, giving a typical size comparison such as bacteria at roughly 1–5 µm against eukaryotic cells at roughly 10–100 µm.
• Identifies cytoplasm and a cell membrane as present in bacterial cells, with the membrane controlling movement of substances into and out of the cell.
• States that the cell membrane is surrounded by a cell wall, and that this wall is outside the membrane and provides support and protection.
• Explains that the genetic material is not enclosed in a nucleus, so bacterial cells have no nucleus.
• Describes the main genetic material as a single DNA loop, and identifies plasmids as one or more small rings of DNA that may also be present.
• Distinguishes plasmids from the main DNA loop, noting that plasmids are separate small rings and may carry additional genes such as antibiotic-resistance genes.
• Compares a named prokaryote with a named eukaryote, for example Escherichia coli against a human cheek cell, to show the absence of a nucleus in the prokaryote.

Common mistakes and how to put them right

• Saying bacterial cells have no genetic material at all.

  Put it right: They do have genetic material, but it is a single DNA loop not enclosed in a nucleus, and plasmids may also be present.

• Confusing the cell wall with the cell membrane, or placing the wall inside the membrane.

  Put it right: The membrane is the selectively permeable boundary around the cytoplasm, and the wall surrounds the membrane on the outside.

• Describing plasmids as the main genetic material or as a nucleus.

  Put it right: Plasmids are small additional DNA rings, separate from the single main DNA loop, and are not a nucleus.

• Claiming all bacteria have plasmids.

  Put it right: Plasmids may be present, so use language such as there may be one or more plasmids rather than stating they are always present.

Exam tips

• When asked to compare cell types, structure your answer as paired points, for example eukaryotic cells have a nucleus whereas prokaryotic cells do not, and eukaryotic cells are much larger than prokaryotic cells.
• Use precise terms: write single DNA loop and plasmids rather than vague phrases such as DNA floating around, and name the wall as a cell wall outside the cell membrane.
• If a question asks why a bacterium can still carry out life processes without a nucleus, link each structure to its job: cytoplasm for reactions, membrane for control of entry and exit, wall for support, DNA loop and plasmids for genetic information.