Classification of living organisms: Bacteria (true bacteria)

The idea

Bacteria (true bacteria) are single-celled prokaryotes placed in their own domain in the three-domain system. Each cell has no nucleus and no membrane-bound organelles; its genetic material is a single circular DNA molecule free in the cytoplasm, and it may also carry small circular plasmids. Ribosomes are present, but they are smaller than eukaryotic ribosomes. Most bacteria have a cell wall, and many have a slime capsule, flagella for movement or pili for attachment. A practical method to understand their classification is to compare a bacterial cell with a plant cell: both may have a cell wall, but only the plant cell has a true nucleus, chloroplasts and mitochondria. This structural contrast, along with biochemical differences from Archaea, is central to their classification into a distinct domain.

What earns the marks

• Bacteria are prokaryotic: they have no true nucleus and no membrane-bound organelles.
• Their genetic material is a single loop of DNA in the cytoplasm, often with additional small rings called plasmids.
• Bacteria have ribosomes, but these are smaller than those found in eukaryotic cells.
• Most bacteria have a cell wall, and some have capsules, flagella or pili.
• Bacteria form a domain distinct from Archaea and Eukaryota based on biochemical differences.
• When comparing with eukaryotes, bacteria lack mitochondria and chloroplasts.

Common mistakes and how to put them right

• Stating that bacteria have a nucleus.

  Put it right: State that the circular DNA lies free in the cytoplasm.

• Confusing plasmids with the main chromosome.

  Put it right: Describe plasmids as small additional circular DNA molecules.

• Claiming bacteria have mitochondria for respiration.

  Put it right: Note that respiration occurs in the cytoplasm and on the cell membrane, as mitochondria are absent.

Exam tips

• Label a bacterial cell diagram with circular DNA, plasmid, ribosome, cell wall and cell membrane.
• Use the term 'prokaryotic' and immediately give one structural reason, such as the absence of a nucleus.
• When comparing with eukaryotes, mention at least one similarity (cytoplasm, ribosomes, cell membrane) and one difference (lack of nucleus or mitochondria).