Structure of a bacterium

A bacterium is a single-celled prokaryotic organism. Its cell is simpler than a plant or animal cell because it has no nucleus and no membrane-bound organelles, yet it can still carry out all life processes.

Bacteria are far smaller than eukaryotic cells, a typical bacterium is about a tenth the length of a cheek cell, so their internal structure is only visible under an electron microscope. The DSKP places the bacterium alongside the plant and animal cell so that you can compare the three and explain the difference between prokaryotic and eukaryotic organisation.

The same structure returns later in the syllabus in the chapters on the immune system, where the cell wall is the antigen the body recognises, and on biotechnology, where the plasmid becomes a tool.

Parts and functions

How structure suits function

Because a bacterium is prokaryotic, its genetic material lies free in the cytoplasm as a nucleoid instead of being enclosed by a nuclear membrane, so the cell has no nucleus, mitochondria, chloroplasts or endoplasmic reticulum like a plant or animal cell. This simple structure, combined with a very small cell size, gives the bacterium a large surface area to volume ratio, which lets nutrients and waste diffuse in and out quickly and allows the cell to grow and divide by binary fission at a fast rate.

• Small size, a high surface area to volume ratio means diffusion alone supplies the whole cell, so no transport system is needed.
• Rigid cell wall, bacteria often live in dilute surroundings such as pond water; the wall stops the cell bursting when water enters by osmosis.
• Plasma membrane folds, with no mitochondria, the enzymes of respiration sit on the plasma membrane, and folding it inward increases the surface available.
• Single circular chromosome, a small genome is copied quickly, so binary fission can happen in as little as twenty minutes under good conditions.
• Plasmids, extra genes can be gained or passed on without altering the main chromosome, letting a population adapt fast, for example by acquiring antibiotic resistance.
• Flagellum and pili, the flagellum rotates to move the cell towards nutrients, while pili anchor it to a host surface.

Related processes

Bacteria reproduce asexually by binary fission: the circular chromosome is copied, the cell elongates, the two copies move apart and the cell splits into two identical daughter cells. Because there is no nucleus, this is not mitosis, and a comparison question may ask you to explain that difference.

In the immune-system chapter, molecules on the bacterial cell wall and capsule act as antigens. Phagocytes engulf and digest bacteria, while lymphocytes produce antibodies that bind to the antigens on the surface.

The plasma membrane and cell wall are also the targets of antibiotics, which is why antibiotics kill bacteria but not the human cells around them, which have no cell wall.

In biotechnology, a plasmid is cut open with a restriction enzyme, a useful gene such as the human insulin gene is joined in with ligase, and the recombinant plasmid is put back into a bacterium. As the bacterium divides, every daughter cell carries the gene and produces the protein.

Common exam errors

Why plasmids matter

Plasmids can be copied and passed between bacteria, and in genetic engineering they are used as vectors to carry a useful gene, such as the human insulin gene, into a bacterial cell.

Worked question

Question (in the style of Paper 2 Section A): Diagram Z shows a bacterium. Structure R is a region of the cytoplasm containing a coiled thread of DNA.

Structure S is a small ring of DNA. (a) Name R and S. [2 marks] (b) Give two structural differences between this cell and an animal cell. [2 marks] (c) Explain why structure S is useful in genetic engineering. [2 marks]

Model answer: (a) R is the nucleoid; S is a plasmid. (b) The bacterium has a cell wall, while an animal cell has none; the bacterium has no nucleus (its DNA is not enclosed by a nuclear membrane), while the animal cell has a nucleus.

(Also accepted: no mitochondria; circular rather than linear DNA; smaller ribosomes.) (c) A plasmid is a small circular DNA that can be cut with a restriction enzyme and joined to a foreign gene; when it is returned to the bacterium it is copied every time the cell divides, so the gene is cloned along with it.

Marking note: in part (b) each difference must state both sides, what the bacterium has and what the animal cell has. Writing "cell wall" alone is one side of the comparison and does not earn the mark.

Source:SRC-DSKP-EN

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