IGCSE Biology 0610: Placing Organisms into the Five Kingdoms, and Viruses

A microbiologist collects four unidentified samples from a woodland stream and records the following features under a microscope.

Sample A: a single cell with no nucleus, surrounded by a cell wall. Sample B: made of many thread-like cells woven together; each cell wall contains chitin; no chlorophyll is present; the sample is growing on, and absorbing nutrients from, a piece of decaying wood. Sample C: a single cell containing a nucleus and green chloroplasts; it swims using a single whip-like tail. Sample D: far smaller than samples A to C; consists only of a protein coat surrounding a strand of genetic material; shows no signs of activity unless it is inside a living host cell.

(a) State the kingdom to which sample A belongs. [1]

(b) State the kingdom to which sample B belongs. [1]

(d) Explain why sample D cannot be placed into any of the five kingdoms. [2]

Part (a): Sample A

Sample A is a single cell surrounded by a cell wall but with no nucleus. The absence of a nucleus (and of other membrane-bound organelles) is the key feature used to place an organism in the prokaryote kingdom, regardless of whether it also has a cell wall.

Part (b): Sample B

Sample B is made of many cells, but two features rule out the plant kingdom: its cell walls contain chitin, not cellulose, and it has no chlorophyll. Instead, it feeds by absorbing nutrients directly from the decaying wood around it. These features, a chitin cell wall, no chlorophyll, and absorption of nutrients from organic matter, place sample B in the fungus kingdom.

Part (c): Sample C

Sample C is a single cell, but unlike sample A it contains a nucleus, so it is a eukaryotic cell. It also contains green chloroplasts, meaning it can photosynthesise, yet it is not part of a multicellular plant body. A single-celled eukaryotic organism like this, which does not fit neatly into the animal, plant or fungus kingdoms, is placed in the protoctist kingdom.

Part (d): Sample D

Sample D is described as consisting only of a protein coat surrounding a strand of genetic material, with no cytoplasm, cell membrane or other cell contents. Because it is not made of a cell, it cannot be assigned to any of the five kingdoms, since the five-kingdom system is a way of grouping cellular organisms. This is why sample D, a virus, is treated as lying outside the five-kingdom classification altogether.

Final answers

(a) Sample A: prokaryote
(b) Sample B: fungus
(c) Sample C: protoctist
(d) Sample D cannot be placed in any kingdom because it is not made of a cell, consisting only of a protein coat and genetic material, and the five-kingdom system classifies only organisms made of cells.