Biology
Kingdom Prokaryotae
4,143 words, 10 sections. The first two are free.
The opening sections
Definitions first, then the reasoning. Each section is short enough to read in one sitting.
One warning first: the official PMDC syllabus table does not name this chapter, so it carries no learning outcome and no authority of its own. The papers disagree completely. Bacteria have been asked 191 times across 68 official papers between 2000 and 2025. A student hunting for this material in the official unit list will find the prokaryote content sitting inside the related official unit on cell structure; it is examined material the papers have never stopped asking. Mastered, you can name the envelope from the outside in, place a bacterium in a Gram group from a description, name a flagellar arrangement from a picture, and read which phase of the growth curve a graph shows.
Every bacterial cell has a cell membrane, cytoplasm, ribosomes and chromatin bodies. Most have a cell wall as well, and the wall gives the cell its shape. Capsule, slime, flagella, pili, fimbriae and granules are the optional parts, present in some bacteria and absent in others, so a question naming one of them is usually testing whether you know it is not universal.
What a bacterium never has is a membrane. There is no nuclear membrane, so the DNA lies in a distinct region of the cytoplasm, the nucleoid. There are no membrane-bound organelles either, no mitochondria and no chloroplasts, and the only organelle a prokaryotic cell is credited with is the ribosome. Inclusions are not organelles at all: glycogen, lipid droplets, crystals and pigments are non-living components with no membrane and no metabolism, and glycogen is the bacterial storage carbohydrate.
The structure examiners ask about most is the mesosome, an infagination of the cell membrane rather than of the cell wall, associated with DNA replication and cell division. It is also credited with respiration and the export of enzymes, performing mitochondrial functions. The plasmid sits alongside: a small circular, self-replicating, double-stranded molecule present in many bacteria in addition to the chromosome, carrying fewer genes than the chromosome, not essential for growth and metabolism, and replicating autonomously rather than controlling chromosomal replication.
8 more sections in this chapter
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