Summary

This chapter examined the cell as the fundamental structural and functional unit of every living organism, beginning with its historical discovery by Robert Hooke in 1665 and building up, through the work of Schleiden, Schwann and Virchow, to the modern Cell Theory: that all organisms are made of cells, that the cell is the basic unit of life, and that every cell arises only from a pre-existing cell by division.

All cells fall into one of two structural plans. Prokaryotic cells -- bacteria, cyanobacteria and mycoplasma -- have no true, membrane-bound nucleus and no membrane-bound organelles; their genetic material lies in direct contact with the cytoplasm as a nucleoid, and their protein synthesis depends on smaller 70S ribosomes. A generalised prokaryotic cell is bounded by a three-layered cell envelope -- glycocalyx, cell wall and plasma membrane -- often further modified into mesosomes, flagella, pili and fimbriae, and its cytoplasm additionally carries plasmids and unbound inclusion bodies.

Eukaryotic cells -- found in protists, fungi, plants and animals -- possess a true nucleus, enclosed by a double nuclear envelope perforated with nuclear pores, containing chromatin (which condenses into chromosomes during division) and a nucleolus responsible for ribosomal RNA synthesis. Plant cells and animal cells share this basic eukaryotic plan but differ in several respects: plant cells possess a rigid cellulose cell wall, plastids, and typically one large central vacuole, and are connected to neighbouring cells by plasmodesmata, whereas animal cells generally possess centrioles and comparatively more prominent lysosomes and lack a cell wall and plastids. …