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1 The Cell as the Basic Unit of Life
Michael Wink
Heidelberg University, Institute of Pharmacy and Molecular Biotechnology (IPMB), Im Neuenheimer Feld 329, 69120 Heidelberg, Germany
The base unit of life is the cell. Cells constitute the base element of all prokaryotic cells (cells without a cell nucleus, e.g. Bacteria and Archaea) and eukaryotic cells (or Eukarya) (cells possessing a nucleus, e.g. protozoa, fungi, plants, and animals). Cells are small, membrane‐bound units with a diameter of 1–20 μm and are filled with concentrated aqueous solutions. Cells are not created de novo, but possess the ability to copy themselves, meaning that they emerge from the division of a previous cell. This means that all cells, since the beginning of life (around 4 billion years ago), are connected with each other in a continuous lineage. In 1885, the famous cell biologist Rudolf Virchow conceived the law of omnis cellula e cellula (all cells arise from cells), which is still valid today.
The structure and composition of all cells are very similar due to their shared evolution and phylogeny (Figure 1.1). We see an astonishing constancy in fundamental structures and mechanisms. Owing to this, it is possible to limit the discussion of the general characteristics of a cell to a few basic types (Figure 1.2):
Bacterial cells
Plant cells
Animal cells
Figure 1.1 Tree of life – phylogeny of life domains.
Figure 1.2 Schematic structure of prokaryotic and eukaryotic cells. (a) Bacterial cell, (b) plant mesophyll cell, and (c) animal cell.
Nucleotide sequences from 16S rRNA, amino acid sequences of cytoskeleton proteins, and characteristics of the cell structure were used to reconstruct this phylogenetic tree. Prokaryotes are divided into Bacteria and Archaea. Archaea form a sister group with eukaryotes; they share important characteristics (Tables 1.1 and 1.2). Many monophyletic groups can be recognized within the eukaryotes (diplomonads/trichomonads, Euglenozoa, Alveolata, Stramenopilata [heterokonts], red algae and green algae/plants, fungi and animals; see Tables 6.3–6.5 for details).
Table 1.1 Comparison of important biochemical and molecular characteristics of the three domains of life.
| Character | Prokaryotes | Eukaryotes | |
|---|---|---|---|
| Archaea | Bacteria | ||
| Organization | Unicellular | Unicellular | Unicellular or multicellular |
| Cytology | |||
| Internal membranes | Rare | Rare | Always (Table 1.2) |
| Compartments | Only cytoplasm | Only cytoplasm | Several (Table 1.2) |
| Organelles | No | No | Mitochondria; plastids |
| Ribosomes | 70S | 70S | 80S (mt, cp: 70S) |
| Membrane lipids | Ether lipids | Ester lipids, hopanoids | Ester lipids, sterols |
| Cell wall | Pseudopeptidoglycan, polysaccharides, glycoproteins | Murein (peptidoglycan), polysaccharides, proteins | PL: polysaccharides, cellulose F: chitin A: no |
| Cytoskeleton | FtsZ and MreB protein | FtsZ and MreB protein | Tubulin, actin, intermediary filaments |
| Cell division | Binary fission | Binary fission | Mitosis |
| Genetics | |||
| Nuclear structure | Nucleoid | Nucleoid |
Membrane‐enclosed
|
