Prokaryotes

We surveyed some differences between prokaryotes and eukaryotes in chapters 2-3. Prokaryotes are much smaller than eukaryotic cells. They have much smaller genomes, far less elaborate structures and smaller repertoires of responses to stimuli. They do not grow and differentiate as animal and plant cells do. They are machines for reproducing as quickly as conditions allow. When conditions do not allow, many bacteria turn into quiescent "suspended animation" forms known as endospores.

In short, prokaryotes are simpler than eukaryotes. However, the characterisation of "livingness" still applies. Bacteria respond to external stimuli. Some of these stimuli promote transcription of particular genes, and the equipment for detecting and processing the stimuli obviously consists of gene products. Transcription and protein manufacture are very rapid, so the delay between stimulus reception and synthesis of a new gene product is considerably shorter than it is in eukaryotic cells. Nevertheless there is still a time lapse. The duration of delay might not affect our characterisation, so long as it is finite; we shall consider this claim at the end of the present section.

Prokaryotes have internal states, as defined in chapter 6, though these are simpler than the internal states of eukaryotic cells. They have interconnected metabolic pathways. They regulate their compositions. They have structures including cell membrane and wall and storage granules. They presumably transport materials between locations within the cell. Metabolism is needed to construct and maintain the cell membrane; the cell membrane is needed to organise metabolism; and perturbations of "cellular homeostasis" change metabolism and membrane organisation. "Simpler than eukaryotic cells" does not mean "simple" in the sense that a non-living physical system is simple. Prokaryotic internal states involve hundreds of different proteins, and even a single protein is a complicated piece of equipment.

Responses to stimuli can change the internal state of a prokaryote directly, as in eukaryotes; and by definition, the signalling pathway intermediates are aspects of the internal state. So there is a dialogue between internal state and stimulus-response. Inevitably, there is also a dialogue between internal state and gene expression, though the regulation of gene expression and its dependence on internal state are less sophisticated than we described in chapters 7-8. Also, prokaryotes respond to signals from one another and from other sources, so they "communicate" in broadly similar ways to eukaryotic cells.

In short, our general characterisation of "livingness" applies to prokaryotes. But it is interesting to consider the shorter delays (the intervals between ti, t2 and t3 in Fig. 10-1). In multicellular eukaryotes these time-delays, notably between gene expression and internal state, are valuable because they allow for differentiation and apoptosis; but no such processes occur in prokaryotes. They also allow for the eukaryotic cell cycle; but prokaryotic replication needs no comparably elaborate cell-division apparatus or succession of events. Therefore, prokaryotes probably do not require the same time-delays as eukaryotic cells. We suggest, therefore, that the shorter time delays do not imply a mismatch between our characterisation of "livingness" and the nature of prokaryotes. Rather, they point to something fundamental in the distinction between prokaryotes and eukaryotes.

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