A characterisation of life

We suggest that "livingness" is characterised at the cell level by a three-way dialogue among:-

  • the internal state;
  • the set of all responses to external stimuli; and
  • the pattern of gene expression.

"Internal state" encompasses cell structure, metabolism, and internal transport, locked together by reciprocal dependence at any moment. "Responses to stimuli" include all the cell's signalling pathways. "Pattern of gene expression" means the rate (zero or finite) at which each gene is being transcribed.

At each of the three corners of the main triangle in Fig. 10-1, details can change from moment to moment. Fluctuations in the internal state affect stimulus-response and gene expression more or less immediately (t1). Altered responses to stimuli affect internal state and gene expression after a slight delay (t2). Changes in gene expression pattern affect internal state and stimulus-response after a longer delay (t3). This dialogue-with-delays can have various consequences:- near-constancy, i.e. restoration of the status quo after a perturbation;

  • a cyclic sequence of changes, e.g. resulting in successive cell divisions (the cell cycle);
  • a progressive sequence of changes towards some final cell state (differentiation);
  • a progressive sequence of changes resulting in the controlled death and demolition of the cell (apoptosis).
Fig. 10-1: diagram summarising a characterisation of the living state.

In some instances, a change in the internal state might include the production of (new) chemical signals, which are sent out to other cells. It might include the formation of new receptors and signalling pathways. Thus, a cell's capacity to affect and to be affected by other cells can change with time. These changes of capacity underpin embryo development and many other features of multicellular life. On the other hand, the cell's signalling capacities might not change with time, or they might change only in reaction to a disturbance that they seek to correct; thus, homeostasis is maintained.

We have spent several chapters establishing this account of "livingness". Our emphasis has been on eukaryotic cells, particularly the cells of animals such as humans. In this chapter we shall consider two main questions:-

  • 1) Is the account sufficiently general? Does it apply to plants, fungi and single-celled eukaryotes as it does to animals? Does it apply to prokaryotes? Unless the answers are "yes", the characterisation needs to be reconsidered.
  • 2) On the other hand, is it sufficiently restrictive? Does it apply to anything non-living? If so, it needs to be modified.

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