Читать книгу Astrobiology - Charles S. Cockell - Страница 124

The modes of reproduction that we have just examined allow populations of cells to grow. In multicellular eukaryotes, this process is controlled by a network of genes that lead to cell differentiation, whereby initially unspecialized cells change into the whole panoply of cell types in a given organism, such as skin, liver, and heart cells. These cells are under the control of genes that produce hormones influencing cell differentiation and development. These pathways encompass an entire field of science that deals with cell developmental biology. We won't delve into this field any more here, but you are encouraged to find out more. Cell developmental biology provides many insights into the origin and evolution of plants and animals, and particularly the evolution of cell differentiation and multicellularity. However, in Chapter 15 we consider the rise of multicellularity again, why this happened on Earth, and whether it is inevitable.

Perhaps the best characterized growth patterns are to be found in the prokaryotic cells (Figure 5.20). Generally prokaryotes do not differentiate, although some fungi and slime molds do exhibit primitive differentiation. Some bacteria have specialized cell structures such as the heterocysts in cyanobacteria, which are specialized cells for fixing atmospheric nitrogen gas from the atmosphere into biologically available nitrogen compounds.

Figure 5.20 The major phases of growth in a population of prokaryotes.

The life cycle of a population of prokaryotic cells, for example, a culture of bacteria studied in the laboratory, is quite simple and follows some well-established patterns illustrated in Figure 5.20. In the first stage, the lag phase, the cells grow slowly. This represents the phase during which the cells are beginning to reproduce in the presence of newly available energy and nutrients. Following this stage, when the cells have adjusted to the new environment, they enter the logarithmic phase or exponential phase when growth is rapid. The organisms are not always growing according to an exact mathematical logarithmic function. The true growth rate depends on the energy available. In the next stage, the cells run out of some essential nutrient or energy supply, and they enter the stationary phase. Following the stationary phase, the cells begin to die as they enter the death phase.