The Growth of Populations of Cells
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.
5.11 Moving and Communicating
One often sees prokaryotes described in the literature as “primitive.” In the sense that they are simpler than eukaryotic cells, this description is not wrong. However, prokaryotic cells have remarkable features including the ability to move and communicate. We now investigate these characteristics. They are of relevance to understanding the evolution of life on Earth for two reasons. First, they show that the prokaryotes that inhabit the planet today are likely much more complex than the first cells that emerged on Earth. We should therefore be careful when we think of these organisms as “primitive” and recall that there has been at least 3.5 billion years of evolution since life emerged. At the same time, many features across cell types are conserved and seem ancient, such as the genetic code, whose commonality across life suggests that certain cellular structures reflect early events in cellular evolution. Second, learning about these cellular features is an important part of grasping that prokaryotes are not just simple bags of fluid that take in nutrients and excrete waste; single-celled structures can evolve remarkably complex behaviors. This tells us that there is potentiality for great complexity in even the simplest cellular structures.
5.11.1 Movement in Prokaryotes
Cells are not necessarily sessile. For many microorganisms, moving around is essential. By changing location, they can get access to new energy resources and nutrients or move away from toxins (chemotaxis).
Movement is most commonly achieved in microbes with the use of a flagellum (plural flagella), a remarkable and exquisite cell structure that works by rotation (Figure 5.21), driven by the energy molecule, ATP. Swimming speeds from 2 to 200 μm s−1 in water have been reported in microbes.
Figure 5.21 Microbial movement and flagella. The microbe Salmonella, stained to show the flagella. Each organism is about 1 μm long.
Source: Reproduced with permission of Centers for Disease Control.
Below is shown the structure of a bacterial flagellum in a Gram-negative bacterium. The structure is a motor embedded in the cell membranes and driven by ATP. It comprises a number of protein subunits which are labeled.
Source: Reproduced with permission of wikicommons.
A flagellum can be rotated at 200 revolutions per second. The flagella are typically about 15–20 μm long. The main “whip” of the flagellum is composed of a hollow cylindrical structure, about 20 nm (nanometers) in diameter, which is anchored to a rotating mechanism by a specialized structure called the hook. The chemical motor that rotates the flagellum is situated across the cell membrane and is made up of over 20 different proteins. To build the flagellum, new proteins are passed from the base of the flagellum, up through the hollow core, and added to the end of the filament, just underneath a cap protein at the tip. The flagellum is probably one of the most remarkable structures that have evolved in microorganisms.
Discussion Point: Rotating Structures in Nature – Why Don't Animals Have Wheels?
Figure 5.22 Tumbleweeds move by rolling, but why don't we see macroscopic rotating structures in nature?
Source: Reproduced with permission of EriKolaborator.
The flagellum is remarkable from many perspectives, but one intriguing feature is that it is one of the few rotational structures in nature. The ATP synthase, which makes the energy storage molecule ATP, is another rotating structure which is introduced in the next chapter. We could ask at the larger scale: Why aren't there more rotating structures in nature? One question that biologists often ask is: Why don't animals have wheels? Let's briefly examine this question. Wheels are not very good on rough surfaces, such as most planetary surfaces, and they cannot easily climb vertical surfaces higher than their radius, making them less useful than legs. Of course, tumbleweeds (Figure 5.22) and some spiders roll across plains and down slopes, respectively, but they are not genuine rotating structures within an organism. One major problem with rotating structures in multicellular life is evolving a system to rotate an appendage without it forming knots in muscles, blood vessels, and other attached structures. One answer could be to make a rotor, like the flagellum. However, the flagellum relies on small protein units embedded in a membrane, and it relies on the diffusion of ATP to provide the energy. It is not immediately obvious how one could scale this up into a rotating macroscopic structure, since diffusion, because it is too slow, would be limiting to structures on centimeter or larger scales. This then brings us back to using blood vessels and other means to transport the necessary nutrients and energy, which itself returns us to the problem of how to make rotating blood vessels, etc. You might like to consider these ideas for yourself. Why are rotating structures in life on Earth limited to molecular machines? Why don't we see large-scale rotational structures in multicellular animals? Is this just a chance evolutionary outcome or are there
