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The life of a cell is simple in theory, and complex in practice. The evolutionary purpose of your cell is to grow, ultimately amassing enough resources to produce all the macromolecules needed to make another cell. How best to grow, though, depends on the environment and what fuel is available. Rather than focusing on the chemical reactions of metabolism, which are well covered in other texts, in this chapter we will examine a few architectural features that enhance growth in various conditions. In a world of scarce resources and fierce competition, any adaptation that lets your cell grow more efficiently can have a big effect on its success. And often, these adaptations are visible in the structure of the cell.
You have already seen how the shape of your cell can allow it to gather nutrients from the environment more efficiently. For example, prosthecate bacteria like this Caulobacter crescentus use long, thin extensions to increase surface area relative to volume, allowing them to absorb more nutrients. The extra surface area of a stalk increases a cell’s ability to absorb nutrients, but it also adds more membrane, diluting membrane proteins and increasing the time it takes them to diffuse around the cell. C. crescentus stalks get longer throughout their lifetime, so the situation only gets more extreme with age. To solve this problem, you might want to separate the envelope of the stalk from the envelope of the rest of the cell. C. crescentus has evolved a structure to do just this, called a stalk band. These protein structures form diffusion barriers for the membranes and periplasm, but not the cytoplasm, so nutrients can still diffuse into the cell body. Each cell division produces another band in the elongating stalk, so you can tell a cell’s divisional age by counting its bands.