Within all living cells is the machinery to produce DNA, RNA, and protein, which themselves are composed of the six elements hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus. These elements are plentiful in the nonliving environment. However, it is not easy for these elements to be incorporated into biological molecules. N2 and CO2 in the atmosphere are particularly unreactive (Alberts et al. 2014).
Biosynthesis requires that nitrogen and carbon dioxide be fixed, making N and C available to living organisms. Many cells must rely on other classes of cells to achieve this goal. For instance, beginning with plants, nitrogen-fixing bacteria are often tasked with supplying their need for nitrogen compounds. Plants can fix CO2 from the atmosphere. Animals, then, rely on plants for their supplies of both carbon and nitrogen compounds (Alberts et al. 2014).
What you see here is that in even the most basic aspects of biochemistry, living cells can differ widely. Amazingly, a single composite cell can be formed from cells with complementary needs and associations, in some cases. In many other cases, close associations may develop between cells of this type (Alberts et al. 2014).
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