Symbiosis means “living together,” and often refers to two organisms that coexist, each one supplying something that the other needs. With the theory of endosymbiosis biologists have proposed that some organelles—the mitochondria and the plastids—arose not by an infolding of the cell membrane but by one cell ingesting (but not digesting) another cell, giving rise to a symbiotic relationship. Eventually, the ingested cell lost its autonomy and some of its functions. In addition, many of the ingested cell’s genes were transferred to the host’s DNA. Mitochondria and plastids in today’s eukaryotic cells are the remnants of these symbionts, retaining some specialized functions that benefit their host cells.

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Consider the case of the plastid. About 2.5 billion years ago some prokaryotes (the cyanobacteria) developed photosynthesis. The emergence of these prokaryotes was a key event in the evolution of complex organisms because they increased the O₂ concentration in Earth’s atmosphere (see Key Concept 25.2).

According to the endosymbiosis theory, photosynthetic prokaryotes also provided the precursor of the modern-day plastid. Cells without cell walls can engulf relatively large particles by phagocytosis (see Figure 5.10). In some cases, such as that of phagocytes in the human immune system, the engulfed particle can be an entire cell, such as a bacterium. Plastids may have arisen by a similar event involving an ancestral eukaryote and a cyanobacterium (Figure 5.23B).