In the two billion years that eukaryotes have been on earth, they have evolved into some of the most dramatic and interesting creatures, such as platypuses, dolphins, giant sequoias, and the Venus flytrap. Not all eukaryotes are multicellular, however. Many fungi are unicellular, and the Protista (or protists) are a huge group of eukaryotes, nearly all of which are single-celled organisms visible only with a microscope. Nonetheless, because all prokaryotes are single-celled and thus invisible to the naked eye, every organism that we see around us is a eukaryotic organism, including all plants and animals (FIGURE 3-5).

Figure 3-5Diversity of the eukaryotes. Every organism that we can see without magnification is a eukaryotic organism.

Eukaryotic cells are about 10,000 times larger than prokaryotic cells in volume. They possess numerous structural features that make it easy to distinguish eukaryotes from prokaryotes under a microscope (FIGURE 3-6). Chief among the distinguishing features of eukaryotic cells is the presence of a nucleus, a membrane-enclosed structure that contains linear strands of DNA. In addition to a nucleus, eukaryotic cells usually contain in their cytoplasm several other specialized structures. Many of these structures, called organelles, are enclosed separately by their own lipid membranes.

Figure 3-6Comparison of eukaryotic and prokaryotic cells.

The physical separation of compartments within a eukaryotic cell means that the cell has distinct areas in which different chemical reactions can occur simultaneously. In the mostly non-compartmentalized interior of a prokaryotic cell, random molecular movements quickly blend the chemicals throughout the cell, reducing the ease with which different reactions can occur simultaneously. FIGURE 3-7 illustrates a generalized animal cell and a generalized plant cell. Because they share a common, eukaryotic ancestor, they have much in common. Both can have a plasma membrane, nucleus, cytoskeleton, and a host of organelles, including rough and smooth endoplasmic membranes, Golgi apparatus, and mitochondria. Animal cells have centrioles, which are not present in most plant cells. Plant cells have a rigid cell wall (as do fungi and many protists) and chloroplasts (also found in some protists). Plants also have a vacuole, a large central chamber (only occasionally found in animal cells). We explore each of these animal and plant organelles in detail later in this chapter.

Figure 3-7Structures found in animal and plant cells.

When you compare a complex eukaryotic cell with the structurally simple prokaryotic cell, it’s hard not to wonder about the origin of eukaryotic cells. We can’t go back two billion years to watch the initial evolution of eukaryotic cells, but there is considerable evidence for some of what occurred. In particular, the endosymbiosis theory provides the best explanation for the presence of two organelles in eukaryotes: chloroplasts in plants and algae, and mitochondria in plants and animals. Chloroplasts enable plants and algae to convert sunlight into a more usable form of energy. Mitochondria help plants and animals harness the energy stored in food molecules. (Chapter 4, on energy, explains the details of both of these processes.)

According to the theory of endosymbiosis, two different types of prokaryotes may have set up close partnerships with each other. For example, some small prokaryotes capable of performing photosynthesis (the process by which plant cells capture light energy from the sun and transform it into the chemical energy stored in food molecules) may have come to live inside a larger “host” prokaryote. The photosynthetic “boarder” may have made some of the energy that it captured in photosynthesis available for use by the host.

After a long while, the two cells may have become more and more dependent on each other, until neither cell could live without the other (they became “symbiotic”) and they became a single, more complex organism. Eventually, the photosynthetic prokaryote evolved into a chloroplast, the organelle in plant and eukaryotic algae cells in which photosynthesis occurs. A similar scenario might explain how another large host prokaryote engulfed a smaller prokaryote unusually efficient at converting food and oxygen into easily usable energy and this smaller prokaryote evolved into a mitochondrion, the organelle in plant and animal cells that converts the energy stored in food into a form usable by the cell (FIGURE 3-8).

The idea of the role of endosymbiosis in the evolution of eukaryotes is supported by several observations. .

• 1. Chloroplasts and mitochondria are similar in size to prokaryotic cells and divide by splitting (fission), just like prokaryotes.
• 2. Chloroplasts and mitochondria have ribosomes, similar to those found in bacteria, that allow them to synthesize some of their own proteins; this ability is not found in other organelles, which rely on proteins made by cytoplasmic ribosomes.
• 3. Chloroplasts and mitochondria have small amounts of circular DNA, similar to the circular DNA in prokaryotes and in contrast to the linear DNA strands found in a eukaryote’s nucleus.
• 4. Analysis of chloroplast and mitochondrial DNA has revealed that it is highly related to bacterial DNA, much more closely than it is related to eukaryotic DNA.

The best current theory about the origin of the other organelles in eukaryotes is a process called invagination. The idea is that the plasma membrane around the cell may have folded in on itself to form the inner compartments, which subsequently became modified and specialized (see Figure 3-8).

Figure 3-8How did eukaryotic cells become so structurally complex? Two theories.