1.3 Eukaryotic Cells and Evolution

Diversity in Eukaryotic Cells

Before discussing the evolution of eukaryotic cells, we must quickly review their structure and diversity. In addition to the common components of all cells (a plasma membrane, cytoplasm, DNA, and ribosomes), there are a few components of eukaryotic cell structure we can consider. Each eukaryotic cell has a nucleus and various other membrane-bound organelles, a cytoskeleton, and (usually) some kind of extracellular component. All eukaryotic cells can be identified by the presence of a nucleus and mitochondria, which play prominent roles in genetic regulation and energy production, respectively. However, there are some striking differences in cell structure across phylogenetic kingdoms (Figure 1.8). For example, plant cells and other photosynthetic eukaryotes have chloroplasts, while animal and fungal cells lack these organelles. Plant and fungal cells both have an extracellular (outside the plasma membrane) cell wall, however, the composition differs. Plant cell walls are made primarily of cellulose, while fungal cell walls are made primarily of chitin. Animal cells have no external cell wall but do have an extracellular matrix composed of proteins and carbohydrates.

There are also differences among different cell types within multicellular organisms (such as yourself!). For example, the structure and composition of human liver cells are very different from that of neurons or skeletal muscle cells (Figure 1.9). These differences in structure reflect the various ways that eukaryotic cells have evolved to survive and thrive in different environments.

Limits to Cell Size

Eukaryotic cells are also large, at least relative to their prokaryotic cousins. At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm (Figure 1.10). The prokaryotes’ small size allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out of the cell. This is not the case in eukaryotic cells, which are too large for intracellular transport to effectively occur via diffusion.

Small size, in general, is necessary for all cells, due to the relationship between cell surface area (area of the cell membrane) and cell volume (amount of cytoplasm). We can think of this most easily if we imagine a cube-shaped cell, with each edge having a length = A. This cube has a surface area of 6A², and a volume is A³. Thus, as the size (length) of a cell increases, its surface area increases as the square of its length, but its volume increases as the cube of its length (much more rapidly) (Figure 1.11). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply to a cell of any approximately spherical shape. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide. Other ways to increase surface area are by folding the cell membrane, becoming flat or thin and elongated, or developing organelles that perform specific tasks. We see the latter in eukaryotic cells, which can grow larger than prokaryotic cells due to their numerous membrane-bound organelles.

Evolution of Eukaryotes

Eukaryotes evolved comparatively late in evolutionary history, approximately 1.8 billion years ago. While eukaryotic cells have diverse sizes and shapes, they are considerably larger than prokaryotic cells. The ability to compartmentalize various cellular processes into specialized organelles allowed eukaryotic cells to overcome some of the limits to cell size (described above) that prokaryotes experience. We don’t fully know how the first ancestral eukaryote developed a nucleus, but there is good scientific support for how eukaryotic cells evolved to contain mitochondria (all eukaryotes) and chloroplasts (plant and algal cells), as described by the theory of endosymbiosis section below.

The Theory of Endosymbiosis

The endosymbiosis theory, like all scientific theories, is stronger than a hypothesis because it has been supported by many experiments and lines of evidence. This theory proposes that eukaryotic cells originated from prokaryotic cells that engulfed and established a symbiotic relationship with bacterial cells. Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives inside the other. The theory of endosymbiosis suggests a series of steps for how this process occurred (Figure 1.12). The precursor to a eukaryotic cell likely had a nucleus and the scaffolding for the endomembrane system (e.g., endoplasmic reticulum-like structures). This cell then engulfed and formed a symbiotic relationship with a mitochondrion-like bacterium, a symbiosis that eventually evolved into a eukaryotic cell with one or more mitochondria that could perform cellular respiration. In one lineage of eukaryotes, a second endosymbiosis occurred, this time with a chloroplast-like bacterium that could perform photosynthesis. This endosymbiosis gave rise to the group of organisms that includes plants, eukaryotic algae, and other photosynthetic eukaryotes.

There are multiple levels of evidence that mitochondria and chloroplasts evolved via endosymbiosis. Scientists have long noticed that modern bacteria, mitochondria, and chloroplasts are similar in size. We also know that modern bacteria have circular DNA and ribosomes, just like mitochondria and chloroplasts. In addition, there are modern bacteria with similar metabolic capabilities to mitochondria and chloroplasts – aerobic bacteria (capable of cellular respiration) and autotrophic bacteria (cyanobacteria; capable of photosynthesis). Mitochondria and chloroplasts are also able to produce copies of themselves through binary fission – the same process most bacteria use to reproduce. All of these pieces of information suggest that mitochondria, chloroplasts, and modern bacteria share a common ancestor, and that early eukaryotes engulfed but did not destroy the ancestors to mitochondria and chloroplasts. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.