In addition to providing a significant nutritional mode, the advent of endocytosis in an ancestor of living eukaryotes also enabled a completely new way to generate cellular change and complexity: endosymbiosis. Put simply, endosymbiosis is the process by which one cell is taken up by another and retained internally, such that the two cells live together and integrate at some level, sometimes permanently. Endosymbiotic interactions have been common in eukaryotic evolution, and many such partnerships persist today (Margulis, 1981). In two cases, however, endosymbiotic events had far-reaching effects on the evolution of life: these are the origins of mitochondria and plastids (chloroplasts).
Mitochondria are generally known as the energy-generating powerhouses of eukaryotic cells, where oxidative phosphorylation and electron transport metabolism takes place (Reichert and Neupert, 2004). They are also involved in several other jobs such as oxidation of fatty acids, amino acid metabolism, and assembly of iron-sulfur clusters (Lill et al., 1999; Lill and Kispal, 2000).
They are bounded by two membranes, the innermost of which is generally highly infolded to form ‘cristae’ that take characteristic shapes, either flat, tubes, or paddle-shapes (Taylor, 1978). The presence of mitochondria is an ancestral trait in eukaryotes (Roger, 1999; van der Giezen and Tovar, 2005; van der Giezen et al., 2005; Williams and Keeling, 2003), although in certain anaerobes and microaerophiles they have radically reduced or transformed functions: in some cases they are not involved in energy production at all (e.g., the ‘mitosomes’ of microsporidia, diplomonads, and archaemoebae, or ‘hydrogenosomes’ of parabasalia, some ciliates, and some chytrid fungi) (Embley, 2006; Müller, 1993; Tovar et al., 1999; van der Giezen et al., 2005; Williams and Keeling, 2003). Mitochondria can be traced back to a single endosymbiosis of an alpha-proteobacterium.
Plastids are the photosynthetic organelles of plants and algae. “Plastid” is a general term for all such organelles, including chloroplasts (in the green lineage), rhodoplasts (in the red lineage), leucoplasts (colourless plastids), etc. Plastids have diverse functions in addition to photosynthesis, including the biosynthesis of amino acids, fatty acids and isoprenoids (Harwood, 1996;
Herrmann and Weaver, 1999; Rohdich et al., 2001). As in the case of mitochondria, plastids in many lineages have been radically reduced or transformed, primarily through the loss of photosynthesis (e.g., the ‘apicoplast’ of Apicomplexa, and the relict plastids of many parasitic algae and plants (Gould et al., 2008; Ralph et al., 2004; Wilson, 2002)).
Plastids can also be traced back to a single endosymbiosis event involving a cyanobacterium and the ancestor of the Archaeplastida (Reyes-Prieto et al., 2007; Rodriguez-Ezpeleta et al., 2005). However, unlike mitochondria, plastids then spread to other eukaryotic lineages by secondary and tertiary endosymbiotic events (Archibald, 2005; Gould et al., 2008; Keeling, 2004; McFadden, 1999).
In these events, one eukaryotic cell took up another eukaryote that already contained a plastid (an alga), and this second, endosymbiotic eukaryote was then reduced and integrated. In most cases all that remains of this alga is the plastid surrounded by the remains of the endosymbiont’s plasma membrane. However, in cryptomonads and chlorarachniophytes a tiny relict of the algal nucleus called a “nucleomorph” is also retained, the study of which helped elucidate the complex evolutionary history of plastids (Archibald, 2005; Douglas et al., 2001; Gilson et al., 2006; McFadden et al., 1997). Other endosymbiotic relationships based on photosynthesis are also known (Johnson et al., 2007; Okamoto and Inouye, 2005; Rumpho et al., 2008), but typically these are not integrated to the extent that they are generally accepted to be ‘organelles’ rather than ‘endosymbionts’. One possible exception is the euglyphid amoeba Paulinella chromatophora, where a cyanobacterium similar to Synechococcus or Prochlorococcus has been integrated to an extent approaching that of canonical plastids (Nowack et al., 2008).
Eukaryotic Cell Cycle
The cell cycle is the life cycle of a cell. During this cycle, it grows and divides. Checkpoints exist between all stages so that proteins can determine whether the cell is ready to begin the next phase of the cycle.
Quiescence, also known as senescence or resting, is a phase in which the cell is not actively dividing. It is also known as Gap 0, or G0. This stage is considered the start of the cell cycle, although it is one that cells can reach and then stop dividing indefinitely, which ends the cell cycle.
Liver, stomach, kidney cells, and neuron are all examples of cells that can reach this stage and remain in it for long periods of time. It can also occur when a cell’s DNA is damaged. However, most cells do not go into the G0 stage at all, and can divide indefinitely throughout the life of an organism.
In interphase, the cell grows and takes in nutrients in preparation for division. Interphase takes up about 90 percent of the cell cycle. It consists of three parts: Gap 1, Synthesis, and Gap 2.
- Gap 1 (G1) is also known as a growth phase. The cell gets larger and increases its stock of proteins, along with organelles such as the energy-producing mitochondria.
- Synthesis (S) is the phase in which DNA replicates. During synthesis, the chromosomes replicate so that each chromosome is made up of two sister chromatids. At the end of this phase, there is double the amount of DNA in the cell.
- Gap 2 (G2) is another growth phase. The cell becomes even larger in order to prepare for mitotic division.
Mitosis, or M phase, is when the cell begins to organize its duplicated DNA for separation into two daughter cells. The chromosomes separate so that one of each chromosome goes into each daughter cell. This results in the daughter cells having identical chromosomes to the parent cell. Mitosis itself is divided into prophase, metaphase, anaphase, and telophase, which mark various points in the DNA separation process. Mitosis is then followed by a process called cytokinesis, during which the cell separates its nuclei and other organelles in preparation for division and then physically divides into two cells.
Examples of Eukaryotic Cells
Plant cells are unique among eukaryotic cells for several reasons. They have reinforced, relatively thick cell walls that are made mostly of cellulose and help maintain structural support in the plant. Each plant cell has a large vacuole in the center that allows it to maintain turgor pressure, which is pressure from having a lot of water in the cell and helps keep the plant upright. Plant cells also contain organelles called chloroplasts which contain the molecule chlorophyll. This important molecule is used in the process of photosynthesis, which is when a plant makes its own energy from sunlight, carbon dioxide, and water.
Like plant cells, fungal cells also have a cell wall, but their cell wall is made of chitin (the same substance found in insect exoskeletons). Some fungi have septa, which are holes that allow organelles and cytoplasm to pass between them. This makes the boundaries between different cells less clear.
Animal cells do not have cell walls. Instead, they have only a plasma membrane. The lack of a cell wall allows animal cells to form many different shapes, and allows for the processes of phagocytosis “cell eating” and pinocytosis “cell drinking” to occur. Animal cells differ from plant cells in that they do not have chloroplasts and have smaller vacuoles instead of a large central vacuole.
Protozoa are eukaryotic organisms that consist of a single cell. They can move around and eat, and they digest food in vacuoles. Some protozoa have many cilia, which are small “arms” that allow them to move around. Some also have a thin layer called a pellicle, which provides support to the cell membrane.
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes (Bacteria and Archaea), which have no membrane-bound organelles. Eukaryotes belong to the domain Eukaryota or Eukarya. Their name comes from the Greek εὖ (eu, “well” or “true”) and κάρυον (karyon, “nut” or “kernel”). Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, and in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may also be multicellular and include organisms consisting of many cell types forming different kinds of tissue. Animals and plants are the most familiar eukaryotes.
Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells. These act as sex cells (gametes). Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental chromosomes resulting from genetic recombination during meiosis.
The domain Eukaryota is monophyletic and makes up one of the domains of life in the three-domain system. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things. However, due to their generally much larger size, their collective worldwide biomass is estimated to be about equal to that of prokaryotes. Eukaryotes evolved approximately 1.6–2.1 billion years ago, during the Proterozoic eon.