and can allow it to change shape, such as when it is involved in tissue formation or movement toward nutrients and resources. The cytoskeleton plays a particularly important role in cell division, providing a scaffold during the separation of chromosomes. It is also used as a network for moving cell components around within the cell. Components travel along the microtubules like a railway track, attached to molecules such as kinesin. This is an adenosine triphosphate (ATP)-requiring process.
The cytoskeleton may not be completely unique to eukaryotes. Prokaryotes have been shown to have simple cytoskeletal structures made up of proteins such as crescentin, which seems to form a ring structure, providing shape to some bacterial cells. These observations show that although in gross characteristics eukaryotic cells seem very different from prokaryotic cells, they share common characteristics that likely reflect their common origins.
Eukaryotic cells can contain a number of other organelles in which separate tasks are undertaken. For example, plant cells contain chloroplasts, the site of photosynthetic reactions, which we explore in more detail in Chapter 6. Eukaryotic cells contain the endoplasmic reticulum, which is split into two types. “Rough” endoplasmic reticulum is the site of protein synthesis, “smooth” endoplasmic reticulum, the site of lipid synthesis. The Golgi body (or Golgi apparatus) is involved in packaging proteins from the endoplasmic reticulum, particularly proteins that are to be excreted from the cells.
Animal cells have lysosomes, which are organelles containing enzymes that allow the cell to break down engulfed molecules as a source of food. Secretory vesicles are involved in the excretion of hormones and other chemical messengers.
A particularly important organelle within the eukaryotes is the mitochondrion (plural mitochondria), found in most eukaryotic cells. They are ATP-producing organelles, the site of aerobic respiration, although they also have roles to play in cell signaling and differentiation. Although plants trap energy in chloroplasts, they still use mitochondria to break down the glucose they produce in photosynthesis as a source of energy for the cell. The mitochondria are important because they are the energy-yielding factories, if you like, of the eukaryotic cell and in many ways define the eukaryotic cell. By tapping into aerobic respiration, which is much more energy-yielding than oxygen-free modes of acquiring energy, and by having many mitochondria, the eukaryotic cell was able to harness much greater quantities of energy to allow for greater complexity and for the energy-intensive processes we associate with multicellular life. In that sense, the acquisition and taming of the mitochondrion by the earliest eukaryotic cell likely allowed for the revolution we associate with the emergence of complex multicellular life. You might like to consider this paragraph again when you have investigated energy acquisition in cells, discussed in the next chapter.
The diversity of cellular structures made possible by the plethora of component parts allows for a wide variety of cell types. For example, the human body contains over 200 types of cells specialized for functions as diverse as passing electrical signals (neurons) or providing an external barrier (skin cells). Together these cells comprise the ∼40 trillion cells that make up the human organism. Some of these cells last for a lifetime (such as some neurons), and some, such as white blood cells, can last for just a day. It is now recognized that this is not just a random case of cells dying when they get damaged or otherwise compromised. Many cells have a programmed cell death or apoptosis, which can be triggered by cell damage or when the cells are no longer required. These pathways presumably evolved to prevent rogue cells from causing damage to the organism.
5.7.1 Endosymbiosis
Where did these organelles in eukaryotes come from? It was long suspected that maybe some of them had begun their origins as independent cells. For example in the mitochondria, the physically circular nature of their DNA and the content of the genetic code suggested that they were once bacteria that were acquired by eukaryotes, eventually becoming dependent on the host cell (Figure 5.17). The ancestral bacterium is thought to have been an alphaproteobacterium, a subphylum of bacteria. This process of endosymbiosis, championed by biologist Lynn Margulis (1938–2011), explains the origin of chloroplasts in eukaryotic photosynthetic organisms such as plants and algae. The chloroplasts were once independent photosynthetic cyanobacteria.
Figure 5.17 A schematic illustration of the concept of endosymbiosis. Chloroplasts and mitochondria were acquired initially as independent prokaryotic cells.
Endosymbiosis has even been invoked to explain the emergence of the eukaryotic nucleus. Another explanation is that the nucleus was a part of the original eukaryotic membrane that split off to form a separate structure (the exomembrane hypothesis). The origins of the eukaryotic nucleus remain an intriguing problem in cell biology.
5.8 The Reproduction of Cells
In cellular organisms (the prokaryotes and in most eukaryotic cell division) cells divide by mitosis (called binary fission in the prokaryotes; Figure 5.18). During mitosis, DNA is replicated, generating two cells with exactly the same genetic composition as the original cell. All prokaryotes divide in this way. It is also referred to as asexual reproduction. In prokaryotes, the rate of cell division can be sufficiently high that a cycle of DNA replication can be begun before the previous one is completed. Mitosis is used in multicellular eukaryotes for replicating cells such as skin cells.
Figure 5.18 The process of mitosis or “binary fission.”
In many eukaryotes, including animals, plants and fungi, an additional form of replication is achieved, referred to as meiosis (Figure 5.19). Meiosis, put simply, is the process of making sex cells that can come together to make new individuals – this defines sexual reproduction. It seems logical, then, that the central process of meiosis is to make cells with half the genetic complement, so that when the mother's and father's sex cells come together, they produce a full complement again.
Figure 5.19 The process of meiosis.
Meiosis can be explained by reference to typical animal cells involved in this process (Figure 5.19). Animal cells contain two sets of chromosomes. As a consequence, they are called diploid cells. One set of chromosomes has come from the mother and one set from the father. In the first stage of meiosis, these sets of chromosomes are replicated in a diploid cell (Figure 5.19a). This process occurs before mitosis or meiosis. In mitosis these chromosomes would just separate into daughter cells, creating identical cells to the parent cell. However, in doubling up the chromosomes in meiosis, the cell has entered into the stage called Meiosis I. In the next step, the sets of doubled up chromosomes line up (Figure 5.19b) and then exchange genetic information (Figure 5.19c), generating chromosomes that are no longer identical to one another. This event is unique to meiosis and is called cross-over. By crossing over, segments of the chromosomes are mixed, generating variation. These chromosomes can now be divided into two new cells (Figure 5.19d). In Meiosis II, the next stage of meiosis, these cells are again divided in an identical way to mitosis, generating four cells (Figure 5.19e). These are the sex cells. These sperm or eggs (gametes) contain half the genetic complement and are called haploid cells. They can join together in sexual reproduction in which chromosomes from the mother and father come together to generate new adult diploid cells, which begin the process again in the new individuals.