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A LOOK AT CELLS AND TISSUES

With very few exceptions, all living things are either a single cell or an assembly of cells. This chapter will begin to describe what a cell is, and further chapters will say much more. However, to begin with, we can briefly describe a cell as an aqueous (watery) droplet enclosed by a lipid (fatty) membrane. Cells are, with a few notable exceptions, small (Figure 1.1), with dimensions measured in micrometers (μm, 1 μm = 1/1000 mm). They are more or less self‐sufficient: a single cell taken from a human being can survive for many days in a dish of nutrient broth, and many human cells can grow and divide in such an environment. In 1838 the botanist Matthias Schleiden and the zoologist Theodor Schwann formally proposed that all living organisms are composed of cells. Their “cell theory,” which nowadays seems so obvious, was a milestone in the development of modern biology. Nevertheless, general acceptance took many years, in large part because the plasma membrane (Figure 1.2), the membrane surrounding the cell that divides the living inside from the nonliving extracellular medium, is too thin to be seen using a light microscope. Microorganisms such as bacteria, yeast, and protozoa exist as single cells. In contrast, the adult human is made up of about 30 trillion cells (1 trillion = 1012), which are mostly organized into collectives called tissues.

 ONLY TWO TYPES OF CELL

Superficially at least, cells exhibit a staggering diversity. Some have defined, geometric shapes; others have flexible boundaries; some lead a solitary existence; others live in communities; some swim, some crawl, and some are sedentary. Given these differences, it is perhaps surprising that there are only two types of cell (Figure 1.2). Prokaryotic (Greek for “before nucleus”) cells have very little visible internal organization so that, for instance, the genetic material, stored in the molecule deoxyribonucleic acid (DNA), is free within the cell. These cells are especially small, the vast majority being 1–2 μm in length. The prokaryotes are made up of two broad groups of organisms, the bacteria and the archaea (Figure 1.3). The archaea were originally thought to be an unusual group of bacteria but we now know that they are a distinct group of prokaryotes with an independent evolutionary history. The cells of all other organisms, from yeasts to plants to worms to humans, are eukaryotic (Greek for “with a nucleus”). These are generally larger (5–100 μm, although some eukaryotic cells are large enough to be seen with the naked eye; Figure 1.1) and structurally more complex. Eukaryotic cells contain a variety of specialized structures known collectively as organelles, embedded within a viscous substance called cytosol. Their DNA is held within the largest organelle, the nucleus. The structure and function of organelles will be described in detail in subsequent chapters. Table 1.1 summarizes the differences between prokaryotic and eukaryotic cells.

 Figure 1.1. Dimensions of some example cells. 1 mm = 10−3 m; 1 μm = 10−6 m; 1 nm = 10−9 m.

Cell Division

One of the major distinctions between prokaryotic and eukaryotic cells is their mode of division. In prokaryotes the circular chromosome is duplicated from a single replication origin by a group of proteins that reside on the inside of the plasma membrane. At the completion of replication the old and new copies of the chromosome lie side by side on the plasma membrane which then pinches inwards between them. This process, which generates two equal, or roughly equal, daughter cells is described as binary fission. In eukaryotes the large, linear chromosomes, housed in the nucleus, are duplicated from multiple origins of replication by enzymes located in the nucleus. Sometime later the nuclear envelope breaks down and the replicated chromosomes are compacted so that they can be segregated without damage during mitosis. We will deal with mitosis in detail in Chapter 14. For the moment we should be aware that although it is primarily about changes to the nucleus, mitosis is accompanied by dramatic changes in the organization of the rest of the cell. A new structure, the mitotic spindle, is assembled specifically to move the chromosomes apart whilst other structures are dismantled so that their components can be divided among the two daughter cells following cell division.

VIRUSES

Viruses occupy a unique position between the living and nonliving worlds. On the one hand they are made of the same molecules as living cells. On the other they are incapable of independent existence, being completely dependent on a host cell for reproduction. Almost all living organisms have viruses that infect them. Human viruses include polio, influenza, herpes, rabies, smallpox, chickenpox, HIV, and SARS‐CoV‐2, the causative agent of COVID‐19. Viruses are submicroscopic particles consisting of a core of genetic material enclosed within a protein coat called the capsid. Some have an extra membrane layer called the envelope. Viruses are inert until they enter a host cell, whereupon their genetic material directs the host cell machinery to produce viral protein and viral genetic material. Viruses often insert their genome into that of the host, an ability that is widely made use of in molecular biology research (Chapter 8). Bacterial viruses, bacteriophages, are used by scientists to transfer genes between bacterial strains. As we will see, human viruses are used as vehicles for gene therapy.

 Figure 1.2. Organization of prokaryotic and eukaryotic cells.

 Figure 1.3. The tree of life. The diagram shows the currently accepted view of how the different types of organism arose from a common ancestor. Many minor groups have been omitted. Distance up the page should not be taken as indicating complexity or how “advanced” the organisms are. All organisms living today represent lineages that have had the same amount of time to evolve and change from the last universal common ancestor.

ORIGIN OF EUKARYOTIC CELLS

Prokaryotic cells are simpler in their organization than eukaryotic cells and are assumed to be more primitive. According to the fossil record, prokaryotic organisms precede, by at least 1.5 billion years, the first eukaryotes that appeared some 2 billion years ago. It seems highly likely that eukaryotes evolved from prokaryotes, and the most likely explanation of this process is the endosymbiotic theory. The basis of this theory is that some eukaryotic organelles originated as free‐living bacteria that were engulfed by larger cells in which they established a mutually beneficial relationship. For example, mitochondria would have originated as free‐living aerobic bacteria and chloroplasts as photosynthetic cyanobacteria. The endosymbiotic theory provides an attractive explanation for the fact that mitochondria and chloroplasts contain their own DNA and ribosomes both of which are more closely related to those of bacteria than to all the other DNA and ribosomes in the same cell. The case for the origin of other eukaryotic organelles is less persuasive. Nevertheless, while it is clearly not perfect, most biologists are now prepared to accept that the endosymbiotic theory provides at least a partial explanation for the evolution of the eukaryotic cell from prokaryotic ancestors.

 TABLE 1.1. Differences between prokaryotic and eukaryotic cells.

a The tiny chromosomes of mitochondria and chloroplasts are exceptions; like prokaryotic chromosomes they are often circular.

b  The S value, or Svedberg unit, is a sedimentation rate. It is a measure of how fast a molecule moves in a gravitational field, and therefore in an ultracentrifuge.

Example 1.1 Sterilization by Filtration

Because even the smallest cells are larger than 1 μm, harmful bacteria and...