1.3Cells and Cell Membrane
Cells are small membrane-bound (cell membrane) structures. They vary in size from less than one micrometer in diameter to tens of micrometer. Animal ova are a few millimeters in size, and birds’ eggs can be a few centimeters in size. The body of an adult human is composed of 60 trillion cells, and the bodies of huge trees and elephants are also composed of numerous cells.
The basic essential components of cells are the cell membrane and genetic material, and their various properties in different types of organisms will be discussed in Chapters 6–12. Although the cell membrane is important in separating the internal environment of the cell from its external environment, this does not imply that life exists exclusively within the cell’s interior. In Chapter 11, we will see that the cell membrane controls the import and export of substances, with a difference created in concentration of ions and metabolites inside and outside the cell. Cell death ensues when the concentration difference between the inside and outside of the membrane disappears and equilibrium is reached. This indicates that life of a cell is also dependent on the environment external to the cell membrane. The cell wall and extracellular matrix (ECM) are located outside the cell membrane. In many bacterial and plant cells, cell shape is maintained by the cell wall. The cell discovered by Robert Hooke in the 17th century was in fact the cell wall of a plant cell (Column Fig. 1-1). The space outside the cell membrane of plant cells is called the apoplast and is important in ion storage and as a pathway for transport of active substances. Furthermore, the external environment of the cells inside the multicellular organism acting as the internal environment of this organism provides a special environment. In animals, intercellular adhesion and cellular movement are induced by the interactions between cells and ECM.
Genetic material contains the basic information to control every phenomenon that occurs within the cell; it is analogous to the hard disc of a computer. The genetic material in organisms is a microscopic filamentous material (DNA). The most surprising thing is that total self-replication of DNA is possible because of its complementary structure (see Chapter 7). Information in a computer is determined by the arrangement of zeroes and ones, while the genetic information in DNA is expressed by the arrangement (sequence) of the four elements A, T, C, and G (nucleotides or bases). DNA in a human cell is divided into 46 chromosomes with a total of six billion bases. Because humans have two homologous copies of each chromosome, excluding the sex chromosomes, there are approximately three billion bases per set (we will designate this number as “N”). As there are four types of bases, these bases can be arranged in numerous ways (4N). Moreover, in terms of information volume (binary logarithm), there are 2N bits or 750 megabytes (one byte equals eight bits). It is surprising when one considers that this volume is less than that in a 1 GB USB flash drive. However, the portion of human DNA carrying genes for proteins is estimated to be only 1.3%; thus, the amount of information coded is a small percentage of the total amount (see Chapter 7).
One may wonder how DNA exists within cells. In eukaryotic cells, DNA exists in combination with proteins in a structure called chromatin and is enclosed by a nuclear membrane, while prokaryotic cells do not have a nuclear membrane (Fig. 1-2). Previously, the term “chromosome” was used only to indicate the genetic material of eukaryotes, apart from it being used to indicate condensed chromosomes during cell division. However, this term is now also used for genomic DNA in prokaryotic cells (including bacterial cells) because it is a condensed structure, too. . The word genome is used here to indicate the total genetic information of an organism. The molecular structure and replication mechanism of DNA and the organization of the genome will be discussed in Chapters 6 and 7. A detailed explanation of the genome concept will be presented in Chapters 10 and 24.
Who discovered the cell? In 1665, the English physicist Robert Hooke observed the cell wall of a dried plant, Quercus suber (cork oak), and called it a cell (Column Fig. 1-1). Then, who was the first to observe a living cell? Many researchers made such observations. The German botanist Matthias Schleiden collected the observations of previous researchers and proposed that plants are composed of cells in 1838. Cell theory is considered to have been established the following year when the German zoologist Theodor Schwann proposed that animals are also composed of cells. At that time, the cell nucleus was considered an important feature of the cell theory, and chloroplasts were already known as cell organelles.
Today, we unquestioningly accept the cell theory because we have been taught since childhood that organisms are composed of cells. However, there are many unusual organisms in nature. Some unicellular organisms have a structure just as complex as if they were miniature versions of multicellular organisms. Let us examine a Paramecium cell (Column Fig. 1-2). Although this cell is covered on all sides by fine cilia (resembling hair on animal bodies) and possesses various granules and contractile vacuoles (resembling the organs of animals), it is still considered unicellular because of its single nucleus and reproduction through cell division. In fact, there is a macronucleus (amplifies essential genes for cell activities but degenerates during reproduction) and micronucleus (transmitted during reproduction) within the nucleus proper. It may be impossible to fit all organisms into a single cell framework because even plant and animal cells are fairly different from each other (see Fig. 1-2). Bacterial cell structure was believed to be extremely simple when it was observed using electron microscopes in the 1960s. However, the recent observation using fluorescent-labeled proteins and high sensitivity fluorescence microscopy has shown that membranes, appeared to be homogeneous, actually exhibit functional differentiation. Clearly, there is much left to be discovered. If we try to combine and unify all information of cells of various organisms, we would simply end up pointing to the existence of the nucleus (genetic material) and cell membrane.
The Paramecium is approximately 200 µm long. Visible structures include contractile vacuoles as two white spheres; the macronucleus as the slightly black region toward the bottom; and the cell mouth from just left of the center, diagonal to the bottom right corner of the diagram. The body surface is covered by countless cilia, although the focal plane of this photomicrograph makes them difficult to observe. Slightly longer cilia are visible at the very bottom. Thus, it is a single Paramecium cell that has such a complicated structure.