2.4The Molecules That Constitute Cells
To understand cells, let us first study the main molecules that constitute them. Table 2-2 shows typical subcellular molecular substances. Obviously, molecular compositions differ slightly between E. coli and human cells, and they also differ among the cells of each tissue of human cells. However, the proportions of molecular compositions do not differ greatly.
The water content of cells is so high that they can be considered flooded; water constitutes about 70% of cells. Why do cells contain so much water? Water is a polar molecule, and thus, dissolves many substances, such as ions and proteins. Furthermore, although water molecules are small, they bind together by hydrogen bonding, and consequently, their melting point, boiling point, and specific heat become higher than those of other small molecules. This stable nature of water is thought to be important for formation and maintenance of stable life in the environment of the earth's surface.
Proteins are the second most abundant subcellular substances after water. Twenty amino acids form the raw materials of proteins. Although proteins are very important molecules, there are nine "essential amino acids" that humans cannot produce. Therefore, we obtain them by eating other organisms. These 20 amino acids bind in various combinations to form various proteins (Fig. 2-3).
These proteins have various lengths, but are usually about 100–1000 amino acids long in humans. If proteins containing 100 amino acids formed with free use of the 20 amino acids, theoretically, an astronomical number (20100) of proteins could be produced, yet the total number of known proteins in humans is not more than about 100,000.
Amino acids are arranged appropriately to form spatial structures. Sections of amino acid chains fold and form structures such as helices or sheets, and the entire protein is folded into a tertiary structure. Proteins also bind with each other to form complex quaternary structures. These spatial structures are important for protein function, and if the spatial structures are degraded by heat or some other agent, then the proteins cease to function. Using these spatial structures, proteins perform various central roles as enzymes, structural proteins, cytoskeletons, receptors, etc., in cell function. This is one of the reasons that most human genetic diseases are caused by changes in protein function. As described below, amino acid sequences of proteins are deciphered from the genetic information in the DNA (see Chapters 3 and 4).
Organ and Cell Transplantations
Organs have complex structures and functions; consequently, it is currently impossible to create a perfect artificial organ (see Chapter 5). Therefore, when organ function deteriorates and threatens a patient's life, medical treatment is performed in which the patient's affected organ, such as his/her heart, liver, lung, or kidney, is replaced by a healthy organ donated by another person. This treatment is called organ transplantation. Organ donors can be living or deceased, depending on the type of organ transplanted. Organs such as kidneys can be donated by a healthy person, whereas organs such as the heart and cornea can only be obtained from a person who is brain dead or has died of cardiac arrest.
Cell transplantation is a medical treatment in which cells with specific properties are transplanted. Cells with innate or acquired deteriorations of function are replaced by cells donated from the recipients themselves or from a third person. For example, type I diabetes is a disease that occurs when β cells, which function in insulin production in the pancreas, are destroyed causing a shortage of insulin. It is possible to produce and supply insulin in the body of type I diabetes patients by transplanting β cells from the islets of Langerhans. Furthermore, in case of abnormalities in the hematopoietic function of bone marrow tissue due to leukemia or similar diseases, normal hematopoietic function is restored by transplantation of hematopoietic stem cells. The transplanted hematopoietic stem cells are established in the bone marrow and produce various blood cells such as white blood cells and red blood cells. This kind of medical treatment has become possible for the first time since the structure and properties of cells and tissue were elucidated.
"Lipid" is a general term for any substance that is insoluble in water. Lipids include a variety of compounds such as glycerolipids, sphingolipids, and steroids. They are important as structural components of membranes in organisms. Biological membranes are formed by double layers of phospholipids, and within these membranes or on their surface layer are proteins, which differ depending on the cell type (Fig. 2-4). Biological membranes form compartments such as organelles, and the proteins of biological membranes are responsible for exchanging substances in and out of these compartments. Neutral fats are one kind of glycerolipid in which all three hydroxyl groups of the glycerol form ester bonds with fatty acids. Neutral fats perform the role of energy storage (see Column in Section 4 of Chapter 8 ［8.4.4］)
Sugars act as energy sources in organisms. Glycogen is a polysaccharide of many glucose molecules and functions as an energy-storing molecule. During energy production, glycogen is cleaved, releasing many glucose molecules with phosphates attached. This glucose breaks down into water and carbon dioxide to produce large amounts of energy (see Chapter 8). Sugars also form a part of the cell structure. For example, the cell walls of plants are made of polysaccharides such as cellulose, and their use as raw materials for biomass and biofuels is anticipated. Moreover, sugars are used as information in living organisms. And familiar examples of this attribute of sugars can be illustrated in the discussion of ABO blood types. In people with blood type A, acetylgalactosamine (A) is present at the termini of sugar chains that protrude from outside of the membranes of red blood cells. In people with blood type B, galactose (B) is present on the termini of the sugar chains. On the other hand, no such sugar is present on the termini of sugar chains in people with blood type O.
Nucleic acids are compounds that consist of a base, a pentose sugar (i.e., a sugar formed from five carbons), and a phosphate (Fig. 2-5A). There are two pentose sugars, ribose and deoxyribose, and five nucleotide bases, adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U). Deoxyribonucleic acid (DNA) is a chain of nucleotides in which each nucleotide consists of a deoxyribose sugar bound to a phosphate and one of the four bases A, G, C, or T (Fig. 2-5B). Ribonucleic acid (RNA) is also a chain of nucleotides in which each nucleotide consists of a ribose sugar and a phosphate bound to one of the four bases A, G, C, or U. Other examples of nucleic acids are adenosine triphosphate (ATP, see Chapter 8, Figure 8-4), which supplies energy in enzyme reactions of organisms, and cyclic AMP (cAMP, see Chapter 4), which functions as a signal mediator.