Fundamentals of Biochemistry - Filippovich, Y. B. 1999

Water and Mineral Metabolism
Water Metabolism

Water content and distribution in the Organism and Cell. As noted earlier (see Chapter I), water accounts for about ³/₄ of the Earth's biomass. Its total amount in living organisms is approximately 5 times greater than the volume of water in all the rivers of the globe. The water content varies significantly across different organisms and, particularly, among various animal and plant Tissues. For instance, biological fluids (such as tree sap, Blood, Lymph, saliva, and animal gastric juice) contain 88 to 99% water, whereas in plant wood or animal Bone tissue, this figure drops to 20—24%.

The younger the organism or organ, the higher its water content. A prime illustration of this is the gradual dehydration of Human and Animal bodies during Aging, which is accompanied by characteristic Skin wrinkling. Similarly, There is a progressive decrease in moisture content in plants as they mature. Furthermore, in woody plants, apical (young) leaves are always significantly richer in water than basal (older) leaves.    

The majority of water in an organism is localized within its Cells, a fraction known as intracellular water. Conversely, water concentrated in the extracellular space or residing within biological fluids is termed extracellular water. For example, in The Human Body, ²/₃ of water is intracellular and ¹/₃ is extracellular. Apparently, the water content in cells correlates to some extent with the intensity of their metabolic and life processes. Thus, the water level in actively dividing cells reaches 80% and in some cases even 90%.

For a long time, it was believed that, at least in plant cells, the bulk of water was concentrated in the vacuole. However, more precise measurements have shown that only about ¹/₃ of intracellular water resides in the vacuole, while the remainder is localized in the protoplasm and Cell wall. Among subcellular structures, the nucleolus and lipid inclusions contain the least amount of water; nuclear sap, the mitochondrial matrix, and hyaloplasm are quite rich in water, whereas its content is low within the lipoprotein structures of The Endoplasmic reticulum.

State of water. In a simplest bacterial cell, there are about 100,000 water molecules per nucleic acid molecule, approximately 40,000 per protein molecule, and roughly 1,500 water molecules per lipid molecule (see Chapter I). Thus, organic molecules within The Cell are constantly surrounded by water molecules and, naturally, interact with them. One must also not overlook the interaction of water molecules with one another as well as with inorganic cations and anions.

The water molecule is polar and possesses a specific dipole moment. Consequently, water molecules orient themselves relative to one another, forming a structured system. Upon entering the electric field of an ion, water molecules form a Hydration shell around it. This interaction is accompanied by the disruption of the water's own Structure (the disordering effect). If the newly emerging order in the arrangement of water molecules around the ion is less than that in structured water itself, so-called negative hydration is observed; this means that water molecules near the ion possess greater mobility than in pure water. Specifically, negative hydration is characteristic of K, Rb, Cs, Cl, and I ions, whereas positive hydration is typical for Na, Li, Ca, and Ba ions. Similar phenomena also occur around the cationic and anionic centers of organic molecules, including macromolecules. Thus, a significant portion of water molecules in the cell exists in a bound state due to their involvement in hydration processes.

Hydrogen Bonds play an equally important role in Structuring water itself and in its interaction with macromolecules and micromolecules. Thanks to these bonds, yet another fraction of water molecules within the cellular content is converted into a bound form.

It is hypothesized that near the hydrophobic regions of macromolecules, water organizes into an ice-like structure that helps maintain the Tertiary Structure of various Biopolymers, especially Proteins. Naturally, this water should also be classified as bound water.

Finally, water can be entrapped within individual macromolecules during The formation of their tertiary structure, and particularly within supramolecular biocomplexes, which are so characteristic of the cellular interior.

The foregoing describes instances of water transitioning into a bound state at THE MOLECULAR LEVEL. However, a deeper investigation into the state of water within subcellular structures reveals some new possibilities. For instance, the high porosity of Ribosomes is well known; in their Native State, they contain 2.7 times more water than dry ribosomes. The protein-lipid membranes of Mitochondria, Lysosomes, the endoplasmic reticulum, and other subcellular structures isolate considerable volumes from the intracellular space, where water is concentrated alongside other substances. The nuclear envelope performs a similar function. Thus, a significant amount of water is also bound within the composition of subcellular structures, sometimes referred to as immobile water.

Consequently, there is every reason to distinguish between two categories of water in the organism: free and bound. Depending on the type of binding, the latter is held by the organism with greater or lesser force. Accordingly, bound water is further subdivided into weakly bound water (water of the diffusion layers of hydration shells, structured water, etc., i.e., water that acts as a solvent and freezes at temperatures close to 0° C) and tightly bound water (water of the 1st hydration shell, enveloping the molecule like a "stocking," almost incapable of acting as a solvent and freezing at temperatures of —25° C or even —269° C). It is quite possible that intracellular water continually undergoes protein-regulated pulsation transitions from an ordered to a disordered state, which is interconnected with the expulsion of spent metabolites (waste products) from the cell and the uptake of essential substances.

From a modern perspective, body water cannot be viewed as an inert medium simply filling the space between macromolecules, subcellular particles, cells, etc. On the contrary, it must be regarded as a structural element of both the cellular contents and the organism as a whole. Only in this light can its immense significance in life processes and its ability—in the words of A. Szent-Györgyi—to be the "matrix of life" be fully appreciated and understood.

The Role of water in life processes. Numerous experiments have established that water plays a vital role in the life of an organism. For instance, the complete loss of fats and a 50% reduction in protein content resulting from starvation are not as dangerous as the loss of 20% of water. The lifespan of starving dogs, for example, can be increased 10-fold if they are supplied with water.

This is because water performs highly essential and diverse Functions in the body. Along with Other Compounds, it acts as a primary component in forming a unified intracellular structure, which helps achieve the fine Organization of processes characteristic of living matter. Being an integral part of subcellular structures, water is largely capable of regulating their functional activity: for example, the intensity of Oxidative Phosphorylation in mitochondria depends on their degree of Swelling, while the maintenance of ribosome structure and Protein Synthesis capability depends on their water saturation, and so forth. The principles of water binding with BIOLOGICALLY ACTIVE SUBSTANCES underlie the regulation and self-regulation of A number of biochemical and physiological processes. Therefore, as A. Szent-Györgyi rightly points out, "one cannot speak of proteins, Nucleic Acids, Nucleoproteins, and water as if they were two separate systems. They form a single system that cannot be divided into components without destroying its essence." For example, only at a water content of 0.6 g per 1 g of dry DNA does the latter retain its native properties.

Being the primary medium in which molecules of various biopolymers are dispersed within the cell, water directly participates in the formation of protoplasmic sols and gels. The transition from sols to gels and vice versa is frequently accompanied by thixotropy (from the Greek *thixis*—Touch, and *tropos*—turning, change), i.e., the liquefaction of a gel under METABOLISM/18.html">The Influence of mechanical forces (such as shaking) and its subsequent resolidification. This is accompanied by a dramatic change in viscosity, which is closely linked to a number of physiological and biological phenomena: Muscle contraction, the differentiation of certain intracellular structures, Cell Division, protoplasmic streaming, and the like.

Possessing low viscosity, high mobility, and the capacity to dissolve a wide range of inorganic and Organic compounds, water performs transport functions in the body. It also serves to eliminate Metabolic waste products from the organism. This Transport of substances occurs both on a large scale—via specialized transport systems (the circulatory and lymphatic systems of animals, xylem and phloem conducting bundles of plants)—and in small doses across cellular and intracellular membranes.

In living nature, water serves as the medium in which numerous and diverse chemical processes take place. The high Dielectric Constant of water ensures the electrolytic dissociation of substances capable of splitting into ions. However, The Role of water in biochemical processes is not limited to its solvent function; in many cases, it is a direct participant in Chemical Reactions. Examples include Hydrolysis, hydration and dehydration, oxidation, and many synthesis reactions that proceed with either the absorption or release of water.

The high thermal conductivity of water and the significant heat absorption during its evaporation form the basis by which plants and animals regulate and stabilize their body Temperature. The sufficiently high heat capacity of water also contributes to this process.

Water metabolism. Through evaporation via the skin and exhaled air, as well as through excreted urine and feces, a human loses an average of 2,600 ml of water per day. Experiments have shown that ⁶/₇ of this water loss is replenished by dietary intake. For a long time, the water deficit of approximately 350 ml remained unaccounted for. Later, however, it was discovered that the human body obtains this amount of moisture As a result of water metabolic production. This water, generated through the body's metabolic processes, is termed endogenous water, in contrast to exogenous water entering the body from the outside. In the Embryogenesis OF THE sunn pest, for example, ¹/₄ of the water, and during the transformation of the silkworm pupa into a moth, almost all the water is of endogenous origin.

Endogenous water is synthesized as a result of The oxidation of organic compounds. It has been calculated that the Complete oxidation of 100 g of fats yields 107.1 g of water, CARBOHYDRATES yield 55.5 g, and proteins yield 41.3 g. A certain level of endogenous water production is characteristic, apparently, of All living organisms.

Water exchange in plants is no less—and seemingly even more—vigorous. Thus, during the growing season, a single corn or sunflower plant transpires up to 200 kg of water. On a hot day, The amount of water passing through a leaf is twice the mass of the leaf itself. Under arid conditions, the velocity of the upward water stream in wood reaches 25 m/min.

Within the organism, water is delivered to tissues and removed from them as part of biological fluids. The penetration of water into cells and back out occurs through pores in cell membranes having a diameter of 0.3–0.4 nm. Several hypotheses have been put forward regarding The Mechanism of water penetration through The cell membrane. It is believed that water transport is active, driven by the interaction of its dipolar molecules with the polar substances that make up the pore wall. Another view is that the exchange of water molecules between the intracellular and extracellular spaces occurs via free diffusion. It is also suggested that osmotic phenomena play a decisive role in water entering and exiting the cell. Be that as it may, this process proceeds quite vigorously, and the water half-exchange time in the cells of various tissues is 30–90 s, which significantly exceeds the half-exchange time of organic molecules or ions in analogous experiments.

Regulation of Water metabolism. Certain cations specifically influence the retention and release of water by Cells and Tissues. For instance, Na⁺ induces the accumulation of water in cells and tissues, whereas K⁺ and Ca²⁺ exert the exact opposite effect.

In the animal organism, Hormones exert a powerful influence on water balance: the diuretic hormone secreted by the anterior pituitary promotes enhanced water excretion from the body via urine (diuresis), while the antidiuretic hormone (vasopressin—see p. 449) produced by the posterior pituitary enhances water reabsorption in the renal tubules and sharply reduces diuresis. This regulation is thus carried out at the organ level—the Kidneys. However, the very ratio of the aforementioned hormones in the body is apparently regulated by the Central Nervous system.

There are additional mechanisms for regulating water metabolism mediated by tissues and Organs. For example, water depots are located in the skin and Liver of animals and humans, where water reserves accumulate during excessive intake. In plants, reserve water is concentrated in intercellular spaces, and its rate of evaporation is regulated by the stomatal apparatus.