Fundamentals of Biochemistry - A. A. Anisimov 1986

Water and Salt Metabolism
Mineral Substances

The ash remaining after the combustion of a living Organism accounts for 3–5% of the total body weight in vertebrates, 0.5–3% in plants, and even less—0.4–2%—in microorganisms. Individual Tissues and Organs differ significantly in their ash element content. For instance, in vertebrate Bone tissue, these elements make up about 17%; in dry, defatted tooth tissue, up to 55%; and in Muscles and Blood Plasma, less than 1% of the wet weight. In plants, Mineral Substances are abundant in leaves (10–15% of dry weight), significantly less so in roots and seeds (3–5%), and exceptionally scarce in wood (1%). Bacteria exhibit wide variations in ash element content depending on growth conditions. For example, in Vibrio cholerae, this range spans from 6 to 26% of dry weight, whereas under standard nutrient media and conventional conditions, it is 3–10%.

Mineral elements are present in living organisms in various forms: 1) tightly bound to organic substances (S in Proteins, P in Nucleic Acids, Fe in Hemoglobin, Zn and Cu in certain enzyme molecules); 2) as insoluble deposits (Ca and P in bones); 3) in a dissolved state within tissue fluids and the Cytosol (cations K⁺, Na⁺, Mg²⁺, Са²⁺, anions Cl-, SO^2-₄, PO^-3₄).

The Significance of inorganic salts in the life of any organism is immense and multifaceted. They generate a specific osmotic pressure in individual tissues, organs, and fluids, acting as a crucial physiological factor that governs the distribution of Water and dissolved substances across tissues. Higher animals are particularly sensitive to shifts in osmotic pressure; through evolution, they have developed mechanisms that maintain the constancy of osmotic pressure in blood plasma, Lymph, and extracellular fluid.

Thus, the Osmotic Pressure of human blood plasma fluctuates within a fairly narrow range (7.7–8.1 atm). This stability is maintained by a specialized regulatory system where the Kidneys and Sweat Glands play the primary role. In contrast, in marine invertebrates, the internal osmotic pressure depends on the osmotic pressure of the surrounding water. If seawater is diluted with fresh water, their internal pressure decreases. In plants, the difference between the osmotic pressure of The Cell sap and the turgor pressure of The Cell wall determines the cell's "suction pressure" and The rate of water and nutrient uptake. Plants exhibit wide variations in osmotic pressure depending on growing conditions. For instance, in freshwater Algae, epidermal cell pressure ranges from 1 to 3 atm; in field crops, it is 5–10 atm; and in desert and halophytic plants, it reaches 80–100 atm. However, for every plant species, there are specific physiological limits for osmotic pressure fluctuations. Alongside mineral salts, this pressure is also determined by sugar and amino acid content.

By forming buffer systems, certain mineral salts help maintain a stable pH in body tissues and fluids. For example, in humans, blood and tissue pH varies within very narrow margins (pH 7.3–7.4) despite the continuous production of A wide variety of acids. Acidification of the blood down to pH 6.8 results in death.

The vital activity of warm-blooded animals can likewise only proceed within a narrow window of internal environment fluctuations (pH 7.4–7.0). Humans and animals utilize three main buffer systems: phosphate, bicarbonate, and protein. Phosphates constitute the primary urinary buffer. In blood and tissues, phosphates are relatively scarce, so the capacity of the phosphate buffer there is low. The bicarbonate buffer plays a paramount role in the blood, where its buffering capacity is high (25–35% of the total buffer capacity). The protein buffer system is universal and found in many tissues.

Structure/19.html">The Importance of A number of mineral elements stems from their presence within biologically vital compounds: Mg in the chlorophyll molecule, Fe in hemoglobin, S in proteins, P in Nucleic Acids and certain proteins, and I in the thyroid hormone. Many metal cations are also integral components of individual Enzymes.

The ability of mineral elements to interact with major biopolymer molecules—proteins and nucleic acids—determines their influence on the spatial conformation of macromolecular compounds. The concentration and COMPOSITION OF THE ionic environment can play a major role in this regard, sometimes independently of The formation of chemical bonds between ions and Biopolymers. By affecting the conformation and PHYSICOCHEMICAL PROPERTIES OF biologically crucial compounds, membranes, and other subcellular structures, inorganic ions thereby regulate their Functions—catalytic, hormonal, transport, structural, and others. Well-known Examples include The Effect of Ionic strength on the conformation of RNA, DNA, and proteins, The Role of Mg²⁺ in ribosome functioning, and the part played by Ca²⁺ in regulating adenylate and guanylate cyclase systems, etc.

Mineral elements are especially vital to the enzymatic machinery of any living organism. The action of inorganic ions on Enzymes can be direct or indirect. In direct action, ions either become part of the enzyme molecule or the enzyme-substrate complex, or they act as Allosteric Effectors and nonspecific agents that influence the physicochemical properties and conformation of enzymes without being obligatory components. Many enzymatic reactions proceed only in the presence of specific ions.

The indirect action of inorganic ions on enzymes can be mediated through changes in: 1) the physicochemical Properties of the Cytoplasm and The structure of cellular water; 2) the Structure and properties of Biomembranes, given that many enzymes are membrane-bound; 3) the levels of specific enzyme substrates; 4) The activity of enzymatic METABOLISM/35.html">Protein Biosynthesis.

The role of ash substances in organismal physiology is linked to various other phenomena and processes. For instance, Ca₃(РO₄)₂ imparts structural strength to bone tissue; Na₂CO₃ participates in The transport of СO₂ from respiring tissues to the pulmonary alveoli. Trace Elements hold a special place among mineral substances. Although they occur in living organisms in minute quantities (10^-6–10^-12%), they are utterly indispensable, as their absence leads to severe Metabolic Disorders. This is because trace elements activate numerous enzymatic processes (either as constituents of the enzymes themselves or as their activators) and are required for the synthesis of certain Vitamins and Hormones.

Trace elements include B, Mn, Zn, Cu, Mo, Co, Ni, Li, Se, I, Cl, Br, As, and several others. Cu, Zn, and Mo are components of various enzyme molecules. Mn activates Tricarboxylic Acid Cycle enzymes, certain Nitrogen metabolism enzymes, and enzymes involved in The biosynthesis of auxin—a critical phytohormone—while also promoting Vitamin C synthesis in plants. I is a constituent of the THYROID HORMONES thyroxine and triiodothyronine, and Co is part of the vitamin B₁₂ molecule. Br takes part in the biosynthesis of Pituitary Hormones.

In humans and vertebrates, among cations, Ca is present in the highest amounts (about 15 g/kg of body weight, predominantly in bones), followed by K (~ 3.5 g/kg), Nа⁺ (~ 1.5 g/kg), Mg (~ 0.5 g/kg), and Fe (~ 0.04 g/kg). Among metalloids, P (~ 10 g), Se (~ 2.2 g), and Cl (~ 1.5 g) predominate. An adult human's daily requirement for mineral salts is 4–6 g Na, 2–4 g Cl, 2–3 g K, 0.7–0.8 g Ca, 1.5–2 g P, and 0.015–0.020 g Fe. Requirements for Ca and P increase during Pregnancy and in children under 8 years of age. Dietary NaCl intake is driven by dietary habits and vastly exceeds physiological requirements. The absorption of soluble salts occurs throughout the Small Intestine without quantitative limitations. Changes in the osmotic pressure and ionic status of blood and tissue fluids are prevented by increased renal excretion and the consumption of excess water (it is a well-known fact that thirst spikes sharply after consuming salty foods).

Unutilized salts are eliminated via urine, feces, and sweat. Working in hot industrial environments, engaging in long marches, or participating in strenuous sports leads to profuse sweating, which can cause significant "demineralization." In such cases, drinking fluids with small additions of NaCl is recommended. Inadequate Dietary intake of mineral elements leads to severe pathologies. Reduced iodine levels in drinking water cause endemic goiter. Deficiencies in Cu and Co result in Various Forms of anemia. Mineral Metabolism is closely intertwined with hormone regulation; for example, the Parathyroid glands participate in controlling Ca metabolism, while Adrenal Cortex Hormones regulate Na and K levels. Salt metabolism is likewise closely tied to water balance. Severe pathological conditions involving dehydration are also accompanied by demineralization, specifically chloride depletion.

Plant tissues are particularly characterized by a high potassium content (25–35% K₂O of total ash weight). They also contain substantial amounts of P (7–10% P₂O₅) and Ca (3–30% CaO). Cereal straw is very rich in Si (exceeding 40% of total ash weight), whereas cereal grain is rich in P (up to 50%, largely in the form of phytin). The amount of calcium in ash typically increases with age; in the bark of an old oak tree, it accounts for over 90% of the total ash.

Experiments in which plants were grown on aqueous mineral salt solutions with individual elements omitted have established that higher plants require seven macroelements for survival: N, P, K, S, Ca, Mg, and Fe. Trace elements are also necessary, though in such minuscule amounts that their presence in tap water and as impurities in macroelement salts is generally sufficient. When crops are grown in field soils, N, P, and K reserves are depleted fairly quickly; consequently, nitrogen, phosphorus, and potassium fertilizers serve as the primary mineral fertilizers. The Food Program adopted by the May (1982) Plenum of the CPSU Central Committee envisaged a substantial increase in mineral fertilizer production in our country.

In bacteria, the constant ash elements include P, K, Na, Mg, Ca, Fe, S, and Cl. Salts of these elements are typically incorporated into bacterial culture media in measurable quantities—0.1–1%. Microbial Cells are especially rich in phosphorus (10–45% P₂O₅ of total ash, and up to 75% in Mycobacterium tuberculosis). Cu, Si, Zn, Co, and Mn are present in trace amounts and function as trace elements.