Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Components of Biomolecular Complexes
Elemental Composition of Living Organisms

Only 40 of the 92 naturally occurring chemical elements are found in living organisms. Moreover, of these 40 elements, just 27 are essential for biological activity, while the remaining ones (such as bismuth, lead, tin, cadmium, antimony, and thallium) act as environmental contaminants.

The elemental composition of organisms, even those differing vastly in complexity, varies significantly less than the composition of any Organism differs from that of the lithosphere.

Four chemical elements—C, O, H, and N—account for approximately 98% of an organism's body weight. Since all Organic compounds are built from these elements, they are referred to as organogens.

The remaining elements that make up living systems are classified into Macronutrients (K, S, P, Cl, Mg, Na, Ca, and Fe), which constitute 0.01–0.1% of body weight, and micronutrients (Zn, Cu, I, F, Mn, B, Br, Co, Mo, Si, Ba, Se, V, Cr, and Ni), which make up less than 0.01%.

Despite their minute quantities, micronutrients perform indispensable Functions (for instance, serving as components of Enzymes); consequently, a deficiency in any of them leads to severe Metabolic Disorders. An excess of micronutrients is equally dangerous due to the likelihood of their participating in non-specific Chemical Reactions.

Most inorganic cellular substances exist in the form of salts. The concentrations of cations and anions within a Cell and in its surrounding environment typically differ markedly.

For example, inside an animal cell, the concentration of K⁺ ions is 140 mM, whereas Na⁺ ranges from 5 to 15 mM. In contrast, the extracellular or external environment is poor in potassium (5 mM) and has a relatively high sodium concentration (145 mM). This concentration gradient between The Cell and its environment serves the specialized function of establishing a Membrane Potential across The Plasma Membrane, which is actively maintained as long as the cell remains alive. Following cell death, the ionic concentrations inside and outside the cell rapidly equilibrate.

Water uptake by Cells, as well as the buffering capacity of Cells and Tissues, largely depends on salts. Cell membranes are permeable to water molecules while remaining impermeable to large molecules and ions.

If the water content of the surrounding medium is higher than that inside the cell, water naturally flows into the cell to equalize the concentration gradient (a process known as osmosis). This phenomenon underlies, for example, the absorption of water by plant roots.

Inorganic ions (such as Ca²⁺ and Mg²⁺) participate in regulating the catalytic activity of many enzymes by binding to them and thereby activating them.

Other ions (Cu²⁺, Fe²⁺, and Zn²⁺) form an integral part of the active centers of certain enzymes.

The Ca²⁺ ion plays a crucial role in Nerve Impulse propagation and Muscle contraction, while inorganic phosphate is an essential component of the intracellular ATP-dependent energy transfer system.

Inorganic salts are found in cells not only in dissolved form but also in a solid state, where they perform a structural function. Specifically, the strength and hardness of Bone tissue and mollusk shells are provided by insoluble hydroxyapatite, whose composition can be roughly described by the formula Ca₅(РO₄)₃OН.

In aqueous solutions, biologically vital ions such as K⁺, Na⁺, Ca²⁺, Mg²⁺, and Cl^- do not exist as isolated ions; rather, they are always hydrated—surrounded by polar water molecules that interact electrostatically with the ion (Figure 40).

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Figure 40 — Hydrated magnesium cation