ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 3. THE MOLECULAR LEVEL OF ORGANIZATION OF ANIMAL LIVING MATTER

2. Inorganic compounds: water and mineral salts

The elements that make up living organisms can either be components of various inorganic (Water and mineral salts) and Organic compounds (Proteins, CARBOHYDRATES, fats, Nucleic Acids, Hormones, Vitamins), or exist in the form of ions (K , Na , Ca² , Mg , Cl^-, H₂PO₄^-).

The most important inorganic substance in living organisms is water — Н₂О. Water is the primary medium in which METABOLISM and Energy conversion take place. The water content in most living organisms is 60-70%, and in some (for example, jellyfish) — up to 98%. Water forms The basis of the internal environment of living organisms (Blood, Lymph, intercellular fluid).

The exceptional importance of water for living systems is due to The Structure of its molecules. A water molecule (Н₂О) consists of two hydrogen atoms covalently bonded to an oxygen atom by strong polar covalent bonds. O is a stronger nonmetal than H, which causes the shared electron pairs in the water molecule to be shifted toward it. Therefore, although the water molecule is overall uncharged, a negative charge accumulates near the O atom, and a positive charge near the H atoms. The water molecule is polarized and is a dipole (having both positive and negative charges). Opposite poles of neighboring water molecules attract each other, forming Hydrogen Bonds. These are relatively weak bonds, 15-20 times weaker than covalent ones. It is these bonds that determine the special significance of water for life. Water molecules in a liquid are bound to each other and to the molecules of dissolved substances by hydrogen bonds. The energy of these bonds is low, so they are quickly broken and easily reformed.

Because water molecules are dipoles, they have the unique property of dissolving polar substances, which include ionic compounds: salts, acids, and bases (note: non-ionic compounds include alcohols and sugars). Water molecules seem to "pull apart" the molecules of polar substances. This increases the reactivity of the dissolved substances, as their molecules or ions gain The ability to move freely. Substances that can dissolve in water are called hydrophilic (from the Greek hydor — water, phileo — to love).

Substances that do not interact with water and therefore do not dissolve in it are called hydrophobic (from the Greek hydor — water, phobos — fear). Hydrophobic substances include almost all fats and some proteins.

Water is also involved in the Regulation of the thermal state of organisms. It has a high heat capacity, meaning the ability to absorb heat with minimal changes in its own Temperature. Thanks to this, water prevents sudden temperature changes in Cells and in the Organism as a whole during significant fluctuations in the environment. During water evaporation, organisms expend a lot of heat, thereby protecting themselves from overheating. Due to its high thermal conductivity, water ensures an even distribution of heat among body Tissues, circulating through the cavities of Organs and the body.

In addition to water, organisms also contain other inorganic compounds — mineral salts.

They exist in a dissociated (dissolved) state in the form of cations (K⁺, Na⁺, Ca²⁺, Mg²⁺, etc.) and anions (Cl^-, HCO₃^-, H₂PO) , SO₄^2- and

etc.) or exist as solid compounds (CaCO₃, Ca₃(PO₄)₂).

The differing concentrations of K and Na outside and inside cells generate an electrical potential difference across The Cell's Plasma Membranes. This enables the Transport of substances across membranes.

Calcium compounds (CaCO₃, Ca₃(PO₄)₂) are Components of the Extracellular matrix of Bone tissue, as well as the shells of Mollusks, Protozoans (foraminiferans), and the carapaces of crustaceans. In protozoans such as radiolarians, the intracellular Skeleton is composed of SiO₂ (silicon dioxide, or silica) or SrSO₄ (strontium sulfate).