MPSC Notes - Water Properties and Mineral Salts Part -1

Water Properties and Mineral Salts Review

1. What is the approximate percentage (in mass) of water in the human body? Is this percentage expected to be larger in the adult or in the old individual?

Approximately 65% of the human individual mass is water. The brain, for example, has around 90% of water in mass, the muscles, 85%, and the bones have between 25% and 40% of water.

Younger adult individuals have proportionally more water in mass than older individuals.

2. What are the main biological functions of water?

Water is the fundamental solvent for chemical reactions of living beings; it is the main means of substance transportation in the cell and between cells and tissues and it is responsible for the maintenance of adequate temperature for the functioning of the organism. Water is also the reagent or the product of many biochemical reactions, like photosynthesis, cellular respiration,peptide bond for protein formation, etc.

3. Water has key participation in organic reactions. What are examples of two types of organic reactions in which water is respectively incorporated or liberated in the products of these reactions?

Photosynthesis is a biochemical process in which water is incorporated into organic molecules. In the reaction, the hydrogen atoms from water go to the produced glucose and the oxygen atoms from water form the molecular oxygen liberated: carbon dioxide + water + light = glucose + molecular oxygen. Aerobic respiration is an example of biochemical reaction in which water is produced: glucose + molecular oxygen = carbon dioxide + water.
4. Is water a polar or a non-polar molecule? What is the consequence of that characteristic for the function of water as solvent?

Water is made of two atoms of hydrogen attached by covalent bond with one central atom of oxygen making an angular spatial structure. The hydrogen atoms “lend” electrons to the oxygen and consequently this atom becomes more negative while the hydrogens become more positive. The spatial geometry of the water molecule makes it a polar molecule, with negative and positive poles.

Water is an excellent solvent for polar substances because the electrical activity (attraction and repulsion) of its poles helps the separation and the mixing of these substances, giving them more movement and thus increasing the number of molecular collisions and the speed of chemical reactions. On the other hand, water is not good as a solvent for non-polar substances.

Polarity is one of the water properties.5. Which kind of polarity do water-soluble and fat-soluble substances respectively have?

Water-soluble substances are polar molecules, i.e., they have electrically charged areas. These molecules get the description “water-soluble” because they are soluble in water, a polar molecule too.

Fat-soluble substances are non-polar molecules, i.e., they are electrically neutral. They get the description “fat-soluble” because they dissolve other non-polar substances.

6. What is the importance of water for enzymatic activity?

Enzymes, biological catalysts, depend on water to reach their substrates and bind to them. There is no enzymatic activity without water. In addition, enzymes depend on adequate pH interval to work and the pH is a consequence of the liberation of hydrogen cations (H+) and hydroxyl anions (OH-) by acids and bases in water solution.

7. Can the heat capacity of water be considered small or large? What is the biological significance of that characteristic?

From Thermology it is known that the quantity of exchanged heat (Q) is equal to the mass (m) multiplied by the specific heat of the substance (c) multiplied by the variation of temperature (T), Q = m.c.ΔT., and that heat capacity is Q/T, hence, m.c. Heat capacity, however, relates to a specific body, since it considers mass, whereas specific heat relates to the general substance. Therefore it is more correct to refer to specific heat in this problem.

Water has a specific heat of 1 cal/g.oC which means that 1 oC per gram is changed in its temperature with the addition or subtraction of 1 cal of energy. This is a very elevated value (for example, the specific heat of ethanol is 0,58 cal/g oC, and mercury, a metal, has a specific heat of 0,033 cal/g. oC) making water an excellent thermal protector against variations of temperature. Even if sudden external temperature changes occur, the internal biological conditions are kept stable in organisms which contain enough water.

High specific heat is one of the most important water properties.

8. What are the main water properties that make water special for life?

The water properties that make water biologically important are molecular polarity, thermal stability (elevated specific heat), fusion and ebullition points that allow water to be liquid in most environments, acid-base neutrality, small molecular size and low chemical reactivity. (Compared to other substances, like ethanol or hydrogen sulfide.)

9. What are ions? What are the two types of molecules into which ions are classified?

Ions are atoms or substances electrically charged by means of loss or gain of electrons.

The two types of ions are the cations and the anions. Cations are ions with positive total electric charge and anions are ions with negative total electric charge. 

10. Which are the main positive ions found in living beings?

The main cations found in living beings are the sodium cation (Na+), the potassium cation (K+), the calcium cation (Ca++), the iron cations (Fe++, Fe+++), the magnesium cation (Mg++), the zinc cation (Zn++) and the manganese cation (Mn++).

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