Water & pH lecture-2

Water Water is essential for life, it covers 2/3 of the earth's surface and every living thing is dependent upon it. The human body is comprised of approximately 70% water, and it is a major component of many body fluids including blood, urine, saliva, and others.

A. Structure of Water A water molecule is composed of two hydrogen atoms covalently bonded to an oxygen atom. With a total atomic weight of 18 amu (Dalton). The hydrogen atoms form an angle of about 105°, so the molecule is “bent”. The structure of the electrons surrounding water is tetrahedral, resembling a pyramid.

A water molecule is formed when two atoms of hydrogen bond covalently with an atom of oxygen. In a covalent bond electrons are shared between atoms. In water the sharing is not equal, the oxygen atom attracts the electrons more strongly than the hydrogen.

This gives water an asymmetrical distribution of charge. Molecule that have ends with partial negative and positive charges are known as polar molecule. Water is a polar solvent, it readily dissolves most biomolecules, which are generally charged or polar compounds.

Each hydrogen bears a partial positive charge, while the oxygen atom bears a partial negative charge. As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another, called a hydrogen bond.

The bent geometry of water and the attraction between water molecules gives rise to unique properties that are essential for its role in living organisms and the environment.

B. Ionization of Water Consider a glass of pure water: In a tiny fraction of the water molecules, one of the hydrogen nuclei is completely pulled off the molecule. This forms two ions: A hydrogen ion (H+) And a hydroxyl ion (OH–) This is caused by the attraction of the water molecules for each other.

Water molecules are continuously splitting into ions and rejoining to form water molecules. Thus, water molecules exist in equilibrium with hydrogen ions and hydroxide ions.

In chemically pure water, the number of H+ and OH– ions are the same. Certain chemical substances, when dissolved in water, can change the amounts of H+ or OH –. Acid: A substance that increases the amount of H+ (and decreases the amount of OH–) or it can be defined as substance that can donate hydrogen ions (protons). Base: A substance that increases the amount of OH– (and decreases the amount of H+) or it can be defined as substance compounds accept hydrogen ions. Neutral substance: A substance that does not change the amounts of H+ and OH– (so H+ remains equal to OH–)

Acidity and alkalinity are represented by a value called “pH” pH = -log[H+] The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. Acids: pH value is less than 7 Bases: pH value is greater than 7 Neutral substances: pH value is equal to 7 Each pH value represents a 10-fold change in the amount of H+ in the solution. So a substance with pH = 5 has a 10 times greater amount of H+ than a substance with pH = 6.

The acidity of the aqueous solution is usually reported using the logarithmic pH scale. Buffers usually maintain the pH of the extracellular fluid between 7.35-7.45 pH values in the cell and in the extracellular fluid are kept constant within narrow limits. In the blood, the pH value normally ranges only between 7.35 and 7.45

The pH value of cytoplasm is slightly lower than that of blood at 7 In lysosomes pH is 4.5–5.5 Extreme values are found in the stomach pH 2 and in the small bowel > 8. Since the kidney can excrete either acids or bases, depending on the state of the metabolism, the pH of urine has a particularly wide range of variation 4.8–7.5.

Many biomolecules are amphoteric in aqueous solution that is they can accept or donate protons. For example, at physiologic pH all the amino acids have a positive group and a negative group. Thus they can act as an acid or a base. Such substances are called Ampholytes.

Importance of pH Measurement of pH is one of the most important and frequently used procedures in biochemistry. The pH affects the structure and activity of biological macromolecules; 1. For example, the catalytic activity of enzymes is strongly dependent on pH. 2. Measurements of the pH of blood and urine are commonly used in medical diagnoses.

C. Solvent Properties of Water Solution: A mixture of two (or more) different substances in which the particles of one substance are completely interspersed with the particles of the other substance. Solvent: The substance that is present in the largest amount. Solute: The substance that are present in smaller amount.

Hydrophilic substances: Substances that can be dissolved in water. Water molecules are attracted to ions or to other molecules that have partial positive and negative charges. Examples of hydrophilic substances: Sodium chloride (table salt): This substance consists of sodium ions and chloride ions. Sucrose (table sugar): This substance is a compound with many -OH groups in its structure, with many partial positive and negative charges.

Hydrophobic substances: Substances that cannot be dissolved in water but can be dissolved in nonpolar solvents such as chloroform and benzene. Water molecules have difficulty interacting with uncharged molecules, these substances tend to separate from water. Example of a hydrophobic substance: Cooking oil: The molecules of cooking oil have long chains of carbon atoms bonded to hydrogen. The atoms do not have the “bent” geometry of water, so there are no partial charges to attract water molecules. Therefore, oil and water don’t mix!

Amphipathic substances: Substances in which part of the molecule is hydrophobic, and part of the molecule is hydrophilic. When amphipathic substances are mixed with water, its molecules form into clusters called micelles: with the hydrophilic part on the outside of the micelle in contact with water. and the hydrophobic part on the inside of the micelle, away from the water .

Amphipathic substances (cont.): Example of an amphipathic substance: Soap: Soap molecules have an ionic group attached to one end, and an oily hydrocarbon chain attached to the other end. When soap is mixed with water, it forms micelles that trap oily dirt molecules.

D. Thermal Properties of Water Molecules are in constant motion due to the heat energy (kinetic energy) they contain. Phases of matter: Solid Limited movement of molecules; non-fluid. Liquid Molecules can move freely around each other; fluid. Gas Molecules have greatest freedom of movement; substance can expand to fill the available space.

Water has unusual thermal properties because of the attraction of water molecules for each other. 1-Water has relatively high boiling point. 2-Water remains in a liquid state over a wide temperature range. 3-Water has a high heat capacity: it can absorb a large amount of heat with a small change in temperature. 4-The solid form of water (ice) is less dense than the liquid, so ice floats on water.

E. Monomers and Polymers An organic molecule that serves as a “building block” to build larger organic molecules. Polymer: An organic molecule composed of two or more monomer units linked together by covalent bonds.

Condensation reaction: Polymers are often formed by the process of condensation. In this process, two hydrogen atoms and an oxygen atom are removed from two monomer units. And a covalent bond forms between the monomers.

Hydrolysis reaction: Polymers are often broken down by the process of hydrolysis. In this process, a water molecule is inserted between the monomer units of a polymer to split the polymer into its monomer units.