Water and its role in controlling biological processes

Life on earth originates in ancient ocean after water has reduced the temperature on earth to the permissive level to support vital biological activities. Most of the biological reactions operate in aqueous environment and use water as one of the reactant. It is making ~70% content of the organism and controls the enzymatic activity of major metabolic pathways. In higher mammals, it has been used as a medium for material transfer between different organs and a reactant to neutralize and facilitates excretion of toxic metabolites out of the body. The enormous capacity of water to perform multitasking, is the sole basis of life activities and lies in its unique structure and ability to interact with biological molecules.

Structure of Water: Water has a chemical formula of H₂ O with central oxygen covalently attached to two hydrogen atoms (Figure 2.1, A). It has two lobes of unpaired electron opposite to the hydrogen atom called as “lone pairs”. The H-O-H bond angle is 104.5⁰ and over-all molecule adopt partial tetrahedral geometry. In the water molecule, oxygen is more electronegative than hydrogen, as a result sharing electrons are placed towards oxygen and gives partial positive charge to hydrogen (δ⁺) and negative charge to oxygen (2δ^-). Hence, water molecule develops a dipole and form hydrogen bonding involving electronegative oxygen of first molecule and positively change hydrogen of next molecule. In this manner, each water molecule can be in hydrogen bonding interactions to 4 neighboring molecules (Figure 2.1, B). As a result, water molecules exist in three different state of matter; liquid, solid (Ice) and gas (vapor). In liquid state, water molecules are under dynamic hydrogen bonding interaction to the neighboring molecules where as in solid state (Ice) molecules present in one plane are in hydrogen bonding interaction with molecules present in same plane and neighboring plane giving a quasi crystal like packing (Figure 2.1, C).

Water Ionization and pH: Water weakly ionizes reversibly into hydrogen ion (H+) and hydroxide ion (OH-) as shown in Eq. 2.1. But H+ thus formed gets hydrated to form hydronium ion (H3O+) due to hydrogen bonding between two water molecule which allows instantly hydration of ionized proton (Eq 2.2).