Water and Buffer

The most abundant substance in living systems.
Makes up 70% or more of the weight of most organisms.
Living organisms depend on water for their existence.
Physical & chemical properties of water make it fit to support life:
High boiling point  water remains liquid in most seasons.
Ice less dense than water  floats on liquid water and water freezes from top to bottom. So a good insulator: a frozen layer of ice serves as a blanket that protects creatures below.
Ubiquitous solvent in cells.
Excellent solvent of polar and ionic substances.
Medium where most cell’s metabolic reactions take place.
Ionization of water and its acid-base reactions important for the functions of proteins and nucleic acids.
The Shapes of proteins and nucleic acids and structure of biological membranes are consequence of their interaction with water.

Water has a simple structure.
Oxygen has 6 electrons in the outer shell: 1s2, 2s2, 2p4.
Sp3 hybridization  H—O—H bond angle is 104.5.
The net charge of water molecule is zero. But O—H bond is polar because O is more electronegative than H. Sharing of electrons between H and O is unequal.
The charge on O = -0.82 and on H = +0.41. This charge separation produces permanent dipoles.