Water, the Solvent for Life

The human body is 66% water by weight, according to Hill and Kolb. Water is the universal solvent for life, referred to by Nobel Laureate A. Szent-Gyorgy as "the matrix of life".That water serves as the solvent for sodium chloride (salt) and other substances so that the fluids of our bodies are similar to sea water. This leads Hill and Kolb to refer jokingly to us as "walking bags of sea water". Water serves to suspend the red blood cells to carry oxygen to the cells. It is the solvent for the electrolytes and nutrients needed by the cells, and also the solvent to carry waste material away from the cells.

With water as the solvent, osmotic pressure acts to transport the needed water into cells. With cells bathed in the interstitial fluid, diffusion contributes to carrying needed molecules into the cells. When more complex mechanisms control the transport of molecules across the membranes into and out of cells, the presence of water as the surrounding medium and solvent is essential.

Water as a Polar Molecule

Hydrogen forms covalent bonds with oxygen to form the H₂O molecule, but oxygen has a higher electronegativity than hydrogen, so it takes a larger portion of the shared electron charge.

This polar nature of the water molecule is crucial to it's stability in liquid form at the temperatures needed for life, and for its activity as a solvent for biologically related molecules. A measure of the polar nature of the water molecule is its dipole moment. The nature of the internal interactions in water is described in terms of dipolar bonding.

Hydrogen bonding between the water molecules in the liquid state contributes to its viscosity, its density, and surface tension. The strength of this internal bonding contributes to the high specific heat of water.

Hydrogen Bonds and Solvent Action

The polar nature of the water molecules causes them to be attracted to molecules such as sodium chloride which have ionic bonds.

The strength of the attraction to the polar water molecules is sufficient to separate the Na+ and Cl- into free ions. Such ions are important in biochemistry where they are transported in cellular membrane processes and in nerve functions such as action potentials.

The molecules of sugars have OH groups which can interact with the charges of water molecules to create hydrogen bonds. The forces of these hydrogen bonds can separate molecules of sugar, so most sugars are readily soluble in water. This is particularly important with glucose so that it can move freely in the blood vessels to provide energy to the body.