If we hope to understand function in biological systems, we must first understand structure. At a simple level, we can divide molecules up according to their affinities for water – hydrophobic (limited solubility in water), hydrophilic (soluble in water) and amphiphilic (have characteristics of both hydrophobicity and hydrophilicity). Hydrophobicity in biological molecules arises largely because carbon-hydrogen bonds have electrons that are fairly evenly shared (not unlike carbon-carbon bonds). By contrast, the electrons between the oxygen and hydrogen of water are not equally shared. Oxygen has a greater electronegativity, so it holds them closer than hydrogen does. As a consequence, oxygen has what we call a partial negative charge and hydrogen has a partial positive charge.
Virtually all of life on Earth is built upon the biochemistry that arises from the molecular properties described in the preceding paragraph. The biomolecules referred to as lipids are largely water insoluble because they have predominantly carbon-hydrogen bonds with few ionic or hydrogen bond characteristics.