Water attracting molecules are called hydrophilic. Water repelling molecules are called hydrophobic. Amino acids simultaneously carry positive and negative charges. Such substances are called zwitter ions. Lipids are compounds of C, H, O but the ratio of H and O is more than 2 : 1, that is, the ratio of oxygen is lesser than carbohydrates. Lipids are insoluble in water but soluble in non-polar solvents such as benzene, chloroform etc.

The most common lipid found in a cell is phospholipid. It contains a hydrophilic (polar) head and a hydrophobic (non- polar tail).