Final answer:
Osmium tetroxide primarily reacts with fatty acids in cells, especially targeting phospholipids in cell membranes, making it a tool for staining lipid bilayers in electron microscopy.
Step-by-step explanation:
Osmium tetroxide primarily reacts with fatty acids (lipids) when it is used in cellular studies. It reacts by fixing and staining the lipid bilayers, thus highlighting them in cell preparations. Osmium tetroxide acts as an excellent staining agent because it binds to the double bonds present in the unsaturated fatty acids, which are abundant in the phospholipids of cell membranes. This creates a high contrast for those areas in electron microscopy. It does not primarily react with DNA, RNA, proteins, or carbohydrates in the context it's used for.
Regarding the other parts of the question, water is not one of the four major classes of organic compounds; the correct choices would be nucleic acids, proteins, carbohydrates, and lipids. Moreover, in macromolecules, carbon is a fundamental component that allows the formation of the versatile structures necessary for life.