Final answer:
Osmium tetroxide is the chemical commonly used for both fixation and staining during transmission electron microscopy (TEM) preparation, due to its ability to stabilize biological structures and enhance contrast by its electron-dense nature.
Step-by-step explanation:
Among the chemicals typically used for both fixation and staining in the sample processing for transmission electron microscopy (TEM), osmium tetroxide stands out. Fixation is crucial in microscopy to preserve the structure of the cells and stop metabolic reactions. For TEM, not only does fixation stabilize the sample, but the staining step is also essential. Samples are stained with heavy metals such as uranyl acetate or osmium tetroxide. These substances contain heavy metal atoms that are electron-dense and enhance contrast by interacting differently with various components of the cell, thereby visualizing the structures more clearly in the electron microscope.
Osmium tetroxide is unique as it serves the dual purpose of fixing and staining the sample. Its reactive nature allows it to cross-link and stabilize many biological molecules, particularly lipids, hence fixing the biological structures firmly in place. Furthermore, osmium is a heavy metal, which makes it electron-dense and thereby suitable for providing the needed contrast in electron microscopy.