Why are cells lysed?

For immunoprecipitation assays, western blots, and similar procedures, use of cell lysates (the cytosolic component of lysed cells, separated via centrifuging) is standard protocol and gives dependable results. By destroying the protective outermost cell membranes and releasing the contents, the components of the cells are much more acessible and homogenous, making many experiments possible. Some procedures make use of the organelle pellet instead, which is usually composed of either the cell nuclei or mitochondria, for studies targeting cell metabolism.

For detection and tracking, biomolecules of interest will often be tagged with a probe either prior or post-lysis. 

Of all of these applications, possibly the most prominent is that of immunohistochemical (IHC) assays which employ the use of selected antibodies to detect, and with some techniques even amplify, a signal of interest. Lysing the cells is necessary for these procedures, since the cell membrane would otherwise block access of the antibodies to the biomolecules being studied.  

For more information on antibody labeling or other applications/products involving cell lysate, see 'Additional Resources' below.