= Cell biology
<Cell biology> studies the organization, activities and life cycles of <biological cells>. It connects molecular mechanisms with spatial structure: how <biological membranes> partition a cell, how material moves between compartments, how signals change cellular behavior, and how cells grow and divide. A list of molecules alone does not establish where they act or how their activities are coordinated.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter1, Cells and Genomes; cellular organization and common molecular machinery]{ref}
Consider a <protein> that appears on one side of an epithelial cell. Measuring its total abundance asks how much is present. Imaging its location asks where it is present. Disrupting its delivery and testing transport asks whether that location is needed for a particular function. These are complementary experiments; an increase in abundance need not establish correct targeting or increased physiological activity.
A classic cell-biological experiment used viruses as probes of membrane domains. In polarized epithelial monolayers, electron microscopy distinguished apical influenza budding from basolateral <vesicular stomatitis virus> budding. The observations revealed organized regions of the host <cell membrane>, while the particles provided experimentally visible reporters.<Asymmetric budding of viruses in epithelial monlayers: a model system for study of epithelial polarity by E. Rodriguez Boulan and D. D. Sabatini>[1978, abstract and asymmetric budding observations]{ref}
The field combines such spatial observations with biochemical and genetic perturbations. A mechanism becomes more convincing when it explains a measurable cellular change after a defined intervention, rather than merely giving a plausible interpretation of a static image.
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