cell—or factors that may impede or facilitate uptake—varies
depending on receptor type. One common factor among all
organisms is the presence of a cellular membrane that separates
the cytoplasm (cell interior) from the external environment.
Most contaminants must pass through this membrane before
deleterious effects on the cell or organism occur. (In some
instances, it is possible for contaminants to exert a toxic
effect without penetrating the cell membrane such as β-lactam
antibiotics, which damage bacterial cell walls and cause cell
lysis.) Uptake generally requires contaminant transfer to and
through a released state. In the case of bacteria, physical
features (e.g., the cell wall) can isolate their cellular
membrane from contact with particulate material, such that
contaminants must be dissolved in the aqueous phase before they
can be taken up. However, there are exceptions to the notion
that bioavailability is directly dependent on solubility. For
example, contaminant-laden particles that undergo phagoc