Final answer:
The molecule observed outside the cell and within cytoplasmic vesicles likely entered the cell through endocytosis, a form of active vesicle transport in which the cell membrane envelopes material to internalize it.
Step-by-step explanation:
The most likely explanation for finding a molecule on the outside of the cell and in vesicles in the cytoplasm is that this substance moved into the cell by endocytosis. Endocytosis is a process through which cells ingest material by enveloping it in a portion of their cell membrane, creating a vesicle that is then internalized. If large molecules, such as proteins, need to enter a cell, vesicle transport is necessary, since these molecules cannot pass through the plasma membrane because of their size. Vesicle transport is an active transport process, requiring energy, and it includes two principal mechanisms - endocytosis and exocytosis.
During endocytosis, the cell membrane can either form a vesicle around the substance to internalize it, which is then processed within the cell, or in some cases, fuse with a lysosome to break down the material. Phagocytosis and pinocytosis are two forms of endocytosis; phagocytosis involves the engulfment of large particles, while pinocytosis involves the encapsulation of liquids or small particles. These internalized vesicles can be observed via electron microscopy within the cytoplasm, supporting the hypothesis that endocytosis has occurred.