Mature molecules of tropocollagen are found in the:
a. Cisternae of the rough endoplasmic reticulum of a fibroblast
b. Extracellular environment of a fibroblast
c. Golgi apparatus of a plasma cell
d. Secretory vesicles of a fibroblast
e. Smooth endoplasmic reticulum of a macrophage
ANS: B
Tropocollagen molecules, which have the ability to spontaneously self-assemble into large colla-gen fibers, are only formed by proteolytic cleavage of procollagen when secreted outside of the cell. Otherwise, they would assemble within a cell and expand to completely disrupt the cell. See 3-6.
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