Here, there are receptors on one cell and the ligand that they experience is not a dissolved molecule, but actually a molecule that's attached to another cell.

这个细胞上有许多受体,与之结合的配体并非是可溶性分子,而是一个附着在其他细胞上的分子

耶鲁公开课 - 生物医学工程探索课程节选