1. cell adhesion can often be blocked in vitro by treating cells with specific agents. Which of the following substances would be expected to interfere with cell adhesion mediated by selectins? with cell adhesion medaited by L1 molecules? The substances are trypsin, which digests proteins; a peptide containg RGD; neuramainidase, which remves sialic acid from an oligosaccharide; collagenase, which digests collagen; hyaluronidase, which digests hyaluronic acid; EGTA, which binds Ca2+ ions in the medium.

2. suppose you found that molecule A, which had a molecular mass of 1500 daltons, was able to penetrate the channels of a gap junction, but molecule B, whose molecular mass was only 1200 daltons, was unable to diffuse between the same cells. how might these molecules be different so as to explain these results?

3. Propose some mechanism that might explain how tobacco mosaic virus was able to alter the permeability of a plasmodesma. How might you be able to test your propsal?