Chapter 7

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 mediated by L1 molecules? The substances are trypsin, which digests proteins; a peptide containing \(\mathrm{RGD}\) neuraminidase, which removes sialic acid from an oligosaccharide; collagenase, which digests collagen; hyaluronidase, which digests hyaluronic acid; EGTA, which binds \(\mathrm{Ca}^{2+}\) ions in the medium.

Mice lacking a gene for fibronectin do not survive early embryonic development. Name two processes that might be disrupted in such embryos.

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 \(\mathrm{B}\), 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?

In what ways are the extracellular matrices of animals and the cell walls of plants similar in construction?

It was noted that two different autoimmune diseases, one producing antibodies against a component of hemidesmosomes and the other producing antibodies against a component of desmosomes, both cause severe blistering of the skin. Why do you think these two conditions have such similar symptoms?

Follicle-stimulating hormone (FSH) is a pituitary hormone that acts on the follicle cells of the ovary to trigger the synthesis of cyclic AMP, which stimulates various metabolic changes. FSH normally has no effect on cardiac muscle cells. However, when ovarian follicle cells and cardiac muscle cells are grown together in a mixed culture, a number of the cardiac muscle cells are seen to contract following addition of FSH to the medium. How might this observation be explained?

Why do you think animal cells are able to survive without the types of cell walls that are found in nearly every other group of organisms?

Certain of the cell junctions occur as encircling belts, while others occur as discrete patches. How do these two types of structural arrangements correlate with the functions of the respective junctions?

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 proposal?