The proteins in the cell membrane
a. remain in fixed postions
b. allow small molecules that can dissolve lipids to pass into the cell
c. transport certain molecules into or out of the cell
d. prevent osmosis from taking place.

C. Channel proteins bind to molecules too large to diffuse into or out of the cell, and allow them to pass.

Six amino acids have side chains that are polar but not charged. these are serine (ser), threonine (thr), cysteine (cys), asparagine (asn), glutamine (gln), and tyrosine (tyr). these amino acids are usually found at the surface of proteins, as discussed in the proteins 2 module.