The population in the United States in 2000 was about 281 million people. In 2015, it was 321 million people. From 2000 to 2015, the population increased

The population from 2000 to 2015 increased by 40 million.

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.