Protein digestion begins in the mouth with the physical breakdown of foods by chewing, which increases the surface area for chemical digestion. Chemical digestion of proteins begins with the denaturation of proteins in the stomach and continues with the subsequent breakdown of the resulting polypeptides into amino acids.
Which of the following statements about protein digestion are true? Check all that apply.

a) Most of the hydrolysis of polypeptides takes place in the small intestine.
b) Pepsin is active only in an acidic environment.
c) The primary structure of a protein is destroyed by hydrolysis.
d) Some proteins are broken down into amino acids by the process of chewing.
e) Hydrochloric acid in the stomach denatures proteins and activates some enzymes.
f) The tertiary and secondary structures of proteins unfold due to the action of the enzymes in the gut.