Answer
Secondary structures are held together by hydrogen bonds along the polypeptide backbone. Tertiary structures are held together by the interactions of R groups from the various amino acids, both covalent and non-covalent interactions.
Work Step by Step
See Fig. 5.18, and you can clearly see the R-group interactions visualized for secondary structure, and from that you can extrapolate that the same would be true for tertiary structure as well.
