Biochemistry: Break (down)...starch, proteins...

No, down is part of the verb phrase. And it probably should be "into" rather than "to". Read a relevant textbook in English and you will find that

Another vote for "breaks down" and "into".

To 'break' means to forcefully divide something. To 'break down,' in this context, means to render something into smaller constituent parts by some process, or other. If you say 'he broke the chair,' then he smashed it. If 'he broke down the chair,' he deliberately took it apart.

And 'into' is far better.