2. In this chapter, you learned that some pathogens must attach to host cells to cause disease. To accomplish this, molecules on their surface (adhesins or ligands) recognize and attach to molecules (receptors or integrins) on the surface on certain host cells.
So a chromatic note, is any note you play that is not in the scale you are playing. They are also a great way to practice, especially if you are going through the pentatonic scale. Try adding one of these outside chromatic notes to your scale, and see how it sounds above the chord you are playing over.
Usually chromatic scales are spelled with sharps when ascending and flats when descending. As noted above, composers will often choose certain notes from this scale to use more frequently than others, thereby providing the listener with a sense of a home note, referred to as the tonic.