Professional employee, as used in this subpart, means any person meeting the definition of employee employed in a bona fide . . . professional capacity given in 29 CFR part 541. The term embraces members of those professions having a recognized status based upon acquiring professional knowledge through prolonged study. Examples of these professions include accountancy, actuarial computation, architecture, dentistry, engineering, law, medicine, nursing, pharmacy, the sciences (such as biology, chemistry, and physics), and teaching. To be a professional employee, a person must not only be a professional but must be involved essentially in discharging professional duties.