The aim of assessment (tests) is for me the instructor to see how much and how well the student have learned the a subject or concept. All concepts taught are important but some are weighed more heavily. for instance the process of Photosynthesis is is tested more thoroughly than basic plant morphology. This is not to take away from plant morphology which can be a course on its own.
There is a fundamental problem when we (instructors) favor these difficult subjects and often explain them in much more detail. Students start to see the components and processes as memorization tasks rather than a concept. Thus come test time, many students can regurgitate fine details but fail to explain the broader concept. When the broad principle is no longer in the minds of students then these minute details that they have memorized are worthless.
How then do we teach kids the principles and the details? given that almost all instructors are limited by time, one must choose to focus on the details or the concepts.