Hi Emily - good question, but please note, there is no need to panic about these kinds of details - MCAT biology (just like all the other subjects) is NOT like the biology you were expected to know in first year....indeed, you almost need to approach the BS exam like you would VR - analyze, critique and reason with the information that the MCAT tells you in the passage and question stems. You are gaurenteed to always see new material on the exam (which is INTENTIONAL)....the MCAT writers are interested in seeing how you use this information, not if you have memorized minute details such as the classification system of prokaryotes....

To quickly answer your specific question, the most basic and reasonable classification system is based on carbon and energy source. For instance, in the carbon source category, autotrophs use/fix CO2 and heterotrophs use organic matter. In the energy source category, you either have phototrophs (which utilize light for energy) or chemotrophs (which oxidize organic or inorganic matter). Technically, chemotrophs could either be organotrophs (oxidize organic matter) or lithotrophs (oxidize inorganic matter). However, I have NEVER seen an MCAT question that would expect you to know all the details on page 58 - so please don't panic....it is much more important that you understand the material presented rather than memorize all the fine details...

Good luck and I hope this helped!