I ask the forebearance of MCAT students, whose patience I imagine I am testing by my decision to include some discussion of plants in these interdisciplinary discussions. First of all, the course is free, so I am entitled to some eccentricities. Of course I know that anything more than a rudimentary factual knowledge base about plant biology is not required for the MCAT. There are a number of reasons I decided to go against the advice of every other person giving advice in the field of MCAT preparation and talk about plants, not a huge amount, but enough to make students anxious. I admit that this is partly due to my own interest, but there are better reasons that decided the issue. Firstly, you really will not ever understand oxidative metabolism in the ecological context without the paired understanding of photosynthesis. That is a good reason. When you can look out and see the flow of energy through the biosphere at the molecular level of the living forms around you, you will recast your surroundings into a profound coherence. This is not only satisfying, in itself, but it is the state of mind that leads to high MCAT scores. Students and teachers often think that the easiest way to learn a body of material to pare it down to its barest essentials and only focus on the core. This is true to an extent. However, conceptual understanding comes from clusters of association and underpinnings.
Secondly, plant biology gives us good field on which to practice pure science comprehension. If you run into an interdisciplinary discussion that goes into excruciating detail, you can know by default that the details will not be on the test. But the MCAT is not even about details, for the most part. Practice decoding. Put the discussion into the context of more fundamental principles. You are exercising your brain for MCAT passages.
Anyway, here's what I wanted to say about plastids. They are present in all plant cells. They include leucoplasts and chromoplasts. Leucoplasts are storage bins. Chromoplasts are pigment containing plastids. Chloroplasts are the chlorophyll containing chromoplasts where photosynthesis takes place. Like mitochondria, plastids contain prokaryotelike ribosomes and circular strands of DNA. Like mitochondria, plastids are most likely descendents of prokaryote symbionts.