I think more importantly, you need to know how write well, and be able to express yourself clearly and coherently. (Crucial for science writing)
I don't think they expect people to throw these huge scientific words in, and explaining first year uni science.... from what I've heard from others, It changes every year so I would be ashame if all that study did go to waste.
I think JMSS want to see what you are capable of, not what you already know. It wouldn't hurt exposing yourself to some science study, but the key here is they wanna see how you think. How you can approach the prompt, logically, rationally, creatively etc. They want to see potential.
So I think writing essays, make sure your structure is solid. Try it out in test conditions as from what I hear, they give you like 30 mins (or was it 15?) to write an essay.