I don't like to talk about cysteine.

I don't understand the "memorize the pathways" style of teaching.

I'm not one of those people who says "Why memorize anything when you can look it up?" That doesn't generally work because (1) you need to know that a fact exists at all before you can look it up, (2) a lot of problem-solving is done by your subconscious, which of course can't look up anything, and (3) often you can't see the big picture until you have learned enough of the pieces, even though learning the pieces seems like arbitrary memorization while you still don't know enough of them.

However, I still don't see any point in memorizing lists of arbitrary alphanumerical protein names. Knowing the pathway's purpose, inputs and outputs, and any key intermediates is sufficient. I can't think of any scenario where a pathway isn't the focus of your research but being able to recall the names of all the enzymes and the order in which they act (as opposed to looking them up) is useful in practice.

(But maybe I'm the one who is ignorant of the practical applications of that knowledge... All I can say is that there has been no need for it during the course of my career so far.)

Fluorine is extremely rare in biological systems. (I was going to say "never" but I looked it up and apparently there are a couple of exotic compounds that have it.) However, fluorine is a component of many man-made drugs and poisons. Halogens are generally not incorporated directly into bio-molecules (with exceptions, the chief one being iodine in thyroid hormones) but chlorine plays an essential role in all living things as a free, negatively charged ion.

Some heavier, metallic elements in the form of ions are necessary for the function of many enzymes, but biological systems can't work with chemically bound metals the way that human technology can. I looked up what the heaviest element with a biological role is and the answer is apparently tungsten (although I've never come across an enzyme incorporating tungsten during the course of my work) but even heavier metals can act as poisons by taking the place of lighter, catalytically active metals in enzymes.

It can be fun to look at the Wikipedia article of some weird element that never seems to be mentioned and see what strange uses humans have actually found for it.

If it’s too big to be created in self sustaining fusion it’s too big to give a shit about.

God must really love hydrogen.