One subject I've had in my list to write about (once I spent the time to find some good sources) was why some leaves are edible while others aren't—completely aside from the question of toxicity, there are lots of plants that we simply can't digest. I hadn't yet got around to digging into the subject when I ran across the answer recently, along with loads of other interesting information about food chemistry.
A few weeks ago I discovered a free, online course offered by McGill University via edX, on the subject of food and nutrition. Despite being offered by the chemistry department there, it doesn't require more than high school chemistry and the ability to use a 4-function calculator as prerequisites, and I'm not even sure if it needs high school chemistry. You should probably know the difference between an atom and a molecule, and at least recognize the Periodic Table of the Elements.
I was too late to sign up for the credit version, where the assignment deadlines are enforced, but there is a non-credit, audit version (which I'm doing) where you still have access to all the video lectures, discussions, and mini-quizzes.
So back to edible vs. non-edible leaves.
In the lesson on carbohydrates, (week 4, lesson 1) they showed the chemical structure of starch vs. cellulose (video 8). Both are long strings of glucose connected by oxygen atoms, but the way they're strung together is different—and that's it. That's the difference between plants we can digest and plants we can't. (Plants we can digest still have cellulose in them and we pass that through our system no problem—but we don't get any nutrition out of it.)
(Screenshot from Food for Thought, week 4/lesson 1/video 8. Requires free course registration to view.)
They look very similar at first glance, but if you look closely, they have an important difference: every second glucose segment is upside down in the cellulose chain.
We have the enzymes necessary to digest starch. We don't, but cows and other ruminants do, have the enzymes necessary to digest cellulose.
Enzymes are complicated things which have a very specific shape, and can fit around molecules of a very specific shape. So, an enzyme that fits the shape of starch in order to cut it down to its component glucose molecules will simply not fit the different shape of cellulose, even though the components are all the same.
So that was short and sweet. Also, check out the course, it's fascinating. (Keep in mind you can adjust the playback speed of the videos. I found my attention wandering because the instructors speak kind of slowly; running them at 1.5x speed makes it easier for me to keep from wandering. You can also back up and repeat sections if you don't catch it the first time through, or pause to look at the diagrams, because it's a video.)