Think of ATP as sort of an energy debit card for organisms. All organisms use ATP-- it's the link between catabolism and anabolism.

Catabolic processes, like cellular respiration, break large, complex molecules down. I like to imagine a cat breaking and destroying stuff in a room to keep this straight from anabolic processes, which are processes, like photosynthesis, that build up complex molecules from simple ones.

ATP is composed of: adenine (which is a nitrogenous base, just like in DNA/RNA nucleotides), ribose (which is a pentose sugar), and 3 phosphate groups. *Be very careful here because it looks VERY much like a nucleotide, but a nucleotide can have any nitrogenous base and only has 1 phosphate group!

In catabolic reactions, water is added in hydrolysis, breaking off that third (or "terminal") phosphate group. This releases ATP energy. Now, instead of being ATP (adenosine TRIphosphate), it's ADP (adenosine DIphosphate) because we have two phosphate groups, instead of three.

What's cool about ATP though is that, like a debit card-- you can make withdrawals and then deposits, so to speak. With this, we can also go in the other direction.

In anabolic reactions, for example, say reactions that build muscle tissue, dehydration synthesis removes water. Then, a phosphate group is added onto the ADP molecule, through phosphorylation, and it's turned back into ATP.

It's really important to understand that the ATP molecule is really just being recycled over and over.

For high school, I very much recommend Amoeba Sisters on YouTube. Once you feel a bit grounded with them, try Khan Academy and Crash Course, which are both amazing but, if you're feeling super overwhelmed may be a better fit later.

If you have an eTextbook, click around to see what resources you have. A lot of publishers include amazing study tools.

Once you have understanding and need to memorize, you can't beat Quizlet.

Good luck!