1. ADP (Adenosine Diphosphate): This is the starting molecule for ATP synthesis. It's like a partially charged battery.

2. Phosphate (Pi): This inorganic phosphate is added to ADP to create ATP.

3. Energy Source: This can be in various forms, but the most common one is:

* Glucose: This sugar is broken down through cellular respiration, releasing energy that is used to attach the phosphate to ADP.

Here's a simple analogy:

Imagine ADP as a partially charged battery. The phosphate is like the electricity you need to fully charge it. The energy source (like glucose) is the power outlet that provides the energy for the charging process.

The process of ATP synthesis is called phosphorylation. There are several ways this can happen, but the most common is oxidative phosphorylation which takes place in the mitochondria using a series of reactions that utilize the energy from the breakdown of glucose.