Energy and ATP: Powering the Cell

Energy cannot be created or destroyed, only converted from one form to another, and every energy conversion loses some usable energy as heat -- two facts, drawn from the laws of thermodynamics, that explain a great deal of biology, including why food chains (Unit 11) lose so much energy at each step. Cells do not run directly on the chemical energy stored in glucose; instead, they convert that energy into a more immediately usable form, adenosine triphosphate, or ATP. An ATP molecule is built from an adenine base, a ribose sugar, and a chain of three phosphate groups; the bond holding on the third phosphate group is a high-energy bond that releases a burst of usable energy when broken, converting ATP into ADP (adenosine diphosphate) plus a free phosphate group. Cells constantly recharge ADP back into ATP, making ATP less like a warehouse of stored energy and more like a rapidly-spent-and-refilled currency.

Two complementary processes, covered in detail over the next two weeks, generate and use this energy currency at the largest scale in the living world. Photosynthesis, occurring in chloroplasts, converts light energy into the chemical energy stored in glucose, releasing oxygen as a byproduct. Cellular respiration, occurring mainly in mitochondria, converts the chemical energy in glucose into usable ATP, releasing carbon dioxide and water as byproducts. Notice that the overall equations for the two processes are near mirror images of each other -- the products of one are essentially the reactants of the other -- which is exactly what allows plants, animals, and the atmosphere itself to stay in a long-term working balance.