"A man is rich in proportion to the number of things he can afford to let alone." -- Henry David Thoreau

I. Energy in Cells

1. Remember that energy is stored in covalent bonds.
   1. A chemical reaction that is exothermic releases more energy than it takes in; an endothermic reaction requires more energy to run than it gives off.
   2. We've seen how enzymes can cause exothermic reactions to happen spontaneously, by lowering the activation energy.
   3. But enzymes by themselves cannot force an endothermic reaction to happen -- there must also be a source of energy!
2. As we've already seen, several types of molecule can be used for energy storage over the "long term" (usually sugars, starches, and lipids)
3. The main short-term energy carrier in living systems is a nucleotide molecule, ATP (adenosine triphosphate)
   
   ATP (top: shown as a formula. bottom: space-filling "Ping-Pong-ball" model)
   1. ATP can break a bond between two of the phosphate groups (groups of phosphorus and oxygen atoms) that it carries. This produces a molecule of ADP (adenosine diphosphate) and a phosphate ion (PO₄, often just abbreviated as P_i).
   2. The reaction ATP -> ADP + P_i is exothermic -- it releases energy.
   3. It can be coupled to endothermic reactions that cells must carry out to live.
      1. EXAMPLE: Muscle consists of two basic proteins, both of which form long, parallel filaments: actin and myosin. (There's other proteins involved as well, but for our purposes we need not know about them.)
      2. Myosin binds by its "head" to actin.
      3. When a molecule of ATP comes into contact with a myosin molecule, it breaks down into ADP and phosphate. . . and transfers the bond energy to the myosin. . .
      4. . . . and causes the myosin molecule to change its shape! This lets it "ratchet" one notch down along the actin molecule.
4. Analogy with personal finance:
   1. ATP is like cash in your pocket -- you can use it to buy almost anything quickly and easily, and most people always have some cash on hand, but it's not efficient to carry enormous wads of cash.
   2. Most people store their wealth in more stable forms (bank accounts, savings bonds, IRAs, home equity, etc.) and convert those to cash only as they need it.
   3. In the same way, most cells and other organisms store energy in the form of starches, sugars, or lipids, and convert those into ATP only as needed.