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Study Questions

What do playing Wii, watching Dancing With the Stars, and doing your homework have in common? At first glance, not much...since two of those things are fun, and one isn't. Don't worry; we won't judge you on which one of those is the most unpleasant for you. But, they do have something in common: they all require energy. This energy is not just the energy that powers light bulbs and televisions—it is also the energy that powers cells in your body.

Most of us know that we need to eat food for energy, unless you're a plant, in which case you just soak up the Sun and carbon dioxide. If you are a plant, congrats on learning how to read. How does food become useful to cells in the body? Cells can't process pizza, mac and cheese, or Doritos in their natural or intact forms, so the body needs to break them down a bit. Note that we use the term "natural" loosely here. The process of converting food into useful energy at the level of the cells is called cellular respiration.

Cellular respiration is a misleading name: usually "respiration" refers to breathing. In this case, though, cellular respiration refers to converting glucose, a simple sugar (carbohydrate), into energy for cells.

Cellular respiration is a bit like an old-school arcade game. At the arcade, you start with a dollar, change it into tokens (we said old school!), play the game, and get 38 tickets that you can turn in for, at most, one crappy bouncy ball at the exchange counter. Never mind our bitter flashbacks to arcade trips in the early '90s. Carry on.

Your starting input, the dollar bill, is like a glucose molecule, and you end up with 38 tickets, or adenosine triphosphate (ATP) molecules. Glycolysis is the name we would give to the part of the process where you take your dollar bill (glucose) to the change machine and get tokens. You then feed these tokens into the pinball machine, called the citric acid cycle, and it spits out a couple tickets (ATP molecules). Let's just say that, along earning with your tickets, you win a bonus game of Skee-Ball called oxidative phosphorylation (come to think of it, this is not the coolest arcade we have ever been to...), which gives you even more tickets, or ATP molecules. Tickets can be exchanged for prizes at the arcade counter, and in the cell, ATP provides the energy for work—the best prize of all!

Not sold on our analogy? Fine.

So, how exactly does ATP provide energy for the cell? ATP is a source of chemical energy that can be used to

  • Drive chemical reactions
  • Transport substances across membranes
  • Do mechanical work

Chemical energy is the energy released in a chemical reaction, like when certain chemical bonds are broken. One molecule of ATP has three phosphate groups that are bound together. When a bond between phosphates is broken, energy is released. This energy can be used to power another chemical reaction. When a reaction that requires energy is paired with a reaction that releases energy, like the breaking of a phosphate bond in ATP, we call this process energy coupling.

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