11. ATP-Powered Proteins (637)

A cell is an active, bustling place. Enzymes are performing chemical reactions. Macromolecules--DNA, RNA, proteins, lipids, and carbohydrates--are being built and broken down. Molecular motors are carrying organelles around the cell. DNA damage is being repaired. And the cell membrane is selectively letting molecules and ions in and out.

Everything is moving. Countless processes are unfolding at once.

All of this requires energy. A lot of it.

This, then, raises a fundamental question: What powers all of this activity?

A natural answer is: food. The food we eat--and, therefore, that our cells eat--consists of molecules held together by chemical bonds.

When we eat, we turn the carbohydrates and lipids in our food into nutrients, especially sugars and fatty acids. The nutrients enter the bloodstream and are delivered to the cells in our body.

Then, inside the cell, the nutrients are further broken down. Energy becomes available as they’re converted into more stable chemical forms.

That energy is converted into adenosine triphosphate (ATP) molecules.

ATP is the cell’s most immediate and widely used energy carrier. It converts chemical energy into physical movement and work.

ATP is how the cell turns chemistry into motion.

To understand how ATP works, we need to look at the molecule itself.

The ATP molecule

ATP is built from a familiar component: adenosine, which is an adenine base bound to a ribose sugar. This is the same basic unit found in RNA--but now carrying three phosphate groups attached in a row to the sugar.

These phosphates--especially the last of the three--give ATP its energy-transferring role. The three phosphates also give it its name: adenosine triphosphate.

When ATP's outer phosphate is removed, what's left is an ADP (adenosine diphosphate) and a free phosphate ion. This reaction also releases usable energy.

ATP acts like a resettable trigger.

Once the ATP is cleaved into ADP and a phosphate ion, the cell uses more energy from nutrients to re-load the third phosphate, returning the molecule to the more energetically powerful ATP form.

And the cycle continues, the same cycle driving innumerable tiny events.

The cell's energy currency

Think of ATP as the cell's energy currency. To see what that means, consider human currency.

We carry money and use it to make things happen--buying goods, paying for services.

As they say, "money makes the world go 'round."

The cell also needs a currency--something that can cause motion and action. It needs a chemical currency. That's ATP.

In most cells, the production of ATP from the breakdown of nutrients occurs in the mitochondria. Metabolically active cells like heart muscle cells might have 5,000-8,000 mitochondria. Less active cells like skin cells might have a few to a few hundred.

Once produced in the mitochondria, ATP molecules then diffuse through the cell or are used locally.

Making it real

The notion of "energy being released" is abstract. So let me offer a couple of analogies to ATP functioning that might make the idea more tangible.

One way to think about ATP is as a compressed, or loaded, spring. When the outermost phosphate bond is broken, it’s like letting go of the spring. The stored tension suddenly converts into motion, reshaping the protein or other molecule to which the ATP is attached.

Or think of a kick. In many cases, the release of ATP is like a tiny molecular kick. The newly freed phosphate doesn't just drift away. If the ATP was bound to a protein, it can change its shape, like a nudge that forces the protein into a new position. That shape change is what drives action.

ATP turns stored chemical energy into motion. It powers the countless small movements that, taken together, bring the cell to life.

In the next chapter, we’ll slow down and examine how one of these tiny, local interactions produces a visible, macroscopic effect: muscle contraction.