Adenosine Diphosphate: Definition & Function

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Adenosine diphosphate isn't the star of the cell when it comes to energy, but it is certainly important! Read on to learn how adenosine diphosphate helps our cells have energy to do everything they need to do.

Cellular Work

You might not really think of the cells in your body as things that work. When was the last time you saw a kidney cell in a cubicle, finishing up a weekly expense report? Still, cells have many jobs to perform, from making proteins, sending nerve impulses, moving muscles, and more. Just as you need energy for your body to do your work, a cell needs energy to do its work.

How Do Cells Get Energy?

To perform work, cells need energy. They can't use food (like sugar or protein) directly as energy sources. Instead, cells use two highly-related compounds: adenosine triphosphate (ATP) and adenosine diphosphate (ADP).

These two molecules are almost identical. Both are composed of one adenine molecule, a sugar molecule, and phosphate groups. As the names suggest, ATP has three phosphate groups and ADP has two.

A common analogy to explain how cells use energy is to think about money. A paycheck that is made out to you can't be used to purchase items from the store. You have to take the check to a bank and have it turned into cash. You can then take the cash to the store and purchase what you want. Food is like the paycheck your cells receive, and ATP is the cash that they can spend to do work.

The ATP/ADP Cycle

ATP and ADP work together to provide cellular energy. ATP, as we mentioned, is like cash that can be used by a cell to perform work. To get energy, one of the phosphate groups is broken off from the ATP molecule, releasing energy. This leaves one free phosphate molecule and one ADP molecule left over. They're like the change you receive from a purchase. If you put a dollar bill into the vending machine and select a beverage that costs $0.75, you'll get a quarter back.

Now, what would happen if you took your quarter, and added to a group of three other quarters? You would, once again, have a dollar. Your cells are able to recombine pieces to make new ATP molecules. During a process called cellular respiration, the cell uses energy from food, including sugar, proteins, and fats, and connects a free phosphate molecule onto an ADP molecule, creating ATP. The ATP will then be used for more cellular work. It will again leave behind ADP and a phosphate. This recycling of ADP into ATP, and ATP breakdown into ADP, is called the ATP/ADP cycle.

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