What is ATP? | Work of adenosine triphosphate (ATP)

What is ATP? Work of adenosine triphosphate

 what is ATP? We love it. You’ll find ATP in so much of our science art…whether it’s a mitochondrion or cell transport…or cellular respiration or fermentation, you’ll see ATP mentioned. So why the big deal? Why is it all over the place? Many times students will get in their mind that it is an energy currency of some kind.

 When I first started studying biology, I noticed that in textbooks it’s often represented as like this starburst thing or thunderbolt, and you know, I guess in my mind I imagined it was like some big blast of energy that helped the cells do things. And by doing things, I mean that we need ATP to do many cellular processes.

Examples include active transport such as when a cell is trying to move something against its concentration gradients. Or its role in muscle contraction with the actin and myosin cross-bridge…ATP is critical for many types of cell signalling; you need your cells to be able to communicate. Those are all just some examples.

But what is ATP? How do we get it? And…how does it work? 

So what is ATP?

If you remember the four major biomolecules, ATP would fit in with the nucleic acids. Yes, like DNA and RNA. ATP is a nucleotide derivative so it has those three parts you’d see in DNA or RNA nucleotides: phosphate, sugar, base, but it actually has 3 phosphates. ATP is short for its full name, adenosine triphosphate. This fancy name is helpful as it tells you that it contains the nitrogenous base known as adenine, and three phosphates---hence the “tri” in adenosine triphosphate. Its sugar is ribose.

How do you get ATP?

 All cells need ATP and so they need processes that can be used to generate it. But the process can differ. It might involve oxygen such as aerobic cellular respiration. It might not involve oxygen such as anaerobic respiration or fermentation. During cellular respiration, plants break down the glucose they MADE from photosynthesis to make ATP. During cellular respiration, animals break down the glucose they CONSUMED to make ATP. And it's not just plants and animals; bacteria, fungi, protists, and archaea---they all need to make ATP.

 but one thing that we do want to mention about making ATP is that it is important to understand it is part of a cycle. With the ATP cycle, you have ATP, which can be hydrolyzed, releasing energy and losing one of its phosphates in this process. A process like cellular respiration can provide the energy needed to add a phosphate to ADP in order to regenerate ATP again, which is important as ATP can be used quickly. This brings us to how ATP is able to work.

how does ATP work?

 It’s not just about ATP being hydrolyzed and releasing energy. It’s more than that. So when ATP is hydrolyzed, meaning here it involves the addition of water, it’s not really that the bond between this second and third phosphate itself is a super-strong bond. It’s actually more than the bond between the second and third phosphate contributes to this ATP being unstable. These phosphates with their negative charges don’t like being arranged like this. The change from ATP losing its third phosphate to become the more stable ADP is an exergonic reaction and releases free energy.

 A popular example for understanding ATP is to use the spring illustration. Like a wire spring. Consider how you might compress the spring---ATP would be modelled by that compressed spring---and then you would let it go until it just goes into this relaxed state, which would be represented by ADP. When ATP is hydrolyzed, if the energy was just released, it will likely not be useful for a cell if it’s not actually coupled to something that needs it. Thankfully, the energy release can be coupled to endergonic processes that the cell needs to do. This can occur when the phosphate from the ATP is transferred to a molecule that is going to be acted upon.

For example, this cell transport protein here is supposed to move some kind of molecule against its concentration gradient. Recall if it was passive transport, these molecules would be moving from high to low concentration, but inactive transport thanks to ATP, this protein can move them against the gradient. When the phosphate is transferred to this protein, we say the protein has been phosphorylated. Sounds powerful. We can say, in our example, that this protein is more reactive and less stable in this form, this phosphorylated intermediate state.

 When it reverts into its original, more stable shape, it can assist in moving them the other direction. So from marvelling at the beating of a single cilia hair, or chromosomes being separated in cell division, or binding the correct amino acid on a tRNA, I could go on---we hope that little ATP symbol will mean something every time you see it.