ATP-The Cell's Energy Molecule

Structure of ATP

ATP, or adenosine triphosphate, is going to be a very prevalent molecule as we discuss two processes that sustain life as we know it: cellular respiration and photosynthesis. As a result, it is important to familiarize ourselves with its structure. The structure of ATP is fairly straightforward, consisting of 3 parts: adenine, a ribose sugar, and three phosphate groups.

Before we dive deeper into ATP's structure, let's think about its name, specifically the triphosphate. 'Tri' means three, so ATP contains three phosphates. This aspect of its structure is quintessential to its function. You do not need to memorize the adenine or ribose portions - just worry about the three phosphate groups.

You will recall that ATP is referred to as the cell's primary battery or energy carrier. If energy needs to be used, it is generally brought by ATP.

Draining and Charging the Battery

In order to unleash all of that amazing and useful energy held within ATP, a cell must detach one of those phosphate groups. Once the phosphate group is removed, energy is released and used by the cell.

Wait, we just learned that the 'TP' in ATP stands for three phosphates. So if we lose one, we only have two! This means that the molecule would now be referred to as ADP, or adenosine diphosphate. If ATP is our charged battery, ADP is our dead, or drained, battery.

So using the energy of ATP yields ADP, a loose phosphate group, and some released energy. But cells don't like to waste things. So, if more energy is obtained later (through cellular respiration for instance, which we will explore soon), ADP and a phosphate group (P) can be 'charged' back to full energy (ATP). This is the cycle ATP molecules undergo. The individual reactions are:

Draining the Battery: ATP + water --> ADP + P + energy (released)

Charging the Battery: ADP + P + energy --> ATP + water

ATP Powering Enzymes - Active Transport and More

Remember that ATP's utility to a cell is that is a battery - it carries energy for the cell to use to perform some job. Also recall that enzymes are the 'doers' of the cell. So if a job needs to get done, an enzyme will be doing it. If energy is needed to perform said job, ATP will provide that energy.

In one of these diagrams, you can see a very important enzyme, ATP Synthase, that we will encounter quite a lot of in the next topics. ATP Synthase spans a membrane, and you can see that it is allowing a proton (H+) to pass through it, going from the upper side to the lower side of that membrane. This proton (H+) is traveling down its concentration gradient, so no additional energy is required. However, ATP synthase is able to harness the kinetic energy of the H+ ions moving to power the construction of ATP (charging the battery).

The purpose behind showing you this diagram is primarily to introduce you to how we will be representing these processes. If an enzyme is catalyzing a reaction, such as the ones shown, we will often draw the enzyme and draw an arrow showing the reactants on the flat end of the arrow and the products on the pointy end of the arrow.

This process could be shown in reverse if an enzyme was using energy, as shown below.

The Importance of ATP

Every single day, you produce and use roughly your own body weight in ATP (according to many meticulous studies). That is about 200 trillion molecules of ATP every single day. We use it so quickly after producing it that there is roughly only 60 grams (a little more than 2 ounces) of it within you at any given moment. - Source:The Body: A Guide for Occupants by the excellent Bill Bryson

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