Every single function and movement of the body requires energy. The energy currency of the body (and all living organisms) is an energy-carrying molecule known as adenosine triphosphate or ATP for short.
Although some people may recall hearing about ATP from their days in high school biology class, the specifics of how ATP functions and how it is made remains unclear to most people.
So, what is ATP? How is ATP made? What is ATP made of? Where is ATP made? Most importantly, what does ATP do?
In this article, we will return to some of the concepts of high school biology in a useful way for everyday exercise and performance by discussing what ATP is, how ATP is made, and how ATP functions in the body.
We will cover:
- What Is ATP?
- How Does ATP Work?
- How Is ATP Produced?
- Why Is ATP Important?
Let’s jump in!
What Is ATP?
ATP, which stands for adenosine triphosphate, is a large biomolecule made of a nitrogenous base (adenine) and a sugar molecule (ribose), which together create adenosine.
The triphosphate component refers to the three phosphate molecules bonded to the adenosine.
The ATP molecule was first discovered in 1929 by German chemist Karl Lohmann who isolated the molecule as he was studying muscle contractions.
However, it took another decade for ATP to be fully understood. In 1939, Fritz Lipmann won the Nobel Prize for establishing that ATP has “energy-rich phosphate bonds” and is the universal carrier of energy in all living cells.
How Does ATP Work?
The ATP molecule stores energy in the phosphate bonds. As phosphate molecules are cleaved off, usable energy is released.
When one phosphate is removed, the molecule is known as adenosine diphosphate (ADP), and then as another phosphate is removed, the molecule becomes adenosine monophosphate (AMP).
The highest energy configuration is ATP; as phosphates are removed, energy is released, and the molecule carries less stored energy.
ATP and ADP are constantly in a cycle going back and forth between the higher energy state of ATP and the low energy molecule ADP as a phosphate molecule is either cleaved off or added back on.
In this way, the ATP/ADP Molecule can be seen somewhat like a rechargeable battery in that when the battery is “full,” the molecule is in its triphosphate state as ATP with three phosphate molecules, carrying the maximum amount of energy the molecule can hold for cellular work.
As energy is needed by the cell, one phosphate molecule is removed, so the molecule is “drained“ of some of its battery power. In the diphosphate state, the two phosphate molecules, the adenosine diphosphate, ADP, is the low-energy form of the molecule.
Then, after food is consumed, the body can convert energy in food into stored energetic phosphate bonds.
Another phosphate can be added onto the “low battery“ state of a DP and convert the molecule back to the fully charged ATP.
A single ADP/ATP molecule can go through this cycle of losing and gaining a phosphate molecule or essentially being charged and uncharged countless times during its lifecycle.
How Is ATP Produced?
So, how is ATP made?
The body produces ATP molecules through a process known as hydrolysis.
The energy that comes from the carbohydrates, proteins, and fat molecules from the foods and beverages we consume can be used to form the energy-carrying molecule ATP.
The primary food source used to make ATP is carbohydrates because glucose, a simple sugar that carbohydrate molecules get broken down into, is the main fuel source for the mitochondria in our cells.
Mitochondria are organelles, or little cellular components, that convert caloric energy from food into cellular energy, or ATP, via a process termed cellular respiration.
Essentially, mitochondria are able to extract the caloric energy found in food that is contained within the bonds of the sugar molecules and convert it into usable energy for the cells, which, again, is the ATP molecule.
There are different types of cellular respiration, which is the process by which this energy conversion occurs. Once cellular respiration occurs in the presence of oxygen, it is known as aerobic respiration.
Aerobic respiration occurs through a pathway known as the Krebs or citric acid cycle and the electron transport chain.
When insufficient oxygen is available, such as during high-intensity exercise, cellular respiration can still occur, but it uses different pathways, termed anaerobic metabolism, or more specifically, glycolysis and the ATP/PC system.
Glucose molecules begin the conversion to ATP with glycolysis, a series of chemical reactions that ultimately breaks down glucose molecules into smaller molecules known as pyruvate and four ATP molecules.
As long as glycolysis takes place in the presence of oxygen, the pyruvate molecules then enter the Krebs cycle or citric acid cycle, which further breaks down the remaining portion of the sugar molecules into electron carriers.
Electron carriers are special molecules that fuel the synthesis of ATP.
They enter the electron transport chain (ETC), which is another aerobic cellular respiration pathway. This pathway pumps positively-charged protons throughout the inner membrane of the mitochondria, which ultimately drives a relatively massive amount of ATP production compared to the earlier stages and phases of the cellular respiration process.
Although most of the ATP produced in the human body is made in the mitochondria using aerobic respiration, ATP can also be produced anaerobically, without oxygen, both in the body and in other living organisms such as animals, plants, and even some bacteria.
For example, in the human body, when you are doing a vigorous workout, there can be insufficient oxygen to produce energy aerobically. In these cases, anaerobic glycolysis occurs, which means that instead of pyruvate, all the end product of the chemical reactions that comprise the glycolysis cycle, lactate is the final end product.
Lactic acid fermentation then produces ATP anaerobically. However, the Krebs cycle and the electron transport chain cannot occur without sufficient oxygen, so the potential yield of ATP for each glucose molecule that is broken down is significantly reduced.
Additionally, the anaerobic glycolysis pathway produces hydrogen ions, which are acidic, and decrease the pH in your muscle tissue. A lower pH can cause a burning feeling that can be associated with high-intensity exercise such as sprinting or HIIT workouts.
According to research, when ATP synthesis occurs via aerobic cellular respiration within the mitochondria, approximately 32 ATP molecules are produced per molecule of glucose that is oxidized.
Typically, it is thought the body relies on the hydrolysis of 100 to 150 moles of ATP per day to support all of the various functions. Because one mole is equal to 6.022 × 10²³ molecules, this represents a massive demand of ATP by the body per day.
Although the majority of ATP is produced through cellular respiration pathways, ATP can also be produced through beta-oxidation (in which fat molecules are broken down for energy) and ketosis (in which ketones are burned for energy).
Additionally, plants, algae, and certain bacteria can make ATP by converting sunlight rather than food energy into cellular energy (ATP) via photosynthesis.
These organisms don’t even have mitochondria. They use chloroplasts to carry out a cellular respiration function.
Why Is ATP Important?
ATP is vital for sustaining the life of any organism, including the human body. Without ATP, cells would not be able to carry out their functions, muscles would not be able to contract, digestion would not occur, the heart could not beat, etc.
ATP is required to harness the energy found in the foods that we eat, so without ATP, even if you were eating, you would be unable to make use of the potential energy found in the carbohydrates, proteins, and fat calories consumed.
ATP can be equated to the gasoline in a car or the battery power on your smartphone. If you’re down to zero and do not have any gas or battery power, the vehicle will not run, and your phone will not be usable.
It’s important to note that in addition to needing actual caloric macronutrients (carbohydrates, protein, and fat) to produce ATP, micronutrients such as the B vitamins and minerals like copper, magnesium, manganese, and phosphorus are necessary to produce ATP.
As long as you are consuming a well-balanced diet with enough calories for the amount of physical activity you are doing, staying well hydrated, and getting enough rest, your body should have all of the necessary resources it needs to make ATP and support the constant recycling ATP/ADP cycle to power your essential functions of living as well as your voluntary physical activity and exercise.
To understand your body’s daily calorie needs, you can refer to our article, What Is TDEE? Total Energy Expenditure, Explained.