What Is ATP?

Adenosine triphosphate (ATP) is an energy-carrying molecule known as "the energy currency of life" or "the fuel of life," because it's the universal energy source for all living cells.

Every living organism consists of cells that rely on ATP for their energy needs. ATP is made by converting the food we eat into energy. It's an essential building block for all life forms. Without ATP, cells wouldn't have the fuel or power to perform functions necessary to stay alive, and they would eventually die. All forms of life rely on ATP to do the things they must do to survive.

This article explains how adenosine triphosphate works, how it's made, why ATP is so important to cellular processes, and what makes it vital to all life forms.

How ATP Works

ATP is made of a nitrogen base (adenine) and a sugar molecule (ribose), which create adenosine, plus three phosphate molecules. If adenosine only has one phosphate molecule, it’s called adenosine monophosphate (AMP). If it has two phosphates, it’s called adenosine diphosphate (ADP).

Although adenosine is a fundamental part of ATP, when it comes to providing energy to a cell and fueling cellular processes, the phosphate molecules are what really matter. The most energy-loaded composition for adenosine is ATP, which has three phosphates.

ATP was first discovered in the 1920s. In 1929, Karl Lohmann—a German chemist studying muscle contractions—isolated what we now call adenosine triphosphate in a laboratory. At the time, Lohmann called ATP by a different name. It wasn't until a decade later, in 1939, that Nobel Prize–winner Fritz Lipmann established that ATP is the universal carrier of energy in all living cells and coined the term "energy-rich phosphate bonds."

Lipmann focused on phosphate bonds as the key to ATP being the universal energy source for all living cells, because adenosine triphosphate releases energy when one of its three phosphate bonds breaks off to form ADP. ATP is a high-energy molecule with three phosphate bonds; ADP is low-energy with only two phosphate bonds.

Much like a rechargeable battery with a fluctuating state of charge, ATP represents a fully charged battery, and ADP represents "low-power mode." Every time a fully charged ATP molecule loses a phosphate bond, it becomes ADP; energy is released via the process of ATP becoming ADP.

On the flip side, when a phosphate bond is added, ADP becomes ATP. When ADP becomes ATP, what was previously a low-charged energy adenosine molecule (ADP) becomes fully charged ATP. This energy-creation and energy-depletion cycle happens time and time again, much like your smartphone battery can be recharged countless times during its lifespan.

How ATP Is Made

The human body uses molecules held in the fats, proteins, and carbohydrates we eat or drink as sources of energy to make ATP. This happens through a process called hydrolysis.

After food is digested, it's synthesized into glucose, which is a form of sugar. Glucose is the main source of fuel that our cells' mitochondria use to convert caloric energy from food into ATP, which is an energy form that can be used by cells.

ATP is made via a process called cellular respiration that occurs in the mitochondria of a cell. Mitochondria are tiny subunits within a cell that specialize in extracting energy from the foods we eat and converting it into ATP.

Mitochondria can convert glucose into ATP via two different types of cellular respiration:

• Aerobic (with oxygen)
• Anaerobic (without oxygen)

Aerobic cellular respiration transforms glucose into ATP in a three-step process, as follows:

• Step 1: Glycolysis
• Step 2: The Krebs cycle (also called the citric acid cycle)
• Step 3: Electron transport chain

During glycolysis, glucose (i.e., sugar) from food sources is broken down into pyruvate molecules. This is followed by the Krebs cycle, which is an aerobic process that uses oxygen to finish breaking down sugar and harnesses energy into electron carriers that fuel the synthesis of ATP. Lastly, the electron transport chain (ETC) pumps positively-charged protons that drive ATP production throughout the mitochondria’s inner membrane.

ATP can also be produced without oxygen (i.e., anaerobic), which is something plants, algae, and some bacteria do by converting the energy held in sunlight into energy that can be used by a cell via photosynthesis.

Anaerobic exercise means that your body is working out "without oxygen." Anaerobic glycolysis occurs in human cells when there isn't enough oxygen available during an anaerobic workout. If no oxygen is present during cellular respiration, pyruvate can't enter the Krebs cycle and is oxidized into lactic acid. In the absence of oxygen, lactic acid fermentation makes ATP anaerobically.

The burning sensation you feel in your muscles when you're huffing and puffing during anaerobic high-intensity interval training (HIIT) that maxes out your aerobic capacity or during a strenuous weight-lifting workout is lactic acid, which is used to make ATP via anaerobic glycolysis.

During aerobic exercise, mitochondria have enough oxygen to make ATP aerobically. However, when you're out of breath and your cells don’t have enough oxygen to perform cellular respiration aerobically, the process can still happen anaerobically, but it creates a temporary burning sensation in your skeletal muscles.

Why ATP Is So Important

ATP is essential for life and makes it possible for us to do the things we do. Without ATP, cells wouldn't be able to use the energy held in food to fuel cellular processes, and an organism couldn't stay alive.

As a real-world example, when a car runs out of gas and is parked on the side of the road, the only thing that will make the car drivable again is putting some gasoline back in the tank. For all living cells, ATP is like the gas in a car's fuel tank. Without ATP, cells wouldn't have a source of usable energy, and the organism would die.

A Word From Verywell

Eating a well-balanced diet and staying hydrated should give your body all the resources it needs to produce plenty of ATP. Although some athletes may slightly improve their performance by taking supplements or ergonomic aids designed to increase ATP production, it's debatable that oral adenosine triphosphate supplementation actually increases energy.

Always use common sense and talk to a healthcare provider before spending money or ingesting supplements that make potentially hyped-up marketing claims about increasing energy by boosting ATP production.