ATP (adenosine triphosphate) is an organic molecule found throughout living cells. Organisms have to be able to move, reproduce and find nourishment.

These activities take energy and are based on chemical reactions inside the cells that make up the organism. The energy for these cellular reactions comes from the ATP molecule.

It is the preferred source of fuel for most living things and is often referred to as the "molecular unit of currency."

The ATP molecule has three parts:

1. The adenosine module is a nitrogenous base made up of four nitrogen atoms and an NH2 group on a carbon compound backbone.
2. The ribose group is a five-carbon sugar in the center of the molecule.
3. The phosphate groups are lined up and linked by oxygen atoms on the far side of the molecule, away from the adenosine group.

Energy is stored in the links between the phosphate groups. Enzymes can detach one or two of the phosphate groups liberating the stored energy and fueling activities such as muscle contraction. When ATP loses one phosphate group it becomes ADP or adenosine diphosphate. When ATP loses two phosphate groups, it changes to AMP or adenosine monophosphate.

The respiration process at the cellular level has three phases.

In the first two phases, glucose molecules are broken down and CO2 is produced. A small number of ATP molecules are synthesized at this point. Most of the ATP is created during the third phase of respiration via a protein complex called ATP synthase.

The final reaction in that phase combines half a molecule of oxygen with hydrogen to produce water. The detailed reactions of each phase are as follows:

A six-carbon glucose molecule receives two phosphate groups from two ATP molecules, turning them into ADP. The six-carbon glucose phosphate is broken down into two three-carbon sugar molecules, each with a phosphate group attached.

Under the action of coenzyme NAD+ , the sugar phosphate molecules become three-carbon pyruvate molecules. The NAD+ molecule becomes NADH, and ATP molecules are synthesized from ADP.

The Krebs cycle is also called the citric acid cycle, and it completes the breakdown of the glucose molecule while generating more ATP molecules. For each pyruvate group, one molecule of NAD+ becomes oxidized to NADH, and the coenzyme A delivers an acetyl group to the Krebs cycle while releasing a carbon dioxide molecule.

For each turn of the cycle through citric acid and its derivatives, the cycle produces four NADH molecules for each pyruvate input. At the same time, the molecule FAD takes on two hydrogens and two electrons to become FADH2, and two more carbon dioxide molecules are released.

Finally, a single ATP molecule is produced per one turn of the cycle.

Because each glucose molecule produces two pyruvate input groups, two turns of the Krebs cycle are needed to metabolize one glucose molecule. These two turns produce eight NADH molecules, two FADH2 molecules and six carbon dioxide molecules.

The final phase of cell respiration is the electron transport chain or ETC. This phase uses oxygen and the enzymes produced by the Krebs cycle to synthesize a large number of ATP molecules in a process called oxydative phosphorylation. NADH and FADH2 donate electrons to the chain initially, and a series of reactions builds up potential energy to create ATP molecules.

First, NADH molecules become NAD+ as they donate electrons to the first protein complex of the chain. The FADH2 molecules donate electrons and hydrogens to the second protein complex of the chain and become FAD. The NAD+ and FAD molecules are returned to the Krebs cycle as inputs.

As the electrons travel down the chain in a series of reduction and oxidation, or redox reactions, the energy liberated is used to pump proteins across a membrane, either the cell membrane for prokaryotes or in the mitochondria for eukaryotes.

When the protons diffuse back across the membrane through a protein complex called ATP synthase, the proton energy is used to attach an additional phosphate group to ADP creating ATP molecules.

ATP is produced at each stage of cellular respiration, but the first two stages are focused on synthesizing substances for the use of the third stage where the bulk of ATP production takes place.

Glycolysis first uses up two molecules of ATP for the splitting of a glucose molecule but then creates four ATP molecules for a net gain of two. The Krebs cycle produced two more ATP molecules for each glucose molecule used. Finally, the ETC uses electron donors from the previous stages to produce 34 molecules of ATP.

The chemical reactions of cellular respiration therefore produce a total of 38 ATP molecules for each glucose molecule that enters glycolysis.

In some organisms, two molecules of ATP are used to transfer NADH from the glycolysis reaction in the cell into the mitochondria. The total ATP production for these cells is 36 ATP molecules.

In general, cells need ATP for energy, but there are several ways the potential energy from the phosphate bonds of the ATP molecule are used. The most important features of ATP are:

• It can be created in one cell and used in another.
• It can help break apart and build complex molecules.
• It can be added to organic molecules to change their shape. All these features impact how a cell can use different substances.

The third phosphate group bond is the most energetic, but depending on the process, an enzyme may break one or two of the phosphate bonds. This means the phosphate groups become temporarily attached to the enzyme molecules and either ADP or AMP is produced. The ADP and AMP molecules are later changed back to ATP during cellular respiration.

The enzyme molecules transfer the phosphate groups to other organic molecules.

ATP is found throughout living tissues, and it can cross cell membranes to deliver energy where the organisms need it. Three examples of ATP use are the synthesis of organic molecules that contain phosphate groups, reactions facilitated by ATP and active transport of molecules across membranes. In each case, ATP releases one or two of its phosphate groups to allow the process to take place.

For example, DNA and RNA molecules are made up of nucleotides that may contain phosphate groups. Enzymes can detach phosphate groups from ATP and add them to nucleotides as required.

For processes involving proteins, amino acids or chemicals used for muscle contraction, ATP can attach a phosphate group to an organic molecule. The phosphate group can remove parts or help make additions to the molecule and then release it after changing it. In muscle cells, this kind of action is carried out for each contraction of the muscle cell.

In active transport, ATP can cross cell membranes and bring other substances with it. It can also attach phosphate groups to molecules to change their shape and allow them to pass through cell membranes. Without ATP, these processes would stop, and cells would no longer be able to function.