Adenosine triphosphate (ATP)

Original Editor - Lucinda hampton

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Introduction

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For every cell in your body, the source of energy that keeps everything going is called ATP. Adenosine triphosphate (ATP) is the biochemical way to store and use energy.

ATP is the most abundant energy-carrying molecule in your body. It harnesses the chemical energy found in food molecules and then releases it to fuel the work in the cell.

ATP is a common currency for the cells in your body. The food you eat is digested into small subunits of macronutrients. The carbohydrates in your diet are all converted to a simple sugar (glucose). that first needs to be converted to ATP (the energy form that will work in the cell).

  • Through an intricate chain of chemical reactions glucose is converted into ATP. This conversion process is called cellular respiration or metabolism.
  • Like the exchange of money from one currency to the next, the energy from glucose takes the form of temporary chemical compounds at the end of each reaction. Glucose is changed into several other compounds before its energy settles in ATP.[1]

ATP

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Image R: ATP, 3 different presentations ie "sticks" method,"balls and sticks", the R representation of what the spatial representation looks like in real life. Light blue stands for carbon, dark blue for nitrogen, red for oxygen (hydrogen has been omitted).

Chemically, ATP is an adenine nucleotide bound to three phosphates.

  • There is a lot of energy stored in the bond between the second and third phosphate groups that can be used to fuel chemical reactions.
  • When a cell needs energy, it breaks this bond to form adenosine diphosphate (ADP) and a free phosphate molecule, through the process of hydrolysis which is also known as dephosphorylation.

ATP + H2O → ADP + Pi + energy (30.6 KJ/mole)

  • In the above reaction, ADP is Adenosine Di-Phosphate and Pi is inorganic phosphate.
  • The reaction can also be reversed and ADP can be converted to ATP but it will require the same amount of energy which is released during the process i.e. 30.6 KJ.
  • This process is known as condensation or phosphorylation. This takes place because the ATP molecule is a very unstable and gets hydrolyzed very soon. The bonds between the phosphate group in the ATP molecule is weaker than the ADP molecule[2].

ATP Production

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The creation of ATP takes place throughout the body’s cells. The process begins when glucose is digested in the intestines. Next, it’s taken up by cells and converted to pyruvate. It then travels to the cells’ mitochondria. That’s ultimately where ATP is produced.

Without the pathway to ATP production, your body would be full of energy it couldn’t use. ATP is the universal energy carrier and currency. It stores all the power each cell needs to perform its tasks. And like a rechargeable battery, once ATP is produced, it can be used over and over again[1].

Because ATP is so important, the body has several different systems to create ATP. These systems work together in 3 phases (pathways) namely: Glycolysis; Krebs cycle (citric acid cycle); Electron transport chain.

Glycolysis

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This is the first pathway of the human respiration..[2]

Glycolysis is a series of reactions that extract energy from glucose by splitting it into two three-carbon molecules called pyruvates. Glycolysis is an ancient metabolic pathway, meaning that it evolved long ago, and it is found in the great majority of organisms alive today.

In organisms that perform cellular respiration, glycolysis is the first stage of this process. However, glycolysis doesn’t require oxygen, and many anaerobic organisms (organisms that do not use oxygen) also have this pathway.l[3].

  • Glycolysis takes place in the cytosol of a cell, and it can be broken down into two main phases: the energy-requiring phase, and the energy-releasing phase. See Image at R
  • Overall: Glycolysis converts one six-carbon molecule of glucose into two three-carbon molecules of pyruvate. The net products of this process are two molecules of ATP and two molecules of NADH[3]

Krebs Cycle or Citric Acid Cycle

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The Krebs Cycle, also called the citric acid cycle, is the second major step in oxidative phosphorylation. After glycolysis breaks glucose into smaller 3-carbon molecules, the Krebs cycle transfers the energy from these molecules to electron carriers, which will be used in the electron transport chain to produce ATP.

  • The different enzymes involved in the Krebs cycle are found on the inner membrane or in the matrix space of the mitochondria.
  • Within the mitochondrial matrix, the reactions of the Krebs cycle adds electrons and protons to a number of electron carriers, which are then used by the electron transport chain to produce ATP.
  • This cycle only occurs under aerobic conditions (since energy-rich molecules like NAD + and FAD can only recover from their reduced form once they transfer electrons to molecular oxygen).
  • The citric acid cycle is the common final pathway for oxidation of all biomolecules; proteins, fatty acids, carbohydrates.
  • The citric acid cycle is a cyclic sequence of reactions consisting of 8 enzyme-mediated reactions.
  • This cycle is also particularly important as it provides high energy electrons/molecules to the electron transport chain for the production of ATP and water.
  • The pyruvate formed at the end of glycolysis is first oxidized to Acetyl CoA, which then enters the citric acid cycle[4].

The Krebs cycle produces 2 molecules of ATP for 1 molecule of glucose. The Krebs cycle also produces eight molecules of NADH and two molecules of FADH2 per molecule of glucose. NADH and FADH2 are later used to produce energy during electron transport phosphorylation[4].

Electron Transport Chain

Image: Mitochondrial electron transport chain

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The series of steps by which the electrons flow to oxygen permiting a gradual lowering of the energy of the electrons. This part of the oxidative phosphorylation stage is sometimes called the electron transport chain.

  • The electron transport chain is located within mitochondria, and the proteins of the electron transport chain span the inner mitochondrial membrane. Special proteins, the ones energized by NADH, are embedded in the membrane of mitochondria. They are continuously producing ATP to power the cell.[1]
  • In aerobic respiration, each molecule of glucose leads to about 34 molecules of ATP (Adenosine triphosphate) being produced by the electron transport chain. This is by far the most productive part of respiration
  • The electron transport chain consists of a series of redox reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the free energy (energy available to do work) of the reactants and products. Any reaction that decreases the overall free energy of a system will happen.
  • ATP synthase is an enzyme found among all domains of life. It is powered by a transmembrane proton electrochemical gradient. This is the result of the series of redox reactions. What the electron transport chain does is produce this gradient.The free energy is used to drive ATP synthesis[5].
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  • The electrons carried through the NADH produced by the glycolysis and citric acid cycle, and the FADH2 produced by citric acid cycle are taken for electron transport phosphorylation. In this process 32 ATP molecules are produced.
  • Hence the total ATP’s produced in an aerobic respiration is 2 + 2 + 32 = 36.[2]

ATP Uses

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The ATP molecule is used for many purposes. ATP is an important molecule in metabolism as it holds a lot of energy which is used in many metabolic processes. eg
  • ATP is an important part of the protein synthesis process.
  • It is used in muscle contraction
  • It is very helpful in the transportation of molecules through membranes. This process is also known as active transport.
  • Also used in sending signals and in the process of DNA synthesis[2].
  • Adenosine triphosphate (ATP) is an important extracellular signaling molecule. ATP acts as a neurotransmitter in both peripheral and central nervous systems. In the peripheral nervous system, ATP is involved in chemical transmission in sensory and autonomic ganglia. In the central nervous system, ATP, released from synaptic terminals, induces fast excitatory postsynaptic currents.[6]

ATP concentration

Normally cellular ATP concentration is maintained in the range of 1 to 10 mmol/L, with a normal ratio of ATP/ADP of approximately 1000.

  • Totally quantity of ATP in an adult is approximately 0.10 mol/L.
  • Approximately 100 to 150 mol/L of ATP are required daily, which means that each ATP molecule is recycled some 1000 to 1500 times per day.
  • Basically, the human body turns over its weight in ATP daily.[6]

References

  1. 1.0 1.1 1.2 Ask the scientist ATP Available from:https://askthescientists.com/cellular-energy-production/ (accessed 10.1.2021)
  2. 2.0 2.1 2.2 2.3 Biology wise ATP Available from:https://biologywise.com/what-is-atp (accessed 10.1.2021)
  3. 3.0 3.1 Khan academy Glycolysis Available from:https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/glycolysis/a/glycolysis (accessed 10.1.2021)
  4. 4.0 4.1 Define Biology Krebs cycle Available from:https://definebiology.com/krebs-cycle/ (accessed 10.1.2021)
  5. Kidzsearch ETC Available from:https://wiki.kidzsearch.com/wiki/Electron_transport_chain (accessed 10.1.2021)
  6. 6.0 6.1 Science direct ATP Available from:https://www.sciencedirect.com/topics/medicine-and-dentistry/adenosine-triphosphate (accessed 10.1.2021)