Formation of ATP: Adenosine triphosphate, or ATP, is the primary molecule at the cellular level that stores energy for future reactions or allows the cell to withdraw energy to carry out reactions during times of need. Through the breakdown of food, organisms obtain energy that is stored as ATP. Plants and certain autotrophic organisms also capture and convert light energy in the form of ATP during light reactions. ATP is formed by adding an adenine base to a ribose sugar, a ribose sugar attached to three phosphate groups. Two high-energy bonds, called phosphoanhydride bonds, connect the three phosphate groups.

During a process known as hydrolysis, one phosphate group is removed by breaking a phosphoanhydride bond, releasing energy, and converting ATP into adenosine diphosphate (ADP). As with ADP, energy is released when a phosphate group is removed, forming adenosine monophosphate (AMP). This energy can make unfavourable reactions in a cell favourable by transferring it to other molecules. The AMP can then be reprocessed into ADP or ATP by forming new phosphoanhydride bonds. ADP, ATP, and AMP are continuously converted in the cell by biological reactions. It was Lipmann who discovered the ATP cycle (ATP-ADP system). Lipmann is thus considered the father of the ATP cycle. In this article, we will learn in detail about the formation of ATP.

Formation of ATP During Respiration Process

When the glucose molecule has been broken down in the presence of oxygen in several stages to yield NADH₂, FADH₂, ATP, CO₂, and H₂O. Further, NADH₂ and FADH₂ have been oxidised to produce more ATP, and this process is called the electron transport system or oxidative phosphorylation.

A process of oxidative phosphorylation takes place in the mitochondrial inner membrane. It involves five protein complexes that remove electrons from NADH and FADH₂, regenerating NAD⁺ and FAD⁺ and creating a proton gradient across the membrane used to drive ATP synthesis.

Complex I

Complex I is formed of NADH with flavin mononucleotide (FMN) and an enzyme-containing iron-sulfur (Fe-S). It catalyses the oxidation of NADH. As two electrons pass through complex I, four protons are passed from the mitochondrial matrix into the intermembrane space.

Complex II

Complex II receives an electron from only FADH_2, and it does not pass any proton. Ubiquinone (Q) connects the first and second complexes to the third.

Complex III

Complex III is composed of cytochrome b, another Fe-S protein, and cytochrome c proteins; this entire complex is also called cytochrome oxidoreductase. This complex pumps 2 protons for every 2 electrons from the matrix of the mitochondria to the intermembrane space of mitochondria—the electron moves to cytochrome c to complex IV.

Complex IV

Complex IV is composed of cytochrome proteins c, a, and a₃. It also contains two heme groups and three copper ions. Cytochromes hold the oxygen molecule between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen picks up the two hydrogen ions from the matrix to produce water.

Complex V

Complex V is composed of ATPase or ATP synthase. It has two phases, F₀-F₁; F₀ is present in the membrane of the mitochondria, and F₁ is towards the matrix side. The proton moves from intermembrane space to the matrix through F₀-F₁. Every two protons will lead to the formation of 1 ATP.

Formation of ATP During Photosynthesis Process

ATP formation occurs in the thylakoid membrane of the chloroplast. It can be best explained by the chemiosmotic hypothesis.

Chemiosmotic Hypothesis

Peter Mitchell gave the chemiosmotic hypothesis to explain the mechanism of ATP synthesis. The proton gradient is developed across the membrane and the accumulation of protons inside the thylakoid membrane for ATP synthesis.

Mechanism

1. Splitting of water molecules occurs in the inner side of the thylakoid membrane. The protons produced accumulate within the lumen of the thylakoids.
2. As the electron moves from the photosystem, protons are also transported across the membrane. During proton movement, the primary acceptor of the electron on the outer side of the membrane transfers its electron to an H carrier (PQ-plastoquinone) rather than an electron carrier. The proton is transported from the stroma to the lumen of the thylakoid through the PQ.
3. Thylakoid membranes contain an enzyme called NADP reductase that is located on the stromal side. For the reduction of NADP⁺ to NADPH + H⁺, protons, as well as electrons, are required.

Hence, the proton concentration decreases into the chloroplast’s stroma. The accumulation of protons inside the lumen creates a proton gradient across the thylakoid membrane and reduces the pH in the lumen. In other words, it is the breakdown of this gradient that leads to the release of energy.

The gradient is released by the movement of protons across the thylakoid membrane and stroma of the chloroplast. This movement occurs via the CF₀ transmembrane channel of the ATPase. The ATPase enzyme consists of two parts: F₁ is embedded in the membrane and forms a transmembrane channel for the movement of protons across the membrane due to facilitated diffusion. The other portion is called F₁, present on the outer surface of the thylakoid membrane towards the stroma. After the gradient breaks down, the F₁ part of the ATPase undergoes a conformational change, allowing several molecules of ATP to be formed.

Summary

ATP or Adenosine triphosphate is an energy-storing molecule that has been utilised in various stages of the life cycle of plants and animals. During the light reaction, the thylakoid membrane involved in the chloroplast creates a proton gradient through the movement of electrons and hydrolysis of water; this gradient is released when the proton moves out through the F₀-F₁ complex leading to the formation of ATP. The NADH and FADH₂ formed by the breakdown of glucose molecules in glycolysis, link reaction, and Kreb cycle is oxidised into the inner mitochondrial membrane by the series of complex proteins. This movement occurs as a result of protons entering the intermembrane space, creating a proton gradient.

Frequently Asked Questions (FAQs) on Formation of ATP

Q.1. How much ATP is produced by the oxidation of one NADH?
Ans: Oxidation of one molecule of NADH will lead to the formation of 3 ATP. 

Q.2. In which stage of photosynthesis ATP is formed?
Ans: ATP is created in the light reaction of photosynthesis. 

Q.3. How much ATP is produced by the oxidation of one FADH₂?
Ans: Oxidation of one molecule of FADH₂ will lead to the formation of 2 ATP.

Q.4. Who has proposed the chemiosmotic hypothesis?
Ans: The chemiosmotic hypothesis was proposed by Peter Mitchell.

Q.5. What is the full form of ATP?
Ans: The full form of ATP is Adenosine triphosphate.

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