Formation of ATP in Plants

Adenine tri-phosphate, known as ATP, is the universal currency of energy in all living organisms. It was first isolated in the early 1930s, having been extracted from muscle tissue. In plant cells, ATP is needed for both respiration and photosynthesis, and is formed during different processes.

In respiration, organic molecules act as fuel. These are broken down in a series of reactions, and chemical potential energy is used to synthesise ATP. The synthesis of ATP involves attaching a phosphate group to ADP. The energy needed for this comes from the oxidation of glucose.

The breakdown of glucose can be split into two parts, glycolysis and the Krebs cycle. In glycolysis, the hexose sugar is converted into pyruvate, which has three carbon atoms. This leads to dehydrogenation, when two hydrogen atoms are removed from the triose by a dehydrogenase enzyme. These hydrogen atoms are taken up by a hydrogen carrier and leads to the synthesis of ATP from ADP and inorganic phosphate.

At the same time, the phosphorylated triose is converted to pyruvate, via a series of steps. Two of these steps are directly linked to the formation of ATP. In both cases, the substrate is at a much higher energy level than pyruvate, and sufficient energy is transferred for the synthesis of ATP.

The pyruvate then enters a mitochondrion, where it is converted into acetyl coenyzme A, a 2-carbon compound. Carbon dioxide is given off and the pyruvate loses a pair of hydrogen atoms which again results in the synthesis of ATP.

During the Krebs cycle, Acetyl coenyzme A (2-carbon molecule) reacts with a 4-carbon organic compound called oxaloacetate to form citrate, with six carbon atoms. A series of reactions follow in which the citrate is gradually converted back to oxaloacetate. Four of these steps involve the removal of a pair of hydrogen atoms (dehydrogenation) leading to the ...

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Altogether, the complete oxidation of one molecule of the hexose sugar can yield a net total of 38 molecules of ATP. 30 are produced by the Krebs cycle whereas only eight are produced in glycolysis.

In the first two stages of respiration, hydrogen atoms are removed at various stages, which lead to the synthesis of ATP during oxidative phosphorylation and the electron transport system. The hydrogen atoms are passed along a series of hydrogen carriers, known as a hydrogen carrier system. The system is a series of coupled redox reactions. When the first carrier accepts hydrogen atoms it becomes reduced, then when the hydrogen atom is transferred to the next carrier, the latter is reduced and the first carrier becomes reoxidised. At each stage, sufficient energy is transferred to produce a molecule of ATP.

The carriers are located in the inner mitochondrial membrane and this has led to the explanation of how ATP is produced. Hydrogen atoms are picked up by the initial carrier (NAD) on the matrix side of the membrane. When hydrogen atoms split into protons and electrons, the electrons are taken up by the cytochromes but the protons are moved across to the other side of the membrane and deposited in the narrow gap. This results in an electrochemical gradient being set up across the inner membrane. This gradient provides the energy for the synthesis of ATP. This explanation for the production of ATP is known as the chemiosmotic theory, put forward by Peter Mitchell.

Another important process which produces ATP in plants is photosynthesis, the fixation of carbon dioxide and reduction to carbohydrate using hydrogen from water. It takes place in three main stages, light harvesting, electron transport and reduction of carbon dioxide. The first two stages require light but the third stage does not. The elctron transport stage is mainly responsible for the production of ATP.

As a result of the transfer of electrons, two products are formed. They are NADPH and ATP. For ATP to be synthesised, ADP and inorganic phosphate must be present. Light is absorbed by both photosystems and excited electrons are emited from the primary pigments of both reaction centres. As a result of electron flow from PS II to PS I in the thylakoid membranes, there is an accumulation of hydrogen ions inside the thylakoid, creating a gradient. The passage of hydrogen ions out of the thylakoids provides the energy for ATP to be synthesised in the poresence of ATPase. This is non-cyclic photophosphorylation.

Soemtimes PS I is both the donor and acceptor of electrons, therefore the electrons follow a different route. The process only involves PS I, and light is absorbed by the photosystem and passed to chlorophyll a. An electron in the chlorophyll a molecule is excited to a higher energy level and is emitted from the chlorophyll molecule. It is then captured by an electron acceptor and passed back into a chlorophyll a molecule via a chain of electron carriers. During this process, energy is released to synthesise ATP. This is known as cyclic photophosphorylation.

In both cyclic and non-cyclic photophosphorylation, energy is derived from an electrochemical gradient resulting form the movement of hydrogen ions across the thylakoid membrane. Therefore both processes are applications of the chemiosmotic theory.

In conclusion, the formation of ATP occurs in multiple reactions in plants, during the processes of both respiration and photosynthesis.