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Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In nearly every living thing on earth, the energy comes from the metabolism of glucose. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.

Phosphorylation

Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows one of the substrates (such as ATP) and reactants to more readily react with each other; in reactions involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (~P). This is illustrated by the following generic reaction:

A + enzyme + ATP   [  enzyme    P ]    B + enzyme + ADP + phosphate ion

When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism.

Substrate phosphorylation

ATP is generated through two mechanisms during the breakdown of glucose. A few ATP molecules are generated (that is, regenerated from ADP) as a direct result of the chemical reactions that occur in the catabolic pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the free energy of the reaction is used to add the third phosphate to an available ADP molecule, producing ATP ( [link] ). This very direct method of phosphorylation is called substrate-level phosphorylation    .

This illustration shows a substrate-level phosphorylation reaction in which the gamma phosphate of ATP is attached to a protein.
In phosphorylation reactions, the gamma phosphate of ATP is attached to a protein.

Oxidative phosphorylation

Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process, chemiosmosis, which takes place in mitochondria ( [link] ) within a eukaryotic cell or the plasma membrane of a prokaryotic cell. Chemiosmosis , a process of ATP production in cellular metabolism, is used to generate 90 percent of the ATP made during glucose catabolism and is also the method used in the light reactions of photosynthesis to harness the energy of sunlight. The production of ATP using the process of chemiosmosis is called oxidative phosphorylation    because of the involvement of oxygen in the process.

This illustration shows the structure of a mitochondrion, which has an outer membrane and an inner membrane. The inner membrane has many folds, called cristae. The space between the outer membrane and the inner membrane is called the intermembrane space, and the central space of the mitochondrion is called the matrix. ATP synthase enzymes and the electron transport chain are located in the inner membrane
In eukaryotes, oxidative phosphorylation takes place in mitochondria. In prokaryotes, this process takes place in the plasma membrane. (Credit: modification of work by Mariana Ruiz Villareal)

Career connections

Mitochondrial disease physician

What happens when the critical reactions of cellular respiration do not proceed correctly? Mitochondrial diseases are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation but not the other steps of respiration. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial diseases, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disease.

Section summary

ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced. Energy derived from glucose catabolism is used to convert ADP into ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.

Questions & Answers

what is electrochemical gradient?
Joneth Reply
what is mitosis?
Aliness Reply
(cytology) the division of cell nucleus in which the genome is copied and separated into two identical halves.it is normally follow by cell division
Adekunle
it Is the division of the mother cell into two daughter cells,genetically identical to each other and to their parent cell
Joneth
it is a study of living organisms
Prudence Reply
What are other types of cell
ATAMA Reply
plant and animal cell
Jessy
prokaryotic and eukaryotic cell
Joneth
meaning inside and outside cells
Joneth
external feature of earthworm
Ajas Reply
what is the reason behind that before F2 generation is no segregation?
asmamaw Reply
what is mitosis and meiosis
Rabiu Reply
hypothesis theory law
Tamba Reply
what is hypothesis theory law
Tamba
Briefly describe the process of mitosis and meiosis.
Lilian Reply
what is chromosome
Deborah Reply
Chromosome is the thread-like structure containing DNA and found in the nucleus of a cell. DNA contains gene which has genetic information.
Lilian
if a cell is killed by strain why then is it use
uchenna Reply
what is an atom
Finda Reply
an atom is the must smallest element in the world.
Young
please help me friends. I wont a good example of lab report based on carbohydrate,lipid,reducing and non reducing sugar.
Ng
that can't be split by any chemical means
uchenna
No an atom is the smallest particles of an element which can take part in a chemical reaction
Solayemi
An atom is the smallest indivisible particle of an element which can take part in a chemical reaction.
Samuel
sure atoms can't be splited
Solayemi
atom can't be split
john
what is molecule
john
Molecules consists of one or more tiny particles called atom
Rofiah
atoms joined together by covalent bonds
Bad
an atom is a particle that cannot be divided further by a chemical reaction
Bad
how do earthworms excrete
Ndlovu
what is genotype
Precious Reply
What is the difference between longitudinal section and vertical section
dela Reply
what is a cell
Young Reply
A cell is the basic unit of Life
Sondja
And what is life...?
ShAmy
it's being.
ernest
how many layers does the eye have
Gift
Three
Winifred
Cell is the basic unit of life
dela
what is virus
Khalid
building block of life
Stanisla
what is diffusion
Precious

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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