This generates some ATP through substrate phosphorylation catalyzed by two enzymes: PGK and
Pyruvate kinase. Pyruvate is then oxidised further in the mitochondrion.
In the mitochondrion, pyruvate is oxidised by pyruvate dehydrogenase to acetyl-CoA, which is
fully oxidised to carbon dioxide by the Krebs cycle. Fatty acids are also broken down to acetyl CoA by
beta-oxidation and metabolised by the Krebs cycle. Every turn of the Krebs cycle produces an ATP
equivalent (GTP) through substrate phosphorylation catalyzed by Succinyl-CoA synthetase as well as
reducing power as NADH. The electrons from NADH are used by the electron transport chain to generate
a large amount of ATP by oxidative phosphorylation coupled with ATP synthase.
The whole process of oxidising glucose to carbon dioxide is known as cellular respiration and is
more than 40% efficient at transfering the chemical energy in glucose to the more useful form of ATP.
ATP is also synthesized through several so-called "replenishment" reactions catalyzed by the
enzyme families of NDKs (nucleoside diphosphate kinases), which use other nucleoside triphosphates as a
high-energy phosphate donor, and the ATP:guanido-phosphotransferase family, which uses creatine.
ADP + GTP ATP + GDP
In plants, ATP is synthesized in chloroplasts during the light reactions of photosynthesis. Some of
this ATP is then used to power the Calvin cycle, which synthesizes triose sugars.
If a clot causes a decrease in oxygen delivery to the cell, the amount of ATP produced in the mitochondria
will decrease.
ATP is used for many cell functions including transport work moving substances across cell
membranes. It is also used for mechanical work, supplying the energy needed for muscle contraction. It
supplies energy not only to heart muscle and skeletal muscle, but also to the chromosomes and flagella to
enable them to carry out their many functions. A major role of ATP is in chemical work, supplying the
needed energy to synthesize the multi-thousands of types of macromolecules that the cell needs to exist.
ATP is also used as an on-off switch both to control chemical reactions and to send messages. The
shape of the protein chains that produce the building blocks and other structures used in life is mostly
determined by weak chemical bonds that are easily broken and remade. These chains can shorten,
lengthen, and change shape in response to the input or withdrawal of energy. The changes in the chains
alter the shape of the protein and can also alter its function or cause it to become either active or inactive.
The ATP molecule can bond to one part of a protein molecule, causing another part of the same
molecule to slide or move slightly which causes it to change its conformation, inactivating the molecule.
Subsequent removal of ATP causes the protein to return to its original shape, and thus it is again functional.
The cycle can be repeated until the molecule is recycled, effectively serving as an on and off switch.
