Respiration refers to a set of reactions that occur in organisms to break down respiratory substances (such as glucose) into smaller inorganic molecules.
The processes of respiration release energy.
Some of this energy is used to make ATP.
ATP is the universal energy carrier found in all living cells.
When energy is needed in the body, ATP breaks down into ADP and a phosphate group by hydrolysis which releases energy.
Some biological processes in the body that require this energy are:
The structure of ATP consists of a ribose sugar, adenine and three phosphates.
The first stage of respiration is called glycolysis.
Glycolysis occurs in the cytoplasm of the cell.
Glycolysis is the breakdown of glucose into two molecules of pyruvate.
The stages of Glycolysis:
Therefore the products of Glycolysis are:
The link reaction occurs in the mitochondria.
The link reaction is the conversion of pyruvate into acetyl CoA.
Pyruvate is converted into Acetyl CoA by the removal of a molecule of CO2 and removal of its Hydrogen atoms which are accepted by the coenzyme NAD, reducing it.
The products of the link reaction are:
The Krebs Cycle occurs in the matrix of the mitochondria.
The Krebs Cycle is a series of reactions that break down Acetyl CoA into CO2 and Hydrogen atoms.
The Krebs cycle undergoes two cycles per glucose molecule, because one molecule of glucose produces two molecules of pyruvate.
The products of the Krebs Cycle per molecule of glucose are:
Oxidative Phosphorylation occurs in the inner membrane of the mitochondria.
Oxidative Phosphorylation is the production of ATP from the Hydrogen atoms produced in Glycolysis, the Link Reaction and the Krebs Cycle.
Oxidative Phosphorylation is the final stage of respiration.
In oxidative phosphorylation, the reduced NAD and reduced FAD from the previous stages of respiration give up their Hydrogen atoms.
The Hydrogen atoms are split into H+ ions and electrons.
The electrons are passed along the electron transport chain, which is a series of proteins in the inner membrane of the mitochondria.
As the electrons pass along it, they release energy.
This energy is used to pump H+ ions into the intermembrane space.
This creates a concentration gradient of H+ ions.
The H+ ions move along their concentration gradient, through an ATP synthase enzyme, powering it which causes it to produce ATP from ADP and a phosphate.
Oxygen acts as the final electron and hydrogen acceptor, forming water.
The products of oxidative phosphorylation are:
Anaerobic respiration is the breakdown of glucose in the absence of oxygen.
If oxygen is absent, then there is no final electron or hydrogen acceptor at the end of oxidative phosphorylation.
Therefore, the concentration of H+ ions builds up in the matrix, and reduces the H+ ion gradient across the inner membrane to the point at which oxidative phosphorylation stops.
Reduced NAD and reduced FAD can't unload their Hydrogen atoms, and can't be reoxidised.
So, the Krebs cycle and Link reaction stops.
Therefore, the only stage of respiration that can occur in the absence of oxygen is glycolysis.
However, the reduced NAD still needs to be reoxidised for use in glycolysis.
In plants and fungi, reduced NAD is reoxidised by fermentation:
In animals, reduced NAD is reoxidised by lactate fermentation:
Lactate is produced in muscle tissue, and is then carried in the blood to the liver. When oxygen is available, the lactate may be either converted to pyruvate to then enter the Krebs cycle by the link reaction, or recycled to glucose and glycogen.
If lactate was not removed from the muscle tissues, the pH would be lowered, inhibiting the action of many enzymes.
Glucose is the main respiratory substrate, but other substrates can be used for respiration.
However, blood cells and brain cells can only respire with glucose.
Other Carbohydrates can be used for respiration.
A monosaccharide can be converted to glucose by isomerase enzymes and then respired.
A disaccharide can be converted to a monosaccharide by hydrolysis and then converted to glucose by isomerase enzymes and then respired.
Lipids can be respired.
Lipids are broken down into fatty acids and glycerol.
Glycerol can be converted to triose phosphate and then respired.
Fatty acids can be converted to Acetyl CoA and then respired.
Proteins can be respired.
Proteins are broken down into amino acids.
Some amino acids can be converted to pyruvate and then respired.
Other amino acids can be converted to Acetyl CoA and then respired.