Cellular respiration is a set of biochemical reactions that takes place in most cells. It involves the splitting of pyruvic acid (produced by glycolysis) into carbon dioxide and water, along with the production of adenosine triphosphate (ATP) molecules. In other words, cellular respiration involves a metabolic process by which cells reduce oxygen and produce energy and water. These reactions are essential for cellular nutrition.
Cellular respiration may be divided into four stages:
The first step of cellular respiration (glycolysis) occurs in the cytosol. Oxygen is not essential for glycolysis. The remaining three reactions occur in mitochondria. Oxygen is essential for these reactions.
“The breakdown of glucose up to the formation of Pyruvic acid is called glycolysis.” Glycolysis can take place both in the absence (anaerobic condition) or in the presence of oxygen (aerobic condition). In both cases, the product of glycolysis is pyruvic acid. The specific enzymes of glycolysis, ATP, and coenzyme NAD (nicotine amide adenine dinucleotide) are essential for glycolysis.
The breakdown of glucose takes place in a series of steps. Each step is catalyzed by a specific enzyme. All the enzymes are present in dissolved form in the cytosol.
Glycolysis can be divided into two phases: a preparatory phase and an oxidative phase.
In this step, energy is used for the breakdown of glucose. It has the following steps.
These molecules are isomers of each other. They can be easily interconverted by an enzyme.
This is a very crucial phase of glycolysis. The following reactions take place in this phase.
The oxidation of PGAL is an energy-yielding process. Thus a high-energy phosphate bond is created in this molecule.
The pyruvic acid is equal to half the glucose molecule. This glucose was oxidized by losing two electrons (H+)
Pyruvic acid (pyruvate) is the end product of glycolysis. It is a three-carbon compound. It does not enter into the Kerbs cycle directly. Firstly, the pyruvic acid is changed into a 2-Carbon Acetic Acid molecule. One carbon of pyruvate is released as CO2 (de-carboxylation). This acetic acid enters the mitochondria.
It unites with Coenzyme — A (CoA) and forms acetyl CoA (active acetate). More hydrogen atoms are transferred to NAD during this reaction.
Acetyl CoA enters into Kerb’s cycle. It was discovered by Hans Kerbs. The Krebs cycle is a series of chemical reactions. It completes the oxidation process. There are the following steps in the Kerbs cycle:
It is the last step of the Krebs cycle. Oxaloacetate is the same original 4 — carbon compound from which the Kerbs cycle was started. Now this oxaloacetate is ready to combine with another molecule of acetyl CoA to start another cycle.
NADH and H+ are produced during the Kerbs cycle. The NADH transfers its hydrogen atoms to the respiratory chain. This respiratory chain is also called an electron transport chain. This respiratory chain is present in the inner membrane of the mitochondria. The electrons (hydrogen atoms) are transferred in a series of oxidation steps. These hydrogen atoms finally react with molecular oxygen to form a molecule of water.
Following oxidation-reduction substances take part in the respiratory chains.
These act as intermediates during the transport of electrons. They contain the Haem group. It is related to the prosthetic group of enzymes. The valency of the iron atom is changed during this transport. Haem is the same iron-containing group that is present in the hemoglobin (oxygen-carrying pigment).
The electron (H+) passes through the following acceptors during the respiratory chain.
“The synthesis of ATP molecule in the presence of oxygen is called oxidative phosphorylation.” Normally, oxidative phosphorylation takes place during the respiratory chain. Three ATP molecules are formed during three steps of the respiratory chain. This process can be expressed by the following equation:
The Pi is inorganic phosphate. The molecular mechanism of oxidative phosphorylation is associated with the respiratory chain. These respiratory chains are present in the inner membrane of mitochondria.
The mechanism of oxidative phosphorylation is chemiosmosis (like photosynthesis). In this mechanism of chemiosmosis, the ATP molecule is synthesized during the transport of electrons through the electron transport chain.
The inner membrane of the mitochondria is folded into cristae. These cristae have F1 particles. The protons (H+) are pumped from the matrix into intermembrane space through this inner membrane. They come back from the intermembrane space into the matrix and pass through the F1 particles. The F1 particles contain an ATP synthase enzyme. So it uses the energy of the proton to synthesize the molecules of ATP.
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