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:
- Pyruvic acid oxidation
- Curbs cycle or citric acid cycle.
- Respiratory chain.
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.
I. Glycolysis (Cellular Respiration 1st Stage)
“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.
- The first step in glycolysis is the transfer of a phosphate group from ATP to glucose. As result, a molecule of glucose-6-phosphate is formed.
- The glucose-6-phosphate is changed into its isomer fructose-6-phosphate. This reaction is catalyzed by an enzyme.
- Another molecule of ATP transfers its one phosphate group to fructose-6-phosphate. Now, this molecule becomes fructose — 1, 6-bisphosphate.
- An enzyme splits this molecule of fructose — 1, 6-bisphosphate. Two molecules each having three carbon atoms are formed. One is called 3-phospho- glyceraldehyde PGAL (or Glyceraldehyde-3-phosphate G3P). The other molecule is dihydroxyacetone phosphate (DAP).
These molecules are isomers of each other. They can be easily interconverted by an enzyme.
Oxidative Phase (Payoff Phase)
This is a very crucial phase of glycolysis. The following reactions take place in this phase.
- Two electrons are removed from the molecule of the 3-phoglyceraldehyde (PGAL). These electrons (H+) are transferred to NAD. This is an oxidation-reduction reaction. In this case, the PGAL is oxidized by donating electrons and the NAD is reduced by receiving electrons. An inorganic molecule reacts with the PGAL. It becomes 1, 3-biphosphoglycerate (BGP).
The oxidation of PGAL is an energy-yielding process. Thus a high-energy phosphate bond is created in this molecule.
- The high-energy phosphate molecule is transferred to a molecule of ADP. This ADP becomes ATP. The end product of this reaction is 3-phosphoglycerate (3-PG).
- In this step, the 3- PG is converted into 2- Phosphoglyceric acid (2PG).
- A molecule of water is removed from 2-PG and Phosphoenolpyruvate (PEP) is formed.
- The Phosphoenol pyruvate gives up its high-energy phosphate group. This phosphate molecule reacts with ADP. So the second molecule of ATP is formed. The product of this reaction is Pyruvate or Pyruvic acid (C3H4O3).
The pyruvic acid is equal to half the glucose molecule. This glucose was oxidized by losing two electrons (H+)
II. Pyruvic Acid Oxidation (Cellular Respiration 2nd Stage)
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.
III. Krebs Cycle (Cellular Respiration 3rd Stage)
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:
- The acetyl CoA unites with the oxaloacetate to form citrate. In this reaction, a molecule of CoA is released and one molecule of water is used. Oxaloacetate is a 4 — carbon acid. So the citrate has 6-carbon atoms.
- In this reaction, the citric acid is changed into its isomer called iso-citric acid.
- The iso-citric acid is oxidized with the help of NAD. It also releases a molecule of CO2. As a result, α – ketoglutarate is formed.
- Further oxidation of α – ketoglutarate takes place. It produces NADH + H+. It also undergoes de-carboxylation and adds a molecule of CO2. A molecule of water is also released. [wp_ad_camp_1] As a result, a molecule of succinate is produced. Free energy is released during this reaction. This energy is used for the synthesis of the ATP molecule.
- In this step, the succinate is oxidized to form fumarate. The oxidizing agent in this reaction is Flavin adenine dinucleotide (FAD) Two hydrogen atoms are released during this reaction. They combine with FAD to form FADH2.
- The fumarate combines with a molecule of water to form malate.
- The malate undergoes oxidation to form oxaloacetate. It releases hydrogen atoms (electrons). These hydrogen atoms combine with NAD to form NADH2.
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.
IV. Respiratory Chain (Cellular Respiration 4th Stage)
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.
- A coenzyme catted Coenzyme Q
- A series of cytochrome enzymes
- Molecular oxygen (O2)
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 NADH transfers its electron to Coenzyme Q. So this NADH is oxidized. This oxidation releases energy. This energy is used for the synthesis of a molecule of ATP from ADP and inorganic phosphate.
- The Coenzyme Q is then oxidized by Cytochrome b.
- The cytochrome b is oxidized by Cytochrome c. This step also releases energy for the synthesis of ATP.
- The cytochrome c then reduces a complex of two enzymes called cytochrome a & a3. This complex is commonly called cytochrome a.
- The cytochrome complex is oxidized by an atom of oxygen and a molecule of water is formed. This oxidation releases energy. This energy is used for the synthesis of the third molecule of ATP. So, the electrons are reached the bottom end of the respiratory chain. Oxygen is the most electronegative substance. It is the final acceptor of the electron.
“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.