The cellular respiration may be divided into four stages:
- Pyruvic acid oxidation
- Kerbs cycle or citric acid cycle.
- Respiratory chain.
The first step of cellular respiration (glycolysis) occurs in the cytosol. The oxygen is not essential for glycolysis. The remaining three reactions occur in mitochondria. The oxygen is essential is 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 the glycolysis is pyruvic acid. The specific enzymes of the glycolysis, ATP and coenzyme NAD (nicotine amide adenine dinucleotide) are essential for glycolysis.
The breakdown of glucose takes place in 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 oxidative phase.
In this step, energy is used for the breakdown of glucose. It has 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 of each having three carbon atoms are formed. One is called 3-phospho- glyceraldehyde PGAL (or Glyceraldehye-3-phosphate G3P). The other molecule is dihydroxy acetone phosphate (DAP).
These molecules are isomers of each other. They can be easily interconverted by an enzyme.
Oxidative Phase (Payoff Phase)
This is very crucial phase of the glycolysis. Following reactions take place in this phase.
- Two electrons are removed from the molecule of the 3-phophoglyceraldehyde (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 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-phospho glycerate (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 Phosphoenol pyruvate (PEP) is formed.
- The Phosphoenol pyruvate gives up its high energy phosphate group. This phosphate molecule reacts with ADP. So a second molecule of ATP is formed. The product of this reaction is Pyruvate or Pyruvic acid (C3H4O3).
The pyruvic acid is equal to half 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 Kerbs cycle directly. Firstly, the pyruvic acid is changed into 2-Carbon Acetic Acid molecule. One carbon of pyruvate is released as CO2 (de-carboxylation). This acetic acid enters into the mitochondria. It unites with Coenzyme — A (CoA) and forms acetyl CoA (active acetate). More hydrogen atoms are transferred to NAD during this reaction.
III. Krebs Cycle (Cellular Respiration 3rd Stage)
Acetyl CoA enters into Kerbs cycle. It was discovered by Hans Kerbs. The Krebs cycle is a series of chemical reaction. It completes the oxidation process. There are 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. The 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 – ketoglutrate is formed.
- Further oxidation of α – ketoglutrate takes place. It products NADH + H+. It also undergoes de-carboxylation and adds a molecule of CO2. A molecule of water is also released. As a result, a molecule of succinate is produced. Free energy is released during this reaction. This energy is used for the synthesis of 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 (electron). These hydrogen atoms combine with NAD to form NADH2.
It is the last step of the Krebs cycle. Oxaloacetate is same original 4 — carbon compound from which 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 Kerbs cycle. The NADH transfers its hydrogen atoms to the respiratory chain. This respiratory chain is also called 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 chins.
- A coenzyme catted Coenzyme Q
- A series of cytochrome enzymes
- Molecular oxygen (O2)
These act as intermediates during the transport of electrons. They contain Haem group. It is related to prosthetic group of enzymes. The valency of the iron atom is changed during this transport. Haem is the same ion containing group which is present in the haemoglobin (oxygen carrying pigment).
The electron (H+) passes through following acceptors during 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 form 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 a 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 third molecule of ATP. So, the electrons are reached at 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 respiratory chain. Three ATP molecules are formed during three steps of the respiratory chain. This process can be expressed by following equation:
The Pi is an inorganic phosphate. The molecular mechanism of the 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, ATP molecule is synthesized during transport of electron through electron transport chain.
The inner membrane of the mitochondria is folded into cristae. These cristae have F1 particles. The protons (H+) are pumped from matrix into intermembrane space through this inner membrane. They come back from the intermembrane space into matrix and pass through the F1 particles. The F1 particles contain an ATP synthase enzyme. So it uses the energy of proton to synthesize the molecules of ATP.