The Mechanism of Cellular Respiration

Cellular respiration:

It is divided into few stages

Glycolysis

Pyruvic acid oxidation

Krebs’s cycle /citric acid cycle

Respiratory chain

• 1.       Glycolysis:

Glycol means “Glucose” & lysis means “breakdown”. So this is the process of glucose break down & formation of Pyruvic acid. It can take place in absence/ presence of oxygen, both resulting in same product. A series of steps involved in Glycolysis require specific enzymes.

Glycolysis can be divided into two phases:

        i.            Preparatory Phase

      ii.            Oxidative Phase

Preparatory phase

The first step in Glycolysis in the transfer of phosphate group from ATP to the 6^th carbon atom of glucose; as a result a molecule of glucose 6 phosphate is formed. An enzyme catalyzes the conversion of glucose 6 phosphate to its isomer, fructose 6 phosphate. At this stage another ATP molecule transfer a second phosphate group at 1^st carbon atom of the glucose. The product is fructose-1, 6-biphosphate. The next step in Glycolysis is the enzymatic splitting of fructose-1, 6-diphosphate into 3-phospoglyceraldehyeand dihydroxy acetone phosphate. These two molecules are isomers and are readily interconnected by enzymes.

Oxidative Phase

In this phase two electrons or two hydrogen atoms are removed from the molecule of 3-phosphoglyceraldehde (PGAL) and transferred to a molecule of NAD. During this reaction of second phosphate group is donated to the molecule, which resulted in the formation of 1, 3-diphosphoglyceric acid (DPGA). The oxidation of PGAL is an energy yielding process. At the very next in Glycolysis ATP is formed. The end product of this reaction is 3-phosphoglyceric acid. In the next step 3-phosphoglyceric acid is converted to 2phosphoglyceric acid. From 2-phosphoglyceric acid a molecule of water is removed and the product is phosphoenol Pyruvic acid (PEP). PEP then gives up its high-energy phosphate to convert a second molecule of ADP to ATP. The product is Pyruvic acid (C₃H₄O₃).

• 2.       Pyruvic acid Oxidation:

Pyruvic acid, the end product of Glycolysis does not enter the Krebs cycle directly. The Pyruvic acid is first changed into 2-carbon acetic acid molecule. One carbon is released as Co₂. Acetic acid on entering the mitochondrion unites with coenzymes A (CoA) to form acetyl CoA. In addition more hydrogen is transferred to NAD.

• 3.       Krebs’s Cycle:

Sir Hans Krebs discovered Krebs’s cycle. It starts after the formation of Acetyl CO-A. Krebs cycle takes place in mitochondrial membrane & comprises of following steps:

         i.            The union of acetyl CoA with oxaloacetate to form citrate. In this process molecule of CoA is regenerated and one molecule of water is used. Oxaloacetate is a 4-carbon acid with two carboxyl groups. Citrate thus has 6-carbon atoms and three carboxyl groups.

       ii.            In the next reactions NAD mediated oxidation takes place and citrate is changed into Ketoglutarate.

      iii.            Ketoglutarate is then oxidized & decarboxylated simultaneously. Thus a new product Succinate is formed. One ATP molecule is also synthesized.

     iv.            The next step in the Krebs cycle is the oxidation of Succinate to fumarate. Once again, two hydrogen atoms are moved, but this time the oxidizing agent is a coenzyme called flavin adenine dinucloetide (FAD), which is reduced to FADH₂.

       v.            With the addition of another molecule of water, fumigate is converted to malate.

     vi.            Anther NAD mediated oxidation of malate produces oxaloacetate, the original 4-carbon molecule.

• 4.       Respiratory Chain

NADH formed in Krebs’s cycle transfers its hydrogen atom to the electron carriers of respiratory chains. This transfer brings about transport of electron down to all carriers resulting in a series of reduction oxidation process & ultimately releasing O₂ & water is formed. Electron carriers are

         i.            CoenzymeQ

       ii.            Series of cytochromes enzymes

      iii.            Molecular oxygen

There are two types of Aerobic Respiration

• 1.       External Respiration

In this stage the organisms take the air (containing oxygen) into their bodies. This stage includes the transport of oxygen obtained from the inhaled oxygen to each cell of the body.

• 2.       Internal Respiration

The second stage, which is called internal respiration, consists of the oxidation of glucose, amino acid and fatty acids etc., with molecular oxygen. This respiration is also known as cellular respiration is also known cellular respiration as it occurs within cells.

In the internal or cellular respiration glucose and other compounds are passed through such enzymatic reactions, which release the chemical energy gradually in small amounts, with the help of which ATP molecules are.

There are two method of respiration in the organisms.

Anaerobic Respiration

Some organisms oxidize their food without using any molecular oxygen. This is known as anaerobic respiration.

In this type of respiration considerably less amount of energy is produced as compared with the other type of respiration. It is also called fermentation.

In anaerobic respiration, a glucose molecule is broken down into two molecules of lactic acid with a release of only 47,000 calories of energy.

Glucose→2Lactic acid+ Energy (47,000 calories)

        i.            Holic Fermentation

In primitive cells and in some eukaryotic cells such as yeast, Pyruvic acid is further broken down by alcoholic fermentation into alcohol (C2H5 OH) and

CO₂

2(C₃H₄O₃)       2(C₂H₅OH)  +    2CO₂

Pyruvic acid       Alcohol

      ii.            Actic acid fermentation

In lactic acid fermentation, each Pyruvic acid molecule is converted in to lactic acid C₃ H₆ O₃ in the absence of oxygen gas.

2(C₃H₄O₃)     +      4H      2(C₃H₄O₃)

Pyruvic acid                          Lactic acid

Both alcoholic and lactic acid fermentation yield about 2% of the energy present within the chemical bonds of glucose, which is converted into adenosine triphosphate (ATP).

Respiration

Definition: Living organisms need energy, which is provided by the phenomenon of respiration. It is the process by which organism’s breakdown complex compounds containing carbon to get a maximum of usable energy. Generally respiration means the exchange of respiratory gases (CO2 and O2) between the organism and its environment. This exchange is called external respiration, which is followed by cellular respiration. Cellular respiration is the process by which energy is made available to cells in a step-be-step breakdown of C-chain molecules in the cells.

Aerobic Respiration

In most of the higher and larger organisms, the glucose etc. is oxidized by using molecular oxygen. This type of respiration is known as aerobic respiration.

In aerobic respiration a mole of glucose is oxidized completely into carbon dioxide and water releasing enormous amount of energy. One glucose molecule in this respiration produces 686, 000 calories of energy. Aerobic respiration thus produces 20 times more energy than the anaerobic respiration.