Cellular respiration , Structure of ATP and types of fermentation

Gas exchange is the process of obtaining oxygen either directly from the air as in the case of unicellular organisms or by a respiratory system as in the case of multicellular organisms and releasing CO₂ as a final product of respiration .

Cellular respiration

Cellular respiration is the process by which the living organism’s cells extract the energy stored in the chemical bonds of food molecules , especially sugars ( glucose ) that are manufactured by the plants or eaten by animals , then this energy is stored in the form of ATP  molecules to be used in performing the different activities .

The cellular respiration starts by the oxidation of glucose molecule , The glucose molecule is considered as an excellent example to study the steps of breaking down the food molecules , as it is used commonly by the majority of living organisms to produce energy more than any other molecules of available food , Most stages of oxidation process of glucose molecule occur inside the mitochondria .

Glucose ( as well as other carbohydrates ) is considered as a form of stored energy , also it is a form by which the energy transfers from one cell to another and from one living organism to another .

Structure of ATP

In order to understand how ATP performs its function , we have to study its structure , The molecule is built up of three sub-units which are :

1. Adenine : a nitrogenous base ( it has the properties of a base ) .
2. Ribose : a 5-carbon pentose sugar .
3. Phosphate group : each molecule of ATP has three phosphate groups linked together by bonds .

ATP molecules are considered as the universal currency of energy in the cell , as any energy required by a cell needs ATP which changes into ADP ( adenosine diphosphate ) , an amount of energy ( which is about 7-12 kcal/mole ) is released .

Aerobic cellular respiration

It is the principal route for obtaining the energy in the majority of living organisms in the presence of oxygen , The oxidation of one mole of glucose produces energy of 38ATP , this can be illustrated by the following equation .

C₂H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O  + 38 ATP

The oxidation of glucose molecule takes place in three major stages :

1. Glycolysis occurs in the cytosole of cell .
2. Krebs cycle occurs inside the mitochondria .
3. Electron transport chain occurs inside the mitochondria , as it the mitochondria contain the respiratory enzymes , water , co-enzymes , phosphate groups , Electron carries molecules or Cytochromes that carry the electrons at different energy levels , where the hydrogen atoms are removed from the carbon skeleton of glucose molecule to be passed to the co-enzymes which act as the hydrogen carries ( as NAD⁺ and FAD ) .   

The most important co-enzymes are :

NAD⁺ which is reduced into NADH

NAD⁺ + H₂ → NSDH + H⁺

FAD which is reduced into FADH₂

FAD + H₂ → FADH₂

Glycolysis

Glycolysis occurs in the cytosole ( non-organelle part cytoplasm ) of the cell , It takes place in both aerobic and anaerobic respiration to produce energy , where the stages of glucose breakdown occur in the absence ( or lack ) of oxygen , so , these reactions are called anaerobic respiration .

The glycolysis of glucose occurs as the following : One molecule of glucose is broken down into two molecules of pyruvic acid ( 3-carbon ) through a group of reactions .

1. Glucose is converted into glucose 6-phosphate .
2. Glucose 6-phosphate is converted into fructose 6-phosphate , then fructose 1,6-diphosphate which forms two molecules of PGAL ( phosphoglyceraldehyde ) to be oxidized into two pyruvic molecules .
   

These reactions are accompanied by ( for each molecule of glucose ) :

Reduction of two molecules of the co-enzymes NAD⁺

2 NAD⁺ → 2NADH

Production of two molecules of ATP in the cytosole of the cell .

The equation of reaction :

C₂H₁₂O₆ → 2 C₃H₄O₃ + 2 ATP                 ( Anaerobic respiration , respiratory enzymes )

The energy produced :

Two molecules of ATP which are not enough to perform all the vital activities in the living organisms , Therefore , in the presence of oxygen , the pyruvic acid passes into the mitochondria to produce more energy , this takes place in two stages which are Krebs cycle and Electron transport chain .

The importance of glycolysis : The production of two molecules of ATP , The obtaining of pyruvic acid which is used in both aerobic and anaerobic respiration .

Krebs cycle

The first scientist who described this cycle was Sir Hans Krebs in 1937 , He won Noble prize in 1953 , It occurs inside the mitochondria , Before the entrance into Krebs cycle , the following reaction occurs :

Each molecule of pyruvic acid is oxidized in the presence of co-enzyme A into acetyl co-A and produces : 2 molecules of NADH , 2 molecules of CO₂ .

The acetyl groups resulting from the breaking down of fat molecules or protein molecules can combine with the co-A to join Krebs cycle .

Krebs cycle takes place in many stages , which are :

1. Each molecule of acetyl co-A joins Krebs cycle , where its co-A splits off to repeat its role .
2. The acetyl group ( 2C ) combines with a 4-carbon compound ( oxaloacetic acid ) , to form a 6-carbon compound ( citric acid ) .
3. The citric acid passes through three intermediate compounds , It starts by the ketoglutaric acid ( 5C ) , then succinic acid ( 4C ) , then malic acid ( 4C ) and at the end of reactions the citric acid is formed again , therefore Krebs cycle is called the citric acid cycle .

During one cycle , all these compounds are produced :

1. Two molecules of CO₂ .
2. Three molecules of NADH .
3. One molecules of FADH₂ .
4. One molecule of ATP .

Krebs cycle is repeated twice ( one for each molecule of acetyl group ) , Krebs cycle does not need oxygen , because all the electrons and protons are removed during the oxidation of carbon atoms and received by NAD⁺and FAD molecules .

The importance of Krebs cycle :

The oxidation of carbon atoms through a group of reactions by removing the electrons through the intermediate compounds which are received by the co-enzymes ( NAD⁺and FAD ) and then these co-enzymes transfer them to the cytochromes to release the energy and form ATP molecules .

Electron transport chain

The terminal stage of aerobic respiration that starts at the end of Krebs cycle , It occurs inside the mitochondria , Electron transport chain occurs in many stages as the following :

Hydrogen and high-energy electrons carried by NAD⁺and FAD are transported over certain sequences of co-enzymes called cytochromes ( electron carriers ) that are present inside the inner membrane of mitochondria .

Cytochromes are sequences of co-enzymes present in the inner membrane of mitochondria that carry the electrons at different energy levels , The cytochromes carry the electrons at different energy levels , These high-energy electrons are passed from one molecule of cytochrome to another , during that the energy is released to form ATP from ADP and a phosphate group , This process is called oxidative phosphorylation .

Oxidative phosphorylation is the process by which ATP is formed from ADP and a phosphate group by using the energy released during the passage of electrons ( that are carried on cytochromes ) from a high-energy level to a low-energy level .

Two electrons combine with 2H⁺ and one oxygen atom to form a water molecule , So , oxygen is considered as the last receptor in the electron transport chain .

2é + 2H⁺ + ½ O₂ → H₂O

Each NADH molecule produces three molecules of ATP , while each FADH₂ molecule produces two molecules of ATP .

The importance of electron transport chain : Releasing the energy stored in NADH and FADH₂ through the passage of electrons over a sequence of cytochromes and using produced energy to form ATP from ADP .

Calculation of ATP

In aerobic respiration , each molecule of glucose produces 38ATP :

Two molecules in the cytoplasm of cell ( produced during the glycolysis ) , 36 molecules in the mitochondria ( during the respiration stage ) .

Anaerobic cellular respiration

Anaerobic respiration  ( Fermentation ) is a process by which the living organism obtains energy from the food molecule ( glucose ) in the absence or lack of oxygen by the help of special enzymes and this produces a small quantity of energy ( 2ATP molecules ) .

Stages of anaerobic respiration ( fermentation ) .

1. Glucose is decomposed into two molecules of pyruvic acid with the production of two molecules of NADH , Two molecules of ATP .
2. Pyruvic acid is converted into lactic acid or ethyl alcohol according to the type of cell in which it was formed and this is called fermentation .

Types of fermentation

Types of fermentation are Acidic fermentation and Alcoholic fermentation

Acidic fermentation

As in animal muscular cells and bacteria , In the muscle fibers , when the muscles exert vigorous efforts or exercises , they consume most of the oxygen in their cells and tend to convert the pyruvic acid into lactic acid through its reduction by its combing with the electrons on NADH , This is known as a muscular fatigue .

C₆H₁₂O₆ → 2 C₃H₆O₃ + 2 ATP      ( Acidic fermentation )

If oxygen is available , lactic acid is converted into pyruvic acid again , and then into acetyl co-A , In case of bacteria , the pyruvic acid is converted into lactic acid in the absence of oxygen and many of dairy industries depend on this type of fermentation as cheese , butter and yogurt industries .

Alcoholic fermentation

In yeast and some plant cells : Pyruvic acid is reduced into ethyl alcohol and carbon dioxide , This is used in the industry of some products .

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP     ( Alcoholic fermentation )

Seeds of angiosperms have the power to respire anaerobically , if they are kept under the anaerobic conditions .

Practical activity : Alcoholic fermentation 

Procedures :

1. Put a sugary solution ( or molasses diluted with water by a ratio of 1 : 2 ) in a conical flask .
2. Add a piece of yeast and mix it thoroughly .
3. Close the flask with a stopper of rubber through which a delivery tube passes and dip the free end of tube into a beaker containing lime-water .
4. Leave the apparatus in a warm place for several hours .

Observation :

1. Gas bubbles are seen on the surface of solution in the flask .
2. The release of alcohol odour from the flask .
3. Lime-water has become turbid .

Conclusions :

1. Yeast does anaerobic respiration, So, CO₂ is produced that causes the turbidity of lime-water and the sugary solution turns into alcohol.
2. Yeast does anaerobic respiration in the absence of O₂ and this is called alcoholic fermentation.

 Role of the respiratory system in the excretion process in man & Respiration in plant

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