Cellular respiration , Structure of ATP and types of fermentation

From Online Sciences - December 27, 2017

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 organisms 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 :

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 :

The most important co-enzymes are :

NAD+which is reduced into NADH


FAD which is reduced into FADH2



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 .

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

Reduction of two molecules of the co-enzymesNAD+


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 :

Krebs cycle takes place in many stages , which are :
The importance of Krebs cycle :

Electron transport chain

Calculation of ATP

Anaerobic cellular respiration

Stages of anaerobic respiration ( fermentation ) .

Types of fermentation

Acidic fermentation
Alcoholic fermentation

Practical activity : Alcoholic fermentation


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