Types of respiration present in all living organisms are:
1. Anaerobic Respiration 2. Aerobic Respiration
All living organisms require energy to carry out life processes. This energy comes from food. However, processes carried out in cells cannot use the energy locked in stored food, fats, etc., directly.
Cellular processes get usable energy from a process called respiration.
Respiration commonly involves the use of oxygen to break down carbohydrates and other organic molecules, giving usable energy, carbon dioxide and water in the process.
All organisms breathe—a process in which they take in oxygen and give off carbon dioxide. This is called external respiration. Internal respiration, or cellular respiration, takes place inside every living cell.
In this process, carbohydrates and other organic molecules are broken down in successive steps to produce energy, which is used to make a compound called adenosine triphosphate (ATP). Cellular processes get energy from ATP. ATP is often called the ‘energy currency’ of the cell. The amount of ATP in a cell indicates how energy-rich it is.
Where does this energy come from? During photosynthesis, carbon dioxide and water combine with the help of the energy from the sun to form carbohydrates. Energy gets stored in the bonds of the carbohydrates.
In respiration, these bonds are broken to release energy and give back carbon dioxide and water. This energy then gets stored in the bonds of ATP. These bonds get easily broken to release energy when required by the cells.
We can study cellular respiration by taking the example of the complete oxidation of glucose. This molecule is oxidized and broken down gradually in two distinct stages. The first stage is called glycolysis, which involves anaerobic respiration.
This takes place in the cytoplasm of the cell. The second stage involves aerobic respiration, which takes place inside the mitochondria of the cell.
The overall reaction can be represented as follows:
C6H12O6 (glucose) + 6O2 → 6CO2+ 6H2o+ Energy
1. Anaerobic respiration:
Partial oxidation of food in the absence of oxygen, resulting in the release of some amount of energy, is called anaerobic respiration. Anaerobic means without oxygen or in the absence of oxygen, while aerobic means with oxygen or in the presence of oxygen.
Glucose has six carbon atoms joined to each other by covalent bonds. Hydrogen and oxygen atoms are also attached to these carbon atoms. In anaerobic respiration of glucose, some hydrogen atoms are removed from it, resulting in its oxidation. (The addition of oxygen or the removal of hydrogen is oxidation.) At the end of a series of reactions, glucose gets converted into two molecules of private, which contains three carbon atoms. These reactions also produce two molecules of ATP.
The oxidation of glucose in a series of reactions leading to the formation of private is called glycolysis.
Glycolysis means ‘splitting of sugar’. It takes place in all organisms, in the cytoplasm of the cell. It is the first stage of respiration—both aerobic and anaerobic. After glycolysis, its product (private) gets converted into different compounds depending on whether further reactions take place in the presence or absence of oxygen.
Glycolysis is the last energy-producing stage in case oxygen is absent or in low supply, and in cells that lack mitochondria. After glycolysis, further anaerobic reactions produce different products like lactic acid or ethanol (ethyl alcohol) in different situations.
This step completes the anaerobic respiration of glucose. Anaerobic respiration resulting in the formation of these products is also called fermentation.
Examples of lactic acid fermentation and alcohol fermentation are given below:
In a low supply of oxygen, yeast converts pyruvate to ethanol and carbon dioxide. Certain bacteria (which lack mitochondria) convert pyruvate to lactic acid. When our muscles are overworked, blood is unable to supply oxygen fast enough for producing energy through aerobic means. In this low-oxygen condition pyruvate gets converted to lactic acid. Accumulation of excess lactic acid in the muscles causes pain.
In aerobic respiration, a different path is followed after glycolysis. In the presence of oxygen, in cells that have mitochondria, pyruvate is oxidized further in a number of steps to produce more energy, carbon dioxide and water.
2. Aerobic respiration:
The complete oxidation of food yielding carbon dioxide, water and energy in the presence of oxygen is called aerobic respiration.
Aerobic respiration takes place inside the mitochondria. After glycolysis, pyruvate enters the mitochondria and is oxidized in a series of reactions. The products of these reactions include ATP, carbon dioxide and water. The number of molecules of ATP formed in aerobic respiration is 38. Hence the energy made available is much greater than in the case of anaerobic respiration.
Inside the mitochondria, when an inorganic phosphate group (PO 3-4, represented here as Pi) gets attached to a compound called ADP (adenosine diphosphate), a molecule of ATP (adenosine triphosphate) is formed.
ADP + Pi →ATP
The bond holding the last (terminal) phosphate group is easily broken when ATP reacts with water. In the process, energy is produced. This energy is used to drive cellular processes that are endothermic (i.e., processes that absorb energy). Processes like protein synthesis, contraction of muscles, etc., get energy from ATP.