Aerobic Respiration: How does the cell get ATP? Does the cell also undergo respiration? Can a cell respire in the presence or absence of oxygen? The answer to all such questions is cellular respiration. Cellular respiration is the process where a cell breaks down glucose to produce energy in the form of ATP.

Cellular respiration can take place in the presence or absence of molecular oxygen. Aerobic respiration is a type of cellular respiration that takes place in the presence of oxygen, while anaerobic respiration is a type of cellular respiration that takes place in the absence of oxygen. In this article, we will learn about the definition of aerobic respiration, its steps, significance, and much more.

Definition of Cellular Respiration

The process of breakdown of primary metabolites (like glucose, protein, fatty acids, etc.)  in the cell with the release of energy in the form of ATP is called cellular respiration. Cellular respiration takes place in the living cells of organisms.

Cellular respiration is of two types, i.e. aerobic respiration and anaerobic respiration.

1. Aerobic respiration: It is a process when glucose is broken down to carbon dioxide in the presence of oxygen to produce energy in the form of ATP.
2. Anaerobic respiration: It is a process when glucose is broken down in the absence of oxygen. It is also called fermentation.

Difference Between Respiration and Breathing

Respiration and breathing are two different types of processes that occur simultaneously inside the body, where the former (respiration) is concerned with the production of energy, involves the breakdown of nutrients and converts it into energy, while the latter (breathing) is relatively associated with the process of inhalation and exhalation of oxygen and carbon dioxide.

Energy is an essential factor, which is related to the work done by the body. Within the body of all types of living beings like microorganisms, plants, animals, the energy requirement is met by two types of chemical reactions that take place within the cell. These chemical reactions are of two types, one is called aerobic respiration and the other is called anaerobic respiration, which we discussed above.

What is Aerobic Respiration?

Aerobic respiration is a process in which the cells utilize oxygen for the degradation of primary metabolites and release energy. It takes place in the cytoplasm and mitochondria of the cell and produces ATP (Adenosine Triphosphate).

Aerobic Respiration Equation

In aerobic respiration, oxygen is used in the complete breakdown of glucose with the formation of carbon dioxide and water as end-products. The equation of aerobic respiration is given below:

\({{\bf{C}}_{\bf{6}}}{{\bf{H}}_{{\bf{12}}}}{{\bf{O}}_{\bf{6}}} + {\rm{ }}{\bf{6}}{{\bf{O}}_{{\bf{2}}\;}} \to {\bf{6C}}{{\bf{O}}_{\bf{2}}} + {\rm{ }}{\bf{6}}{{\bf{H}}_{\bf{2}}}{\bf{O}}{\rm{ }} + {\rm{ }}{\bf{38}}{\rm{ }}{\bf{ATP}}\)

Glucose + Oxygen ? Carbon dioxide + Water + Energy

Steps Involved in Aerobic Respiration

The aerobic respiration process has three essential steps:

1. Glycolysis or EMP Pathway
2. Krebs Cycle or Citric Acid Cycle or TCA Cycle
3. Electron Transport Chain or Terminal Oxidation or Oxidative Phosphorylation

Glycolysis

The highlights of Glycolysis or EMP Pathway are:

1. Origin of the word: The word glycolysis is derived by the combination of two Greek words,  Glykos meaning sugar and lysis meaning breakdown or dissolution.
2. Definition: Glycolysis is a process in which glucose is broken down and converted into pyruvic acid in presence of certain enzymes.
3. This pathway is also known as EMP Pathway, as it was discovered by three German scientists, Embden, Meyerhof, and Parnas.
4. Location of Glycolysis: Cytoplasm of the cell
5. Type of pathway: It is an anaerobic oxidative process because it occurs in the absence of oxygen, and there is a loss of hydrogen.
6. Overall equation: \({\bf{Glucose}} + {\bf{2}}\,{\bf{ADP}} + {\bf{2}}{\rm{ }}{\bf{Pi}} + {\bf{2}}\,{\bf{NA}}{{\bf{D}}^ + } \to {\bf{2}}{\rm{ }}{\bf{Pyruvate}} + {\bf{2}}\,{\bf{ATP}} + {\bf{2}}\,{\bf{NADH}} + {\bf{2}}\,{{\bf{H}}^ + }\)
7. It is a common pathway for both aerobic respiration and anaerobic respiration.

Krebs Cycle

The highlights of Krebs Cycle are

1. Definition: It is a cyclic aerobic process taking place in the matrix of mitochondria to break down pyruvic acid into carbon dioxide in the presence of certain enzymes.
2. Location of Krebs cycle: It takes place in the matrix of mitochondria.
3. Citric acid is the first product of this cycle. Thus, it is also called a citric acid cycle (tricarboxylic acid cycle).
4. Since this pathway was discovered by Hans Krebs, it is also called Krebs cycle.
5. Type of pathway: It is a cyclic, aerobic, oxidative, and biochemical pathway. It serves both catabolic and anabolic processes, it acts as an amphibolic pathway.
6. Overall equation: \({\bf{2}}\,{\bf{Pyruvicacid}}{\rm{ }} + \,\,{\bf{8}}\,{\bf{NA}}{{\bf{D}}^ + }\; + {\rm{ }}{\bf{2}}\,{\bf{FAD}} + {\bf{4}}\,{{\bf{H}}_{\bf{2}}}{\bf{O}} + {\bf{2}}\,{\bf{ADP}} + {\bf{2}}\,{\bf{Pi}} \to {\bf{6}}\,{\bf{C}}{{\bf{O}}_{{\bf{2}}\;}} + {\bf{8}}\,{\bf{NADH}}{\rm{ }} + {\bf{8}}\,{{\bf{H}}^ + }\; + {\rm{ }}{\bf{2}}\,{\bf{FAD}}{{\bf{H}}_{\bf{2}}}\; + {\rm{ }}{\bf{2}}\,{\bf{ATP}}\)

Electron Transport Chain or Terminal Oxidation or Oxidative Phosphorylation

The highlights of electron transport are:

1. Definition: Oxidative phosphorylation is the process of ATP production with the help of energy released during the oxidation of coenzymes. It is the last step in aerobic respiration.
2. Phosphorylation is defined as the process of formation of ATP during a process.
3. Location of Oxidative Phosphorylation: It occurs in the inner membrane of the mitochondria.
4. Type of pathway: It is a linear, aerobic, and oxidative pathway, where oxygen is the terminal acceptor of electrons.
5. The electrontransport chain consists of several electron carriers through which electron moves in sequence and ultimately forms molecular oxygen.
6. Electrons and H⁺ ions are released from \({\rm{NADH  +  }}{{\rm{H}}^{\rm{ + }}}\) and \({\rm{FAD}}{{\rm{H}}_{\rm{2}}}{\rm{,}}\) which are in turn formed during glycolysis and Krebs cycle.
7. \({\rm{1 NADH}}\) produces \({\rm{3 ATP}}\) molecules by this pathway.
8. \({\rm{1 FAD}}{{\rm{H}}_{\rm{2}}}\) produces \({\rm{2 ATP}}\) molecules by this pathway.
9. Net gain from this pathway is \({\rm{34 ATP}}\) molecules.

Differences Between Glycolysis and Krebs Cycle

Glycolysis	Krebs Cycle
Glycolysis takes place in the cytoplasm.	Krebs cycle takes place in the matrix of mitochondria.
Oxygen is not necessary for glycolysis.	In the Krebs cycle, oxygen is necessary.
It happens in aerobic and anaerobic respiration.	It happens only in aerobic respiration.
It is a linear pathway.	It is a cyclic pathway.
A partial breakdown of glucose takes place converting it to two molecules of pyruvic acid.	A complete breakdown of glucose takes place, where one pyruvic acid gives three carbon dioxide molecules.
It does not evolve carbon dioxide.	It evolves carbon dioxide.

Examples of Aerobic Respiration

The examples of aerobic respiration are:

1. Multicellular organisms like plants and animals produce energy by aerobic respiration.
2. The internal organs of the human body like the brain, heart, liver and red muscle fibres perform aerobic respiration.

Do All Human Cells Carry Out Aerobic Respiration?

No, all human cells do not carry out aerobic respiration. In RBCs, mitochondria are absent and hence they cannot carry out aerobic respiration. Similarly, white muscle fibres and muscles during strenuous activity do not receive adequate oxygen and thus undergo anaerobic respiration.

Aerobic Respiration in Plants

Plants do not have specialized organs for respiration like animals and humans. The respiration process in plants occurs using glucose produced during photosynthesis and oxygen to create energy for the plant’s growth. Respiration is quite the opposite of photosynthesis. They use carbon dioxide to produce glucose and oxygen and can be used as the source of energy later.

Respiration in plants occurs in the leaves, stems, and roots of the plant, whereas photosynthesis occurs only in the leaves and stems.
In respiration, plants exchange gases through stomata and lenticels. There are two types of respiration in plants:

1. Cellular respiration: The respiration occurring in the normal cells of the body to generate energy.
2. Photorespiration: The respiration carried out mainly in \({{\rm{C}}_3}\) plant cells when the concentration of oxygen is high, carbon dioxide is low and intensity of sunlight is very high. Photorespiration decreases the photosynthetic activity of such plants.

Significance of Aerobic Respiration

Aerobic respiration plays a significant role in releasing a lot of energy which helps in the survival of life. These are the following importance of aerobic respiration:

1. It releases a large amount of energy in comparison to anaerobic respiration.
2. It carries out a complete breakdown of glucose into carbon dioxide.

Difference between Aerobic and Anaerobic Respiration

There are substantial differences between both types of respiration:

1. The breakdown of glucose in the presence of oxygen to produce a large amount of energy is called aerobic respiration ; Whereas the
   breakdown of glucose in the absence of oxygen to produce energy is called anaerobic respiration .
2. The chemical equation for aerobic respiration is glucose + oxygen gives carbon dioxide + water + energy whereas the equation for anaerobic respiration is glucose, giving lactic acid + energy
3. Mitochondria in aerobic respiration cytoplasm occurs while anaerobic respiration only occurs in the cytoplasm.
4. High amounts of energy are produced and 38 ATP is released at a time in aerobic respiration; Less amount of energy is produced and 2 ATPs are released at a time in anaerobic respiration.
5. The end products in aerobic respiration are carbon dioxide and water, while lactic acid (animal cells), carbon dioxide
   and ethanol (plant cells) are the end products in anaerobic respiration.
6. Aerobic respiration requires oxygen and glucose to produce energy whereas anaerobic respiration does not require oxygen but uses
   glucose to produce energy.
7. The steps involved in aerobic respiration are – 1. Glycolysis – also known as Embden-Meyerhof-Parnass (EMP) pathway; 2. Respiratory chain (electron transport and oxidative phosphorylation); 3. Tricarboxylic acid cycle (TCA), also known as citric acid cycle or Krebs cycle, whereas anaerobic respiration involves only two steps, which is 1. Glycolysis and 2.Fermentation.
8. Aerobic respiration refers to the complete process of combustion , whereas it is incomplete in anaerobic respiration.
9. A production of aerobic respiration energy is a long process , whereas anaerobic respiration relative to a faster process is.
10. Examples of aerobic respiration occur in many plants and animals (eukaryotes) while anaerobic respiration occurs in human muscle
    cells (eukaryotes), bacteria, yeast (prokaryotes), etc.

We have summarised the difference between aerobic and anaerobic respiration in the table below:

Basis for Comparison	Aerobic Respiration	Anaerobic Respiration
definition	The breakdown of glucose in the presence of oxygen to produce a large amount of energy is called aerobic respiration.	The breakdown of glucose in the absence of oxygen to produce energy is called anaerobic respiration.
chemical equation	Glucose + oxygen gives carbon dioxide + water + energy	Glucose gives lactic acid + energy
happens in	Mitochondria from the cytoplasm.	Takes place in the cytoplasm itself.
energy produced	A high amount of energy is produced.	Small amount of energy production.
number of ATP released	38 ATP.	2 ATP.
final product is	Carbon dioxide and water.	Lactic acid (animal cells), carbon dioxide and ethanol (plant cells).
need it	oxygen and glucose to produce energy.	It does not require oxygen but uses glucose to produce energy.
contains	1. Glycolysis – also called Embden-Meyerhof-Parnas (EMP) pathway. 2. Respiratory chain (electron transport and oxidative phosphorylation). 3. The tricarboxylic acid cycle (TCA), also known as the citric acid cycle or the Krebs cycle.	1. Glycolysis. 2. Fermentation
combustion process	Full	incomplete.
action Type	It is a long process for the production of energy.	It is a faster process than aerobic respiration.
example	Aerobic respiration occurs in many plants and animals (eukaryotes).	Anaerobic respiration occurs in human muscle cells (eukaryotes), bacteria, yeast (prokaryotes), etc.

Summary

Every living organism on this earth needs the energy to carry out various life processes, whether plant, animal, or human. Respiration is the process that is required to produce energy. Aerobic respiration is a part of respiration in which the cells utilize oxygen to degrade primary metabolites to produce energy.

The process of aerobic respiration in cells takes place in three steps, i.e. glycolysis, Krebs cycle, and oxidative phosphorylation. Glycolysis does not require oxygen, while the Krebs cycle and oxidative phosphorylation requires oxygen.

FAQs

Q.1. How does aerobic respiration differ from anaerobic respiration?
Ans: Aerobic respiration takes place in the presence of oxygen and anaerobic respiration takes place in the absence of oxygen. 

Q.2. What is aerobic respiration and anaerobic respiration?
Ans: Aerobic respiration: It is a process when glucose is broken down to carbon dioxide in the presence of oxygen to produce energy in the form of ATP.
Anaerobic respiration: It is a process when glucose is broken down in the absence of oxygen. It is also called fermentation.

Q.3. Name the first product formed in the Krebs cycle.
Ans: The first product formed in the Krebs cycle is citric acid, hence it is also called the citric acid cycle. 

Q.4. How many ATP are produced in aerobic respiration?
Ans: 38 ATP molecules are produced during aerobic respiration.

Q.5. Name the pathway that is common between aerobic and anaerobic respiration.
Ans: Glycolysis or EMP pathway is the common pathway between aerobic respiration and anaerobic respiration.  

Q.6. Which step of aerobic respiration produces maximum ATP?
Ans: Oxidative phosphorylation produces maximum ATP, i.e. 34 ATP molecules are formed in this step.

Q.7. What is the role of oxygen in aerobic respiration?
Ans: Oxygen is responsible for accepting electrons in the electron transport chain.