Definition of Respiration

Respiration could be defined as a metabolic biochemical process that takes place in all living cells of an organism where they produce energy by intake of oxygen and liberation of carbon dioxide from the oxidation of various organic substances. The energy produced is in the form of Adenosine-triphosphate or ATP which is also known as the energy molecule.

The different types of respiration:

Two types of respiration processes have been observed

  1. Aerobic respiration 

  2. Anaerobic respiration

Aerobic Respiration

Aerobic respiration is a type of cellular respiration that takes place in the presence of oxygen and produces energy. It is observed in both plants and animals and the end product of this type of respiration is water and Carbon dioxide (CO2).

Have a look at the reaction which takes place during aerobic respiration.

Glucose (C6H12O6)  + Oxygen 6(O2) → Carbon-dioxide 6(CO2)  +  Water 6 (H2O) + Energy (ATP)

As observed, in this type of respiration, Glucose molecules are split in the presence of oxygen and the end or by-products which are released are CO2, water and energy in the form of ATP. The total energy released in this reaction is 2900KJ which is used to produce ATP molecules. It is observed in all types of multicellular organisms. 

Aerobic Respiration in Plants:

In plants, aerobic respiration starts when oxygen or O2 enters in plant cells through the stomata found in the epidermis of leaves. Then the process of photosynthesis is carried out which produces food for the plant body and releases energy. The chemical reaction that takes place is below:

  Carbon-dioxide 6(CO2)  + Water 6 (H2O) → Glucose  (C6H12O6)   +  Oxygen 6(O2)         

Anaerobic Respiration

Anaerobic respiration is another type of cellular respiration that takes place in the absence of oxygen and produces energy. In this process,  glucose breaks down without the help of oxygen and the by-products produced are alcohol, CO2 and energy or ATP. The process takes place in the cytoplasm of a cell. The chemical reaction of this process is as follows:

Glucose(C6H12O6) → Alcohol 2(C2H5O H) + Carbon dioxide 2(CO2) + Energy (ATP )

Aerobic respiration could be divided into 

  1. Lactic acid fermentation

  2. Alcoholic fermentation

Lactic acid fermentation: It is the type of anaerobic fermentation where glucose is split in the absence of oxygen to produce two molecules of lactic acid and two molecules of ATP.

The Chemical Reaction is :


During exercise, body muscles use O2 to produce ATP faster than they can be supplied.  When this happens, muscle cells carry out glycolysis faster than they can provide oxygen to the electron transport chain. This is why lactic acid fermentation takes place in our cells after a long session of exercise which makes our muscles sore.

Alcoholic fermentation: In this type of anaerobic respiration, glucose is split in the absence of oxygen two molecules of ethyl alcohol or ethanol, two molecules of Carbon dioxide and two molecules of ATP. The chemical reaction is :

Glucose(C6H12O6) → Alcohol 2(C2H5O H) + Carbon dioxide 2(CO2) + Energy (ATP )

Anaerobic respiration could be observed in human muscle cells, bacteria, methanogens and other prokaryotes.

The Different Stages of Respiration:

Cellular respiration takes place through a range of metabolic pathways. Glucose is broken down to water, carbon dioxide and some amount of ATP. More ATP is produced later in a process known as oxidative phosphorylation which is powered by the movement of electrons through the electron transport chain. Below is a summary of the different phases of cellular respiration:

Glycolysis: In this process, a six-carbon sugar, glucose undergoes various chemical transformations. The end product is pyruvate and a three-carbon organic molecule. Two molecules of ATP and NADH are also generated.

Pyruvate oxidation: Each pyruvate enters the mitochondrial matrix, and is converted to a two carbon molecule that is bound with Coenzyme- A. The whole compound is known as acetyl CoA.  The by-products in this reaction are carbon dioxide and NADH.

Citric Acid Cycle: The Acetyl CoA combines with a four-carbon molecule which undergoes a series of chemical reactions also known as the Citric Acid cycle or the Krebs cycle.  In the end, the initial four-carbon molecule is regenerated with the addition of FADH2, NADH, carbon dioxide and two molecules of ATP. Krebs cycle is known as the tricarboxylic acid cycle as well.

Oxidative Phosphorylation: The FADH2 and NADH generated in the Krebs cycle donates their electron to oxygen through various electron carriers via the electron transport chain in a process known as oxidative phosphorylation. The reaction takes place in the mitochondrial matrix. As electrons move down the chain, there's a release of energy that is used to pump protons out of the matrix and it forms a gradient. The protons then again flow back in the matrix with the help of the enzyme ATP synthase which produces ATP. At the end of the chain, oxygen accepts the electrons and protons to form water.

In the end, 38 molecules of ATP are generated from one molecule of glucose. It should be mentioned that glycolysis can also take place in the absence of oxygen and the process is known as fermentation.

The Significance of Respiration:

Respiration is the energy supplying and releasing process in all living organisms by converting food energy into metabolically usable forms of chemical energy. The process releases energy in a controlled step-by-step manner so it could be properly used for all cellular activities such as metabolism and cell division. Various intermediate products of glycolysis and the citric acid cycle are used in the synthesis of many organic compounds which are used in cellular metabolism. In a plant, the carbon dioxide required for photosynthesis is replenished by the carbon dioxide released in respiration and the oxygen needed for respiration is replenished with the oxygen released in photosynthesis. Therefore respiration and photosynthesis are complementary to each other and maintain the balance of carbon dioxide and oxygen in nature. Respiration also conserves energy. One glucose molecule releases 673Kcal of the energy of which 456 Kcal is conserved in 38 molecules of ATP.

FAQ (Frequently Asked Questions)

Q1: What are the differences between Aerobic and Anaerobic Respiration?


Aerobic Respiration

Anaerobic Respiration

Aerobic respiration is observed in the presence of oxygen.

Anaerobic respiration is observed in the absence of oxygen.

Glucose is broken down into carbon dioxide, water and energy.

Glucose is broken down into lactic acid or ethyl alcohol and energy.

Aerobic respiration takes place in the cytoplasm of the mitochondria.

Anaerobic respiration takes place in the cell cytoplasm

38 molecules of ATP are produced in aerobic respiration

2 molecules of ATP are produced in anaerobic respiration.

The stages in aerobic respiration are

  • Glycolysis

  • Pyruvate oxidation

  • Citric acid cycle

  • Electron transport and oxidative carboxylation

  • The stages in anaerobic respiration are

  • Glycolysis

  • Fermentation

Aerobic respiration can be observed in eukaryotic cells (plants and animals)

Anaerobic respiration takes place in human muscle cells, prokaryotes.

Q2: What is Respiration in Humans?

Ans: Respiration in human can be referred to as a metabolic process which is carried out in 4 stages

Glycolysis: where two molecules of ATP and pyruvate is generated

Pyruvate oxidation: Acetyl CoA is produced which is used in the citric acid cycle.

Citric acid cycle: Two molecules of ATP are generated again

Oxidative phosphorylation: The last step where up to 34 molecules of ATP are generated.

Overall from one molecule of glucose 38 molecules of ATP are generated which are used as a source of energy for various cellular processes.

Q3: What are the Key Features of Aerobic Respiration?


  •  Aerobic respiration uses oxygen to breakdown glucose, fatty and amino acids to produce ATP

  • Pyruvate is converted to Acetyl CoA which is used in the Krebs cycle inside the mitochondrial matrix.

  • The Krebs cycle takes place two times for each molecule of glucose

  • The inner mitochondrial matrix has an arrangement of protein complexes which ensures that the electrons pass from one reacting molecule to the other.

  • A total of 38 molecules of ATP is produced from one molecule of glucose

Q4: What is an ATP?

Ans: ATP or Adenosine-Triphosphate is an organic compound composed of adenine, ribose sugar and phosphate groups and is utilised to provide energy for many biochemical Processes within a cell. It is known as the energy currency of a cell.