Combustion and Respiration: Combustion is the process of burning something. Any carbon-containing substance reacts with oxygen to produce carbon dioxide gas, water vapour, along with energy in the form of heat and light during combustion. Based on this concept, respiration is also a combustion process because here also hydrocarbons react with oxygen to form carbon dioxide gas, water vapour, along with energy.

The basic difference between combustion and respiration or cellular respiration is the starting material, i.e., in combustion, hydrocarbons like methane, propane, etc., burn, while in cellular respiration, the hydrocarbon that burns is ‘glucose’. We can see that if we set fire on a piece of paper, it rapidly starts burning and releases lots of heat and light in the form of fire. But have you ever thought about what would happen if such rapid combustion occurred inside our body! No, it is impossible to burn glucose externally inside the cells of our body. This is the major difference between combustion and respiration, i.e., the speed of reaction and burning.

Though combustion is a rapid process, respiration occurs very slowly in a controlled manner and in a multistep chemical reaction inside the body. Respiration is also the chemical process by which organic compounds release energy. But this energy is in the form of Adenosine Triphosphate (ATP) that hydrolyses to Adenosine Diphosphate (ADP) and Phosphoric acid (Pi) and releases energy. Both combustion and respiration are exothermic reactions as they release energy.

What is Combustion?

Combustion is a redox chemical reaction between a fuel that acts as a reductant and atmospheric oxygen as an oxidant that results in the formation of gases like carbon dioxide, water vapour and energy accompanied by flame and smoke. Based on the availability of oxygen gas, combustion reaction is classified into two types:

1. Complete Combustion – When a combustion reaction takes place in the presence of a sufficient amount of oxygen, it is known as complete combustion that leads to the formation of carbon dioxide gas along with a blue flame. In LPG combustion used in our kitchen, blue lame is the result of complete combustion.

During combustion, a hydrocarbon like methane \(\left({{\text{C}}{{\text{H}}_4}} \right)\) react with oxygen gas to form carbon dioxide, water vapour and energy in the form of heat, flame, and smoke. The chemical reaction involved in combustion is given below:
\({\text{C}}{{\text{H}}_4} + 2{{\text{O}}_2} \to {\text{C}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}} + {\text{Energy}}\)

2. Incomplete Combustion – When this combustion takes place in an insufficient amount of oxygen, it is known as incomplete combustion that leads to the formation of carbon monoxide gas along with a yellow-orange flame. The chemical reaction for incomplete combustion is:

\(4{\text{C}}{{\text{H}}_4} + 5{{\text{O}}_2}\left({{\text{limited}}} \right) \to 2{\text{CO}} + 8{{\text{H}}_2}{\text{O}} + 2{\text{C}} + {\text{Energy}}\)

What is Respiration?

Respiration is the biological oxidation of food materials or respiratory substrates inside a cell, using oxygen gas and producing carbon dioxide and water as by-products along with the release of energy, i.e., ATP (adenosine triphosphate) that helps to carry out various metabolic activities inside our body. This type of respiration occurring in the presence of oxygen is known as aerobic respiration.

\({{\text{C}}_6}{{\text{H}}_{12}}{{\text{O}}_6} + 6{{\text{O}}_2}\left({{\text{Cellular}}\,{\text{Respiration}}} \right) \to 6{\text{C}}{{\text{O}}_2} + 6{{\text{H}}_2}{\text{O}} + {\text{ATP}}\)

Respiratory substrates may include carbohydrates, proteins, fats, and organic acids.

Both plants and animals undergo cellular respiration. Plants trap sunlight and, in the presence of sunlight, perform photosynthesis in chlorophyll to form glucose as food. This glucose is then broken into mitochondria (the powerhouse of the cell) to form carbon dioxide and water along with energy as ATP. This breakdown of glucose to release energy in the presence of oxygen is known as respiration.

Respiration is not Breathing!

We must not get confused between respiration and breathing. They are two different processes occurring in living organisms. The process of inhaling and exhaling is breathing. In contrast, respiration is the breakdown of glucose into carbon dioxide and water as by-products along with the release of energy, i.e., ATP in the presence of oxygen.

Similarities between Combustion and Respiration

Respiration in cells resembles ordinary combustion in the following aspects:

1. Both respiration and combustion are a type of redox reaction that is accompanied by the breakdown of chemical bonds.
2. Oxygen is used in both cases.
3. Both are a form of burning. Combustion is burning, and respiration can be called “slow-burning”.
4. The end products of both respiration and combustion are carbon dioxide, water vapour and release of energy.

Difference between Combustion and Respiration

Though there are few similarities between respiration and combustion, they are not the same at all. Some fundamental differences between these two processes are given below:

Combustion	Respiration
Combustion releases a huge amount of energy in a single step.	Respiration is a multistep process and releases a limited amount of energy stepwise in a controlled manner.
Most of the energy changes into heat and some into light and flame.	Each energy-releasing step in respiration is coupled with the synthesis of ATP (Adenosine Triphosphate) as energy packets.
Combustion takes place at ignition temperature.	Respiration takes place at room temperature.
Combustion is a fast reaction.	Respiration is a slow reaction.
For combustion, hydrocarbons act as fuel.	For respiration, food acts as fuel.

Heat of Combustion

The heat released during the combustion process under standard conditions is known as ‘Heat of combustion.

It is expressed in calorific value. The amount of energy liberated on complete combustion of \(1\,{\text{kg}}\) fuel is known as its calorific value. It is always measured in kilojoule/kilogram \(\left({{\text{kJ}}/{\text{kg}}}\right).\) Based on this calorific value, fuel efficiency can also be determined. For example, if two fuels of equal quantities are burnt in the presence of oxygen, the one having a higher calorific value will be considered as a more efficient fuel than the one having a lower calorific value. This calorific value is very important when the fuels are chosen for internal combustion engines. The fuels having higher calorific value will help the internal combustion engines work efficiently.

The heat of combustion is expressed in the following terms:

1. \({\text{Energy}}\,{\text{mol}}{{\text{e}}^{ – 1}}\) of fuel
2. \({\text{Energy}}\,{\text{mas}}{{\text{s}}^{ – 1}}\) of fuel
3. \({\text{Energy}}\,{\text{volum}}{{\text{e}}^{ – 1}}\) of fuel

Summary

In short, we can say that combustion and respiration are redox reactions. Both can be assumed as burning. While combustion is a process of burning something, respiration is slow-burning. Hydrocarbons react with oxygen to produce carbon dioxide gas, water vapour and energy in the form of heat and light in combustion. Similarly, respiration is also a combustion process because here also hydrocarbons react with oxygen to form the same by-products along with the release of energy as ATP.

The starting material in combustion is hydrocarbons like methane, propane, etc., which burns while in cellular respiration; the hydrocarbon that burns as a starting material is ‘glucose’. The major difference between combustion and respiration is the speed of reaction and burning. Combustion is a rapid process while respiration is very slow and occurs in a controlled manner and in several steps in the metabolic chemical reaction inside the body. The energy released in respiration is in the form of Adenosine Triphosphate (ATP) that hydrolyses to Adenosine Diphosphate (ADP) and Phosphoric acid (Pi). Both combustion and respiration are exothermic reactions as they release energy. Thus, we can say that combustion and respiration are quite similar to each other.

FAQs on Combustion and Respiration

Q.1. How is combustion related to cellular respiration?
Ans: Both respiration and combustion are two different types of processes, but still, they are related to each other by certain common points such as Both combustion and cellular respiration are a type of redox reaction that is accompanied by the breakdown of chemical bonds. Oxygen is necessary in both cases. Though both are a form of burning, combustion is burning, and respiration can be called “slow burning”. Finally, the end products of both respiration and combustion are similar to carbon dioxide, water vapour and release of energy.

Q.2. Explain respiration and combustion with an example of sugar.
Ans: During respiration, sugar or glucose molecules undergo biological oxidation inside the cell, using oxygen gas and produce carbon dioxide and water as by-products along with the release of energy, i.e., ATP (adenosine triphosphate) that helps to carry out various metabolic activities inside our body. But if we talk about the combustion of sugar, it is simply the burning of sugar that leads to its charring—this form carbon dioxide and water.

Q.3. How is the energy released during combustion and respiration?
Ans: The process by which energy is released during combustion and respiration differs a lot. Combustion releases a huge amount of energy in a single step in the form of heat and some into light and flame. At the same time, respiration is a multistep process and releases a controlled amount of energy in a stepwise manner. Each energy-releasing step in respiration is coupled with the synthesis of energy packets, ATP (Adenosine Triphosphate).

Q.4. Differentiate between respiration and combustion.
Ans: The basic difference between respiration and combustion are as follows: (i) Combustion is a fast reaction, and respiration is a slow reaction. (ii) Combustion releases a huge amount of energy in a single step, and most of the energy converts into heat, light and flame. In contrast, respiration is a multistep process and releases a limited amount of energy stepwise in a controlled manner. Each step in respiration is coupled with the synthesis of ATP (Adenosine Triphosphate) as energy packets. (iv) Combustion takes place at ignition temperature, and respiration takes place at room temperature. (v) Hydrocarbons act as fuel in combustion, and in respiration, food acts as fuel.

Q.5. Why is respiration more efficient than combustion?
Ans: Respiration is the process of oxidation of food like glucose, amino acids, fatty acids, etc., to produce energy. Now, this breakdown does not occur at once; it takes place slowly in various steps. In cellular respiration, one molecule of glucose produces 38 ATP in the form of energy. In contrast, combustion is simply burning, where it releases a huge amount of energy in a single step in the form of heat and some into light and flame. On comparing the amount of energy released during combustion and respiration, we can conclude that respiration is more efficient than combustion.

Q.6. Do respiration and combustion require oxygen?
Ans: Yes, both respiration and combustion require oxygen. Both combustion and cellular respiration are a type of redox reaction that is accompanied by the breakdown of chemical bonds. Oxygen is necessary in both cases.

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