Difference Between Respiration and Combustion

The key difference between respiration and combustion is that the respiration is the oxidation of glucose to release energy or produce ATP, while the combustion is the burning of something by supplying external heat to obtain energy.

Living organisms need the energy to carry out cellular functions. Some processes occur without the use of energy, but most cellular processes utilize energy. Cellular respiration is the process that produces cellular energy, mainly in the form of ATP. Glucose is the primary energy source in this process. During respiration, ATP molecules are generated as a result of the oxidation of glucose molecules in the presence of oxygen. Combustion is also a process that produces energy but in the form of heat. It requires the supply of external heat. Therefore, respiration and combustion differ from each other.

CONTENTS

1. Overview and Key Difference
2. What is Respiration 
3. What is Combustion
4. Similarities Between Respiration and Combustion
5. Side by Side Comparison – Respiration vs Combustion in Tabular Form
6. Summary

What is Respiration?

Respiration is a process which occupies a series of reactions coupled with oxidation and reduction reactions and electron transfer. At the end of respiration, organisms produce energy in the form of ATP (the energy currency of the cells) to utilize for their metabolic processes. Respiration occurs in the presence of oxygen as well as in the absence of oxygen. Based on that, respiration can be aerobic respiration or anaerobic respiration.

During aerobic respiration, oxygen molecules act as the final electron acceptors and reduce to produce water. This creates an electrochemical gradient which drives the ATP synthesis. Aerobic respiration consists of three main phases, in which rearrangement of carbon molecules takes place through a series of enzyme-catalyzed reactions to yield ATP. The first phase common to both aerobes and anaerobes is the glycolytic pathway, which produces two pyruvate molecules from a glucose molecule. Here, this conversion produces two ATP molecules and two NADH molecules.

The second phase is the tricarboxylic acid (TCA) cycle or Krebs cycle, which is the central hub, where intermediates of all metabolic pathways join to contribute towards energy production by producing NADH, FADH2 and two molecules of CO₂  via oxidation-reduction reactions.  The TCA cycle only takes place in aerobes. In both these processes (glycolysis and Krebs cycle), substrate-level phosphorylation takes place to produce energy.

The final stage is the electron transport chain or oxidative phosphorylation which takes place at the inner membrane of mitochondria. This process phosphorylates ADP to synthesize ATP by transferring electrons along the electron transport chain. It uses NADH electron carriers and ATP synthase enzyme for the formation of ATP. Oxidative phosphorylation needs molecular oxygen as the final electron acceptor. Hence, oxidative phosphorylation is feasible only under aerobic conditions, and it produces a higher number of ATP in aerobic organisms.

What is Combustion?

Combustion is the burning of something to produce energy, mainly in the form of heat. It does not occur in living cells. It requires the supply of external heat. Therefore, when the heat is given, it occurs rapidly in an uncontrollable manner, producing heat at once.

Combustion does not need enzymes and other chemicals. In fact, it is a non-cellular process which does not produce any intermediary products. Moreover, combustion takes place in the absence of water, unlike respiration. Burning of fuels to obtain energy is a popular example of combustion. Combustion is a common process in rocket engines.

What are the Similarities Between Respiration and Combustion?

• Both respiration and combustion produce energy.
• They utilize oxygen.
• Heat is produced in both processes.

What is the Difference Between Respiration and Combustion?

Respiration is the process that produces ATP or chemical energy in living cells by breaking glucose molecules. In contrast, combustion is the burning of something to produce energy. So, this is the key difference between respiration and combustion. Moreover, respiration is a cellular process that takes place slowly in a controllable manner, while combustion is a non-cellular process that takes place rapidly in an uncontrollable manner.

The below infographic summarizes the difference between respiration and combustion.

Summary – Respiration vs Combustion

Respiration and combustion are two processes that generate energy. Respiration generates chemical energy mainly in the form of ATP, which can be utilized for cellular functions; combustion produces energy in the form of heat. Furthermore, respiration is a cellular process that takes place with the help of different chemicals such as enzymes, while combustion takes place due to the supply of external heat. Moreover, respiration is a slow and controlled process, while combustion is a rapid and uncontrolled process. So, this is the summary of the difference between respiration and combustion.

Reference:

1. “Combustion.” Wikipedia, Wikimedia Foundation, 28 Sept. 2019, Available here.
2. “Cellular Respiration.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.

Image Courtesy:

1. “CellRespiration” By RegisFrey – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Et baal” By Einar Helland Berger – Own work (CC BY-SA 2.5) via Commons Wikimedia