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Cellular Respiration – Types, Equations and Step

Jul 7, 2022

An Overview

Cellular respiration is a series of metabolic events that occur within cells to transform biological energy from food into a chemical molecule known as adenosine triphosphate (ATP). Metabolism is a sequence of chemical events that occur to keep the cells of an organism alive. These are classified into two types:

  • Catabolism is the method of breaking molecules to get energy.
  • Anabolism is the process through which all molecules necessary by the cells are synthesised.

As a result, cellular respiration is a catabolic activity in which big molecules are broken down into smaller ones, providing energy to fuel cellular functions.


What Is Cellular Respiration? 

The mechanism through which cells turn carbohydrates into energy is known as cellular respiration. Living cells require fuel and an electron acceptor to drive the chemical process of converting energy into a usable form to produce ATP and other kinds of energy to power cellular processes.

Types of Cellular Respiration

  • Aerobic respiration

Eukaryotes undergo cellular respiration in the organelle called mitochondria. Mitochondria is an organelle designed to disintegrate into carbohydrates and produce ATP effectively. Mitochondria are sometimes described as “the cell’s powerhouse” due to their ability to make sufficient amounts of ATP!


Although oxygen is the most potent electron acceptor known in nature, aerobic respiration is extremely efficient. Oxygen “loves” electrons. Its affection for electrons “pulls” them through the mitochondrial electron transport chain.

The unique architecture of the mitochondria, which houses all of the required reactant molecules for cellular respiration in a compact, membrane-bound region within the cell, also contributes to aerobic respiration’s high efficiency.


Most eukaryotic cells can also do anaerobic respiration, such as lactic acid fermentation, without oxygen. However, these mechanisms do not generate enough ATP to sustain the cell’s life functions, and cells will eventually perish or cease to function without oxygen.

  • Fermentation 

The term ‘Fermentation’ was coined for many distinct forms of anaerobic respiration done in the absence of oxygen by many strains of bacteria and archaebacteria and specific eukaryotic cells.


These processes can employ a wide range of electron acceptors and generate a wide range of byproducts. Fermentation can take several forms, including:

  • Alcoholic fermentation

This fermentation is carried out by yeast cells and other cells and results in the production of alcohol and carbon dioxide as metabolites. That’s why brews are fizzy: during fermentation, yeasts produce carbon dioxide gas, which generates bubbles, as well as ethyl alcohol.

  • Lactic acid fermentation

In an oxygen-free environment, human muscle cells and some microorganisms execute this kind of fermentation. Humans use lactic acid fermentation to produce yoghurt. Harmless microorganisms are cultured in milk to create yoghurt. These bacteria generate lactic acid, which gives yoghurt its signature sharp-sour flavour and combines with milk proteins to form a dense, creamy texture.

  • Propionic acid fermentation

This form of fermentation is carried out by bacteria and is employed in the production of Swiss cheese. The unique bitter, nutty flavour of Swiss cheese is due to propionic acid. The holes in a cheese slice are caused by the gas bubbles produced by these microorganisms.

  • Acetogenesis

Acetogenesis is a process of fermentation carried out by bacteria that produce acetic acid as a byproduct. Acetic acid is the distinguishing element in vinegar, giving it its harsh, sour flavour and aroma. Surprisingly, the bacteria that make acetic acid run on ethyl alcohol. To make vinegar, a sugar-containing mixture must first be fermented with yeast to generate alcohol, then with bacteria to convert the alcohol to acetic acid!

  • Methanogenesis 

Methanogenesis is a kind of anaerobic metabolism that only archaebacteria can accomplish. Methanogenesis is the breakdown of a carbohydrate fuel source to create carbon dioxide and methane.


Some friendly bacteria in the gastrointestinal system of the host, cows, and other animals produce methane. Some of these bacteria can digest cellulose, a plant sugar that cannot be broken down by cellular respiration. Cows and other animals may get some energy from these ordinarily indigestible carbohydrates thanks to symbiotic microorganisms!

What Is Cellular Respiration’s Equation?

Aerobic Respiration 

The aerobic cellular respiration equation depicts glucose (C6H12O6) combining with oxygen (O2) and ADP to form carbon dioxide (CO2), water (H2O), and ATP:

C6H12O6 (glucose)+ 6O2 + 36 ADP (depleted ATP) + 36 Pi (phosphate groups)→ 6CO2+ 6H2O + 36 ATP

The carbon molecules of glucose are released as six molecules of carbon dioxide when they have been entirely broken down.

Refer to the above cellular respiration diagram for a better understanding. 

cellular respiration diagram

Lactic Acid Fermentation

During the process of lactic acid fermentation, one glucose molecule is split into two molecules of lactic acid. The chemical energy held in the broken glucose molecules is transferred to bonds formed by a phosphate group and ADP.

C6H12O6 (glucose) + 2 ADP (depleted ATP) + 2 Pi (phosphate groups) → 2 CH3CHOHCOOH (lactic acid) + 2 ATP

Refer to the above cellular respiration diagram for more clarity. 

Lactic Acid Fermentation

Alcoholic Fermentation 

Alcohol fermentation is comparable to lactic acid fermentation. In alcoholic fermentation, oxygen is not the final electron acceptor. So, rather than oxygen, the cell accepts the remaining electrons via a transformed form of pyruvate. This results in ethyl alcohol, which is present in alcoholic drinks. Brewers and distillers make this alcohol using yeast cells, which are highly adept at fermentation.

C6H12O6 (glucose) + 2 ADP (depleted ATP) + 2 Pi (phosphate groups)→ 2 C2H5OH (ethyl alcohol) + 2 CO2 + 2 ATP

Refer to the above given cellular respiration diagram. 

Alcoholic Fermentation

Steps of Cellular Respiration 

The process of cellular respiration involves three following steps:

Step 1: Glycolysis

Glycolysis is the primary function shared by all types of cellular respiration. Now, what happens in glycolysis?

In the process of glycolysis, a sugar molecule is split in half, producing two molecules of ATP.

The cellular respiration equation for glycolysis is as follows:

C6H12O6(glucose) + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3COCOO− + 2 NADH + 2 ATP + 2 H2O + 2H+

The term glycolysis has a Greek origin. ‘Glyco’ means ‘sugar’, and ‘lysis’ means ‘to split.’ Hence, glycolysis is a process that splits a molecule of sugar. 

In the majority of pathways, the process of glycolysis starts with the breakdown of glucose into two molecules of pyruvic acid. These two molecules of pyruvic acid are further processed to release various forms of byproducts. These byproducts include ethyl alcohol and lactic acid.

Step 2: Reduction

The next step of the process is reduction. Reduction means to ‘reduce’ a molecule by adding electrons to it.

In lactic acid fermentation, NADH contributes an electron to pyruvic acid. It results in the byproducts of lactic acid and NAD+. Because NAD+ is critical for the process of glycolysis, this step becomes important for the cell.

In alcoholic fermentation, pyruvic acid goes through an auxiliary step. It loses an atom of carbon in the form of carbon dioxide. During this process, an intermediate molecule, i.e. acetaldehyde, is formed. Acetaldehyde is further reduced to generate ethyl alcohol and NAD+.

Step 3: Krebs Cycle

Aerobic respiration upgrades these processes to the next level. It does not use the intermediate products of the Krebs cycle. Instead, aerobic respiration uses oxygen as the ultimate acceptor of electrons. The electron transport chain processes the protons and electrons bound to the electron carriers (NADH). 

The electron transport chain operating inside the mitochondrial membrane uses the energy from the electrons to drive protons to one side of the mitochondrial membrane. This results in the electromotive force. This force is utilised by the ATP synthase phosphorylate (protein complex) to produce ATP.

Pyruvate Oxidation 

Aerobic respiration will continue if oxygen is present. The pyruvate molecules created after glycolysis in eukaryotic cells are carried into mitochondria, which are the locations of cellular respiration. There, pyruvate is converted to an acetyl group that is taken up and activated by the carrier substance coenzyme A. (CoA). Acetyl CoA is the resultant substance.

Vitamin B5, or pantothenic acid, is used to make CoA. Although the cell can utilize acetyl CoA in a variety of ways, its primary usage is to transport the acetyl group from pyruvate to the subsequent step in the route for the breakdown of glucose. 

Decomposition of pyruvate  

Pyruvate, a byproduct of glycolysis, must go through a number of modifications before it can enter the Citric Acid Cycle, the following step in cellular respiration. Three steps make up the conversion process (in Figure) 


Step 1: Pyruvate undergoes the removal of a carboxyl group, which causes the release of a carbon dioxide molecule into the environment. A two-carbon hydroxyethyl group is linked to the enzyme as a result of this step (pyruvate dehydrogenase). This is the first of the original glucose molecule’s six carbons to be taken out. For every molecule of glucose metabolized, this phase is repeated twice, removing two of the six carbons in total. Keep in mind that two pyruvate molecules are created at the end of glycolysis. 

Step 2: NAD+ is converted to NADH. The hydroxyethyl group is converted to an acetyl group through oxidation, and NAD+ takes up the electrons to produce NADH. Later, to produce ATP, the high-energy electrons from NADH will be needed. 

Step 3: Acetyl CoA is produced by adding an acetyl group to coenzyme A. A molecule of acetyl CoA is created when the enzyme-bound acetyl group is transferred to CoA. 

It should be noted that anytime a carbon atom is removed during the second stage of glucose metabolism, it is bound to two oxygen atoms, resulting in carbon dioxide, one of the main byproducts of cellular respiration. 

Byproducts of Cellular Respiration

The byproducts of cellular respiration are as follows:


The chemical adenosine triphosphate (ATP) is the major product of all cellular respiration. The functions of ATP are as follows:

  • This molecule accumulates the energy created during breathing.
  • It allows the cell to distribute to other sections of the cell. 
  • A variety of biological components use ATP as a source of energy. 
  • ATP is also frequently utilised by carriers, usually proteins that carry chemicals across the cell membrane.
Carbon Dioxide

Carbon dioxide is a ubiquitous byproduct of biological respiration. Carbon dioxide is typically seen as a waste product that must be eliminated. Carbon dioxide produces acidic ions in an aqueous solution. This can significantly reduce the pH of the cell, eventually causing normal cellular activities to halt. Cells must constantly expel carbon dioxide to avoid this.

Other Byproducts 

While all kinds of cellular respiration create ATP and carbon dioxide regularly, different respiration forms depend on different molecules to be the ultimate acceptors of the electrons needed in the process.

Frequently Asked Questions

1. What is the primary energy source in cellular respiration?

A: The principal source for cellular respiration is the molecule glucose. Without it, the entire process would not begin since there would be no pyruvate for use in the Krebs cycle, which produces carbon dioxide and NADH.

2. Is glycolysis occurring in the mitochondria?

A: Mitochondrial glycolysis solely addresses the pay-off phase of glycolysis, in which the three-carbon sugars are transformed to pyruvate, releasing energy and reducing equivalents in the form of ATP and NADH.

3. What is the function of ATP?

A: The functions of ATP include: Ion transport, substrate phosphorylation, nerve impulse transmission, muscular contraction, and chemical synthesis. All these processes need ATP for the production of energy. These and other processes generate significant demand for ATP.


What is cellular respiration? It refers to the oxygen-dependent and electron transport chain–dependent activities that reoxidize coenzymes that have been decreased by fuel oxidation.

It is associated with mitochondrial ROS production and detoxification, oxidative phosphorylation, electrochemical gradient formation across membranes, and thermogenesis. Cells that accomplish high amounts of reductive biosynthesis, despite continued fuel oxidation, reduce it.

Cellular respiration


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