Cellular Respiration, Aerobic & Anaerobic Respiration

Respiration at Cell Level

Living organisms require energy to carry on their essential life activities. This energy is provided within the cells by the phenomenon of respiration. Respiration is the universal procedure by which organisms breakdown complex compounds containing carbon in a manner that permits the cells to collect a maximum of useable energy.

In biology, the term respiration is used in two ways. More familiarly the term respiration means the exchange of breathing gases (CO2 and O2) in between the organism and its environment. This exchange is called external respiration. The cellular respiration is the process by which energy is provided to cells in a step by step breakdown of C-chain molecules in the cells.

Cellular respiration

Cellular respiration is a set of metabolic reactions and procedures that occur in the cells of organisms to transform chemical energy from oxygen molecules or food into adenosine triphosphate and after that release waste products.

Types of Cellular respiration

The most typical fuel used by the cell to offer energy by cellular respiration is glucose. The way glucose is metabolized depends upon the availability of oxygen. Prior to going into a mitochondrion, the glucose molecule is divided to form two molecules of pyruvic acid (3 Carbon molecules). This reaction is called glycolysis (glycolysis literally suggests splitting of sugar), and occurs in the cytosol and is represented by the equation:


This reaction happens in all the cells and biologists think that a similar reaction might have happened in the very first cell that was arranged in the world.

Cell processes pyruvic acid in three significant methods, alcoholic fermentation, lactic acid fermentation, and aerobic respiration. The first two reactions happen in the absence of oxygen and are described as anaerobic (without oxygen). The total breakdown of the glucose molecule happens just in the presence of oxygen, i.e. in aerobic respiration. Throughout aerobic respiration glucose is oxidized to CO2 and water and energy are released.


Aerobic Respiration

” Aerobic respiration is the process of producing cellular energy in the presence of oxygen.”

The chemical equation for aerobic respiration is:


The energy is released by splitting the glucose molecules with the help of oxygen gas. At the end of the chemical reaction, energy, water molecules, and carbon dioxide gas are released as the by-products or end products of the reactions.

The 2900 kJ of energy is released during the procedure of breaking the glucose molecule and in turn, this energy is used to produce ATP– Adenosine Triphosphate molecules which are utilized by the body for various purposes. Aerobic respiration procedure happens in all multicellular organisms consisting of animals, plants, and other living organisms.

During the respiration process in plants, the oxygen gas enters the plant cells through the stomata, which is present in the epidermis (mesophyll cells) of leaves and stem of a plant. With the help of the photosynthesis procedure, all green plants manufacture their food and thus release energy.

Role of Mitochondria in Aerobic Respiration

Mitochondria are large granular or filamentous organelles that are dispersed throughout the cytoplasm of animal and plant cells. Each mitochondrion is consisting of an external enclosing membrane and an inner membrane with intricate folds or cristae that extend into the interior of the organelle.

Mitochondria play a role in cellular respiration by transferring the energy of the organic molecules to the chemical bonds of ATP. A big “Battery” of enzymes and coenzymes gradually release energy from the glucose molecules. Thus, mitochondria are the “Power houses” that produce the energy necessary for numerous cellular functions.


Adenosine Triphosphate and its Significance

Adenosine triphosphate, normally abbreviated ‘ATP’ is a compound found in every living cell and is one of the necessary chemicals of life. It plays a crucial function in the majority of biological energy changes. Traditionally, ‘P’ represents the whole phosphate group.

The second and third phosphate represent the so-called “high energy” bonds. If these are broken by hydrolysis, much more totally free energy is released as compared to the other bond in the ATP molecule. The breaking of the terminal phosphate of ATP releases about 7.3 Kcal of energy. The high energy ‘P’ bond enables the cell to collect a great quantity of energy in a very small area and keeps it ready for use as soon as it is needed.


The ATP particle is utilized by cells as a source of energy for various functions, for instance, synthesis of more complex substances, active transport across the cell membrane, muscular contraction and nerve conduction, etc.

Anaerobic Respiration

The respiration in the absence of oxygen is called anaerobic respiration. It might be:

  • Alcoholic Fermentation: In primitive cells and in some eukaryotic cells such as yeast, pyruvic acid is more broken down by alcoholic fermentation into alcohol (C2 H5 OH) and CO2.


Fermentation is commonly used for the production of alcohol and bread making. When sugar, yeast, flour, and water are integrated to form the dough, yeast breaks down the sugar and produces carbon dioxide, which triggers the bread to rise.

  • Lactic acid fermentation: In lactic acid fermentation, each pyruvic acid molecule is converted into lactic acid C3 H6 O3 in the absence of oxygen gas:


Lactic acid fermentation is used to flavor or maintains dairy products and vegetables, for instance, yogurt and pickles.

This type of anaerobic respiration occurs in muscle cells of human beings and other animals during extreme physical activities and heavy exercises, such as sprinting when oxygen can not be carried to the cells as quickly as it is required. Both alcoholic and lactic acid fermentation yield fewer percentages of energy from the glucose molecules. Only about 2% of the energy present within the chemical bonds of glucose is converted into adenosine triphosphate (ATP).

Biological oxidation

The maintenance of the living system needs a continuous supply of totally free energy which is eventually originated from numerous oxidation-reduction reactions. Except for photosynthetic and some bacterial chemosynthetic processes, which are themselves oxidation-reduction reactions, all other cells depend ultimately for their supply of totally free energy on oxidation reactions in respiratory processes.

Sometimes biological oxidation includes the elimination of hydrogen, a reaction catalyzed by the dehydrogenase attached to specific coenzymes. Cellular respiration is basically an oxidation reaction.