Cellular Respiration – An Overview

Introduction

Cellular respiration is a process in which an organism acquires energy to maintain various functions, such as growth, movement, repair, reproduction and defence against danger. The process is carried out in both autotrophic and heterotrophic organisms and involves a series of metabolic processes within a cell. Cellular respiration is the most critical process that occurs in any organism, but it does not occur in the same manner in every organism. There are different types of cellular respiration. 

Both prokaryotic and eukaryotic cells carry out this process. In prokaryotic cells, it takes place in the cytoplasm and in eukaryotic cells, it initiates in the cytosol and then continues in the mitochondria. The process mainly entails converting adenosine diphosphate (ADP) to adenosine triphosphate (ATP).

In eukaryotes, the process takes place in the following stages:

1. Glycolysis
2. Krebs cycle or citric acid cycle
3. Electron transport chain or oxidative phosphorylation

Cellular Respiration 

-Cellular respiration occurs in the cytosol and mitochondria of a cell in an organism.

 -The first stage i.e. glycolysis, occurs in the cytosol and the rest of the stages occur in the mitochondrion. 

-Mitochondria produce energy which is known as adenosine triphosphate.

-ATP is the primary chemical produced in the cellular respiratory process. Due to their major contribution to the process, mitochondria are known as the “powerhouse of the cell.” 

-Several enzymes present in mitochondria help in the respiration process.

 – Mitochondria organelles are divided into an inner membrane and an outer membrane. Between these two membranes, there is a space called the intermembrane space.

-There are plenty of proteins present in the outer membrane. They are called “porins” and can be permeated by molecules and ions like ATP.

-The inner membrane has some complexes  for ETS (electron transport system)present inside, which help in the final stage of the cellular respiration process – the electron transport chain. 

Importance of Cellular Respiration 

The production of biochemical energy is the chief function of the cellular respiratory process. 

The process is necessary for both prokaryotic and eukaryotic cells because the biochemical energy produced is responsible for powering numerous metabolic processes, such as locomotion, transportation of molecules through membranes and biosynthesis. This is why the cellular respiratory process is one of the most indispensable processes in any organism. 

Types of Cellular Respiration 

The process of cellular respiration is of two types: aerobic respiration and anaerobic respiration. When it takes place with the presence of oxygen, it is called aerobic cellular respiration and when it takes place without the presence of oxygen, it is called anaerobic cellular respiration.

Aerobic Respiration 

In almost all prokaryotes and eukaryotes, aerobic respiration occurs in the cells. The end products of this type of respiration are water, CO2 and energy. Aerobic respiration can only take place with the help of oxygen. It occurs in three stages: glycolysis, citric acid cycle, Krebs cycle and electron transport chain. 

1. Glycolysis: Splitting glucose (sugar, six carbon molecules) into two molecules of pyruvate (three-carbon molecules) is known as glycolysis, which releases energy. Prior to the initiation of glycolysis, the transportation of glucose into the cell takes place, where it is phosphorylated. It occurs in the cytosol in most organisms. 
2. Citric acid cycle: The Krebs cycle or citric acid cycle was discovered by Hans Adolf Krebs in 1937. The pyruvate produced after glycolysis enters into the mitochondrial matrix. The end products of this step are three molecules of NADH, two molecules of carbon dioxide (CO2), one molecule of FADH2 and one molecule of GTP. They are formed through each molecule of pyruvate. The end products of the citric acid cycle are responsible for carrying out the electron transport chain along with oxidative phosphorylation. 
3. Electron transport chain: This stage is also called oxidative phosphorylation, where electrons travel through an electron transport system located in the mitochondria’s inner membrane. This travelling of electrons helps in maintaining a gradient of hydrogen ions. Electrons move from NADH to FADH2 and then to molecular oxygen when protons are pumped from the mitochondrial matrix to the space between the two membranes called the intermembrane matrix. This movement causes a reduction of O2 into H2O. 

Therefore, in aerobic respiration, oxygen is used as the final acceptor of electrons and this is the major role of oxygen in the entire cellular respiratory process. When the electron transport chain in the case of prokaryotes does not need oxygen for this stage, we call it anaerobic respiration. 

In aerobic respiration, cells use the protein gradient’s energy to generate a large amount of ATP. 

Anaerobic Respiration

While studying alcoholic fermentation of glucose by yeast, Pasteur (1860) discovered that it can respire in the absence of molecular oxygen. This process was termed as anaerobic respiration.

Anaerobic respiration is an enzyme controlled partial breakdown of organic compounds (floods) without using oxygen and releasing only a fraction of energy. Therefore, end products are never completely inorganic. It occurs in roots of some waterlogged plants, certain parasitic worms, animal muscles and some microorganisms (e.g., moulds, bacteria). The term anaerobic respiration is often used in connection with higher organisms. In microorganisms, anaerobic respiration is often called fermentation. Fermentation is named after products like alcoholic fermentation, lactic acid fermentation. Buchner (1897) found that fermentation could be caused by mixing sugar solution with yeast extract, instead of living yeast cells. The enzyme complex present in the extract was named zymase. Because of the latter, fermentation is also called zymosis.

In anaerobic respiration electrons are removed from the substrate during oxidation but are not finally transferred to molecular oxygen. The final electron acceptors are frequently compounds such as pyruvic acid or acetaldehyde, which form a part of anaerobic respiratory pathway. The end products are lactic acid or ethyl alcohol and not water.

The mechanism of anaerobic respiration or fermentation resembles that of respiration up to glycolysis. Pyruvate formed at the end of glycolysis is anaerobically broken down to yield various products depending upon the organism and the type of tissue. The two common products are ethyl alcohol and lactic acid.

Conclusion 

Different types of organisms have evolved different biological processes for their cellular respiration. The processes can be aerobic or anaerobic. 

In all organisms, the process begins by glycolysis in the cytoplasm of the cell. Then, it either continues in the mitochondria in the aerobic respiration where the Krebs cycle and electron transport chain occurs or it stays in the cytoplasm in anaerobic respiration where fermentation ensues. 

The cellular respiration process, whether aerobic or anaerobic, produces energy for the sustenance of the organisms.