Aerobic Respiration

Organisms such as prokaryotes and eukaryotes use respiration mechanisms for the breakdown of food that may require environmental oxygen. The process by which mitochondria use to transfer the energy in foods to ATP is known as cellular respiration. In this process, a food molecule breaks down in mitochondria, may consume oxygen and transfer energy to cells (in which it is stored as ATP molecule) and the environment (in the form of heat).

There are two types of cellular respiration - they are aerobic respiration and anaerobic respiration. The cells of animals, plants, and many bacteria need oxygen (O₂) to facilitate energy transfer during cellular respiration. In these organisms, the type of cellular respiration takes place is called aerobic respiration. In aerobic respiration, ATP forms as electrons are harvested and transferred along the electron transport chain, and eventually donated to oxygen gas. Many eukaryotes produce the majority of their ATP from glucose molecules in this manner. The meaning of the word aerobic is with air. Aerobic respiration is thought to have evolved as a modification of the basic photosynthetic machinery. The oxidation of glucose by aerobic respiration in eukaryotes produces up to three dozen ATP molecules. On the other hand, in the case of anaerobic respiration, the organisms do not require oxygen (O₂) for cellular respiration. Alcohol fermentation, lactic acid fermentation etc. are examples of anaerobic respiration.

Cellular respiration is different from normal respiration. Respiration is more commonly referred to as breathing and it is a physical act of inhaling and exhaling process. While cellular respiration is the process that occurs inside cells and that involves the use of oxygen to transfer energy from food to ATP.

There are three pathways that, in combination, are required to form the process of cellular respiration. These pathways are glycolysis, Krebs cycle, and oxidative phosphorylation pathway. Among them, glycolysis and Krebs cycle are required in breaking down of food molecules, while the third pathway i.e. oxidative phosphorylation transfers the energy from the food molecules to ATP. 

Here are some basic points that discuss cellular respiration:

1. Glycolysis occurs in the cytoplasm of the cell. During this process, cells break glucose into pyruvate. Pyruvate is a three-carbon containing compound. After this step, pyruvate is broken down into a two-carbon molecule which is known as acetyl-coenzyme A (acetyl-coA) and carbon dioxide C0₂).

2. Cells use this acetyl-coA and enter into Krebs cycle. This step occurs in the matrix of the mitochondria. In this step, acetyl-coA breaks down into carbon dioxide.

3. In oxidative phosphorylation, there is a transfer of energy from the cells because of the breakdown of food to ATP molecules. Oxidative phosphorylation takes place in the inner membrane which is also termed as the cristae of the mitochondria.

The energy present in a chemical bond can be visualized as potential energy borne by the electrons that make up the covalent bond. Cells are capable of harvesting this energy often to produce ATP. (ATP is also known as the energy currency of the cell.) After this, the energy depleted electron (associated with a proton as a hydrogen atom) is donated to some other molecule. This process is followed by the acceptance of hydrogen atoms by oxygen gas and it forms water, and the process is called aerobic respiration. On the other hand, the process when an inorganic molecule (rather than oxygen) accepts the hydrogen is called inorganic respiration. Similarly, when an organic molecule accepts the hydrogen atom, the process is known as fermentation. The example of aerobic respiration is the breakdown of glucose. 

The change in free energy occurred during this reaction is -720 Kcal per mole of glucose under the conditions found within a cell. Breaking of the six C-H bonds in glucose is responsible for the change in free energy. When glucose is burned, the exact amount of energy is released as heat. However, the energy released while burning cannot be used to perform cellular functions. The cell is capable of harvesting useful energy from the catabolism of food molecules due to its conversion of a portion of the energy into a more useful form.

The process of harvesting energy in the form of ATP from sugar molecules in the presence of oxygen involves a complex series of enzyme-catalyzed reactions that occur in four different steps as mentioned below.

In the first stage, energy is captured by the substrate-level phosphorylation through glycolysis and this step is followed by three stages that carry out aerobic respiration by oxidizing the end product of glycolysis. 

1. Stage One: Glycolysis

Glycolysis is the first step of extracting energy from glucose. Glycolysis reaction is a 10-reaction biochemical pathway. Location of glycolysis is cytoplasm of the cell because the enzymes required to carry out glycolysis are present in the cytoplasm. They are not bound to any membrane or organelle. In this reaction, two ATP molecules are used up during initial steps. However, at the end of the cycle, four ATP molecules are formed by the substrate-level phosphorylation. Hence, there is a net yield of 2 ATP while catalyzing one glucose molecule by glycolysis. 

Additionally, four electrons are captured during the formation of NADH and that can be used in the production of ATP by aerobic respiration. Also, by this reaction, two molecules of pyruvate are formed that still contain most of the energy the original glucose molecule held. This step occurs in both, aerobic as well as anaerobic respiration.

2. Aerobic Respiration: Stage Two: Pyruvate Oxidation

This stage starts with the end product of the first stage. This stage involves the conversion of pyruvate into carbon dioxide, as well as, into a two-carbon molecule called acetyl-coA. The conversion of every molecule of pyruvate results in the reduction of one molecule of NAD+ to NADH. This NADH also could be used for the production of ATP. Hence, by this step, there is a production of 2 NADH molecules.

3. Aerobic Respiration: Stage Three: The Krebs Cycle

Acetyl-coA enters into the Krebs cycle. This cycle is made up of nine different reactions called the Krebs cycle. This cycle is named after the British biochemist, Sir Hens Krebs. This cycle is also known as the citric acid cycle as in the first step of this cycle, citric acid or citrate is formed. It is also known as the tricarboxylic acid cycle as citrate has three carboxyl groups. During the Krebs cycle, two more ATP molecules are extracted by substrate-level phosphorylation. Also, a large number of electrons is removed and that can be used for the reduction of NAD+ and it is converted into NADH that is useful for the production of ATP by electron transport chain. 

4. Aerobic Respiration: Stage Four: Electron Transport Chain

All the reduced electron carriers now enter into this step, the electron transport chain. In this step, the energetic electrons carried by electron carriers like NADH are utilized for the production of a large amount of ATP. 

Electron transport chains are collectively made up of membrane-embedded proteins and organic molecules. The electronic transport chain components are found in the plasma membrane of prokaryotes, whereas in eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. The electron transport chain contains proteins such as Fd (ferredoxin), PQ (plastoquinone), Cyt C (cytochrome C), Q (ubiquinone), and PC (plastocyanin). The enzyme NADP reductase is also present. It is important in the reduction of an electron acceptor molecule and in the generation of NADPH.

While travelling of electrons through the chain, it enters into a lower energy level from a higher energy level. It means it moves from less electron-hungry molecules to more electron-hungry molecules. Hence, this type of transfer of electron is an example of downhill electron transfer. The above-mentioned different protein complexes use the released energy (released during electron transfer) and that turn out into pumping of the proton from the mitochondrial matrix to the intermembrane space. This is particularly responsible for forming a proton gradient.

The reaction like pyruvate oxidation, the reaction of the Krebs cycle, and the production of ATP by electron transport chains take place within many forms of bacteria, and inside the mitochondria of all eukaryotes. (Note- According to researchers, mitochondria are thought to have evolved from bacteria. As you know, plants and algae are photosynthetic and they are able to produce ATP by using sunlight. However, these photosynthetic organisms are also capable of producing ATP by aerobic respiration just like animals and other non-photosynthetic eukaryotes do. 

Summary of Aerobic Respiration

1. Aerobic respiration is the aerobic catabolism of carbohydrate to carbon dioxide, water, and energy.

2. The overall aerobic respiration can be mentioned by following chemical reaction.

  C₆H₁₂O₆+6O₂→6CO₂+6H₂O

3. This type of cellular respiration is seen in aerobes and facultative anaerobes.

4. Aerobic respiration involves four stages- glycolysis, a transition reaction that forms acetyl-CoA, the Krebs cycle, and an electron transport chain.