Cellular Respiration: Definition, Equation, Types of Cellular Respiration

Cellular respiration refers to breathing that occurs at the smallest level of our bodies, the cellular level. Respiration is the process through which the nutrients from consumed food get transformed into usable energy. Each cell needs energy to carry out its activities because it is the structural and functional unit of life. 

All living things engage in the act of respiration to survive. Breathing gives the body extra energy for sprinting. Even though breathing is a crucial component of respiration, breathing as a whole is a phenomenon that keeps our bodies functioning.

Key Terms: Cellular Respiration, ATP, FAD, Aerobic Respiration, Anaerobic Respiration, Citric Acid Cycle, Oxidative Phosphorylation

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What is Cellular Respiration?

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A series of metabolic reactions known as cellular respiration is used to transform chemical energy (in the form of sugar) into a usable form of energy (ATP) inside the cell.

• Catabolic processes, which break down bigger organic molecules into smaller forms, are a part of cellular respiration.
• Several processes that are tailored for the breakdown of particular molecules make up the overall process of cellular respiration.
• All living things undergo cellular respiration, which is the fundamental process of life. The majority of multicellular organisms engage in aerobic respiration as their primary mode of cellular respiration.
• High energy bonds must be broken down for cells to respire, releasing energy in the form of ATP.
• Although technically a combustion reaction, cellular respiration takes place in the cell in a gradual, regulated manner to release energy through a sequence of processes.
• In the presence of potent oxidizing agents like molecular oxygen, the majority of the processes that take place during cellular respiration are redox reactions.
• Different phases of cellular respiration are catalyzed by various sets of enzymes, all of which can be located inside the cell.

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What is ATP? 

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An inorganic substance called adenosine triphosphate (ATP) functions as an energy-carrying molecule by storing the energy generated during chemical reactions.

• The nucleotide molecule ATP has three primary structural components: adenine, a nitrogenous base; ribose, a sugar unit; and three phosphate groups attached to the ribose backbone.
• In contrast to proteins and carbohydrates, ATP functions as a shuttle to release energy during energy-intensive tasks.
• The dissolution of the phosphate bonds to produce ADP or AMP molecules causes ATP to release energy.
• Mitochondria are the cell's powerhouse; thus, the majority of ATP is created there, while some ATP may also be made in the cytoplasm.

Also check:

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What is FAD?

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A metabolic coenzyme known as flavin adenine dinucleotide (FAD) functions as an electron carrier in a variety of enzymatic activities throughout the body.

• FAD has two nucleotide units, one of which has adenine as the nitrogen base and the other of which has flavin units.
• In the body, riboflavin and two ATP molecules are used to create FAD. FMN is produced as a result of ATP's phosphorylation of riboflavin. Then, FAD is created as a result of the transfer of one AMP unit from ATP.
• There are two oxidation states of the FAD involved in cellular respiration, FADH, and FADH₂.
• FAD molecules participate in the exchange of electrons between molecules because of their various oxidation states.

Flavin Adenine Dinucleotide Structure

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Location of Cellular Respiration 

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Individual cells engage in cellular respiration to generate energy for that specific cell. 

• Cellular respiration begins in the cytoplasm of the eukaryotic cell. Because the cytoplasm contains the enzymes needed for glycolysis, the first stage of cellular respiration, glycolysis, occurs there.
• The cell's mitochondria, where the remaining phases of cellular respiration take place, receive the products of the glycolysis process.
• The citric acid cycle and oxidative phosphorylation are among the subsequent processes that take place in the mitochondria's outer and inner membranes.
• Since prokaryotic cells lack distinct cell organelles, all phases of cellular respiration take place in the cytoplasm in these cells.

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Cellular Respiration Equation 

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There may be variations in the processes and reactions involved in various forms of cellular respiration.

Aerobic Respiration Equation 

• ADP, an oxygen molecule, and one glucose molecule combine during aerobic respiration to produce carbon dioxide, water, and energy. The most effective mechanism for cellular respiration, which also yields the most ATPs, is aerobic respiration.
• Aerobic respiration's general equation is:

Anaerobic Respiration Equation 

• The equation used in anaerobic respiration depends on the pathway used. Because they generate fewer ATPs than aerobic respiration, anaerobic pathways are less effective.
• One glucose molecule is converted into ethyl alcohol, carbon dioxide, and energy during the fermentation of alcoholic beverages. There is no oxygen present during the procedure.
• Anaerobic respiration's general equation is:

• One glucose molecule is converted into lactic acid and energy during the lactic acid fermentation process. Even in the absence of oxygen, this respiration still takes place.

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Types of Cellular Respiration 

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Aerobic Respiration 

• Complex, high-energy molecules are broken down during aerobic respiration.
• Carbohydrates are completely oxidized during aerobic respiration to yield the most energy possible.
• Most eukaryotes and some prokaryotes engage in aerobic respiration, the most effective kind of cellular respiration.
• Aerobic respiration uses the oxygen molecule as the last electron acceptor, which effectively produces ATP.
• Because the synthesis of ATP is aided by the double bond in the oxygen molecule, aerobic respiration is more efficient than anaerobic respiration.
• The exchange of oxygen and carbon dioxide gas takes place during the much lengthier process known as aerobic respiration.
• Carbon dioxide, water, and ATP are the byproducts of aerobic respiration when phosphate groups are added to ADP molecules.
• Additionally, during aerobic respiration, which creates ATP via the electron transport chain, additional energy-rich molecules like NADH and FADH₂ are also created.

Anaerobic Respiration 

• Prokaryotic organisms engage in anaerobic respiration, a type of cellular respiration that takes place in the absence of oxygen and results in the production of an acid or alcohol as the final product.
• Other molecules or ions, such as sulfate or nitrate, take the place of oxygen as the ultimate electron acceptor in anaerobic respiration.
• Anaerobic respiration takes several different forms, each of which affects the byproduct.
• Based on the electron acceptors and byproducts, there are several types of anaerobic respiration and fermentation.
• Carbohydrates are broken down during alcoholic fermentation to create alcohol and carbon dioxide as byproducts.
• Lactic acid fermentation is the process by which lactic acid bacteria convert carbohydrates into lactic acid in the absence of oxygen.
• Methane and carbon dioxide are the byproducts of methanogenesis, a special type of anaerobic respiration.
• Prokaryotes living in low-oxygen conditions, such as deep-sea surfaces, are more likely to engage in anaerobic respiration.
• Because the final electron acceptor in anaerobic respiration has a lower reduction potential than oxygen molecules, it is less effective than in aerobic respiration.
• However, the biogeochemical cycles of elements like sulfur, carbon, and nitrogen depend on anaerobic respiration.
• Anaerobic respiration occurs in the cell's cytoplasm because the cytoplasm has the necessary enzymes for the process.

Cellular Respiration

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Cellular Respiration Steps 

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Glycolysis 

• The first stage of cellular respiration consists of a series of ten steps that catabolize glucose molecules to create pyruvate.
• The first stage of glucose metabolism is the common pathway in both aerobic and anaerobic respiration. It is known as glycolysis.
• The breakdown of a six-carbon substance like glucose into two three-carbon compounds (pyruvate) during glycolysis results in the release of two ATP molecules.
• Glycolysis is the first step in aerobic respiration before the citric acid cycle and electron transport chain, which are in charge of producing the majority of ATPs.

A summary of the process of glycolysis can be written as follows:

Pyruvate Oxidation 

• The byproducts of glycolysis, a prevalent route in both aerobic and anaerobic respiration, are pyruvate molecules.
• The presence of oxygen and the state of the metabolism control what happens to pyruvate.
• After being transported from the cytoplasm to the mitochondria, pyruvate molecules undergo oxidation in the presence of oxygen.
• The enzyme pyruvate dehydrogenase complex dehydrogenation the pyruvate produced during glycolysis to produce acetyl Co-A and CO₂. Prokaryotes' cytoplasm and eukaryotes' mitochondrial matrix include the enzyme.
• In the case of aerobic respiration, pyruvate oxidation serves as a bridge between glycolysis and the citric acid cycle.

The overall reaction of pyruvate oxidation can be summed up as:

Citric Acid Cycle 

• The full oxidation of acetyl CoA to produce carbon dioxide and water molecules is known as the citric acid cycle or Kreb's cycle.
• It is the most common pathway for the aerobic metabolism of substances with high energy content.
• The cycle's processes give the electron transport chain the electrons it needs to decrease oxygen while producing ATP.
• This cycle is crucial for the metabolism of numerous macromolecules, including fatty acids and amino acids, in addition to carbohydrates.
• Because energy-dense molecules like NAD⁺ and FAD can recover ATP from their reduced form by transferring electrons to molecular oxygen, the cycle can only take place in the presence of oxygen.
• The citric acid cycle has two basic functions: disposing of carbon and hydrogen atoms and converting chemical potential energy into metabolic energy in the form of ATP.
• A single acetyl Co-A molecule undergoes full oxidation, producing a total of 12 ATPs.
• Only one of the 12 ATP molecules is created by the cycle itself; the other 11 are made when high-energy molecules enter the electron transport chain.
• The citric acid cycle's total reaction can be summed up as follows:

Oxidative Phosphorylation 

• The final stage of aerobic respiration involves a series of redox events to produce ATP molecules.
• The transfer of electrons from the organic component to oxygen during the citric acid cycle results in the release of energy in the form of ATP.
• The chain is made up of several proteins with closely bound prosthetic groups that can accept and donate electrons.
• The amount of energy-dense molecule that transmits the electrons determines how much ATP is produced during oxidative phosphorylation. 
• As electrons are transported across the chain, a variety of chemical groups serve as electron transporters. 

The following is a list of all the reactions in the electron transport chain:

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What is Methanogenesis?

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• The breakdown of carbohydrate molecules into methane and carbon dioxide as byproducts is a distinctive feature of methanogenesis, a kind of anaerobic respiration.
• The majority of methanogenesis-respiring organisms are members of the Archaea domain and coexist closely with anaerobic bacteria.
• Because it is the last stage of the degradation of biomass, methanogenesis is a crucial metabolic activity in these species.
• Methanogenesis is an anaerobic process, where a carbon molecule serves as the terminal electron acceptor. Acetic acid or carbon dioxide are frequently used as electron acceptors in methanogenesis.
• Methanogenesis is crucial for the decomposition of organic matter in addition to aiding in the disintegration of big, complex organic compounds.
• Except for carbon dioxide, all other electron acceptors get depleted during the later phases of bio breakdown. In this scenario, methanogenesis, where carbon dioxide serves as the electron acceptor, breaks down the organic material that is still present.
• Some symbiotic bacteria found in the digestive tract of ruminants also produce methane. This permits the digestion of organic material that the cattle couldn't otherwise break down.

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Things to Remember 

• Cellular Respiration- a series of metabolic processes taking place inside the cells that transform the biological energy from food into ATP.
• Aerobic Respiration - 
• C₆H12O₆ + 6O₂ + 36ADP + 36 Pi → 6CO₂ + 6H₂O + 36 ATP
• Anaerobic Respiration - 
• C₆H₁₂O₆ + 2ADP + 2Pi → 2C₂H₅OH + 2CO₂ + 2ATP
• Cellular respiration steps- 
  • Glycolysis
  • Pyruvate Oxidation
  • Citric Acid Cycle
  • Oxidative Phosphorylation
• Fermentation- Microorganisms carry out fermentation, which produces lactic acid or alcohol as well as the release of carbon dioxide.
• Lactic Acid Fermentation- Glucose is transformed into lactic acid during lactic acid fermentation.
• Alcoholic fermentation- The Carbohydrate molecule is partially oxidized to produce alcohol as a byproduct.
• Methanogenesis- is a form of anaerobic respiration that involves the breakdown of carbohydrate molecules into methane carbon dioxide as a byproduct.