Cellular Respiration Definition
A series of metabolic reactions that takes place within the cell is called cellular respiration. The process involves harvesting biochemical energy from organic molecules, especially glucose is converted into ATP (adenosine triphosphate). ATP is the energy molecule used for various energy-requiring processes within the cell.
What is Cellular Respiration?
Cellular respiration is a very important metabolic process that occurs in all living organisms. The process converts the organic molecules into ATP. ATP is the energy molecule for all living organisms therefore this process has great significance. It takes place in both prokaryotic and eukaryotic cells.
Cellular Respiration Location
Cellular respiration occurs in both prokaryotic and eukaryotic cells. The prokaryotes lack membrane-bound organelle, therefore the process completes in the cytoplasm of a prokaryotic cell. Mitochondria is the site for cellular respiration in the eukaryotic cell. The process completes in 4 stages, which include glycolysis, transition process, citric acid cycle, and oxidative phosphorylation.
How does Cellular Respiration Works?
The process can be either aerobic or anaerobic. Despite this, the name cellular respiration is used for the fact that molecular oxygen is used and carbon-di-oxide is produced as the end product in the reaction. The process is described as anaerobic if the final acceptor is not oxygen.
Several anaerobic bacteria adopted anaerobic respiration and use other molecules instead of oxygen. The fermentation is also an anaerobic reaction, in which pyruvate has not been transported into mitochondrion. It takes another pathway to produce energy where oxygen is not required and is converted into a waste product.
Why is Cellular Respiration Important?
Cellular respiration has great significance because ATP is synthesized by this metabolic process. Many metabolic processes like biosynthesis, locomotion and transportation need biochemical energy. ATP completes the energy needs in the body.
Cellular Respiration Location
As described earlier, the process completes in the mitochondrion of eukaryotes, and in prokaryotes, it completes in the cytoplasm. Let’s see the location by the diagram.
ATP is the stored potential energy form in all organisms. ATP (adenosine triphosphate) is the final chemical product of cellular respiration. Mitochondria is the site of ATP synthesis, therefore also called the “Powerhouse of the cell”.
Mitochondria is a double membrane organelle consists of the outer membrane and inner membrane. The space between these membranes is called intermembrane space. The outer membrane contains pores. Electron transport chain complexes in the inner membrane of mitochondria.
The metabolism of carbohydrates completes by several enzymes- catalyzed reactions. ATP is synthesized at the end of the reaction. The energy needs of the cell are fulfilled by the dephosphorylation of ATP into ADP (adenosine diphosphate).
Role of Oxygen in Cellular Respiration
The process uses molecular oxygen and is therefore considered an aerobic reaction. Molecular oxygen is diatomic and electronegative. It releases chemical energy by pulling electrons towards it. Oxygen combines the potential energy from our food.
For example, Glucose can combine with oxygen, and the high-energy molecules of glucose are transferred to the oxygen. The process releases potential energy stored in the form of ATP.
Cellular Respiration Equations
The process includes several enzyme-catalyzed chemical reactions. For example-
Aerobic respiration: C6H12O6 + 3O2 ⇒ 6CO2 + 6H2O + ATP
Anaerobic cellular respiration:
• Lactic acid fermentation
C6H12O6 ⇒ 2C3H6O3
2ADP+ 2NAD+ + 2Pi ⇒ 2ATP + 2 NADH + 2Pi + 2H2O
• Alcoholic fermentation
C6H12O6 ⇒ 2C2H5OH
2ADP + 2 NAD+ + 2Pi ⇒ 2ATP + 2NADH + 2Pi + 2H2O
Types of Cellular Respiration
Aerobic Respiration: Aerobic respiration is the process where molecular oxygen is used as an electron acceptor. Water and carbon dioxide are the end products of this process.
Lactic Acid Fermentation (Anaerobic Respiration): Lactic acid fermentation is an example of anaerobic respiration. The process includes conversion of pyruvate molecule into lactic acid and releases energy in the form of ATP. Lactic acid fermentation also occurs in muscle cells of humans. The method used to accomplish energy needs during vigorous exercise and lack of oxygen.
Alcoholic Fermentation: It is also called ethanol fermentation. It is the process of converting sugar into ethyl alcohol. Yeast and bacterial species use this method to complete their energy needs. It is also an anaerobic process. The process begins with glycolysis, in which a molecule of glucose breaks down into two molecules of pyruvate. The pyruvate then undergoes fermentation. It forms two molecules of ethanol and two molecules of carbon dioxide.
Methanogenesis: Only some anaerobic bacteria use this method to accomplish their energy needs. The bacteria include Methanobacteriales, Methancoccales, Methanomicrobiales, Methanopyrales, and Methanosarcinales. There are mainly 3 pathways for methanogenesis. Acetoclastic Methanogenesis, Methylotrophic Methanogenesis, Hydrogenotrophic Methanogenesis.
Cellular Respiration Steps
Glycolysis: Glycolysis means “splitting sugar”. The process of splitting sugar molecules into two molecules of pyruvate is called glycolysis. Glycolysis is an anaerobic process that occurs in most organisms. It is also called Embden- Meyerhoff pathway. The Embden –Meyerhoff pathway can be separated into two phases:
1. The investment phase
2. The pay Off pathway
Step 1. In the first step, the glucose is converted into glucose-6- phosphate. The reaction is catalyzed by the enzyme hexokinase phosphorylates.
Step 2. Fructose-6-phosphate is formed by the isomerization of glucose-6-phosphate. The reaction is catalyzed by phosphoglucose isomerase.
Step 3. Phosphofructokinase catalyzes the transfer of the phosphate group and converts glucose-6-phosphate into fructose1, 6-bisphosphate.
Step 4. Fructose 1, 6-bisphosphate converted into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate in the presence of enzyme aldolase.
Step 5. The isomerization of Dihydroxyacetone phosphate into GAP occurs in the presence of triosephosphate isomerase.
Step 6. The oxidation of GAP occurs by coenzyme nicotinamide adenine dinucleotide and phosphorylated by enzyme Glyceraldehyde- 3-phosphate dehydrogenase. The step results in the conversion of NAD to NADH.
Step 7. Phosphoglycerate kinase converts 1, 3 bisphosphoglycerate to 3-phosphoglycerate. The step generates one molecule of ATP.
Step 8. The step is catalyzed by phosphoglycerate mutase. It is the rearrangement step and forms 2-phosphoglycerate.
Step 9. In this step, phosphoenolpyruvate is formed by conversion of 2- phosphoglycerate.
Step 10. The pyruvate kinase transfers a phosphate from PEP and yields ATP from the molecule of ADP and two molecules of pyruvate.
Transition Reaction: It is also called oxidative phosphorylation and occurs in mitochondria. In aerobic respiration, the pyruvate moves into the mitochondrial matrix and is converted into acetyl CoA. The process produces Co2 and NADH.
It is also called the citric acid cycle. The process was discovered by Hans Adolf Krebs in 1937. Krebs cycle completes in the mitochondrial matrix. The cycle produces 2 molecules of CO2, 3 molecules of NADH, 1 molecule of FADH2, and 1 molecule of GTP. The process completes in 8 steps.
Step 1. Acetyl CoA and oxaloacetate take part in the reaction and release CoA group. It produces citrate in the presence of the enzyme citrate synthase.
Step 2. Isocitrate is formed from citrate by the enzyme aconitase
Step 3. Ketone is formed by isocitrate in the presence of coenzyme NAD+. Isocitrate dehydrogenase removes the CO2 group and forms alpha-ketoglutarate.
Step 4. The step includes oxidative decarboxylation by the enzyme alpha-ketoglutarate dehydrogenase. In this step, alpha-ketoglutarate converts into succinyl-CoA.
Step5. Succinyl- CoA is converted into succinate. The process is catalyzed by succinate thiokinase. The process converts ADP into ATP.
Step6. With the oxidation reaction, the succinate is converted into fumarate. The step is catalyzed by succinate dehydrogenase. FAD is used as a coenzyme and forms a molecule of FADH2.
Step7. Fumarate is converted into L- malate by the hydration reaction. The process is catalyzed by the enzyme fumarase.
Step 8. L-malate is oxidized and form oxaloacetate in the presence of enzyme malate dehydrogenase.
Electron Transport Chain and Chemiosmosis
It is the final stage of cellular respiration. The reaction completes in the inner mitochondrial membrane. The process is used to form a gradient of protons that produces ATP. The molecular oxygen is used as an electron acceptor. The complexes involved in the electron transport chain.
The electron transport chain includes four protein complexes. These complexes are complex I, complex II, III, IV, cytochrome C, and coenzyme Q. The complex uses the decrease in free energy to pump hydrogen ions to the intermembrane space. The electrochemical gradient created in this process is used to generate ATP. ATP synthase is used to produce ATP.
Chemiosmosis: Chemiosmosis is the process of the formation of hydrogen ion gradient by the electron transport chain. A British Biochemist names, Peter Mitchell discovered chemiosmosis.
ATP production in cellular respiration: The overall process of cellular respiration produces 34 molecules of ATP. The glycolysis produces 4 molecules of ATP from which 2 molecules are invested in the investment phase. 2 molecules of ATP are produced in the Krebs cycle. The fermentation reaction produces 2 molecules of ATP.
Products of Cellular Respiration
• The glycolysis produces 2 molecules of pyruvate or pyruvic acid, ATP, and NADH.
• The oxidative phosphorylation converts the pyruvate into CoA and produces NADH and Co2.
• The citric acid cycle use acetyl CoA and oxaloacetate and produce oxaloacetate, NADH, ATP, FADH2, and CO2.
• The electron transport chain is the last stage of cellular respiration. It results in the production of NAD, FAD, and ATP.
Cellular Respiration Disorders
Several mitochondrial dysfunctions affect the process of cellular respiration. These disorders can cause mutations in mitochondrial or nuclear DNA. The mutation can cause protein deficiency, complex I mitochondrial disease, and many other disorders.
More than 150 different mitochondrial diseases or disorders are identified that affect cellular respiration. The research studies are ongoing for a better understanding of these mutations or conditions that arise mitochondrial disorders.
Purpose of Cellular Respiration
The main purpose of the process is the metabolism of glucose molecules and produce ATP molecules. ATP is the energy currency for all living organisms. Various cellular activities use ATP as fuel. Therefore cellular respiration is an important process for all living organisms. It is completed either in the presence or absence of oxygen.
Cellular Respiration Citations
- Rejuvenating cellular respiration for optimizing respiratory function: targeting mitochondria. Am J Physiol Lung Cell Mol Physiol . 2016 Jan 15;310(2):L103-13.
- Pathophysiology of Myocardial Infarction. Compr Physiol . 2015 Sep 20;5(4):1841-75.