Cellular respiration describes the metabolic reactions and processes that take place in a cell or across the cell membrane to get biochemical energy from fuel molecules and then release of the cells' waste products. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός Energy can be released by the oxidation of multiple fuel molecules and is stored as "high-energy" carriers. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state The reactions involved in respiration are catabolic reactions in metabolism. For the related metabolic process see Anabolism. Catabolism is the set of Metabolic pathways which break down molecules into The movement of a pair of electrons down the electron transport chain produces enough energy to form 3 ATP molecules from ADP.
Fuel molecules commonly used by cells in respiration include glucose, amino acids and fatty acids, and a common oxidizing agent (electron acceptor) is molecular oxygen (O2). Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology. In Chemistry, an amino acid is a Molecule containing both Amine and Carboxyl Functional groups In Biochemistry, this In Chemistry, especially Biochemistry, a fatty acid is a Carboxylic acid often with a long unbranched Aliphatic tail ( chain) which An oxidizing agent or oxidising agent (also called an oxidant, oxidizer or oxidiser) can be defined as either a Chemical compound An electron acceptor is a chemical entity that accepts Electrons transferred to it from another compound Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the There are organisms, however, that can respire using other organic molecules as electron acceptors instead of oxygen. An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon. Organisms that use oxygen as a final electron acceptor in respiration are described as aerobic, while those that do not are referred to as anaerobic. An anaerobic organism is any Organism that does not require Oxygen for growth and may even die in its presence
The energy released in respiration is used to synthesize molecules that act as a chemical storage of this energy. One of the most widely used compounds in a cell is adenosine triphosphate (ATP) and its stored chemical energy can be used for many processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes. Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy Biosynthesis is a phenomenon wherein Chemical compounds are produced from simpler Reagents Biosynthesis unlike Chemosynthesis, takes place within living The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Because of its ubiquitous nature, ATP is also known as the "universal energy currency", since the amount of it in a cell indicates how much energy is available for energy-consuming processes.
Contents |
Aerobic respiration
Aerobic respiration requires oxygen in order to generate energy (ATP). Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy It is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion to be fully oxidized by the Krebs cycle. Pyruvic acid (CH3COCO2H is an alpha-keto acid. The Carboxylate Anion of pyruvic acid is known as pyruvate. See also Gluconeogenesis, which carries out a process wherein glucose is synthesized rather than catabolized In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. The citric acid cycle, also known as the tricarboxylic acid cycle ( TCA cycle) or the Krebs cycle, (or rarely the Szent-Györgyi–Krebs cycle The product of this process is energy in the form of ATP (Adenosine Triphosphate), by substrate-level phosphorylation, NADH and FADH2. Substrate-level phosphorylation is a type of Chemical reaction that results in the formation of Adenosine triphosphate (ATP by the direct transfer of a Phosphate Nicotinamide adenine dinucleotide, abbreviated NAD+, is a Coenzyme found in all living cells The compound is a dinucleotide since it consists In Biochemistry, flavin adenine dinucleotide ( FAD) is a Redox cofactor involved in several important reactions in Metabolism.
Simplified Reaction: C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) ΔHc -2880 kJ
The reducing potential of NADH and FADH2 is converted to more ATP through an electron transport chain with oxygen as the "terminal electron acceptor". An electron transport chain couples a chemical reaction between an electron donor (such as NADH) and an electron acceptor (such as O2) to the transfer Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. Oxidative phosphorylation is a Metabolic pathway that uses energy released by the oxidation of Nutrients to produce Adenosine triphosphate (ATP This works by the energy released in the consumption of pyruvate being used to create a chemiosmotic potential by pumping protons across a membrane. In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This potential is then used to drive ATP synthase and produce ATP from ADP. Adenosine diphosphate, abbreviated ADP, is a Nucleotide. It is an Ester of Pyrophosphoric acid with the Nucleoside Adenosine Biology textbooks often state that between 36-38 ATP molecules can be made per oxidised glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 32-34 from the electron transport system). Generally, 38 ATP molecules are formed from aerobic respiration. However, this maximum yield is never quite reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into the mitochondrial matrix.
Aerobic metabolism is 19 times more efficient than anaerobic metabolism (which yields 2 mol ATP per 1 mol glucose). They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. See also Gluconeogenesis, which carries out a process wherein glucose is synthesized rather than catabolized The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other
Glycolysis
Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms and does not require oxygen. See also Gluconeogenesis, which carries out a process wherein glucose is synthesized rather than catabolized In Biochemistry, a metabolic pathway is a series of chemical reactions occurring within a cell. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP. Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology. Pyruvic acid (CH3COCO2H is an alpha-keto acid. The Carboxylate Anion of pyruvic acid is known as pyruvate. Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy Four molecules of ATP per glucose are actually produced; however, two are consumed for the preparatory phase. See also Gluconeogenesis, which carries out a process wherein glucose is synthesized rather than catabolized The initial phosphorylation of glucose is required to destabilize the molecule for cleavage into two triose sugars. Phosphorylation is the addition of a Phosphate (PO4 group to a Protein molecule or a small molecule A triose is a Monosaccharide containing three Carbon Atoms There are only two trioses an aldotriose ( Glyceraldehyde) and a Sugar is a class of edible Crystalline substances mainly Sucrose, Lactose, and Fructose. During the pay-off phase of glycolysis, four phosphate groups are transferred to ADP by substrate-level phosphorylation to make four ATP, and two NADH are produced when the triose sugars are oxidized. See also Gluconeogenesis, which carries out a process wherein glucose is synthesized rather than catabolized A phosphate, an Inorganic chemical, is a salt of Phosphoric acid. Substrate-level phosphorylation is a type of Chemical reaction that results in the formation of Adenosine triphosphate (ATP by the direct transfer of a Phosphate Glycolysis takes place in the cytoplasm of the cell. The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called The overall reaction can be expressed this way:
- Glucose + 2 NAD+ + 2 Pi + 2 ADP → 2 pyruvate + 2 NADH + 2 ATP + 2 H2O
Oxidative decarboxylation of pyruvate
The pyruvate is oxidized to acetyl-CoA and CO2 by the Pyruvate dehydrogenase complex, a cluster of enzymes—multiple copies of each of three enzymes—located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes. Pyruvic acid (CH3COCO2H is an alpha-keto acid. The Carboxylate Anion of pyruvic acid is known as pyruvate. Pyruvate decarboxylation is the biochemical reaction that uses Pyruvate to form Acetyl-CoA, releasing reducing equivalents and Carbon dioxide. Pyruvate dehydrogenase complex (PDC is a complex of three Enzymes that transform Pyruvate into Acetyl-CoA by a process called Pyruvate decarboxylation In the Mitochondrion, the matrix contains soluble Enzymes that catalyze the Oxidation of pyruvate and other small organic molecules In the process one molecule of NADH is formed per pyruvate oxidized.
Citric Acid cycle
This is also called the Krebs cycle or the tricarboxylic acid cycle. The citric acid cycle, also known as the tricarboxylic acid cycle ( TCA cycle) or the Krebs cycle, (or rarely the Szent-Györgyi–Krebs cycle When oxygen is present, acetyl-CoA is produced from pyruvate. Acetyl-CoA is an important molecule in metabolism used in many biochemical reactions If oxygen is not present the cell undergoes fermentation of the pyruvate molecule. If acetyl-CoA is produced the molecule then enters the citric acid cycle (Krebs cycle) inside the mitochondrial matrix, and gets oxidized to CO2 while at the same time reducing NAD to NADH. Acetyl-CoA is an important molecule in metabolism used in many biochemical reactions The citric acid cycle, also known as the tricarboxylic acid cycle ( TCA cycle) or the Krebs cycle, (or rarely the Szent-Györgyi–Krebs cycle Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single Nicotinamide adenine dinucleotide, abbreviated NAD+, is a Coenzyme found in all living cells The compound is a dinucleotide since it consists Nicotinamide adenine dinucleotide, abbreviated NAD+, is a Coenzyme found in all living cells The compound is a dinucleotide since it consists NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. Nicotinamide adenine dinucleotide, abbreviated NAD+, is a Coenzyme found in all living cells The compound is a dinucleotide since it consists An electron transport chain couples a chemical reaction between an electron donor (such as NADH) and an electron acceptor (such as O2) to the transfer Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two waste products, H2O and CO2, are created during this cycle. Cellular waste products are formed as a byproduct of Cellular respiration, a series of processes and reactions that generate energy for the cell in the form of
Oxidative phosphorylation
In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. Oxidative phosphorylation is a Metabolic pathway that uses energy released by the oxidation of Nutrients to produce Adenosine triphosphate (ATP An electron transport chain couples a chemical reaction between an electron donor (such as NADH) and an electron acceptor (such as O2) to the transfer In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane An ATP synthase ( is a general term for an Enzyme that can synthesize Adenosine triphosphate (ATP from Adenosine diphosphate (ADP and inorganic Cristae (singular crista) are the internal compartments formed by the inner membrane of a Mitochondrion. It comprises the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidizing the NADH produced from the Krebs cycle. In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP.
Theoretical yields
The yields in the table below are for one glucose molecule being fully oxidized into carbon dioxide. It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state Coenzymes are small organic non- Protein Molecules that carry chemical groups between Enzymes Coenzymes are sometimes referred to as cosubstrates
| Step | coenzyme yield | ATP yield | Source of ATP |
|---|---|---|---|
| Glycolysis preparatory phase | -2 | Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm. | |
| Glycolysis pay-off phase | 4 | Substrate-level phosphorylation | |
| 2 NADH | 4 (6) | Oxidative phosphorylation. Only 2 ATP per NADH since the coenzyme must feed into the electron transport chain from the cytoplasm rather than the mitochondrial matrix. If the malate shuttle is used to move NADH into the mitochondria this might count as 3 ATP per NADH. The malate-aspartate shuttle (sometimes also the malate shuttle) is a biochemical system for translocating electrons produced during Glycolysis (TCA/Krebs Cycle across | |
| Oxidative decarboxylation of pyruvate | 2 NADH | 6 | Oxidative phosphorylation |
| Krebs cycle | 2 | Substrate-level phosphorylation | |
| 6 NADH | 18 | Oxidative phosphorylation | |
| 2 FADH2 | 4 | Oxidative phosphorylation | |
| Total yield | 36 (38) ATP | From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes. | |
Although there is a theoretical yield of 36-38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized due to losses such as the cost of moving pyruvate (from glycolysis), phosphate, and ADP (substrates for ATP synthesis) into the mitochondria. All are actively transported using carriers that utilise the stored energy in the proton electrochemical gradient. In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane
- The pyruvate carrier is a symporter and the driving force for moving pyruvate into the mitochondria is the movement of protons from the intermembrane space to the matrix. A symporter is an Integral membrane protein that is involved in movement of two or more different molecules or ions across a phospholipid membrane such as the Plasma membrane The intermembrane space is the region between the Inner membrane and the Outer membrane of a Mitochondrion or a Chloroplast.
- The phosphate carrier is an antiporter and the driving force for moving phosphate ions into the mitochondria is the movement of hydroxyls ions from the matrix to the intermembrane space. An antiporter (also called exchanger or counter-transporter) is an Integral membrane protein which is involved in Secondary active transport
- The adenine nucleotide carrier is an antiporter and exchanges ADP and ATP across the inner membrane. Adenine nucleotide translocator ( ANT) also known as the ADP / ATP translocator is a Mitochondrial protein The inner membrane is the Biological membrane (phospholipid bilayer of an Organelle or Gram-negative Bacteria that is within an Outer The driving force is due to the ATP (-4) having a more negative charge than the ADP (-3) and thus it dissipates some of the electrical component of the proton electrochemical gradient.
The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H+ are needed to make 1 ATP. Obviously this reduces the theoretical efficiency of the whole process and the likely maximum is closer to 28-30 ATP molecules. [1] In practice the efficiency may be even lower due to the inner membrane of the mitochondria being slightly leaky to protons. [2] Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons. Thermogenin (called Uncoupling protein by its discoverers and now known as uncoupling protein 1 or UCP1) is an uncoupling protein found in the mitochondria When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis. An electron transport chain couples a chemical reaction between an electron donor (such as NADH) and an electron acceptor (such as O2) to the transfer An ATP synthase ( is a general term for an Enzyme that can synthesize Adenosine triphosphate (ATP from Adenosine diphosphate (ADP and inorganic The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in a baby's brown fat, for thermogenesis, and hibernating animals.
Anaerobic respiration
Without oxygen, pyruvate is not metabolized by cellular respiration but undergoes a process of fermentation. See also Fermentation (biochemistry Anaerobic respiration (anaerobiosis refers to the Oxidation of molecules in the absence of Oxygen to produce Fermentation is the process of deriving energy from the oxidation of organic compounds such as carbohydrates using an endogenous electron acceptor which is The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. Cellular waste products are formed as a byproduct of Cellular respiration, a series of processes and reactions that generate energy for the cell in the form of This serves the purpose of oxidizing the hydrogen carriers so that they can perform glycolysis again and removing the excess pyruvate. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. Lactic acid ( IUPAC Systematic name: 2-hydroxypropanoic acid) also known as milk acid, is a Chemical compound that plays a role This type of fermentation is called lactic acid fermentation. Lactic acid fermentation is a biological process by which sugars such as Glucose, Fructose, and Sucrose, are converted into cellular energy and the metabolic In yeast, the waste products are ethanol and carbon dioxide. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single This type of fermentation is known as alcoholic or ethanol fermentation. See also Fermentation (biochemistry Ethanol fermentation is the biological process by which sugars such as Glucose, Fructose, and Sucrose The ATP generated in this process is made by substrate phosphorylation, which is phosphorylation that does not involve oxygen.
Anaerobic respiration is less efficient at using the energy from glucose since 2 ATP are produced during anaerobic respiration per glucose, compared to the 30 ATP per glucose produced by aerobic respiration. This is because the waste products of anaerobic respiration still contain plenty of energy. Cellular waste products are formed as a byproduct of Cellular respiration, a series of processes and reactions that generate energy for the cell in the form of Ethanol, for example, can be used in gasoline (petrol) solutions. Glycolytic ATP, however, is created more quickly. For prokaryotes to continue a rapid growth rate when they are shifted from an aerobic environment to an anaerobic environment, they must increase the rate of the glycolytic reactions. Thus, during short bursts of strenuous activity, muscle cells use anaerobic respiration to supplement the ATP production from the slower aerobic respiration, so anaerobic respiration may be used by a cell even before the oxygen levels are depleted, as is the case in sports that do not require athletes to pace themselves, such as sprinting. Sprints are short running races in athletics. They are roughly classified as events in which top runners will not have to "pace themselves" but can run as fast as
Efficiency of aerobic and anaerobic respiration
Aerobic respiration During aerobic respiration 38 molecules of ATP are produced for every molecule of glucose that is oxidised. C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + 38 ATP The energy released by the complete oxidation of glucose is 2880KJ per mole. The energy contained in 1 mole of ATP is 30. 6KJ. Therefore the energy contained in 38 moles of ATP is 30. 6×38=1162. 8 kJ. Therefore efficiency of transfer of energy in aerobic respiration is=1162. 8/2880=40. 4%.
Anaerobic respiration (1) Yeast (alcoholic fermentation). During alcoholic fermentation, two molecules of ATP are produced. for every molecule of glucose used.
glucose → 2ethanol + 26CO2 (g) +2 ATP
The total energy released by the conversion of glucose to ethanol is 210kj per mole. The energy contained in 2 molecules of ATP is 2×30. 6=61. 2kJ. Therefore efficiency of transfer of energy during alcoholic fermentation is 61. 2/210=29. 1%.
(2) Muscle (lactate fermentation). During lactate fermentation, 2 molecules of ATP are produced for every molecule of glucose used.
glucose → 2 lactate + 2ATP
The total energy released by conversion of glucose to lactate is 150kj per mole. Therefore efficiency of transfer of energy in lactic acid fermentation is 61. 2/150=40. 8%. The amount of energy captured as ATP during aerobic respiration is 19 times as much as for anaerobic respiration. From this point of view Aerobic respiration is more efficient than anaerobic respiration. This is because a great deal of energy remains locked within lactate and ethanol.
Extract from 'Biological Science' by D. J. Taylor, N. P. O Green and G. W. Stout, Cambridge University Press, ISBN 0 521 639239
See also
- Tetrazolium chloride: cellular respiration indicator
References
- ^ Rich PR (2003). Triphenyl tetrazolium chloride, TTC, or simply Tetrazolium chloride is a Redox indicator commonly used in biochemical experiments especially to indicate "The molecular machinery of Keilin's respiratory chain". Biochem. Soc. Trans. 31 (Pt 6): 1095-105. PMID 14641005.
- ^ Porter RK, Brand MD (1995). "Mitochondrial proton conductance and H+/O ratio are independent of electron transport rate in isolated hepatocytes". Biochem. J. 310 ( Pt 2): 379-82. PMID 7654171.
External links
- Chart of Important Metabolic Products
- A detailed description of respiration vs. fermentation
- Kimball's online resource to cellular respiration
- Cellular Respiration and Fermentation at Clermont College
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic