All the living cells obtain energy for cellular processes via the process of cellular respiration. It is the process in which food particles are broken down into smaller molecules and energy is released. During cellular respiration, the carbon atoms present in the various food substances are oxidized to ultimately release carbon dioxide. A large amount of energy is released during these oxidation reactions. This energy is trapped by some metabolic intermediates and is later used to make ATP, the energy currency of the living cells.
Two types of cellular respiration are seen in living organisms; aerobic and anaerobic. Aerobic respiration takes place in the presence of oxygen while anaerobic respiration occurs only in the absence of molecular oxygen.
To understand the differences between these two types of respiration, we will take a look at various aspects of aerobic as well as anaerobic respiration. At the end of this article, you will be able to identify the differences between aerobic and anaerobic respiration. We will mainly focus on the glucose breakdown as it is the most widely used fuel by the living cells.
Common Reactions – Glycolysis
If glucose molecules are used as fuel, glycolysis is common to both aerobic as well as anaerobic respiration. Both types of respirations begin with the breakdown of glucose into two molecules of pyruvic acid.
No oxygen molecules are used in the process of glycolysis meaning that it can occur in both oxygen-rich and oxygen-poor environments. This is the reason why it occurs in both aerobic and anaerobic respiration.
Reactions of Glycolysis
The process of glycolysis takes place in ten steps. They are divided into two phases;
- Energy-investment phase
- Energy-generation phase
Both these phases comprise of five reactions. A brief detail of these reactions is given below.
The five reactions of this phase are as follows;
- In the first reaction, glucose is phosphorylated to form glucose-6-phosphate. This is done by a family of enzymes called hexokinases. Hexokinase I-III can phosphorylate any hexose sugar while Hexokinase IV is highly specific for glucose. It is also known as glucokinase. One ATP molecule is used in this reaction.
- In the second reaction, glucose-6-phosphate is converted to its isomer fructose-6-phosphate. This reaction is catalyzed by an enzyme called phosphoglucose isomerase.
- In the next step, another ATP molecule is used to phosphorylate fructose-6-phosphate. The product of this reaction is fructose-1,6-bisphosphate. This phosphorylation step is catalyzed by phosphofructokinase-1 (PFK-1).
- The fourth reaction involves breaking the fructose-1,6-bisphosphate into two trioses; dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This step is catalyzed by aldolase enzyme.
- The fifth and final step of this phase is the conversion of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate. It is done by triose phosphate isomerase enzyme.
Only glyceraldehyde-3-phosphate can be further processed in glycolysis. Two ATP molecules are invested during this phase. The first and third reactions are irreversible.
This phase also involves five retains that are as follows;
- Glyceraldehyde-3-phosphate is oxidized to form 1,3-bisphosphoglycerate as the oxidation process is coupled with the addition of inorganic phosphate. During this reaction, NAD is also reduced to NADH.
- In the next step, 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate and a molecule of ATP is formed. The reaction is catalyzed by the 3-phosphoglycerate kinase in the reverse direction.
- In the third step, phosphoglycerate mutase shifts the phosphate group from carbon 3 to carbon 3 generating 2-phosphoglycerate.
- In the next step, the enolase enzyme dehydrates the 2-phosphoglycerate molecule to produce phosphoenolpyruvate (PEP).
- In the final step, PEP is converted to pyruvate by pyruvate kinase enzyme. The energy released during this reaction is used to phosphorylate ADP to ATP.
A total of four ATP molecules and two NADH molecules are produced during this phase (using two molecules of glyceraldehyde-3-phosphate). The net products of glycolysis of one molecule of glucose are two ATP molecules, two NADH and two pyruvate molecules.
The further processing of pyruvate molecules depends on the availability of oxygen. It is different in aerobic and anaerobic respiration.
If oxygen is present, pyruvate enters the mitochondria and undergoes further oxidation reactions to ultimately yield carbon dioxide. During aerobic respiration, all the energy present in glucose is released as it is completely oxidized to carbon dioxide.
The aerobic respiration involves the following two processes;
- Synthesis of Acetyl-CoA
- Citric Acid Cycle
A brief detail of these processes is as follows.
Synthesis of Acetyl-CoA
Acetyl-CoA is the substrate for the citric acid cycle. It is obtained by the oxidative decarboxylation of pyruvate inside the mitochondrial matrix. The reaction is catalyzed by pyruvate dehydrogenase enzyme, a multiprotein complex present in the mitochondrial matrix.
Pyruvate dehydrogenase complex has multiple copies of three enzymes;
- Pyruvate dehydrogenase, it decarboxylates the pyruvate molecule
- Dihydrolipoyl transacetylase, it transfers the acetyl group to CoA
- Dihydrolipoyl dehydrogenase, it regenerates the oxidized form of lipoic acid by reducing NAD to NADH2
The final products of this reaction are one acetyl-CoA, one molecule of NADH2, and one carbon dioxide molecule. The acetyl-CoA is further oxidized in the citric acid cycle.
The pyruvate dehydrogenase complex can be inhibited by arsenic poisoning. Arsenic mainly inhibits the dihydrolipoyl transacetylase enzyme, the second enzyme in the pyruvate dehydrogenase complex.
Citric Acid Cycle
It is the final path of aerobic respiration. The two carbon atoms present in the acetyl-CoA are oxidized during these cyclic reactions, resulting in the release of two carbon dioxide molecules. The carbon atoms are removed by the process of oxidative decarboxylation. The energy released during these processes are used to produce high-energy reducing compounds; NADH and FADH2.
The reactions of the citric acid cycle are as follows;
- Acetyl-CoA combines with oxaloacetate to form citrate. The reaction is catalyzed by citrate synthase. The Coenzyme A molecule is released.
- Citrate is converted to its isomer, isocitrate, by aconitase enzyme.
- In the next step, isocitrate undergoes oxidative decarboxylation carried out by isocitrate dehydrogenase enzyme. A molecule of carbon dioxide is released and one NADH molecule is produced. Isocitrate gets converted to alpha-ketoglutarate.
- The alpha-ketoglutarate is subjected to oxidative decarboxylation by enzyme alpha-ketoglutarate dehydrogenase. A molecule of succinyl CoA is produced as a result of this reaction. The second molecule of carbon dioxide is released, and another NAD is reduced to NADH.
- The next step involves the conversion of succinyl CoA to succinate by enzyme succinate thiokinase. The energy released during this reaction is used to phosphorylate GDP to GTP.
- The succinate molecule undergoes oxidation to form fumarate. During this reaction, a molecule of FAD is reduced to FADH2.
- Fumarate is hydrated to malate molecule by fumarase enzyme. A molecule of water is used in this reaction.
- Malate is finally reduced to oxaloacetate by malate dehydrogenase enzyme. Another NAD molecule is reduced during this reaction to NADH.
The citric acid cycle completes with the regeneration of oxaloacetate molecule. This cycle is a series of oxidation reactions that remove carbon atoms from acetyl CoA and utilize the chemical energy to produce high-energy reducing compounds.
The final products of one citric acid cycle are two molecules of carbon dioxide, three molecules of NADH, one FADH2, one GTP, and one CoA.
As stated earlier, aerobic respiration takes place only in mitochondria in the presence of oxygen. Thus, it takes place only in cells having mitochondria. It does not occur in cells that lack mitochondria like mature red blood cells.
In the case of bacteria, enzymes for aerobic respiration are present in the cytosol of the cell. This is the reason why aerobic respiration can take place in bacteria even though they don’t have mitochondria.
Most of the eukaryotes obtain energy for cellular processes via aerobic respiration. This process provides the maximum amount of energy as glucose is completely oxidized to carbon dioxide molecules. The energy released during aerobic respiration is far greater than the energy produced in anaerobic respiration.
The energy yield is calculated in terms of the number of ATP molecules produced in the reaction. The high-energy intermediate like NADH and FADH2 donate electrons to the electron transport chain for the synthesis of ATP.
One NADH provides 3 ATP molecules while one FADH2 provides 2 ATP molecules. NADH from glycolysis provides only 2 ATP per NADH.
The energy yield from one glucose molecule during aerobic respiration is as follows;
Number of ATP produced directly = 2 (glycolysis)
Number of NADH = 2 (glycolysis) + 2 (pyruvate dehydrogenase) + 6 (citric acid cycle)
Number of FADH2 = 2
Number of GTP = 2
Total number of ATP molecules = 2 (direct) + 28 (from NADH) + 4 (from FADH2) + 2 (from GTP) = 36
Thus, the net energy yield in the aerobic respiration of one glucose molecule is 36 ATP.
Anaerobic respiration occurs in the absence of oxygen or when oxygen availability is very poor. It is also termed as fermentation.
Pyruvate can take one of the two routes in the absence of oxygen for further processing. These are;
- Lactic acid fermentation
- Alcohol Fermentation
These two routes are discussed below.
Lactic acid Fermentation
In this type of fermentation, two pyruvate molecules are converted to two molecules of lactic acid. The two NADH molecules produced during glycolysis are used to reduce pyruvate to lactate
The NAD molecules are regenerated during this fermentation process. This regeneration of NAD is essential to keep the glycolysis process running. Glycolysis cannot occur if there is any deficiency of NAD molecules.
Lactic acid fermentation occurs in anaerobic bacteria and some other anaerobic organisms.
It also occurs in human muscle cells during vigorous exercise when the oxygen supply is unable to meet the requirements of the exercising muscle. The muscle shift to anaerobic respiration because they need a continuous supply of ATP that cannot be provided by aerobic respiration due to limited oxygen delivery. The accumulation of lactic acid in muscles is responsible for muscle cramps after severe exercise.
It is another type of anaerobic fermentation during which pyruvate is reduced to ethanol and a carbon dioxide molecule is released. It involves two reactions;
- Pyruvate is decarboxylated to form acetaldehyde in the presence of thiamine pyrophosphate. A carbon dioxide molecule is released during this reaction.
- In the second reaction, acetaldehyde is reduced to ethanol. The reducing power is provided by NADH that was formed in glycolysis.
Alcohol fermentation occurs in yeast and some anaerobic bacteria. This method is also used in the beverage industry to make different alcohol products.
The energy yield of anaerobic respiration is very less than that of aerobic respiration. Only two ATP molecules are obtained from one glucose molecule. The NADH molecules obtained during glycolysis are not used to make TAP, rather they are used as reducing powers in fermentation.
Cellular respiration is the process by which food molecules are oxidized and energy is released for other cellular processes. Two types of cellular respiration are aerobic respiration and anaerobic respiration.
Glycolysis is common to both aerobic and anaerobic respiration. During this process, a glucose molecule is broken down into two pyruvate molecules without using oxygen. Two ATP and two NADH are also produced.
Aerobic respiration involves the complete oxidation of pyruvate molecules in the presence of oxygen.
- Pyruvate is first converted to acetyl CoA by pyruvate dehydrogenase enzyme.
- Acetyl CoA enters the citric acid cycle and is completely oxidized to carbon dioxide.
- It occurs in almost all the eukaryotic cells having mitochondria and aerobic bacteria.
- A total of 36 ATP molecules are obtained from one glucose molecule in this process.
Anaerobic respiration occurs in the absence of oxygen.
- The pyruvate is converted to either lactate (lactic acid fermentation) or ethanol (Alcohol fermentation).
- The reducing power is provided by NADH molecules.
- It occurs in anaerobic organisms or cells that lack mitochondria.
- Muscle cells also shift to this type of respiration if oxygen supply is poor.
- Only two ATP molecules are obtained from one glucose molecule.
- Cox Michael M, Nelson David L (2008). “Chapter 14: Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway”. Lehninger Principles of Biochemistry (5 ed.). W H Freeman & Co. pp. 527–568. ISBN 978-1429222631.
- Dashko, Sofia; Zhou, Nerve; Compagno, Concetta; Piškur, Jure (2014-09-01). “Why, when, and how did yeast evolve alcoholic fermentation?”. FEMS Yeast Research. 14 (6): 826–832. doi:10.1111/1567-1364.12161. ISSN 1567-1364. PMC 4262006. PMID 24824836.
- Tadege, M.; Kuhlemeier, C. (1997-10-01). “Aerobic fermentation during tobacco pollen development” (PDF). Plant Molecular Biology. 35 (3): 343–354. doi:10.1023/A:1005837112653. ISSN 0167-4412. PMID 9349258.
- Cazzulo, Juan José (1992). “Aerobic fermentation of glucose by trypanosomatids”. FASEB Journal. 6 (13): 3153–61. doi:10.1096/fasebj.6.13.1397837. PMID 1397837.
- Portnoy, Vasiliy A.; Herrgård, Markus J.; Palsson, Bernhard Ø. (2008). “Aerobic Fermentation of D-Glucose by an Evolved Cytochrome Oxidase-Deficient Escherichia coli Strain”. Appl. Environ. Microbiol. 74 (24): 7561–7569. doi:10.1128/AEM.00880-08. PMC 2607145. PMID 18952873