Aerobic Respiration : Definition and Types

Aerobic Respiration

All organisms require energy to carry out various activities. Respiration is an important chemical process that releases the energy required for life processes. It can occur both in the presence and in the absence of oxygen. So, depending on oxygen availability, respiration can be aerobic or anaerobic. The following article explains aerobic respiration, its significance, the site of occurrence, and the aerobic respiration equation.

What is Aerobic Respiration? 

Respiration involving molecular oxygen uptake is said to be aerobic respiration. The aerobic respiration equation can be written as follows:

Glucose (C6 H12 O6) + Oxygen 6(O2) → Carbon-dioxide 6(C O2) + Water 6 (H2 O) + Energy (ATP) 

Different Types of Aerobes

The organisms that depend on oxygen are referred to as aerobes. The different types of aerobes are as follows:

• Obligate Aerobes: They strictly require free oxygen to survive. While undergoing cellular respiration, obligate aerobes use oxygen for their metabolism and break down sugar to generate energy. In the electron transport chain, oxygen serves as the terminal electron acceptor. Examples of obligate aerobes are fungi and bacteria, including Mycobacterium tuberculosis, Bacillus, Nocardia asteroids, and Pseudomonas aeruginosa. 
• Facultative Anaerobes: They synthesise energy via aerobic respiration in the presence of oxygen. However, they can also switch to the fermentation process in the absence of oxygen. Examples of facultative anaerobes are bacteria including Staphylococcus spp, Escherichia coli, Salmonella, Listeria spp, and some eukaryotic organisms such as Saccharomyces cerevisiae.
• Microaerophiles: They require oxygen for survival, but environments that contain lower levels of dioxygen than that present in the atmosphere, i.e., < 21% O₂. Examples of microaerophiles include Campylobacter and Helicobacter. 

Mechanism of Aerobic Cellular Respiration

The steps involved in aerobic cellular respiration are as follows:

• Glycolysis
• Oxidation of Pyruvic Acid (Formation of Acetyl Coenzyme A)
• Krebs Cycle
• Electron Transport

Glycolysis

• It is the first step in aerobic respiration that takes place in the cytoplasm. 
• It does not require oxygen and is a common linear pathway for both aerobic and anaerobic respiration. 
• The substrate used is glucose (a 6-carbon compound).
• The end product is two pyruvic acid molecules (3-carbon compounds each), 2 NADH molecules, and one H⁺. 
• It does not produce carbon dioxide.
• Glycolysis consumes 2 ATP molecules and generates 4. So, there is a net gain of 2 ATP molecules.

Glycolysis Pathway

Step 1: First Phosphorylation

Glucose is converted to glucose-6-phosphate in the presence of ATP, Mg⁺⁺, and hexokinase. ATP gets coveted by ADP. 

Step 2: Isomerisation

Glucose-6-phosphate is isomerized to fructose-6-phosphate, which is an isomer of G-6-P. The reaction occurs in the presence of phosphohexose isomerase.

Step 3: Second Phosphorylation

Fructose-6-phosphate is converted to fructose-1,6-diphosphate in the presence of Mg⁺⁺ and the enzyme phosphofructokinase. ATP gets coveted by ADP.

Step 4: Cleavage

The enzyme aldolase helps in breaking down fructose-1,6-biphosphate to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. Both these compounds are interconvertible in the presence of phosphotriose isomerase.

Step 5: Phosphorylation and oxidative dehydrogenation

The glyceraldehyde-3-phosphate turns to 1,3-diphosphoglyceric acid in the presence of NAD⁺, enzyme glyceraldehyde-3-phosphate dehydrogenase, and H₃PO₄. The NAD⁺ is converted to NADH. 

Step 6: First ATP Generation

The 1,3-biphosphoglyceric acid changes into 3-phosphoglyceric acid with the help of Mg⁺⁺ and phosphoglycerokinase enzymes, and the first ATP molecule is generated.

Step 7: Isomerization

The enzymes phosphoglyceromutase and Magnesium ion help in the conversion of 3-phosphoglyceric acid to 2-phosphoglyceric acid. 

Step 8: Dehydration

Enolase and Mg⁺⁺ convert 2-phosphoglyceric acid to 2-phosphoenol pyruvic acid liberating a water molecule. 

Step 9: Second ATP generation

In the final step, 2-phosphoenol pyruvic acid gets converted to pyruvic acid in the presence of pyruvate kinase and a Magnesium ion. ADP gets converted to ATP.

Oxidation of Pyruvic Acid

Under aerobic conditions, pyruvic acid is oxidised via the Krebs cycle. But before entering the cycle, the second step involves the formation of acetyl coenzyme A. The series of steps involved in the conversion is as follows:

• Formation of a complex between thiamine pyrophosphate and pyruvic acid. 
• Decarboxylation of pyruvic acid.
• The acetaldehyde unit left from the decarboxylation process combines with the cofactor lipoic acid and forms an acetyl-lipoic acid complex. 
• The acetyl group is released from lipoic acid to CoA to form acetyl CoA. 

The following equation can represent the reaction:  

Pyruvic acid + CoA + NAD+ → Acetyl CoA + CO2 + NADH + H+In the presence of TPP, lipoic acid, and pyruvic dehydrogenase multienzyme complex.

Krebs Cycle

• It is also called the TCA cycle and the Citric Acid cycle. 
• It takes place in the mitochondria.
• It occurs in aerobic respiration only. 
• The cyclic pathway requires oxygen in the process. 
• The substrate used is Acetyl CoA.
• The end product is oxaloacetic acid (OAA) and carbon dioxide. 
• It does not consume ATP but generates 2 ATP, 2 NADH, 2 FADH, and one H⁺ from 2 pyruvic acid molecules.

Krebs Cycle Pathway

Step 1: Formation of Citric Acid

The first step involves the formation of citric acid by combining Acetyl CoA and oxaloacetic acid in the presence of citrate synthase.

Step 2: Isomerization

Citric acid is converted to its isomer isocitric acid in the presence of aconitase.

Step 3: Decarboxylation and Dehydrogenation

Isocitric acid turns into ɑ-ketoglutaric acid, and a molecule of carbon dioxide is released as ɑ-ketoglutaric acid is a 5-carbon compound. NAD and isocitrate dehydrogenase catalyses the step. 

Step 4: Decarboxylation 

A 4-carbon compound succinyl CoA is formed in the presence of ɑ-ketoglutarate dehydrogenase. One molecule of carbon dioxide and NADH is released in this step.

Step 5: Dehydrogenation 

CoA is removed from succinyl CoA to form succinate. GDP gets converted to GTP, and succinyl CoA synthetase catalyses the reaction. 

Step 6: Dehydrogenation 

Succinate now transforms into fumarate with the help of the enzyme succinate dehydrogenase. The step also involves the reduction of FAD to FADH₂.

Step 7: Hydration 

The addition of H₂O changes fumarate into malate in the presence of fumarase.  

Step 8: Dehydrogenation 

The enzyme malate dehydrogenase converts malate to oxaloacetate while utilising NAD⁺ and releasing NADH and H⁺.

Electron Transport System (ETS)

The ETS is a chain of carriers containing NAD, FAD, conex]zyme Q, and cytochromes localised in the F1 mitochondria particles. Since electrons flow from higher to lower energy levels, every step of ETS lowers the energy level of electrons.

The energy difference is transformed into a phosphate bond by the conversion of ADP to ATP. The oxidation of reduced coenzyme Q releases hydrogen ions. The electrons are passed along the series of cytochromes, and for every pair of electrons passed, three ATP molecules are formed. 

• It uses 10 NADH molecules and gives 10 NAD⁺ molecules.
• The 2 FADH₂ molecules get converted to 2 FAD molecules.
• The process also releases 12 H₂O molecules.
• 34 ADP gets converted to 34 ATP.
• The synthesis of ATP via ETS, wherein oxygen is the terminal acceptor, is known as oxidative phosphorylation. 

How is Aerobic Respiration Different from Anaerobic Respiration?

The following table enumerates the critical differences between aerobic and anaerobic respiration.

Characteristics	Aerobic Respiration	Anaerobic Respiration
Oxygen	Involves the uptake of molecular oxygen.	Does not involve the uptake of molecular oxygen.
Substrate degradation	Complete oxidation of the substrate.	Incomplete degradation of the substrate.
End products	Carbon dioxide Water Energy	Carbon dioxide  Organic compounds like lactic acid and ethyl alcohol Energy
Energy molecules	38 ATP molecules are produced by one mole of glucose.	2 ATP are produced by a mole of glucose.
The site of occurrence in a  cell	Occurs both in the cytoplasm and mitochondria.	Occurs only in the cytoplasm of the cell.
Equation	Aerobic respiration equation: C6H1206 + 6O2 → 6CO2+ 6H2O +673 kcal	Anaerobic respiration equation:C6H1206 + 6O2 →2C2H5OH +  6CO2 + 21 kcal

Conclusion

Aerobic cellular respiration is a series of important processes that convert the chemical energy of organic substances into metabolic energy that living cells can use. It takes glucose as the initial substrate and oxygen and gives energy molecules ATP, water, and carbon dioxide. Besides providing energy, the CO₂ released in the process helps in the maintenance of carbon dioxide balance in nature. 

Frequently Asked Questions

1. What is the significance of the Krebs cycle in aerobic cellular respiration?

A. The Krebs cycle produces hydrogen atoms that ultimately yield the major part of the energy derived from the oxidation of one glucose molecule. Moreover, it is a valuable source of intermediates used to manufacture other substances such as fatty acids, amino acids, and carotenoids.

2. What enzymes are used in glycolysis?

A. The following enzymes are used in the process of glycolysis:

• Hexokinase
• Phosphohexoisomerase
• Phosphofructokinase
• Fructose-1,6-diphosphate 
• Aldolase
• Phosphotriose isomerase
• 3-phosphoglyceraldehyde dehydrogenase
• Phosphoglycerokinase
• Phosphoglyceromutase
• Enolase
• Pyruvate kinase

3. What is the difference between glycolysis and the Krebs cycle?

A. Glycolysis is an anaerobic process that consumes two ATP molecules and results in four ATP molecules. Its end product is pyruvate, and it takes place in the cytoplasm. The Krebs cycle is an aerobic process, and the end product is oxaloacetic acid. It does not consume ATP but provides two ATP molecules. The Krebs cycle occurs in the mitochondria. While glycolysis has a linear pathway, the Krebs cycle has a cyclic pathway.