Aerobic and Anaerobic Respiration, Practice Problems and FAQs

Will the fans, the lights or any electrical equipment at your home work if there’s no electricity in the house? They won’t. Our body is similar to these appliances. Just like these equipment need electrical energy to do their job, our body also requires energy to function. But does our body require electrical energy? No, it requires chemical energy derived from stored chemical molecules such that the energy can be used for different metabolic processes of the body.

But where do we get the energy from? The answer lies in a sentence that we often hear our moms say, “Eat properly or else how will you get the energy to work/study/play?” Yes, we get energy from food. Digested nutrients obtained from food are used as substrates that can be broken down to release energy and the process is known as respiration. While some organisms like us humans require oxygen to respire, there are many that prefer an oxygen free environment for respiration.

In this article we will discuss the two types of respiration based on oxygen requirement, that is aerobic and anaerobic respiration.

Table of Content:

• Cellular respiration
• Aerobic respiration
• Anaerobic respiration
• Practice Problems
• FAQs

Cellular respiration

Cellular respiration is the actual energy releasing biochemical step of respiration. Cellular respiration is an enzymatically regulated multistep biological oxidation of organic substrates inside the living cells that releases energy in small steps for immediate use as well as for temporary storage. Intermediates of cellular respiration are useful in the synthesis of a number of other biochemicals. Only 40% of the respiratory energy is trapped by the cells for various activities. Rest is liberated as heat and is dissipated. The biomolecule best suited for trapping chemical energy is ATP (Adenosine triphosphate) and thus chemical energy released during respiration is stored in the high energy bonds between the phosphate groups of ATP. Whenever the cell requires energy for any activity, ATP is hydrolysed to adenosine diphosphate (ADP) and P_i (inorganic phosphate) and the breaking of the bond to release a phosphate group releases energy that can be used by the cell.

Cellular respiration can occur with or without involving oxygen and hence can be of two types -

1. Aerobic respiration that involves oxygen
2. Anaerobic respiration that does not involve oxygen

Aerobic respiration

Aerobic respiration involves complete oxidation of respiratory substrates in the presence of oxygen to release energy. It involves five steps, out of which the first step, that is glycolysis, occurs in the cytoplasm while the rest of the steps, including oxidative decarboxylation, Krebs cycle, electron transport chain and oxidative phosphorylation, occur in the mitochondria.

The most commonly used respiratory substrate used by cells is glucose. In aerobic respiration, one molecule of glucose is completely oxidised to form six molecules of carbon dioxide, six molecules of oxygen and 686 kcal of energy. Out of this, 40% is stored in the form of 38 molecules of ATP while the remaining 60% is lost as heat.

The overall equation for aerobic respiration can be represented as:

Glycolysis

It occurs in the cytoplasm and involves ten enzyme catalysed reactions that help in breaking down one molecule of glucose into two molecules of pyruvate (CH₃.CO.COOH). This step does not involve oxygen and is common to both aerobic and anaerobic respiration. Two molecules of ATP are used up in glycolysis but four molecules of ATP are generated. So the net gain of ATP molecules during glycolysis is two molecules. This process also involves the reduction of two molecules of NAD⁺ to two molecules of NADH + H⁺ and two molecules of water in two different steps of the process.

So the overall reaction for this process is -

C₆H₁₂O₆ + 2ADP + 2P_i + 2NAD⁺ → 2 CH₃.CO.COOH + 2NADH + H⁺ + 2ATP + 2H₂O

Fig: Glycolysis

Oxidative decarboxylation

The pyruvic acid/pyruvate molecules undergo oxidative decarboxylation after being brought to the mitochondrial matrix. The enzyme catalysing this reaction is the pyruvate dehydrogenase complex which requires NAD⁺, Coenzyme A and Mg²⁺ ion for its function.

In this process, the two molecules of pyruvate formed at the end of glycolysis are decarboxylated to release two molecules of CO₂. Pyruvate molecules are also oxidised by two NAD⁺ which themselves get reduced to two NADH + H⁺. The two carbon acetyl groups, formed as a result of the above reactions, react with Coenzyme A to give two molecules of acetyl CoA as the final product of the reaction.

Fig: Oxidative decarboxylation

Oxidative decarboxylation is considered to be the link between glycolysis and the Krebs cycle and the acetyl CoA molecules act as a substrate for the Krebs cycle. One molecule of glucose leads to the formation of two molecules of acetyl CoA.

Krebs cycle

The Krebs cycle, also known as the tricarboxylic acid cycle or the citric acid cycle, also takes place in the mitochondrial matrix. It is known as the tricarboxylic acid cycle as the first stable product in this cycle is an organic acid with three carboxyl groups, that is citric acid, which is formed when the acetyl coA molecule enters the Krebs cycle and reacts with oxaloacetate. As there are two acetyl CoA molecules produced from each molecule of glucose, two rounds of the Krebs cycle is required for the complete oxidation of glucose through this cycle. Krebs cycle, as the name suggests is a cyclic process including eight enzyme catalysed reactions. Two complete rounds of the Krebs cycle lead to the formation of four molecules of carbon dioxide, six molecules of NADH + H⁺, two molecules of FADH₂ and two molecules of GTP/ATP from two molecules of acetyl CoA.

Thus, starting from glycolysis till the end of two rounds of the Krebs cycle, from the complete oxidation of one molecule of glucose, we get - six molecules of carbon dioxide, 10 molecules of NADH + H⁺, two molecules of FADH₂ and four molecules of ATP.

Fig: Krebs cycle

Electron transport chain (ETC)

This process is the precursor for oxidative phosphorylation which is the last step of aerobic respiration. It helps in building up the proton gradient across the inner membrane of the mitochondria which drives the production of ATP. The electrons released due to the oxidation of NADH and FADH₂ are transported via a series of four electron carriers, Complex I - IV to be ultimately transferred to oxygen which is the terminal electron and H⁺ ion acceptor. When NADH and FADH₂ are oxidised, 2H⁺ and 2e^- are released in the process.

NADH + H⁺ → NAD⁺ + 2H⁺ + 2e^-

FADH₂ → FAD + 2H⁺ + 2e^-

The four complexes involved in the ETC are -

• Complex I or NADH dehydrogenase complex which is composed of a Fe-S complex and FMN (flavin mononucleotide).
• Complex II or Succinate dehydrogenase complex composed of Fe-S and FAD
• Complex III or Cytochrome c reductase composed of Cytochrome b, Cytochrome c, and Fe-S
• Complex IV or Cytochrome c oxidase which is composed of cytochrome c, a and a3 and two copper centres.

Electrons released from NADH are transferred from Complex I to Complex III via ubiquinone and then from Complex III to IV before being passed on to oxygen. Electrons from FADH₂ are transferred to Complex II from where it goes to Complex III, then IV and finally oxygen.

When electrons released from NADH are transferred, for the passage of two electrons through the chain 2H⁺ ions are transported from the mitochondrial matrix into the intermembrane space at three different sites -

• From NADH to FMN
• From cytochrome b to cytochrome c of Complex III
• From cytochrome a to cytochrome a₃ of Complex IV

When electrons released from FADH₂ are transferred, for the passage of two electrons through the chain 2H⁺ ions are transported from the mitochondrial matrix into the intermembrane space at two different sites -

• From cytochrome b to cytochrome c of Complex III
• From cytochrome a to cytochrome a₃ of Complex IV

Oxidative phosphorylation

This process occurs across the inner mitochondrial membrane and helps in production of ATP from the oxidation of NADH and FADH₂. The transfer of H⁺ ions or protons causes a proton gradient between the intermembrane space and the mitochondrial matrix. To break the gradient, two H⁺ ions are brought back from the intermembrane space into the mitochondrial matrix through the F₀ - F₁ channels present in the inner mitochondrial membrane. As the F₀ channel allows the transport of 2H⁺ ions into the mitochondrial matrix, the energy released due to the breaking of the gradient is utilised by the F₁ ATPase enzyme to generate ATP from ADP.

We know that three pairs of H⁺ ions are transferred from the mitochondrial matrix to the intermembrane during the transfer of two electrons released due to oxidation of each NADH molecule. Thus, three ATP molecules are generated while bringing these three pairs of H⁺ ions back from the intermembrane space into the mitochondrial matrix. Similarly, for the transfer of two electrons from each FADH₂ molecule, two pairs of H⁺ ions are sent to the intermembrane space and while bringing them back into the mitochondrial matrix, 2 ATP molecules are generated.

Oxygen acts as the terminal electron acceptor in this process and accepts the H⁺ ions and electrons to be reduced to water.

2H⁺ + 2e^- + ½ O₂ → H₂O

As there are 10 molecules of NADH and two molecules of FADH₂ released due to complete oxidation of one molecule of glucose, thus 12 pairs of electrons are transferred via the ETC which are accepted by six molecules of oxygen to form six molecules of water due to the complete oxidation of one molecule of glucose during aerobic respiration.

Net yield of ATP

Aerobic respiration leads to the generation of 38 molecules of ATP due to complete oxidation of one molecule of glucose. The calculation can be summarised as -

Fig: Respiratory balance sheet

Anaerobic respiration

Incomplete oxidation of a respiratory substrate in the absence of oxygen to release energy is known as anaerobic respiration. It is generally exhibited by lower organisms such as fungi like yeasts and certain bacteria. Anaerobic respiration results in the formation of only two molecules of ATP for the breakdown of per molecule of glucose. Glycolysis is the common phase between aerobic and anaerobic respiration. It results in the formation of two molecules of pyruvate which can then enter into any one of the following pathways - alcoholic fermentation and lactic acid fermentation. Anaerobic respiration occurs in the cell cytoplasm.

Alcoholic fermentation

In the alcoholic fermentation process, pyruvic acid/pyruvate is initially decarboxylated to form acetaldehyde which is then reduced to form ethyl alcohol (C₂H₅OH) and carbon dioxide. The first step is catalysed by pyruvate decarboxylase and the second step is catalysed by alcohol dehydrogenase. This is seen in yeast and certain bacteria.

This can be expressed as -

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

Lactic acid fermentation

In this process, pyruvate is reduced to lactic acid with the help of the lactate dehydrogenase enzyme. This process is exhibited by lactic acid bacteria and by the human muscle cells under the conditions of physical stress due to unavailability of enough oxygen.

C₆H₁₂O₆ → Lactic acid + 2ATP

Difference between aerobic and anaerobic respiration

Practice Problems

1. How many carbon dioxide molecules are released during oxidative decarboxylation of one molecule of pyruvate?

1. 1
2. 2
3. 3
4. 4

Solution: Glycolysis results in the formation of two pyruvate molecules from the breakdown of each molecule of glucose. Each pyruvate molecule is brought to the mitochondrial matrix where it undergoes oxidative decarboxylation. Each pyruvate molecule is decarboxylated to release one molecule of carbon dioxide and is simultaneously oxidised by one NAD⁺ which is itself reduced to two NADH + H⁺. The carbon acetyl group, formed as a result of the above reactions, reacts with one molecule of Coenzyme A to give one molecule of acetyl CoA as the final product of the reaction. Thus, the correct option is a.

Fig: Oxidative decarboxylation

2. The Krebs cycle is also called the tricarboxylic acid because:

1. Acetyl coA enters this cycle by reacting with a tricarboxylic acid.
2. The end product is a tricarboxylic acid.
3. The first stable product is a tricarboxylic acid
4. Many tricarboxylic acids are formed in this cycle

Solution: The Krebs cycle, also known as the tricarboxylic acid cycle or the citric acid cycle, also takes place in the mitochondrial matrix. It is known as the tricarboxylic acid cycle as the first stable product in this cycle is an organic acid with three carboxyl groups, that is citric acid, which is formed when the acetyl coA molecule enters the Krebs cycle and reacts with oxaloacetate. Thus, the correct option is c.

3. How many ATP molecules are generated when NADH is oxidised back to NAD⁺ through the ETC?

1. 1
2. 2
3. 3
4. 4

Solution: Three pairs of H⁺ ions are transferred from the mitochondrial matrix to the intermembrane during the transfer of two electrons released due to oxidation of each NADH molecule through the ETC to oxygen. The transfer of H⁺ ions or protons causes a proton gradient between the intermembrane space and the mitochondrial matrix. To break the gradient, two H⁺ ions are brought back from the intermembrane space into the mitochondrial matrix through the F₀ - F₁ channels present in the inner mitochondrial membrane. As the F₀ channel allows the transport of 2H⁺ ions into the mitochondrial matrix, the energy released due to the breaking of the gradient is utilised by the F₁ ATPase enzyme to generate ATP from ADP. Thus, three ATP molecules are generated while bringing these three pairs of H⁺ ions back from the intermembrane space into the mitochondrial matrix. Thus the correct option is c.

4. Which of the following is the end product of glycolysis?

1. CH₃.CO.COOH
2. CH₃CHO
3. CH₃.CHOH.COOH
4. C₂H₅OH

Solution: Glycolysis occurs in the cytoplasm and involves ten enzyme catalysed reactions that help in breaking down one molecule of glucose into two molecules of pyruvate (CH_3.CO.COOH). Two molecules of ATP are used up in glycolysis but four molecules of ATP are generated. So the net gain of ATP molecules during glycolysis is two molecules. This process also involves the reduction of two molecules of NAD⁺ to two molecules of NADH + H⁺ and two molecules of water in two different steps of the process.

So the overall reaction for this process is -

C₆H₁₂O₆ + 2ADP + 2P_i + 2NAD⁺ → 2 CH₃.CO.COOH + 2NADH + H⁺ + 2ATP + 2H₂O

Thus, the correct option is a.

FAQs

1. What is the extinction point?

Answer: Extinction point is the least percentage of oxygen in the air at which aerobic respiration can take place. It ranges between 3-10%

2. How does cyanide affect the ETC?

Answer: Cyanide is an Electron Transport Chain inhibitor and prevents the flow of electrons from cytochrome a₃ to oxygen. Thus, it is highly toxic.

3. Why is aerobic respiration carried out in so many steps?

Answer: There are multiple advantages to carrying out aerobic respiration in so many steps. These are -

• A stepwise release of bond energy that enables the cells to utilise a major part of it for ATP synthesis.
• ATP energy becomes available in required quantities even in those parts of the cell where respiratory breakdown is not occurring.
• There is less wastage of energy.
• The temperature of the cell does not rise
• Intermediates formed in different steps become available for different biosynthetic pathways.
• Because of several intermediates, many different organic substrates can be used for respiratory breakdown.
• Rate of energy liberation can be adjusted according to requirement through enhanced or decreased availability of enzymes.

4. What is climacteric respiration?

Answer: It is the phase of increased respiration seen in fruits such as apple, banana, etc during their phase of ripening. These fruits showing climacteric rise in respiration are known as climacteric fruits.

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