Aerobic Respiration & Steps Involved, Structure of Mitochondria, Practice Problems, and FAQs

Have you ever gone through a situation, or have you ever wondered what would happen if there is a black out and there is no electricity? It would not only cripple our sight, but also our lives as most of our daily activities such as watching the television, grinding spices in a mixer, washing clothes in a washing machine, purifying water in an RO machine, etc are powered by electricity.

Similarly, what would happen if there is no energy in our body? How would we survive?

Survival will become very difficult because every life sustaining process requires energy. Have you thought about where this energy comes from?

We get energy from the food we eat. This food gets oxidised in the cells and the energy that is released is further used by all body tissues for functioning properly.

This holds true for all living organisms and while some organisms require oxygen for complete oxidation of food to release energy, others only partially oxidise food in the absence of oxygen to release energy. Respiration in the presence of oxygen is known as aerobic respiration and is the mode of energy production for all higher organisms such as plants and animals. In this article we will discuss all that we need to know about aerobic respiration.

Table of contents

• Aerobic respiration
• Structure of mitochondria
• Steps involved in aerobic respiration
• Respiratory balance sheet
• Significance of aerobic respiration
• Practice Problems
• FAQs

Aerobic respiration

Respiration is a process through which energy is released from organic compounds. It occurs in prokaryotes as well as in eukaryotes. Cellular respiration is the process by which cells produce ATP by utilising the chemical energy stored in food. The most commonly used substrate for cellular respiration is glucose which is obtained by the digestion of carbohydrates in the body. Glucose is broken down further in the cells through a series of enzymatic reactions to produce energy in the form ATP (adenosine triphosphate). This process also generates heat energy which keeps the body warm. Cellular respiration can be carried out either in the presence of oxygen or in its absence. In the presence of oxygen, food is broken down completely and the energy yield is more, whereas, in the absence of oxygen, food is broken down partially and hence less energy is yielded.

The stepwise breakdown of respiratory substrates to CO₂ and H₂O, in the presence of oxygen, to yield energy is termed as ‘aerobic respiration’. It occurs with the help of various steps some of which occur in the cytoplasm and some in the mitochondria. The organisms that perform aerobic respiration are known as aerobes. This kind of respiration involves at least 4 major steps-

• Glycolytic breakdown of glucose to pyruvic acid
• Oxidative decarboxylation of pyruvic acid to acetyl CoA
• Krebs cycle
• Terminal oxidation and phosphorylation in the respiratory chain. (ETS)

                                           Fig: Steps involved in aerobic respiration

Structure of mitochondria

Mitochondria are considered the ‘powerhouse of a cell’ because they provide the site and machinery for the ATP synthesis process. The aerobic respiration steps like oxidative phosphorylation, TCA cycle, and electron transport chain occur inside the mitochondria in eukaryotic cells. The structure of mitochondria contains the following important parts-

• Outer membrane
• Inner membrane
• Matrix

Outer membrane

There are transport proteins in it, and it is smooth and continuous. It is selectively permeable that allows the uptake of substrates and release of ATP. It has a few oxidising enzymes but the number is much less compared to the number of enzymes present on the inner membrane.

Inner membrane

The inner membrane of mitochondria is selectively permeable. It regulates the passage of materials into and out of mitochondria.It contains many channel proteins, enzymes, electron carriers, coupling factors and carrier proteins. It folds inwards to form finger-like projections called ‘cristae’ which help to increase the surface area of the inner membrane. The cristae consist of some knob-like particles known as F₀ - F₁ particles or oxysomes.

Oxysomes

The F₁ subunit forms the head piece while the F₀ subunit forms the base piece and the two subunits are joined by a small stalk of around 5 nm length.

The F₀ subunit acts as a proton channel and remains embedded in the inner membrane while the F₁ subunit faces the mitochondrial matrix and functions as ATP synthase enzyme which helps in the synthesis of ATP by oxidative phosphorylation of ADP.

Matrix

It is rich in enzymes and proteins involved in respiration and other metabolic pathways. It has single double stranded circular DNA called mitochondrial DNA (mtDNA). It also has RNA and 70S ribosomes which are similar to prokaryotic ribosomes. Mitochondria are called ‘semi autonomous’ structures as they contain their own genetic material DNA, which is similar to bacterial DNA.

                                                  Fig: Structure of mitochondria

Steps involved in aerobic respiration

There are four steps involved in aerobic respiration:

• Glycolysis
• Pyruvate oxidation
• Krebs cycle
• Electron transport chain

Glycolysis

‘Glycos’ means sugar and ‘lysis’ means splitting. The first step in breaking down glucose to produce energy is called glycolysis. There are ten steps in this metabolic pathway. Because the enzymes required for glycolysis are found in the cytoplasm of the cell, glycolysis takes place there. Two ATP molecules are used in the initial stages of this action. On the other hand, substrate-level phosphorylation leads to the synthesis of four ATP molecules at the conclusion of the cycle. As a result, catalysing the glycolysis of one glucose molecule produces a net of two ATP. Glycolysis occurs in all kinds of living beings like prokaryotes, eukaryotes, and aerobic or anaerobic organisms.

The glycolysis process is divided into a total of 10 steps, which are kept into two phases-

• Preparatory phase-5 steps
• Pay off phase-5 steps

                                       Fig: Phases involved in glycolysis

Preparatory phase

This phase is also known as the investment phase, in which 2 ATP molecules are invested. This phase involves the initial 5 steps of the glycolysis process.

Step 1

The first step involves the conversion of glucose to glucose-6-phosphate with the help of the enzyme hexokinase. ATP is used in this reaction and its third phosphate bond breaks and releases a phosphate group which joins the sixth carbon of the glucose molecule forming glucose-6-phosphate (phosphorylation).

                        Fig: Conversion of glucose to glucose-6-phosphate

Step 2

This step involves the conversion of glucose-6-phosphate to fructose-6-phosphate through isomerisation (conversion of one isomer to another) catalysed by phosphohexose isomerase enzyme.

                                                                    Fig: Isomerisation

Step 3

This step involves the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. The last phosphate group breaks off from ATP to join the first carbon of fructose-6-phosphate. The enzyme catalysing this reaction is phosphofructokinase.

                     Fig: Conversion of fructose-6-phosphate to fructose-1,6-bisphosphate

Step 4

In this step fructose-1,6-bisphosphate breaks into two compounds- dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate(3PGAL) with the help of the aldolase enzyme.

                                 Fig: Conversion of fructose-1,6-bisphosphate

Step 5

Dihydroxyacetone phosphate is then converted into its isomer glyceraldehyde-3-phosphate with the help of the triosephosphate isomerase enzyme. After conversion, there are two molecules of glyceraldehyde-3-phosphate.

                           Fig: Conversion of dihydroxyacetone phosphate

The summary of the preparatory phase is given below:

                                                         Fig: Preparatory phase

Payoff phase

The next five steps come under the payoff phase. In this phase, energy is released in the form of ATP and NADH. As two molecules of glyceraldehyde-3-phosphate are generated from one molecule of glucose in the preparatory phase, thus, two molecules of glyceraldehyde-3-phosphate enter the payoff phase.

Step 6

Two molecules of glyceraldehyde-3-phosphate (PGAL) are oxidised to form two molecules of 1,3-bisphosphoglyceric acid (BPGA) in the presence of glyceraldehyde phosphate dehydrogenase enzyme. Two molecules of NAD + are used that act as electron acceptors and they assist in the oxidation of PGAL by accepting H⁺ ions and electrons. Therefore, in this reaction, NAD⁺ is reduced to NADH⁺.

                                      Fig: Conversion of glyceraldehyde -3-phosphate

Step 7

This is the very first reaction in glycolysis where ATP is released through substrate level phosphorylation. In this reaction, two molecules of 1,3-bisphosphoglyceric acid (BPGA) is converted to two molecules of 3-phosphoglyceric acid(PGA) in the presence of phosphoglycerate kinase. As there are two 1,3-bisphosphoglyceric acids involved, two ATPs are produced.

                                                 Fig: Conversion of BPGA

Step 8

In this step, two molecules of 3-phosphoglyceric acid are isomerised to two molecules of 2-phosphoglycerate or 2-phosphoglyceric acid in the presence of the phosphoglyceromutase enzyme.

                                                       Fig: Isomerisation

Step 9

In this step, two molecules of 2-phosphoglycerate lose water molecules to form two molecules of phosphoenolpyruvate in the presence of enolase enzyme.

                                      Fig: Conversion of 2-phosphoglycerate

Step 10

This is the last step of glycolysis. The energy is released in the form of ATP. Two molecules of phosphoenolpyruvate lose the phosphate group to form two molecules of pyruvate in the presence of the pyruvate kinase enzyme. In the process, two molecules of ADP are phosphorylated to form two molecules of ATP.

                                        Fig: Formation of pyruvate

The summary of the payoff phase is given below:

                                                                   Fig: Payoff phase

Oxidative decarboxylation

The first step of aerobic respiration, taking place inside the mitochondrial matrix, is called oxidative decarboxylation. The two molecules of pyruvic acid, which were generated in glycolysis, enter the mitochondrial matrix. Here, both the molecules of pyruvic acid undergo decarboxylation and oxidation to form the two molecules of a final product called acetyl CoA. Oxidative decarboxylation is the connecting link between glycolysis and Krebs’ cycle. Acetyl CoA produced in this reaction acts as a substrate for TCA or Krebs cycle. The enzyme complex involved in this process is called the pyruvate dehydrogenase complex. This enzyme complex functions only in the presence of the following cofactors - NAD⁺ and coenzyme A and also Mg²⁺.

This process involves -

• Decarboxylation of pyruvate to remove a carboxyl group and form CO₂.
• Oxidation of pyruvate by loss of electrons which are taken up by NAD⁺ to form NADH + H⁺
• Formation of acetyl Co A by the reaction between the two carbon acetyl group, formed due to decarboxylation, and coenzyme A.

                                                 Fig: Oxidative decarboxylation

Kreb’s cycle or TCA cycle

Full form of the TCA cycle is the ‘TriCarboxylic Acid’ cycle. The reason for such a name is that the first stable product formed in the cycle is Citric Acid which has three carboxylic acid groups. Krebs cycle has been named after the scientist ‘Sir Hans Krebs’, who discovered it and won a Nobel prize for it. This cycle takes place inside the mitochondrial matrix where all the relevant enzymes are present. In this cycle, two molecules of acetyl CoA enter, and therefore, two cycles of the Krebs cycle are required for the complete oxidation of one molecule of glucose. This cycle is composed of eight distinct processes and is discussed below:

Step 1

The very first step of the TCA cycle is the condensation of the acetyl group with oxaloacetic

acid (OAA) and water to form citric acid. The enzyme used is citrate synthase and a molecule of CoA is released.

                                                  Fig: Formation of citric acid

Step 2

In this step, citrate is isomerised to isocitrate in the presence of enzyme aconitase.

                                                            Fig: Isomerisation

Step 3

In this step Isocitrate undergoes oxidative decarboxylation to form ∝-ketoglutarate. The enzyme used here is isocitrate dehydrogenase. In this process, NAD⁺ is reduced to NADH, and H⁺ and CO₂ are released.

                                         Fig: Oxidative decarboxylation of isocitrate

Step 4

In this step ∝-ketoglutarate undergoes oxidative decarboxylation to form succinyl CoA in the presence of enzyme ∝-ketoglutarate dehydrogenase. In this process as well, NAD⁺ undergoes reduction to form NADH and H⁺ and CO₂ are released.

                                 Fig: Oxidative decarboxylation of alpha-ketoglutarate

Step 5

Succinyl CoA synthetase catalyses the conversion of succinyl CoA to succinate. In this step, GTP is generated through substrate level phosphorylation.

                                      Fig: Conversion of succinyl CoA to succinate

Step 6

In this step Succinate dehydrogenase (SDH) enzyme is found to be important. It is present attached to the inner mitochondrial membrane that is an integral component of the mitochondrial respiratory chain. In the reaction, succinate gets converted into fumarate in which two hydrogen atoms are removed from the succinate and added to FAD⁺ to form FADH₂.

                                      Fig: Conversion of succinate to fumarate

Step 7

Water is added to fumarate to form another four-carbon molecule known as malate.

                                      Fig: Conversion of fumarate to malate

Step 8

Malate dehydrogenase enzyme catalyses the conversion of malate to oxaloacetate. In this process, NAD⁺ is reduced to NADH. Oxaloacetate is regenerated which again takes part in the TCA cycle as a substrate.

                                    Fig: Conversion of malate to oxaloacetate

The summary of Krebs cycle is given below:

                                                             Fig: Kreb’s cycle

Electron transport chain

The inner mitochondrial membrane contains a group of electron transporters which form the electron transport chain that move electrons from NADH and FADH₂ to molecular oxygen. Throughout the procedure, oxygen is oxidised to produce water and protons are pushed from the mitochondrial matrix to the intermembrane space. The electron transport chain is also known as oxidative phosphorylation. It occurs via four complexes that are discussed below:

Complex I -NADH dehydrogenase complex

Iron-sulphur and FMN (flavin mononucleotide) based enzymes are included in it. Vitamin B₂ is the source of FMN. Two electrons released due to oxidation of NADH to NAD⁺ are transferred to the complex I which then transfers it to Ubiquinone (Q) present in the inner mitochondrial membrane. For every pair of electrons that pass through Complex I, it pumps 2 H⁺ ions (protons) into the intermembrane space from the matrix. Reduced ubiquinone or ubiquinol transfers the electrons received from Complex I to complex III.

                                        Fig: Electron transport through Complex I

Complex II - Succinate dehydrogenase

It is also called succinate dehydrogenase complex and has FAD and a Fe-S complex as the prosthetic group. FADH₂ that bypasses complex 1 is taken up right away by complex II. The chemical ubiquinone (Q) links the Complex II to Complex III and transfers the electrons from the former to the latter.. The hydrophobic centre of the membrane allows the Q molecule to readily travel because it is soluble in water. An electron is supplied straight to the electron protein chain at this phase. The volume of protons pumped across the mitochondria's inner membrane at this point directly correlates with the amount of ATP produced.

                                    Fig: Electron transport through Complex II

Complex III - Cytochrome c reductase

Proteins from Cytochrome b, Cytochrome c, and Fe-S make up the third complex. The heme group is a component of cytochrome proteins. The protons are pumped across the membrane by Complex III. Additionally, it transfers electrons to cytochrome c, from whence they are carried to the fourth complex of proteins and enzymes. Here, Cytochrome c serves as the electron acceptor while Q serves as the electron donor. For the transfer of every two electrons through Complex III, 2H⁺ ions are pumped into the intermembrane space.

                                Fig: Electron transport through Complex III

Complex IV - Cytochrome c oxidase

The fourth complex of the electron transport chain is composed of cytochrome c, a and a₃ and two copper centres. There are two heme groups in this complex and each of them is present in cytochrome c and a₃. A mobile carrier, reduced Cytochrome c ( a mobile carrier) helps to transfer electrons to cytochrome a and a₃ which are then finally transferred to O₂. The enzyme cytochrome c oxidase then catalyses the reduction of O₂ to H₂O.

Until the oxygen content is totally reduced, the cytochromes are in charge of retaining an oxygen molecule between copper and iron. The decreased oxygen in this phase picks up two hydrogen ions from the surroundings to create water.

                                Fig: Electron transport through Complex IV

Transport of protons across the inner mitochondrial membrane

Transfer of electrons from one electron carrier to another via the ETC causes protons to be pumped from the mitochondrial matrix to the intermembrane space, thereby creating a proton gradient.

When NADH + H⁺ is oxidised to NAD⁺, two electrons and two protons are released. For every electron pair that is transferred from- NADH to FMN (Complex I), Cytochrome b to Cytochrome c and Cytochrome a to Cytochrome a₃, two protons or H⁺ ions are transported from the matrix to the intermembrane space. Thus, three pairs of protons are pumped from the matrix to the inter membrane space.

Similarly, oxidation of FADH₂ to FAD releases two electrons and two protons. Every electron pair that is transferred from Cytochrome b to Cytochrome c and Cytochrome a to Cytochrome a₃ requires two protons or H⁺ ions to be transported from the matrix to the intermembrane space. Thus, two pairs of protons are pumped from the matrix to the inter membrane space.

Oxidative phosphorylation

The energy of oxidation-reduction, as the electrons are transported down the energy gradient in the ETC, is utilised for the production of a proton gradient which drives the phosphorylation of ADP to ATP. This process of ATP generation is called oxidative phosphorylation. The process involves Complex V or the ATP synthase complex which is composed of the F₀ -F₁ particles.

                             Fig: Complex V

Mechanism of oxidative phosphorylation

Transport of two electrons released due to oxidation of NADH+H⁺ through the ETC results in pumping of three pairs of protons from the matrix to the inter membrane space which creates a proton gradient. In order to break this gradient, the F₀ channel of Complex V pumps three pairs of protons from the inter membrane space into the matrix and this activates the ATP synthase enzyme of F₁ particle which catalyses the phosphorylation of three ADP molecules to three ATP molecules.

Similarly, for the transport of two electrons from one molecule of FADH₂ through the ETC, two pairs of protons are pumped from the matrix to the inter membrane space which creates a proton gradient. To break this gradient, two pairs of protons are transported by the F₀ channel of Complex V from the inter membrane space into the matrix which in turn drives the synthesis of two ATP molecules from two ADP molecules with the ATP synthase activity of the F₁ complex.

                               Fig: Mechanism of ATP synthesis by Complex V

Respiratory balance sheet

Ideally, one glucose molecule can generate 38 ATPs. However, this is merely a speculative calculation. In practise, this amount of ATP can only be produced under the following circumstances:

• Glycolysis, the TCA cycle, and the ETS pathways should occur in that order, one after the other.
• The mitochondria should receive the NADH produced during glycolysis and undergo oxidative phosphorylation.
• The process doesn't use any of the intermediates to synthesise any other compounds.
• Only the breakdown of glucose should occur. At any of the transitional stages, no additional alternative substrates should be allowed to enter the pathway.

                                   Fig: Respiratory balance sheet

Significance of aerobic respiration

• Aerobic respiration is important because it produces energy for living organisms which in turn is essential for their survival.
• The energy produced from aerobic respiration is essential for the growth and maintenance of all living tissues.
• It plays an important role in the carbon balance in the ecosystem as well as in the global carbon cycle.

Practice Problems

1. What are the end products of aerobic respiration?

1. Sugar and oxygen
2. Water and energy
3. Carbon dioxide and energy
4. Carbon dioxide, water and energy

Solution: The stepwise breakdown of respiratory substrates to CO₂ and H₂O in the presence of oxygen is termed ‘aerobic respiration’. It occurs in the cytoplasm and mitochondria. The glucose is broken down in the presence of oxygen and forms carbon dioxide, water, and energy. The reaction is given below:

C₆H₁₂O₆ + 602 → 6CO₂ + 6H₂O + 38ATP

Hence, the correct option is d.

2. In which of the given reactions a five carbon compound is converted to a four carbon compound?

1. Conversion of ∝-ketoglutarate to succinyl coA
2. Conversion of oxaloacetate to citrate
3. Conversion of isocitrate to ∝-ketoglutarate
4. Conversion of fumarate to malate

Solution: In the fourth step of the Krebs cycle, ∝-ketoglutarate undergoes oxidative decarboxylation to form succinyl CoA in the presence of enzyme ∝-ketoglutarate dehydrogenase. In this process as well, NAD⁺ undergoes reduction to form NADH and H⁺ and CO₂ are released. ∝-ketoglutarate is a five carbon compound which upon removal of carbon dioxide (decarboxylation) yields a four carbon compound that is succinyl CoA. Hence, the correct option is a.

Fig: Oxidative decarboxylation of alpha-ketoglutarate

Oxaloacetate is a four carbon compound which combine with acetyl CoA, that is a two carbon compound to form a six carbon compound known as citrate. Citrate and isocitrate are isomers and hence both contain six carbon atoms. Isocitrate undergoes oxidative decarboxylation to form a five carbon compound known as ∝-ketoglutarate.

3. What is the number of ATP molecules produced by substrate level phosphorylation during the payoff phase of glycolysis?

1. 4
2. 2
3. 6
4. 8

Solution: During the payoff phase of glycolysis, substrate level phosphorylation at two steps leads to the generation of ATP. For the breakdown of each molecule of glucose by glycolysis, two molecules of 1,3 bisphosphoglyceric acid are converted to two molecules of 3 phosphoglyceric acid and the two inorganic phosphate groups released from the substrate in the process are used to phosphorylate two molecules of ADP into ATP. The enzyme catalysing the reaction is phosphoglycerate kinase.

Similarly, in the last step of the payoff phase of glycolysis, two molecules of 2, phosphoenolpyruvate are dephosphorylated in the presence of the pyruvate kinase enzyme to form two molecules of pyruvate. The two inorganic phosphate radicals released are used to phosphorylate two molecules of ADP into ATP. Hence, the correct option is a.

4. Which process is common in aerobic and anaerobic respiration?

1. Krebs' cycle
2. Glycolysis
3. Glycogenolysis
4. ETS

Solution: ‘Glycos’ means sugar and ‘lysis’ means splitting. The first step in converting glucose into energy is called glycolysis. Glycolysis occurs in all kinds of living beings whether they are aerobic or anaerobic organisms because this step is common to both aerobic and anaerobic respiration and occurs in the cytoplasm. The Krebs cycle and ETS take place inside the matrix and on the inner membrane of mitochondria as part of the aerobic respiration process. Hence, the correct option is b.

FAQs

1. Do plants also respire during night time?

Answer: In the course of their nighttime respiration, plants exhale carbon dioxide, take in oxygen, and oxidise food that has been stored. For this reason, it is advised not to sleep under a tree at night. During the day, the rate of photosynthesis exceeds the rate of respiration and hence the carbon dioxide produced as a result of respiration is used up for photosynthesis and only oxygen is released as a byproduct of photosynthesis.

2. Why is aerobic respiration more efficient than anaerobic respiration?

Answer: Aerobic respiration is more efficient than anaerobic respiration because it makes more energy in the form of ATP molecules from glucose. During aerobic respiration, 36 to 38 molecules of ATP are formed from glucose, whereas during anaerobic respiration, only 2 ATP molecules are formed from glucose.

3. Can humans respire without oxygen?

Answer: Respiration in the absence of oxygen, that is anaerobic respiration can occur only in the muscle cells in humans and produces lactic acid and two molecules of ATP. It can only be retained for about three minutes before the muscle cells begin to accumulate significant amounts of lactate. If exercise is continued, excess lactate causes the cellular respiration process to slow down and results in a burning sensation in the muscles.

4. Where does aerobic respiration occur in prokaryotes?

Answer: Prokaryotes do not have membrane bound organelles and hence lack a mitochondria. Thus, in aerobic prokaryotes, the reactions involved in glycolysis occur in the cytoplasm whereas the rest of the steps occur in specialised cell membrane invaginations known as mesosomes.

YOUTUBE LINK: https://www.youtube.com/watch?v=sI-98My_Yz0

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