Respiration in Bacteria: In bacteria, the process of respiration is performed in mesosomes. Mesosomes are folded invaginations in the cell membrane of bacteria. Their function is to increase the surface area required for respiration. They are sites of respiratory activity as they contain respiratory enzymes.

Respiration in Bacteria

With respect to oxygen requirement and mode of cellular respiration, bacteria distinctly belong to two broad categories :

(1) Aerobic respiration

Obligate aerobes : These bacteria grow exclusively in presence of molecular oxygen and fail to survive in its absence, e.g., Bacillus subtilis, Azotobactor, Arthrobactor, Mycobacterium etc.

Facultative anaerobes : The aerobic bacteria which can also survive in absence of oxygen, e.g., Aerobacter, Klebsiella, Pseudomonas, etc.

(2) Anaerobic respiration

Obligate anaerobes : These bacteria grow and multiply in the absence of free oxygen. They fail to survive under aerobic conditions, e.g., Clostridium botulinum.

Facultative aerobes : The anaerobic bacteria which can also survive in presence of oxygen, e.g., Chlorobium limicola.

Cellular Respiration Process

There are several steps that occur in both aerobic and anaerobic respiration in bacteria. The steps in aerobic respiration are:

1. Glycolysis
2. Pyruvate oxidation
3. Citric acid cycle
4. Electron transport chain

The steps in anaerobic respiration include:

1. Glycolysis
2. Fermentation

Cellular Respiration Steps

1. Glycolysis

• Glycolysis is the first step in cellular respiration where the glucose molecule is catabolized to form pyruvate through a series of 10 steps.
• Glycolysis is the initial step of glucose metabolism, which is the common pathway in both aerobic and anaerobic respiration.
• The overall sequence of reaction in glycolysis might differ from one species to another in its regulation and the fate of pyruvate.
• During glycolysis, the six-carbon compound like glucose breaks down into two three-carbon compounds (pyruvate) with the release of 2 molecules of ATP.
• In aerobic respiration, glycolysis is the prelude to the citric acid cycle and electron transport chain, which are responsible for the production of most of the ATPs.
• The product of glycolysis can proceed in one of three different pathways depending on the availability of oxygen and metabolic activities.

A summary of the process of glycolysis can be written as follows:

C6H12O6 + 2ADP + 2Pi + 2NAD⁺        →         2C3H4O3 + 2H2O + 2ATP + 2NADH + 2H⁺

In words, the equation is written as:

           Glucose + ADP + Pi + NAD      →        Pyruvate + Water + ATP + NADH + Hydrogen ions

Steps of Glycolysis by LearningMantras

2. Pyruvate Oxidation

• Oxidation of pyruvate is the second step of aerobic respiration occurring, representing one of the three possible fates of pyruvate molecules.
• Pyruvate molecules are the end products of glycolysis which is a common pathway in both aerobic and anaerobic respiration.
• The fate of pyruvate is determined by the availability of oxygen and metabolic conditions.
• Oxidation of pyruvate occurs in the presence of oxygen after the pyruvate molecules are moved to the mitochondria from the cytoplasm.
• The pyruvate derived from glycolysis is dehydrogenated to yield acetyl Co-A and CO2 by the enzyme pyruvate dehydrogenase complex. The enzyme is found in the mitochondrial matrix of eukaryotes and the cytoplasm of prokaryotes.
• Pyruvate oxidation acts as a link between glycolysis and the citric acid cycle in the case of aerobic respiration.
• During pyruvate oxidation, a total of 3 ATPs are formed (after the entry of NADH to the electron transport chain).

The overall reaction of pyruvate oxidation can be summed up as:

Pyruvate Coenzyme A + NAD        →           Acetyl Co-A + NADH

3. Citric Acid Cycle

• The citric acid cycle or Kreb’s cycle is the process of complete oxidation of acetyl CoA to release carbon dioxide and water molecules.
• It is the most universal pathway for the aerobic metabolism of energy-rich molecules.
• The reactions of the cycle provide electrons to the electron transport chain, which reduces oxygen while generating ATP.
• This cycle is important not only for carbohydrate metabolism but also for other biomolecules like amino acids and fatty acids.
• The cycle can only occur in the presence of oxygen as energy-rich molecules like NAD+ and FAD can retrieve ATP from their reduced form by the transfer of electrons to molecular oxygen.
• There are two main purposes of the citric acid cycle, which include the disposition of carbon and hydrogen atoms and the conversion of potential chemical energy into metabolic energy in the form of ATP.
• A total of 12 ATPs are formed during the complete oxidation of a single molecule of acetyl Co-A.
• Out of the 12 ATP molecules, only one ATP molecule is produced directly from the cycle; the rest are generated after the entry of high-energy molecules into the electron transport chain.

The overall reaction of the citric acid cycle can be summed up as:

CH3CO-SCoA + 3NAD⁺ + FAD + GDP + Pi + 2H2O       →         2CO2 + CoA-SH+ 3NADH + FADH2 + GTP + 2H⁺ 

4. Oxidative Phosphorylation

• Oxidative Phosphorylation or Electron transport chain in the final step of aerobic respiration consists of a chain of redox reactions to synthesize ATP molecules.
• During the process, the electrons generated in the citric acid cycle are transferred from the organic compound to oxygen while simultaneously releasing energy in the form of ATP.
• The transport of electrons occurs between four large protein complexes that are present in the inner mitochondrial membrane.
• The chain consists of a series of proteins with tightly bound prosthetic groups that are capable of accepting and donating electrons by virtue of their multiple oxidation states.
• The number of ATP synthesized during oxidative phosphorylation depends on the energy-rich molecule passing down the electrons. For, e.g., NADH produces 3 moles of ATP, whereas FADH produces 2 moles of ATP.
• Oxidative phosphorylation is essential for the metabolism of all biomolecules as all the metabolic reactions converge at this stage.
• There are different chemical groups that act as electron carriers during the transport of electrons through the chain. Besides, there are four important enzyme complexes that catalyze the transfer.

A summary of the reactions in the electron transport chain is:

NADH + 1/2O2 + H⁺ + ADP + Pi         →          NAD⁺ + ATP + H2O

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By Team Learning Mantras