Why is Cellular Respiration Considered an Efficient Process?

Cellular respiration is the cellular process involved in the generation of adenosine triphosphate (ATP) molecules from the organic nutritional source obtained from the diet. It is a universal process observed in all types of life forms. The glucose (chemical formula C₆H₁₂O₆) molecules are the preferred raw material for cell respiration as it possesses a simple structure and is highly efficient in nature.

What is Cellular Respiration?

The process of cellular respiration is defined as the set of metabolic processes and reactions which help in generation of energy from organic substances such as glucose. This occurs in the organism’s cells where the chemical energy from the nutrients or molecules of oxygen is converted into ATP. The process is found to take place in the presence and the absence of oxygen. The cellular respiration occurring in the presence of oxygen is called aerobic respiration, and the process taking place in the absence of oxygen is called anaerobic respiration.

The ATP molecules are the final products of cellular respiration. These molecules are composed of high-energy phosphate bonds which provide energy for various metabolic activities for the survival and sustenance of the cell. Most of the reactions taking place in cellular respiration are catabolic, where the complex substances are broken down into simple and absorbable forms. The nutrients employed for carrying out cellular respiration encompass fatty acids, amino acids, and carbohydrates. The oxidizing agent identified in the process of cellular respiration is the molecular oxygen, which is taken into the body by the respiratory system. The energy released by the ATP hydrolysis will supply to processes including locomotion, biosynthesis, and molecular transport. 

Types of Cellular Respiration

The process of cellular respiration can happen in the absence or in the presence of molecular oxygen, where the reaction occurring in the presence of oxygen is aerobic and that occurring in the absence of oxygen is anaerobic. The features of both types are as follows:

• When the ATP generation occurs in the presence of molecular oxygen, the raw materials employed for ATP generation include fats, carbohydrates, and proteins. The breakdown of the pyruvate (formed during glycolysis by breakdown of glucose molecule) occurs in the mitochondria only in the presence of oxygen molecules. The process of aerobic respiration results in the formation and elimination of substances such as water and carbon dioxide as metabolic by-products. 
• The organic compound is broken down in the absence of oxygen in anaerobic respiration. The inorganic acceptor molecules are employed for the catabolism of these organic compounds and the acceptor molecules include sulfur, nitrate, and sulfate. These organisms are adapted to thrive in the absence of oxygen, and are usually identified in the hydrothermal vent. The efficiency of energy production is usually lower than aerobic respiration.
• The process of fermentation also takes place in the cell where the pyruvate is transformed into waste products and is discharged out of the cell. The lactic acid is found to be a product of fermentation taking place inside the cell. 
• Aerobic respiration is recognized as 15 times more efficient when compared to that of anaerobic respiration.
• The anaerobic respiration yields only 2 ATP molecules for each glucose molecule, whereas the aerobic respiration yields 38 ATP molecules per glucose molecule.

The Process of Cellular Respiration

Cellular respiration involves three major processes: glycolysis, the citric acid cycle or Krebs cycle, and oxidative phosphorylation or electron transport chain.

• The glycolysis is the primary step in the process and it comprises the breakdown of the carbohydrate for the production of pyruvate. This process can occur in the presence or in the absence of oxygen.
• The citric acid cycle and oxidative phosphorylation are synchronized for the production of the molecules of ATP in the cell and occur in the mitochondria. 

Glycolysis

Glycolysis takes place in the cytoplasm and the process generates two molecules of pyruvic acid as the end product. This process yields a total of two ATP molecules. A total of four ATP molecules are generated during glycolysis, of which two molecules are employed during the preparatory phase of glycolysis. The pay-off phase of the glycolysis marks the transfer of ADP through the substrate-level phosphorylation and generates four molecules of ATP along with two NADH during the pyruvate oxidation. Thus, net ATP production per glucose molecule is two. An ATP is employed in the formation of glucose-6-phosphate. The enzyme glucokinase is important for the formation of glucose 6-phosphate by phosphorylation of sixth carbon of glucose molecule. 

Citric acid cycle

The citric acid cycle which is otherwise termed the Krebs cycle is involved in the production of acetyl-CoA from pyruvate molecules. The process consists of eight essential steps and 18 enzymes. The citrate is the fundamental product of the citric acid cycle formed from oxaloacetate and acetyl-CoA. The citrate further develops the isocitrate, which is further converted into compounds such as α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate. Krebs cycle results in the generation of electron carriers such as NADH (nicotinamide adenine dinucleotide + hydrogen) and FADH₂ (flavin adenine dinucleotide). These electron carriers participate in the electron transport chain or oxidative phosphorylation and result in the synthesis of ATP.

Oxidative phosphorylation

The oxidative phosphorylation process occurs in the mitochondria of the cell and after the citric acid cycle. This process involves the formation of an electron transport chain, which is supported by various enzymatic complexes. All the electrons that enter this transport chain are obtained from FADH₂ and NADH molecules. These are obtained from the initial stages of cellular respiration. The ATP synthase embedded in the inner mitochondrial membrane generates ATP molecules as the end product of oxidative phosphorylation. The hydrogen ion gradient is generated in the mitochondrial region by the process of chemiosmosis (movement of ions across the semipermeable membrane from the lower concentration of ions to higher concentration). The proton pumps present in the mitochondria transport the hydrogen ions across the inner membrane of the mitochondria generating a proton motive force. This force eventually powers the ATP synthase enzyme which synthesizes ATP from ADP (adenosine diphosphate) and inorganic phosphate. 

The Capacity of ATP Generation

• The process of glycolysis results in generation of four molecules of ATP, where two are employed for the preparatory phase. Thus, the net yield of ATP during glycolysis is 2 ATP molecules.
• Krebs cycle or TCA cycle gives two molecules of ATP along with six molecules of NADH and two molecules of FADH₂.  The molecules such as FADH₂ and NADH are the electron carriers which enter the electron transport chin chain.
• The electron transport chain results in the formation of 34 ATP molecules by the conversion of FADH₂ and NADH. Each molecule of FADH₂ yields 2 ATP and each molecule of NADH yields 3 ATP molecules.
• The theoretical efficiency of ATP generation is found to be around 38 molecules.
• But due to the presence of leaky membranes, each glucose molecule produces approximately 29 to 30 ATP.

Context and Applications

The topic is important in exams conducted in the following graduate and undergraduate courses:

• B.Sc. (Bachelors of Science) and M.Sc. (Masters of Science) in Biology
• B.Sc. and M.Sc. in Biotechnology
• B.Sc. and M.Sc. in Microbiology

Related Concept

• Photosynthesis 
• ATP production
• Respiration
• Cell biology