Respiration is a vital biological process through which living organisms release energy from food substances to perform various life activities. During respiration, glucose and other organic compounds are broken down inside cells, and the released energy is stored in the form of ATP, which is known as the energy currency of the cell. Respiration occurs continuously in all living organisms and may take place in the presence or absence of oxygen.
Both types of respiration begin with glycolysis, but their subsequent pathways and energy yields differ significantly. Based on the presence or absence of oxygen, respiration is classified into two main types:
Aerobic Respiration
- Aerobic respiration is the process of complete oxidation of glucose in the presence of oxygen to release energy. In this type of respiration, oxygen acts as the final electron acceptor. Glucose is completely broken down into carbon dioxide and water, and a very large amount of energy is released in the form of ATP.
- Aerobic respiration occurs in most plants, animals, fungi, algae, and many microorganisms. It is the most efficient method of energy production because almost all the energy stored in glucose is extracted.
- The overall equation of aerobic respiration is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)
- In aerobic respiration, approximately 36–38 ATP molecules are produced from one molecule of glucose.
- Aerobic respiration occurs partly in the cytoplasm and partly in mitochondria.
- Glycolysis occurs in the cytoplasm.
- Krebs cycle and Electron Transport Chain occur in mitochondria.
- Mitochondria are therefore called the “powerhouse of the cell” because they are the major sites of ATP production.
Stages of Aerobic Respiration
Aerobic respiration occurs in a series of sequential biochemical reactions, which can be divided into the following major stages:
(a) Glycolysis
Glycolysis is the first stage of respiration and occurs in the cytoplasm of the cell. The term glycolysis means “splitting of sugar.” In this stage, one molecule of glucose containing six carbon atoms is broken down into two molecules of pyruvate, each containing three carbon atoms. During glycolysis:
- Two ATP molecules are consumed.
- Four ATP molecules are produced.
- Net gain of two ATP molecules occurs.
- Two NADH molecules are produced.
(b) Formation of Acetyl-CoA
The pyruvate molecules produced during glycolysis enter the mitochondria, where they are converted into acetyl-CoA through oxidative decarboxylation. During this process:
- Carbon dioxide is released.
- NADH molecules are formed.
- Acetyl-CoA enters the Krebs cycle.
(c) Krebs Cycle
The Krebs cycle, also known as the Citric Acid Cycle or TCA Cycle (Tricarboxylic Acid Cycle), occurs in the mitochondrial matrix. In this cycle, acetyl-CoA combines with oxaloacetic acid to form citric acid, and a series of cyclic reactions occur. During the Krebs cycle:
- Carbon dioxide is released.
- NADH and FADH₂ are produced.
- ATP molecules are formed.
- Oxaloacetic acid is regenerated.
(d) Electron Transport Chain (ETS)
The Electron Transport Chain occurs on the inner membrane of mitochondria. It is the final stage of aerobic respiration and produces the maximum amount of ATP. During this stage:
- Electrons from NADH and FADH₂ pass through a series of electron carriers.
- Energy released during electron transfer is used for ATP synthesis.
- Oxygen acts as the final electron acceptor.
- Water molecules are formed.
Anaerobic Respiration
- Anaerobic respiration is the process of incomplete oxidation of glucose in the absence of oxygen to release energy. In this type of respiration, glucose is partially broken down, resulting in the formation of intermediate products such as ethanol or lactic acid.
- Anaerobic respiration occurs in certain bacteria, yeast, fungi, and muscle cells under oxygen-deficient conditions. It is less efficient than aerobic respiration because only a small amount of ATP is produced.
Types of Anaerobic Respiration
Anaerobic respiration mainly occurs in two forms:
- Alcoholic Fermentation: Alcoholic fermentation occurs mainly in yeast and some microorganisms. In this process, glucose is converted into ethanol and carbon dioxide in the absence of oxygen. This process is widely used in industries for the production of alcohol and bakery products. The reaction is:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy
- Lactic Acid Fermentation: Lactic acid fermentation occurs in muscle cells during strenuous exercise when oxygen supply becomes insufficient. Under such conditions, pyruvate formed during glycolysis is converted into lactic acid. Accumulation of lactic acid in muscles causes muscle fatigue, pain, and cramps. The reaction is:
C₆H₁₂O₆ → 2C₃H₆O₃ + Energy
Importance of Respiration
Respiration is essential for the survival of all living organisms because no life process can continue without energy. The energy released during respiration supports almost every cellular activity occurring inside the body.
- Respiration provides energy for the growth and development of organisms.
- It supplies the energy required for movement and muscular activities.
- It helps in the synthesis of proteins, enzymes, hormones, and nucleic acids.
- It supports active transport across cell membranes.
- It helps in the repair and maintenance of body tissues.
- In animals, respiration helps maintain body temperature.
- It enables cell division and reproduction.
- It supports biosynthetic and metabolic activities.