Respiration in Living Organisms

Respiration is a biochemical process by which living cells produce energy (ATP) through the breakdown of organic molecules, primarily glucose. Unlike breathing (ventilation), which is a physical process, respiration occurs at the cellular level.

1. Significance of Respiration

• Energy Production: The primary purpose is to release energy in the form of Adenosine Triphosphate (ATP) for cellular activities like growth, reproduction, and movement.
• Maintenance of Body Temperature: Some energy is released as heat, helping endothermic animals maintain a constant body temperature.
• Waste Removal: It helps in the excretion of carbon dioxide and water, which are metabolic by-products.

2. The Chemical Process of Respiration

Respiration occurs in two main stages: Glycolysis and the Krebs Cycle.

A. Glycolysis (Anaerobic Stage)

Occurs in the cytoplasm. A molecule of glucose (6-carbon) is broken down into two molecules of pyruvate (3-carbon). It yields a net gain of 2 ATP molecules and does not require oxygen.

B. Krebs Cycle (Aerobic Stage)

Also known as the Citric Acid Cycle, it occurs in the mitochondria. Pyruvate enters the mitochondria and is further broken down into CO₂ and water. This stage produces significantly more energy (34-36 ATP) and requires oxygen.

3. Respiratory Organs and Surfaces

For gaseous exchange to occur, organisms possess specialized surfaces. These surfaces must be thin, moist, highly vascularized (rich in blood supply), and have a large surface area.

• Body Surface: Amoeba, Earthworm.
• Gills: Fish, Tadpoles.
• Trachea (Tracheal System): Insects (e.g., Grasshopper).
• Lungs: Mammals, Birds, Reptiles, Amphibians (adults).
• Stomata: Leaves of plants.
• Lenticels: Woody stems of plants.

4. Mechanism of Gaseous Exchange

In Plants (Stomata)

The opening and closing of stomata are controlled by guard cells. When guard cells take in water (become turgid), they swell and the pore opens. When they lose water (become flaccid), the pore closes. This is often driven by potassium (K+) ion concentration and light intensity.

In Mammals (Inhalation and Exhalation)

• Inhalation (Inspiration): The diaphragm contracts and flattens; external intercostal muscles contract, raising the ribs. Thoracic volume increases, pressure drops, and air enters.
• Exhalation (Expiration): The diaphragm relaxes and becomes dome-shaped; external intercostal muscles relax, ribs move down and in. Thoracic volume decreases, pressure increases, and air is forced out.

5. Aerobic and Anaerobic Respiration

• Aerobic Respiration: Occurs in the presence of oxygen. Formula: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (38 ATP).
• Anaerobic Respiration: Occurs in the absence of oxygen. In muscles, it produces Lactic Acid (leading to fatigue). In yeast, it is called Fermentation, producing Ethanol and CO₂.

6. Experiments in Respiration

• Gaseous Exchange: Using lime water (Calcium hydroxide) to detect CO₂; it turns milky in the presence of expired air.
• Heat Production: Using germinating seeds in a vacuum flask with a thermometer; a rise in temperature indicates respiration.