Respiratory System

Respiration is a biochemical process where glucose is broken down through a series of enzyme-controlled reactions to release energy.

Types of Respiration

1. Internal Respiration: This refers to the oxidation of food substances within cells, leading to the release of energy, carbon dioxide, and water. Oxygen obtained through breathing enables this process. The reaction can be represented as:
   C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
2. External Respiration: This involves inhaling oxygen into respiratory organs (e.g., lungs or gills) and exhaling carbon dioxide and water vapor.

Cellular Respiration

The energy released during respiration supports various activities, including protein, lipid, and protoplasm synthesis; germination; cell division and growth; movement; nerve impulse transmission; active transport; and maintaining body temperature.

Cellular respiration, a process occurring within cells, is categorized into two types:

1. Aerobic Respiration
2. Anaerobic Respiration

Aerobic Respiration

This type of respiration involves the complete breakdown of glucose in the presence of oxygen. The process follows this equation:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Aerobic respiration occurs in two main stages:

Glycolysis

• This series of reactions breaks glucose into a three-carbon molecule called pyruvic acid.
• It occurs in the cytoplasm and does not require oxygen.
• Only a small amount of energy is produced during this stage.

Krebs Cycle (Citric Acid Cycle)

• Pyruvic acid from glycolysis undergoes complete oxidation in the mitochondria, producing carbon dioxide and water.
• Pyruvic acid is first converted into acetic acid, which combines with coenzyme A to form acetyl-CoA.
• Acetyl-CoA reacts with oxaloacetic acid to form citric acid, initiating the Krebs cycle.
• This cycle, occurring in the mitochondrial matrix, is essential for energy production and connects the metabolism of proteins, fats, and carbohydrates.

One molecule of pyruvic acid produces 15 ATP molecules during the Krebs cycle. Since a glucose molecule produces two pyruvic acid molecules, a total of 38 ATP molecules are generated during the complete aerobic process.

Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen, leading to the formation of different end products in plants and animals:

In Animals:

Lactic acid is the main product, represented by the equation:
C6H12O6 → 2C3H6O3 + Energy

In Plants:

Ethanol and carbon dioxide are the primary products, represented by the reaction:
C6H12O6 → 2C2H5OH + 2CO2

How Lactic Acid is Produced in Muscles During Exercise

During exercise, the oxygen supply may become insufficient to meet the energy demands of the muscles. When this occurs, cells produce energy through anaerobic respiration, resulting in lactic acid as a byproduct. The buildup of lactic acid causes muscle fatigue, but it is eventually reduced as oxygen intake returns to normal after exercise. This shortfall of oxygen is referred to as "oxygen debt." It can be repaid through increased oxygen intake, where the person breathes deeply and rapidly to draw more oxygen into the lungs. This oxygen is used to oxidize the lactic acid into carbon dioxide and water.

External Respiration

Respiration is facilitated by an organism's respiratory system. In animals, the following organs are involved in respiration:

1. Body surfaces
2. Book lungs
3. Gills
4. Trachea
5. Lungs

Body Surface

In small animals, gas exchange can occur directly between all body cells. Larger animals, such as amphibians and earthworms, also exchange gases through their body surfaces but require a circulatory system to distribute gases throughout the body. Respiration occurs by diffusion, and gases must dissolve in water before diffusing across the cell membrane. Unicellular organisms use their body surface for gas exchange. Animals that respire through their body surface have moist skin. Breathing, a visible, mechanical action, quickens the rate of gas exchange between the organism and its environment.

Gills

Gills are found in large aquatic organisms such as tadpoles, aquatic snails, and fishes. They are highly branched and vascularized, meaning they are richly supplied with blood capillaries. Gills can be external or enclosed. External gills are found in sea slugs and amphibian larvae, while enclosed gills are found in water snails and crustaceans.

Trachea

The tracheal system is used for respiration in insects. Air enters through spiracles and travels through the tracheal trunk, which branches into a complex network of tracheal tubes that reach every part of the body. At the end of each branch, specialized cells called tracheoles provide a thin, moist surface for gas exchange between atmospheric air and living cells. Oxygen dissolves in the liquid of the tracheole and diffuses into adjacent cells, while carbon dioxide, a byproduct of cellular respiration, diffuses out of the cells and eventually exits the body through the tracheal system.

Respiration in Mammals

The respiratory system of mammals is the most complex among all animals. It consists of a pair of lungs enclosed in the thorax and connected to the outside by a series of branched air tubes.

In humans, air enters through the nose or mouth, both of which lead to the pharynx, a short passageway that branches in two directions. One pathway leads to the digestive system, while the other leads to the larynx (voice box) and the lower respiratory tract. The entrance to the larynx, called the glottis, is covered by the epiglottis, which prevents food from entering the windpipe. For air to enter the larynx, the glottis must remain open.

The trachea branches into two bronchi. Cartilaginous rings in the trachea and bronchi prevent them from collapsing when air pressure is low. Each bronchus leads to a lung, where it branches into smaller tubes called bronchioles. The bronchioles terminate in alveoli, which are richly supplied with blood capillaries and serve as the site of gaseous exchange.

Mechanism of respiration

Process of Inspiration

1. The intercostals muscle contract
2. The rib are moved upward and outward
3. The diaphragm flattens
4. There is an increases in volume and a fall in the pressure of thoracic cavity.
5. Air from outside is drawn into the lungs or alveoli through the nose, trachea, bronchus and bronchioles leading to an increase in the size of the lungs.

Process of expiration

1. The intercostals muscle relax
2. The ribs are moved downward and inward
3. The diaphragm becomes dome shape
4. There is decrease in the volume of thoracic cavity.
5. Air containing waste products like CO2 and water vapour from inside the alveoli or lungs are forced out through bronchioles, bronchi, trachea, and finally to the exterior through the nose.

Respiration in plants

There is no special respiratory organ in plant. Gases move in and out the plant through the stomata and lentils

Plant respiration is limited by the process of diffusion. The opening and the closing of the stomata depends on the guard cells flanking them. When the guard cells are turgid, the cells remain open, but when they become flaccid, the pores close up. When the stomata opens, oxygen diffuses through the intercellular spaces of the leaf into the mesophylls cells. The carbon dioxide diffuses from the mesophyll cells into the intercellular spaces and finally escapes through the stomata into the surrounding air while the water gets mixed up with the water content of the cell.