WBCS Preliminary(Biology): Respiration

1. Respiration is essential for survival of living organisms. It releases energy from the food. Respiration is defined as the catabolic biochemical process during which organic compounds break down in order to release energy. The organic compounds that are broken down are called the substrates. Glucose is the most common substrate. The general equation for respiration is: C₆H₁₂+ 6O₂--->6CO₂+6H₂O+38ATP (Adenosine Triphosphate).
2. Breathing provides the mechanism necessary to take in oxygen and give out carbon dioxide that is a waste gas. It is a physical process and part of respiration. Respiration is a biochemical and physical process.
3. Respiration takes place in the following three stages:
   1. External Respiration: The exchange of gases between the environment and the body is called external respiration or gaseous exchange.
   2. Internal Respiration: The exchange of gases between the body spaces or fluids and the cells is called internal respiration or tissue respiration. The area over which this exchange takes place is called the respiratory surface.
   3. Cellular Respiration: The process of breakdown of food in the cell with the release of energy is called cellular respiration. Cellular respiration takes place in the cells of all organisms.
4. In lower plants (and also protozoans) exchange of gases takes place through the general body surface as they are not highly modified or specialised. Also, the body surface allows the diffusion of gases. Gaseous exchange in higher plants takes place through the stomata in the leaves, lenticels in the stem and general surface of the roots.
   • Stomata are openings generally present on the lower surface of the leaves through which the gases and water vapour diffuse in and out easily. The oxygen diffuses in through the stomata and then enters the leaf cells. Similarly, the carbon dioxide produced by the leaf cells diffuses out through the stomata.
   • In woody stems, the entire surface is covered by bark which is impervious to gases or water. However, there are certain openings or pores in the layer of bark. These are called the lenticels. They are visible slightly more raised than the general surface of the stem. At the base of the lenticels are loosely arranged cells which allow the diffused gases to pass through them.
   • Gases diffuse in and out of the general surface of the roots. The gases are found in the soil surrounding the roots. Plants which grow in salty water show specialised roots called the pneumatophores. These are roots growing out of the surface of water with numerous pores on their surface.
5. In lower animals, gas diffusion takes place through a moist surface membrane, as in flatworms; through the thin body wall, as in earthworms; through air ducts, or tracheae, as in insects; or through specialized tracheal gills, as in aquatic insect larvae. In the gills of fish the blood vessels are exposed directly to the external (aquatic) environment. Oxygen–carbon dioxide exchange occurs between the surrounding water and the blood within the vessels; the blood carries gases to and from tissues. In higher vertebrates, oxygen-poor, carbon dioxide–rich blood from the right side of the heart is pumped into the lungs and flows through the net of capillaries surrounding the alveoli, the cup-shaped air sacs of the lungs.
6. Human Body - Respiratory system: The primary function of the respiratory system is to supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body. The respiratory system does this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide. This exchange of gases is the respiratory systems means of getting oxygen to the blood. Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx (where speech sounds are produced) and the trachea which is a tube that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes which connect to tiny sacs called alveoli. The average adults lungs contain about 600 million of these spongy, air-filled sacs that are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste rich blood from the veins releases its carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs when you exhale.
   • The total lung capacity of an adult human is 5.0 litres
   • Tidal air : Volume of air entering and leaving the lungs during normal breathing (.5 litres)
   • Vital capacity : Volume of air exhaled after a forceful breathing (3.5 litres)
   • Residual air : Volume of air which remains after forceful expiration (1.5 litres)
   • Lung volume and other respiratory volumes can be measured by spirometer.
7. The oxygen is transported to the different parts of the body through the circulatory system. The red blood cells in the blood have pigments called haemoglobin which transport oxygen to the tissues. From the tissues, most of the carbon dioxide is transported as bicarbonate ions in the plasma (outside red blood cells) of the blood.
8. We know Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. The energy released is trapped in the form of ATP (adenosine triphosphate) for use by all the energy-consuming activities of the cell. The process occurs in two phases:
   1. glycolysis, the breakdown of glucose to pyruvic acid
   2. the complete oxidation of pyruvic acid to carbon dioxide and water. This process called aerobic respiration. When the respiration takes place with out oxygen it is known as anaerobic respiration. The end product of anaerobic respiration is carbon dioxide and chemical such as lactic acid and ethanol. When the micro organisms respire anaerobically we call it as fermentation. e.g. fermentation of yeast in the preparation of ethanol.
   3. Breathing Mechanism: The physical movements associated with the gaseous exchange are called breathing. They are controlled by the respiratory centre of the medulla oblongata in the human brain. Thus, the breathing movements are involuntary to a large extent. However, we can control the rate of breathing and the extent of breathing but not for a long time. The respiratory centre is stimulated by the carbon dioxide concentration of the blood. There are two types of physical movements associated with the gaseous exchange. They are:
      • Inspiration or inhalation: During inspiration, the outer intercostal muscles contract, which raises the chest cavity or the ribs. This is accompanied by the lowering of the diaphragm. Together these movements serve to increase the area of the thoracic cavity, which reduces the pressure. The air from outside rushes into the lungs.
      • Expiration or exhalation: The inner intercostal muscles contract bringing the ribs back to the original position and the diaphragm is also raised back by the action of the abdominal muscles. This reduces the space in the chest cavity and increases the pressure. This expels the air out of the lungs.