Class 10 Science


The process by which a living being utilizes the food to get energy is called respiration. Respiration is an oxidation reaction in which carbohydrate is oxidized to produce energy. Mitochondrion is the site of respiration and the energy released is stored in the form of ATP (Adenosine triphosphate). ATP is stored in mitochondria and is released as per need.

Steps of Respiration:

Breaking down glucose into pyruvateThis step happens in the cytoplasm. Glucose molecule is broken down into pyruvic acid. Glucose molecule is composed of 6 carbon atoms, while pyruvic acid is composed of 3 carbon atoms.
Fate of Pyruvic AcidFurther breaking down of pyruvic acid takes place in mitochondria and the molecules formed depend on the type of respiration in a particular organism. Respiration is of two types, viz. aerobic respiration and anaerobic respiration.

Types of Respiration:

Aerobic RespirationThis type of respiration happens in the presence of oxygen. Pyruvic acid is converted into carbon dioxide. Energy is released and water molecule is also formed at the end of this process.
Anaerobic RespirationThis type of respiration happens in the absence of oxygen. Pyruvic acid is either converted into ethyl alcohol or lactic acid. Ethyl alcohol is usually formed in case of anaerobic respiration in microbes; like yeast or bacteria. Lactic acid is formed in some microbes as well as in the muscle cells.

Glucose (6 carbon molecule) ⇨ Pyruvate (3 carbon molecule) + Energy

Pyruvate (In yeast: lack of O2 ) ⇨ Ethyl alcohol + Carbon dioxide + Energy

C6H12O6 ⇨ C2H5OH + CO2

Pyruvate (In muscles: lack of O2 ) ⇨ Lactic Acid + Energy


Pyruvate (In mitochondria: presence of O2 ) ⇨ Carbon dioxide + Water + Energy

C6H12O6 ⇨ 6CO2 + 6H2O + Energy

Pain in Leg Muscles on Running: When someone runs too fast, he may experience a throbbing pain in leg muscles. This happens because of anaerobic respiration taking place in the muscles. During running, the energy demand from muscle cells increases. This is compensated by anaerobic respiration and lactic acid is formed in the process. The deposition of lactic acid causes pain in leg muscles. The pain subsides after taking rest for some time.

Exchange of Gases: For aerobic respiration, organisms need a continuous supply of oxygen, and carbon dioxide produced during the process needs to be removed from the body. Different organisms use different methods for intake of oxygen and expulsion of carbon dioxide. Diffusion is the method which is utilized by unicellular and some simple organisms for this purpose. In plants also, diffusion is utilized for exchange of gases. In complex animals, respiratory system does the job of exchange of gases. Gills are the respiratory organs for fishes. Fishes take in oxygen (which is dissolved in water) through gills. Since availability of oxygen is less in aquatic environment so the breathing rate of aquatic organisms is faster. Insects have a system of spiracles and tracheae which is used for taking in oxygen.

Terrestrial organisms have developed lungs for exchange of gases. Availability of oxygen is not a problem in terrestrial environment so breathing rate is slower compared to what it is in fishes.

Human Respiratory System:

The human respiratory system is composed of a pair of lungs. These are attached to a system of tubes which open on the outside through the nostrils. Following are the main structures in the human respiratory system:

Nostrils: There are two nostrils which converge to form a nasal passage. The inner lining of nostrils is lined by hairs and remains wet due to mucus secretion. The mucus and the hairs help in filtering dust particles out from inhaled air. Further, air is warmed up when it enters the nasal passage.

Pharynx: It is a tube like structure which continues after the nasal passage.

Larynx: This part comes after the pharynx. This is also called the voice box.

Trachea: This is composed of rings of cartilage. Cartilaginous rings prevent the collapse of trachea in the absence of air.

Bronchi: A pair of bronchi comes out from the trachea, with one bronchus going to each lung.

Bronchioles: A bronchus divides into branches and sub-branches, inside the lung. These are called bronchioles.

Alveoli: There are air-sacs at the end of bronchioles. They are called avleoli. Alveolus is composed of a very thin membrane and is the place where blood capillaries open. Alveolus is the site, where oxygen mixes with the blood and carbon dioxide exits from the blood. The exchange of gases, in alveoli, takes place due to pressure differential.

Breathing Mechanism

Mechanism of Inhalation Mechanism of Exhalation

Breathing Mechanism: The breathing mechanism of lungs is controlled by the diaphragm and the intercostalis muscles. Diaphragm is a membrane which separates the thoracic chamber from the abdominal cavity. When diaphragm moves down, the lungs expand and air is inhaled. When diaphragm moves up, the lungs contract and air is exhaled.