Each and every cell in living organisms needs energy. These cells get their energy from oxidation of food.

Definition :- Respiration is a catabolic process in which organic food is oxidise to release energy.

Respiratory Substrate :- The organic food which is oxidise to release energy. Most common respiratory substrate is glucose.

The energy released during cellular respiration is immediately used to synthesise a energy rich compound ATP.

                       ADP+P = ATP

Respiration is an exothermic reaction, it means energy is liberated.

In our body ATP is used for the contraction of muscles, protein synthesis, conduction of nerve impulse, and many other activities.

Steps/Stages of Respiration

There are two steps in respiration :-

1. Glycolysis :- In this glucose molecule is firstly broken down to 2 molecule of three carbon compound, called pyruvic acid (CH3COCOOH).

The site of glycolysis is does not require oxygen.

2. Kreb cycle :- In this step glucose breakdown into carbondioxide, water, energy.

The site of kreb cycle is mitochondria. It require oxygen.

Types of Respiration

1. Aerobic Respiration :- In most of the organisms respiration involves use of molecular oxygen, this type of respiration is called aerobic respiration.

2. Anaerobic Respiration :- In some organisms respiration does not utilize molecular oxygen, this type of respiration is called anaerobic respiration. Exa. In yeast, or in muscle cell

In yeast-

Yeast, is a single celled can respire anaerobically by –

Glucose → Pyruvic acid → 2ethyle alcohol+co2+E

Anaerobic respiration in muscle :- Muscle cells of our body (voluntary/striated muscle) can also respire anaerobically for short time, in them lactic acid is produced instead of alcohol.

Glucose → Lactic acid+Energy

Muscular Cramps :- when you run very fast, the muscle of your legs use lot of energy. This extra energy is produced in our muscle cells by anaerobic respiration. This activity accumulates lot of lactic acid in the muscles and blood. That accumulation of lactic acid in muscle cells causes cramp.

Blood Flow Chart

Differences Between Aerobic and Anaerobic Respiration

Aerobic Respiration

It does not use molecular O2.

It provide less energy.(2 ATP)

It occurs in the cytoplasm only

End Product :- CO2and water

Anaerobic Respiration

It uses molecular O2.

It provides much more energy.(38ATP)

Partly in cytoplasm and partly in mitochondria.

End Product :- Ethanol, CO2,lactic acid.

Respiration in Plants

The gaseous exchange in plants differs from animals in several ways like :-

1. The plants do not have an organised respiratory system.

2. Plants are non motile and have low metabolic requirements,therefore,the respiration in plants occurs at slower rate.

Gaseous exchange in plants-

Plants are multicellular but they have a large surface area in comparison to their volume, as the surface area is large, diffusion alone can supply all the cells of the plant, with required oxygen. Diffusion occurs separately in the leaves, stem, and roots of plants :-

1. In leaves :- Diffusion occurs through stomata in the leaves.

2. Stem :- In woody plants the stem are covered with impermeable bark but the cork cells of bark are packed loosely at places, to allow oxygen to diffuse into the cells underneath. These holes of the stem are called LENTICLES.

3. Roots :- Gaseous exchange in roots generally takes place through root hairs of root epidermis.

Respiration in Animals

Terrestrial animals can breath the oxygen in the atmosphere but aquatic animal need to use the oxygen dissolve in water.

Due to this reason animals living in water expend more energy than lend animals.

Respiratory organs in different group of animals :-




  Cell surface


  Moist skin(Cutaneous resp.)




  Tracheal system

Amphibia, Reptiles, Birds, Man


Respiratory surface

Surface which takes part in exchange of gases is called respiratory surface, which have following characteristics –

Characters :-

1. It should be richly supplied by blood vessels.

2. It should have thin wall which is permeable to gases.

3. It should be moist.

4. It should be near to transport system in higher animals.

Respiration in Fish

In a fish respiratory surface is the surface of their gills.

Due to their unique features gills have a large surface area. They are thin and richly supplied by blood.

Gills are highly supplied by blood capillaries oxygen of the water is taken in quickly. Similarly carbon di oxide is diffuse out. Oxygen is brought to the gills as water flows over them-

Water in → Mouth → Gills → Gill slits → Out

Respiration in Human

In human being and all other higher animals lungs are the respiratory organs.

The respiration in human being can be studied under following steps –

1. Human respiratory system.

2. Breathing mechanism.

3. Transport of gases

Human Respiratory System

It includes :- External nostrils, Nasal passage, Internal nostrils, Pharynx, Larynx, Trachea, Bronchi, Bronchioles, Lungs.

1. External nostrils/external nares :-  Ext. nostrils are a pair of slits at the lower end of the nose. Air enters  the body through the nose or nostrils.

2. Nasal chambers/nasal cavity/nasal passage :- The nasal chambers are a pair of passages in the head above the palate.

The two nasal chambers are separated from each other by the nasal septum.

This passage lined by fine hairs and also produces mucus.

The hair filtered the inhale air and mucus trap dust particles present in inhaled air.

3. Internal nostrils :- Nasal cavity open into it and it leads to pharynx.

4. Pharynx :- Short, Vertical, Tube behind the buccal cavity, which is common passage for air and food.

5. Larynx/voice box :- It is short, tubular cartilaginous is the upper part of trachea. Opening of larynx  is called GULLET.

6. Trachea/wind pipe :- It is long, thin walled tube which is extends downwards through neck. Its walls  are supported by 16-20 “c” shaped cartilaginous rings which prevent them to collapse when air is absent in them.

7. Bronchi(bronchus) :- In the middle of thorax, trachea divides into two tubes called right and left bronchi.

8. Bronchioles :- Bronchus goes to each lung and than further divide into many smaller tubes called BRONCHIOLES.

9. The lungs :- There are two lungs situated in the chest cavity or thorax. Lungs are protected in rib cage

The lungs are covered by two thin membranes called pleura. 

At the end of each bronchiole there are many air sacs or alveoli. Gaseous exchange takes place in alveoli.

Alveoli have all the characteristic of good respiratory surface like

The are thin walled, permeable, moist, highly vascular and have vast surface area.

Breathing Mechanism

Breathing/Ventilation :- Muscular movements which keep the respiratory surface supplied with oxygen.

Organs helps in breathing :-

1. Diaphragm :- The thorax separated from abdomen by a muscular structure called diaphragm.

2. Rib cage.

3. Ribs.

Principle of breathing :- When the volume of thorax is increased the air sucked in and when the volume of thorax is decreased, air squeeze out.

Steps in breathing :-

1. Inspiration/inhalation :- When the diaphragm and the muscles attached to the rib  contract, diaphragm becomes flat and ribs move upward and outward.

Due to this the volume of the chest cavity increased and air pressure decreased therefore, air from outside rushes into the lungs and alveolar sacs get filled with air containing oxygen.

2. Expiration/exhalation :- During expiration diaphragm and muscles attached to ribs relax.

Diaphragm again becomes dome shaped, the rib cage moves inward and downward.

Due to this volume of chest cavity decreased and air pressure increased, therefore the air rich in carbon di oxide rushes out from the lungs.

The average breathing rate in adult man at rest is about 12-18 per minute.

Transport of Gases

As the body size of the animal is very large, only diffusion would not be sufficient to supply the oxygen to all parts. Instead respiratory pigment takes up oxygen from the air in the lungs and carry it to tissues deficient in oxygen. The exchange of gases is done by blood as following –

Transportation of O2 :-

About 97% of the O2 that diffuse into the blood combines with haemoglobin of RBCs forming an unstable compound, oxyhaemoglobin.

The remaining 3% O2 dissolves into water of plasma.

Transportation of CO2 :-

About 70% CO2 transported from the tissues to the lungs in the bicarbonate form in blood plasma.

About 23% co2 combine with haemoglobin to form carbaminohaemoglobin.

This Post Has One Comment

  1. Vanshgrwal

    My Search Of Science Notes Ends Here…..Thank You DoorStep Science.

Leave a Reply