It is a biochemical process of enzyme including oxidative breakdown of organic compounds inside living cells releasing energy in the form of ATP.


The process of intaking O2 & releasing of CO2 known as breathing .

Respiration in Plants

By young roots 

Air occurs in soil interspaces. Root hairs as well as epiblema cells of the young roots are in contact with them. They are also permeable to metabolic gases. Oxygen of the soil air diffuses through root hair-epiblema cells and reaches all internal cells of the young root. Carbon dioxide produced by root cells diffuses in the opposite direction.


By Leaves

  • Leaves and Young Stems. Leaves and young stems are ideally suited to quick exchange of gases. The organs have a covering of nearly impermeable epidermis for reducing loss of water. The epidermis of leaves bears a number of aerating pores called stomata (singular stoma or stomata, Gk. stoma-mouth). Each aerating or stomatal pore is bordered a pair of guard cells. In most of the plants, the guard cells are kidney or bean shaped with inner walls being thicker and less elastic than the outer walls.
  • When the stomata are open, gases diffuse into and out of the leaf as per their concentration gradient. A gas which has come from outside first reaches substomatal chambers. From here, it diffuses to all the intercellular air spaces present in between the mesophyll cells. If the stomata are open during night, oxygen from outside will diffuse into the leaves and young stems while carbon dioxide will diffuse out. It is due to respiratory gas exchange.

Mammalian Respiratory System

The mammalian respiratory system consists of nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli.


  •  It is also called external nares.

Nasal Chamber

  • Nasal septum divides nasal cavity into two nasal chamber by the nasal septum.

Internal nares

  • There are posterior opening of nasal chambers that leads into pharynx.


  • The pharynx provides passage to both air and food.


  • It is the lower part of pharynx and has a slit like aperture called glottis, which can be closed by a leaf like bilobed cartilage epiglottis, during swallowing of food bolus.


  • It is also called voice box or adam’s apple or pomas adami.

Vocal cord

  • In larynx, 2 pairs of vocal cord is found outer pair is false vocal cord whereas, inner pair is true vocal cord when air is forced through the larynx it cause vibration of true vocal cords and  sound is produced.

Trachea (Wind pipe)

  • It is long, tubular structure which runs downward through the neck in front of oesophagus. It is supported by cartilage to prevent collapse.

Primary bronchi

  • These are one pair of small thin walled tubular structure formed by the division of trachea. It further divides and end at alveoli.


  • lungs are present in  thoracic cavity on either side of heart. covered by pleural membrane.
Human Respiration System
Human Respiration System

Mechanism of Respiration

Respiratory centre is in Medulla Oblongeta.

Mechanism of breathing involves Two Phases.

(A) Inspiration  

(B) Expiration


  • Intercostal and phrenic muscles of diaphragm contract to increase thoracic cavity, therefore outside rushes inside.


  • Intercostal & phrenic relex muscles.
  • Due to decrease in thoracic cavity air pressure within lungs increase, the greater pressure within lungs force full air from lungs to outside of body.

Gaseous Exchange

  • Gaseous exchange occur in Alveoli following pressure gradient O2 from high pressure in alveoli diffuse into blood & CO2 from blood in alveoli.

Gaseous Transport

  • O2 is carried by haemoglobin (in RBC). 100 ml of blood can carry ~20 ml of O2 max, CO2 is transported in form of bicarbonates in plasma.

Respiration Asked Questions

What is ATP?

ATP-Adenosine triphosphate is called the energy currency of the cell. ATP is an organic compound composed of the phosphate groups, adenine, and the sugar ribose and functions by providing energy for various biochemical processes within the cells.

Define the Krebs cycle?

The Krebs cycle is also known as Citric Acid Cycle or Tricarboxylic Cycle or TCA cycle. It is the second stage of cellular respiration that occurs in all aerobic organisms to release stored energy for further biological process.

What is Fermentation?

Fermentation is a chain of chemical reactions or a metabolic process through which all living organisms obtain the energy required for the biological processes. It is an Anaerobic Pathway, which is used for producing alcoholic beverages, Yogurt and other food products.

Why do we need energy?

Energy work as fuel for our body. All living species obtain energy from the food they eat and we require energy for different metabolic activities including growth and development, repair, etc.

What is Glycolysis?

Glycolysis is the primary stage of cellular respiration. It is a biochemical reaction, where glucose is oxidized to a simpler organic compound and the series of reactions of the Glycolytic pathway take place in the cytosol of a cell.

What is Cellular Respiration?

Cellular respiration is a set of metabolic reactions that take place in all living cells to release energy by converting biochemical energy from nutrients into adenosine triphosphate- ATP. Based on the oxygen demand, cellular respiration is divided into- Aerobic respiration and Anaerobic respiration.

Define Aerobic and an Aerobic respiration?

Aerobic respiration is a type of cellular respiration, which takes place in the presence of oxygen. This type of respiration is common in all plants and higher animals, including humans, mammals, and birds.
Anaerobic respiration is a type of cellular respiration that takes place in the absence of oxygen and is common in all lower organisms such as bacteria and yeast

Where does cellular respiration occur in plant cells?

In all green plants, cellular respiration takes place in the mitochondria of the cell during the presence of oxygen.

What is the balanced chemical equation for Cellular Respiration?

C6H12O6 (Glucose) + 6O2 (Oxygen) → 6CO2 (Carbon dioxide) + 6H2O (Water) + ATP  (Energy)