NEET Biology Respiration In Plants Introduction and Types of Respiration

Introduction and Types of Respiration

Category : NEET



All organisms require continuous input of energy to carry on life process. These energy comes from cellular activities. All the cellular activities can be grouped into two categories: anabolism (biosynthetic activities of the cell) and catabolism (breaking- up process of the cell). The anabolic activities are endergonic (utilizes energy in cellular activities), while the catabolic activities are usually exergonic (energy releasing process by oxidation of food material). The sum of total catabolic and anabolic reactions occurring at any time in a cell is called metabolism.


Respiration is a vital process, includes the intake of oxygen. Chemically it is catabolic and brings about the oxidation and decomposition of organic compounds like carbohydrate, fat, protein in the cells of plants and animals with the release of energy. Oxidation of organic compounds by respiration, resulting in the release of chemical energies water and carbon dioxide. The overall process may be states according to the following general equation:


In this reaction, six molecules of oxygen taken up and six molecules each of \[C{{O}_{2}}and\text{ }{{H}_{2}}O\]are formed with energy derived from respiration of each molecule of sugar oxidation. The plant cell is able to do chemical work in synthesizing energy- rich materials such as fat and hydrocarbon, osmotic work such as uptake and accumulation of salt and mechanical work such as involved in growth.




Definition of respiration : Cellular respiration is an enzyme controlled process of biological oxidation of food materials in a living cell, using molecular\[{{O}_{2}}\], producing\[C{{O}_{2}}and\text{ }{{H}_{2}}O\], and releasing energy in small steps and storing it in biologically useful forms, generally  ATP.


(1) Use of energy : Cellular activities like active transport, muscle-contraction, bioluminescenes, homothermy locomotion, nerve impulse conduction, cell division, growth, development, seed germination require energy. Main source of energy for these endergonic activities in all living organisms including plants, comes from the oxidation of organic molecules.



The energy released by oxidation of organic molecules is actually transferred to the high energy terminal bonds of ATP, a form that can be readily utilized by the cell to do work. Once ATP is formed, its energy may be utilized at various places in the cell to drive energy- requiring reactions. In these processes, one of the three phosphate groups is removed from the ATP molecule. Thus the role of ATP as an intermediate energy transforming compound between energy releasing and energy consuming reactions.


(2) Significance of respiration: Respiration plays a significant role in the life of plants. The important ones are given below:

(i) It releases energy, which is consumed in various metabolic process necessary for life of plant.

(ii) Energy produced can be regulated according to requirement of all activities.

(iii) It convert in soluble foods into soluble form.

(iv) Intermediate products of cell respiration can be used in different metabolic pathways e.g.

Acetyl- CoA (in the formation of fatty acid, cutin and isoprenoids) ; \[\alpha \]- ketoglutaric acid (in the formation of glutamic acid) ; Oxaloacetic acid (in the formation of aspartic acid, pyrimidines and alkaloids); Succinyl- CoA (synthesis of pyrrole compounds of chlorophyll).

(v) It liberates carbon dioxide, which is used in photosynthesis.

(vi) Krebs cycle is a common pathway of oxidative breakdown of carbohydrates, fatty acids and amino acids.

(vii) It activates the different meristematic tissue of the plant.


(3) Comparison between respiration and photosynthesis: Photosynthesis associated with manufacturing of food, while respiration associated with releasing of energy from this food. Comparison between respiration and photosynthesis is given below:





Occurs only in chlorophyll containing cells of plants.

Occurs in all plant and animal cells.

Takes place only in the presence of light.

Takes place continually both in light and in the dark.

During photosynthesis, radiant energy is converted into potential energy.

During respiration, potential energy is converted into kinetic energy.

Sugars, water and oxygen are products.

\[C{{O}_{2}}and\text{ }{{H}_{2}}O\]are products.

Synthesizes foods.

Oxidizeds foods.

\[C{{O}_{2}}and\text{ }{{H}_{2}}O\]are raw materials.

O2 and food molecules are raw materials.

Photosynthesis is an endothermal process.

Respiration is an exothermal process.

Stores energy.

Releases energy.

It includes the process of hydrolysis, carboxylation etc.

It includes the process of the dehydrolysis, decarboxylation, etc.

Results in an increase in weight.

Results in a decrease in weight.

It is an anabolic process.

It is a catabolic process.

Require cytochrome.

Also require cytochrome.


(4) Exchange of gases in photosynthesis and respiration: Respiration is continually going on in all living cells and oxygen is being continually absorbed and carbon dioxide liberate. The intake of oxygen (Liberated by photosynthesis) and liberation of carbon dioxide (evolved in respiration) takes place through the stomata and lenticels. The real process of respiration consists in the oxidation of organic substances which takes place in the protoplasm of the living cells and the gaseous exchange is an outward manifestation and an accompaniment of respiration. The intensity of gaseous exchange depends upon the intensity of respiration. It is comparatively rapid in meristematic and growing tissues where the formation of new cells and cell wall material requires a large supply of energy and is comparatively slow in mature cells due to the slowness of metabolic activities.



  • Compensation point : It is that value or point in light intensity and atmospheric CO2 concentration when rate of photosynthesis is just equivalent to the rate of respiration in photosynthetic organs so that there is no net gaseous exchange. The value is 2.5- 100 ft candles/ 26.91-1076.4 lux in shade plants and 100-400 ft candles/ 1076.4-4305.6 lux in case of sun plants. It is called light compensation point. There is, similarly, a CO2 compensation point. Its value is 25-100 ppm (25-100\[\mu l.{{l}^{-1}}\]) in C3 plants and 0-5 ppm (0-5 \[\mu l.{{l}^{-1}}\]) in C4 A plant cannot survive for a long at compensation point because the nonphotosynthetic parts and dark respiration will deplete organic reserve of the plant.

    CO2 intake in photosynthesis balanced with \[C{{O}_{2}}\]release in respiration = Compensation point.


(5) Comparison between respiration and combustion : According Lavosier cell respiration resembles the combustion (e.g., burning of coal, wood, oil etc.) in the breakdown of complex organic compounds in the presence of oxygen and production of carbon dioxide and energy, but there are certain fundamental differences between the two processes:


Differences between cell respiration and combustion




Cell respiration



Nature of process

Biochemical and stepped process.

Physico-chemical and spontaneous process.


Site of occurrence

Inside the cells.




Biological control.



Energy release

Energy released in steps.

Large amount of energy is released at a time.



Remain within limits.

Rises very high.



No light is produced.

Light may be produced.



Controlled by enzymes.

Not controlled by enzymes.



A number of intermediates are produced.

No intermediate is produced.



Phases of respiration

There are three phases of respiration:

(1) External respiration: It is the exchange of respiratory gases \[\left( {{O}_{2}}andC{{O}_{2}} \right)\] between an organism and its environment.

(2) Internal or Tissue respiration : Exchange of respiratory gases between tissue and extra cellular environment .

Both the exchange of gases occur on the principle of diffusion.

(3) Cellular respiration: It is an enzymatically-controlled stepped chemical process in which glucose is  oxidised inside the mitochondria to produce energy-rich ATP  molecules with high-energy bonds.

So, respiration is a biochemical process.

Respiratory substrate or Fuel


In respiration many types of high energy compounds are oxidised. These are called respiratory substrate or respiratory fuel and may include carbohydrates, fats and protein.


(1) Carbohydrate: Carbohydrates such as glucose, fructose (hexoses), sucrose (disaccharide) or starch, insulin, hemicellulose (polysaccharide) etc; are the main substrates. Glucose are the first energy rich compounds to be oxidised during respiration. Brain cells of mammals utilized only glucose  as respiratory substrate. Complex carbohydrates are hydrolysed into hexose sugars before being utilized as respiratory substrates. The energy present in one gram carbohydrate is - 4.4 Kcal or 18.4 kJ.


(2) Fats: Under certain conditions (mainly when carbohydrate reserves have been exhausted) fats are also oxidised. Fat are used as respiratory substrate after their hydrolysis to fatty acids and glycerol by lipase and their subsequent conversion to hexose sugars. The energy present in one gram of fats is 9.8 Kcal or 41kJ, which is maximum as compared to another substrate.

The respiration using carbohydrate and fat as respiratory substrate, called floating respiration (Blackmann).


(3) Protein : In the absence of carbohydrate and fats , protein also serves as respiratory substrate. The energy present in one gram of protein is : 4.8 Kcal or 20 kJ. when protein are used as respiratory substrate respiration is called protoplasmic respiration. 


Types of respiratory organism

Organism can be grouped into following four classes on the basis of their respiratory habit -

(1) Obligate aerobes : These organisms can respire only in the presence of oxygen. Thus oxygen is essential for their survival.

(2) Facultative anaerobes : Such organisms usually respire aerobically (i.e., in the presence of oxygen) but under certain condition may also respire anaerobically (e.g., Yeast, parasites of the alimentary canal).

(3) Obligate anaerobes : These organism normally respire anaerobically which is their major ATP- yielding process. Such organisms are in fact killed in the presence of substantial amounts of oxygen (e.g., Clostridium botulinum  and C. tetani).

(4) Facultative aerobes : These are primarily anaerobic organisms but under certain condition may also respire aerobically.

Types of respiration.

On the basis of the availability of oxygen and the complete or incomplete oxidation of respiratory substrate, the respiration may be either of the following two types :  Aerobic respiration and Anaerobic respiration


Anaerobic respiration

It uses oxygen and completely oxidises the organic food mainly carbohydrate (Sugars) to carbon dioxide and water. It therefore, releases the entire energy available in glucose.

\[{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{O}_{2}}\xrightarrow{\text{enzymes}}6C{{O}_{2}}+6{{H}_{2}}O+\text{energy}\](686 Kcal)

It is divided into two phases: Glycolysis, Aerobic oxidation of pyruvic acid.

Other Topics

Notes - Introduction and Types of Respiration
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