Respiration In Plants


Tissue Respiration

Respiration is a series of enzyme controlled oxidation-reduction reactions during which carbohydrates (respiratory substrate) produced during photosynthesis are oxidized to carbon dioxide and oxygen is reduced to water. Energy is released as a result of bond by bond breakage of respiratory substrate. Much of this energy is stored into molecules of adenosine triphosphate (ATP). The process is also called Tissue Respiration since its take place within cells. The process of acquiring oxygen or discharging carbon dioxide from/into environment is called Gas Exchange or External Respiration. The process can be summarized as:


C6H12O6 + 6O2 + 38 ADP + 38 Pi ——————– 6CO2 + 6H2O + 38 ATP

Aerobic & Anaerobic Respiration

Respiration requiring oxygen is called as aerobic respiration while if it occurs in the absence of oxygen then it is known as anaerobic respiration, however it occur in the presence of oxygen. Certain organisms, for example bacterium Closteridium sp respires only in the absence of oxygen. All plants and animals require oxygen and respires aerobically, therefore called Aerobes. The end product of respiration are carbon dioxide and water.

Alcohlic Fermentation

Anaerobic Respiration help to maintain a supply of energy in the absence of oxygen. Plants especially fungi (yeasts) respires anaerobically and the waste products are ethanol and carbon dioxide. This type of anaerobic respiration is called alcohlic fermentation.

C6H12O6 ——————– 2CH3CH2OH + 2CO2 + Energy

In other organisms, for example in certain plants and mammalian muscles, the sole waste product of anaerobic respiration is lactic acid. It is called lactic acid fermentation.

Respiratory Substrate

Any organic plant constituent that is partially or completely oxidized to CO2 and water during the process of respiration is called respiratory substrate. Carbohydrates are the principal respiratory substrates in cells of higher plants. The most important being sucrose and starch. Sucrose along with fructose and glucose are the principal soluble sugars in plant cells. Similarly, sucrose is the main carbohydrate translocated within the plant body. Starch is the chief reserve in plants.

In addition to carbohydrates, other substances serve as respiratory substrates, e.g., castor-oil seeds are rich fat reserves stored in endosperm tissue. During the germination of these seeds the fats are converted to sucrose which are respired by the growing embryo. In some tissues, organic acids may be utilizing as respiratory substrates, for example 4-C malic acid accumulates in leaves of succulents (plants with fleshy leaves belonging to Crassulaceae) during night is oxidized to CO2 and water. Similarly, glycolic acid a 2-C organic acid produced in illuminated leaves of most higher plants, is also used as respiratory substrate.

Proteins are seldom respired except in special cases. In detached leaves the protein degradation has been observed. In seeds high in protein reserves, proteins serve as respiratory substrates during early stages of germination. The proteins are first degraded to amino acids which in turn are converted into intermediates that are oxidized to CO2 and water.

Respiratory Quotient

Respiratory quotient is the ratio of moles of CO2 released by a tissue over a period of time to the moles of O2 taken up.

moles of CO2 released
RQ = —————————————

moles of O2 taken up

The nature of the respiratory substrate in a plant tissue can be determined by calculating the respiratory quotient of the tissue.

  1. If glucose (carbohydrate) is the respiratory substrate, the respiratory quotient would be:

C6H12O6 + 6O2 ——————– 6CO2 + 6H2O

6 moles of CO2
RQ = —————————————    = 1

6 moles of O2


RQ for glucose (carbohydrate) = 1

  1. The fats are poor in oxygen as these are not directly oxidized. These are first hydrolyzed to fatty acids and glycerol. A fraction of oxygen is used up in this process, therefore, the fats require more oxygen for complete oxidation and their RQ is less than unity, i.e., 1.

C57H104O6 + 80 O2 ——————– 57 CO2 + 80 O2

triolein fat

57 CO2
RQ = —————————————    = 0.7

80 O2


RQ for fats = 0.7

  1. Like fats proteins also do not directly take part in respiration. These are first converted into amides or ammonia. Therefore, these require more oxygen for complete oxidation. The RQ value ranges from 0.79 (in case of amides) and 0.99 (in case of ammonia).
  2. In succulents where malic acid is the sole respiratory substrate, the respiratory quotient is:

C57H104O6 + 80 O2 ——————– 57 CO2 + 80 O2

4 moles of CO2
RQ = —————————————    = 0.7

3 moles of O2

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