The Process of Respiration in Plants | Botany

In this article we will discuss about the process of respiration in plants.

In respiration carbo­hydrate is broken down, as a result the potential energy is trans­formed into kinetic form. Though it is a destructive or katabolic process, yet respiration is extremely beneficial, because it releases the necessary energy for performing the life functions.

Plants take in free oxygen from the air which enters through the stomata and lenticels and diffuses through the continuous intercellular space system to reach the living cells. It oxidises sugar, particularly glucose, into carbon dioxide and water vapour, thereby liberating considerable energy.

The reaction may be represented thus:

C₆H₁₂O₆ + 6O₆ = 6CO₂ + 6H₂O + energy (674 cal.)

Though sugar is the main oxidisable material used during the process, other materials and in extreme cases, even protoplasm may be broken down for the liberation of energy. Carbon dioxide and water vapour go out through the stomata and lenticels.

At night when stomata remain closed gaseous exchange takes place through the lenticels. Thus respiration is essentially a process of biological oxidation in which oxygen absorbed from the air breaks down the food (sugar) into carbon dioxide and water, thereby releasing the stored energy.

It should be noted that respiration is not mechanical breathing involving simple inhalation of oxygen and exhalation of carbon dioxide, but a chemical process meant for the liberation of energy for the maintenance of life.

Respiration goes on day and night in all the living cells irres­pective of their position in the plant organs; but the rate is vigo­rous in growing regions like root-tip, stem-tip, flower-buds and germinating seeds.

During, day time when the rate of photo­synthesis is higher than the rate of respiration and when both the processes are going on simultaneously, all the oxygen liberated in photosynthesis is not consumed in respiration, whereas all the CO₂ liberated in respiration is completely consumed in photosynthesis and some more CO₂ must enter the plant for the higher rate of photosynthesis.

As a result, during day time, the exchange of gases between the plant and the atmosphere is that O₂ is liberated and CO₂ is consumed. Thus the effects of respiration are completely masked during day time. During night, however, there is un­doubtedly no photosynthesis, and only exchange of gases between the plant and the atmosphere is the effect of the process of respira­tion—O₂ consumed and CO₂ liberated.

Aerobic and Anaerobic Respiration:

So long there is a normal supply of oxygen, respiration goes on uninterruptedly in all the living cells giving out water and carbon dioxide as end products. This is normal or aerobic respira­tion. If the supply of oxygen is cut off, plants would ultimately perish by suffocation, though death does not come immediately. Respiration continues for a short period. This is anaerobic or intra-molecular respiration.

Anaerobic respiration can only con­tinue for a short time in higher plants. Ultimately the plants die, if they are deprived of CO₂ for longer time, as the energy released in the process is not sufficient for the continuance of the life processes for considerable length of time.

Moreover, the accumulation of alcohol, one of the end products of the process, might reach toxic concen­tration causing permanent harm to the tissues.

Anaerobic respira­tion ceases when the concentration of alcohol produced reaches about 15%. There is really a close similarity between aerobic and anaerobic types of respiration. It is known for certain now that in plants during first stage of respiration, either aerobic or anaerobic, no oxygen is needed.

Zymase, the enzyme (organic catalyst secreted by living cells) extracted from yeast and which has also been found in all other green plants, is the agent responsible for the first part of respiration—which is known as anaerobic cleavage or breakdown. The sugars like glucose are converted by zymase into some other intermediate products.

In presence of oxygen, i.e. in aerobic respi­ration, these intermediate products are further broken down and completely oxidised into CO₂ and H₂O; whereas in absence of oxygen, i.e. in anaerobic respiration, the intermediate products, under further action of zymase, are converted into C₂H₆OH (alcohol) and carbon dioxide, according to the following scheme:

The amount of energy released in anaerobic respiration is much less in comparison to that liberated during aerobic process.

Here is an experiment to de­monstrate anaerobic respiration:

A test-tube is filled with mercury. A few properly soaked seeds axe taken and after removing the seed- coats they are introduced one by one with the help of a bent forceps in the test-tube.

The seed-coats are removed in order to facilitate the exchange of gases. The test-tube is placed in position with the help of clamp. After a day it is noticed that mercury column has come down and the closed end of the tube is filled with a gas.

Now a bit of caustie potash is introduced in the same manner which absorbs the gas and mercury column again rises to fill up the tube, thus proving that the gas evolved is carbon dioxide.