Respiration in Plants: Definition and its Types | Plant Physiology

In this article we will discuss about:- 1. Definition of Respiration 2. Respiratory Substrates 3. Types of Respiration 

1. Definition of Respiration

We know that during photosynthesis, light energy is converted into chemical energy, and is stored in carbohydrate molecules, such as glucose and starch. Organisms make use of such energy for their activities by oxidising these high energy food molecules into simple low energy molecules, i.e., carbon dioxide and water.

The reactions involved in process of oxidation are known as respiration. The compounds that are oxidised during process of respiration are called respiratory substrates.

Technically, Respiration is defined as follows:

This is a process by which living cells break down complex high energy food molecules into simple low energy molecules, i.e., CO2 and H2O, releasing the energy trapped within the chemical bonds

oxygen + glucose -> carbon dioxide + water + heat energy

2. Respiratory Substrates

Respiratory substrates are those organic substances which are oxidised during respiration They are high energy compounds and are called respiratory substrates. They may be carbohydrates, fats and proteins. Carbohydrates, such as glucose, fructose (hexoses), sucrose (disaccharide) or starch, inulin, hemicellulose (polysaccharide), etc., are main respiratory substrates.

Besides, fats are used as respiratory substrates by a variety of organisms as they contain more energy than carbohydrates.

In rare circumstances, when carbohydrate reserves are exhausted, fats and proteins also serve as respiratory substrates. Blackman termed the respiratory oxidation of protoplasmic protein as protoplasmic respiration, while oxidation of carbohydrates as floating respiration.

3. Types of Respiration

There are two main types of respiration:

1. Aerobic Respiration
2. Anaerobic Respiration

1. Aerobic Respiration

This type of respiration leads to a complete oxidation of stored food (organic substances) in the presence of oxygen, and releases carbon dioxide, water and a large amount of energy present in respiratory substrate. Such type of respiration is generally found in higher organisms.

The overall equation is

2. Anaerobic respiration

This type of respiration occurs in complete absence of oxygen. In the absence of free oxygen, many tissues of higher plants, seeds in storage, fleshy fruits, and succulent plants, such as cacti temporarily take to a kind of respiration, called anaerobic respiration. Such respiration generally occurs in lower organisms like bacteria and fungi.

This results in incomplete oxidation of stored food and formation of carbon dioxide and ethyl alcohol, and sometimes also various organic acids, such as malic, citric, oxalic, tartaric, etc. Very little energy is released by this process to maintain activity of protoplasm.

The equation is as follows

This process of oxidation in microbes is known as fermentation. This is quite similar to that of anaerobic respiration in case of higher plants.