The process of plant respiration: Significance, Types and Mechanism


Want to learn about respiration in plants? This article explains in detail about the significance of respiration in plants and the mechanisms such as glycolysis and Kreb's cycle involved in it. It also explains how it is different from photosynthesis.

Introduction

The respiration process is not just restricted to human beings and animals. Just like any other living being, plants also undergo this process. Respiration is a process in plants through which they metabolize the stored food in the usable form of energy required for their growth and various functions. Though respiration in plants can be termed as a katabolic process, it is beneficial for plants.

Plant respiration

Plant respiration is a process in which organic substances which are produced during photosynthesis and stored undergoes oxidation process in the mitochondria of the cell. Organic substances can be carbohydrates, proteins, fats or oils. The entire process is enzyme controlled. There are numerous end products as well as there is a release of energy. Plants require the energy to survive and carry out numerous metabolic processes.

Significance of this mechanism in plants

Photosynthesis in plants is a mechanism in which they produce their food in the presence of sunlight. Food is produced, stored in the form of carbohydrate molecules. Production and storage of energy take place in the process of photosynthesis. However, through the respiration process, the conversion of stored energy into usable form takes place. Plants too just like every living being requires energy for various functions such as for growth, repair, and reproduction. All these functions are dependent on energy. Even the functions such as absorption of water, minerals and salt from the soil, preparation of food through photosynthesis process, cell division require energy which is obtained through respiration. Plant respiration takes place throughout the day. Cells or tissues which are devoid of oxygen may die in its absence. It is the significance of respiration in plants.

How is photosynthesis different from respiration?

Photosynthesis and respiration are two processes in plants that are opposite to each other. Photosynthesis is a mechanism in which plants using carbon dioxide and water in the presence of sunlight prepare their food. They produce the end products oxygen and sugars. Sugar is stored in the form of chemical bonds. This process is conducted in the leaves and stems portions of plants and trees.

The respiration process in plants is where carbohydrates produced during the photosynthesis process is used up to fuel for various functions. Along with carbohydrates, lipids and proteins are also metabolized. Respiration process occurs in the leaves, stems, and root portion of plants. During the process, sugar and oxygen are used up to liberate carbon dioxide, water, and energy. Usable form of energy produced is known as adenosine triphosphate (ATP). In the process, water and carbon dioxide are produced along with the heat.

In the case of the photosynthesis process in plants-
Carbon dioxide in combination with water releases oxygen and glucose.

In the case of respiration in plants-
Glucose in combination with oxygen molecules releases carbon dioxide and water.

When there is the presence of bright sunlight, the rate of photosynthesis is more than the rate of respiration. In the case of dim sunlight, rate of photosynthesis equals the rate of respiration. However, during night time when sunlight is absent, only respiration takes place in plants, and there is no photosynthesis.

Do plants breathe?

Respiration in plants does not mean that plants breathe like humans or animals. Of course, respiration takes place in plants cells. It is similar to the respiration that takes place in animal cells. Just like any other living being, plants need to undergo respiration to survive. If they don't which usually happens under unfavourable circumstances, plants die. However, it does not mean that plants breathe. They undergo respiration to release energy.

Types

Plants undergo photorespiration and dark respiration. The name photo respiration comes as the respiration takes place in the presence of light. It is also commonly referred to as aerobic respiration because it takes place in the presence of oxygen molecules within cell mitochondria. Dark respiration may take place in the presence or absence of light.

  • Aerobic respiration-
    It is normal respiration which takes place in the plants in the presence of oxygen. Plants prepare their food using the photosynthesis process and store it. During aerobic respiration, the stored food in plants undergoes complete oxidation. Water and carbon dioxide are the end products of aerobic respiration. Oxygen is essential as in its absence; plants may not be able to survive and may die.

  • Anaerobic respiration-
    When plants do not receive oxygen, they do not face immediate death. They can survive for a small period through anaerobic respiration. As it takes place in the absence of oxygen, it is called anaerobic respiration. During this process, stored food does not get oxidized completely. Plants can only survive for a short period in anaerobic respiration as the energy produced by them through incomplete oxidation is not enough for their sustenance. Also, alcohol which is the byproduct of the process accumulates and reaches higher concentration. It yields harm and serves as toxic to plant tissues. Anaerobic respiration can be seen in plants such as cacti, stored seeds and fleshy fruits.


Environmental exchange of gases

Oxygen is required for respiration in plants. They absorb it from the environment through stomata, cuticular spaces and lenticels. However, maximum absorption of oxygen takes place during day time through stomata, and during the night time, it takes place through cuticular spaces and lenticels. Once absorbed, oxygen is diffused to different parts of plants, tissues and every cell through the presence of intercellular spaces. Carbon dioxide is produced as the by-product of plant respiration. The carbon dioxide that is generated is released in the environment through the same route from which oxygen gets absorbed. Plants may even retain carbon dioxide for photosynthesis process.

When do plants undergo respiration?

Just like every living being on this earth, plants need the energy to grow, sustain and perform various functions. Thus, respiration in plants takes place throughout the day. However, during the day, plants have to also carry out the photosynthesis process. Thus, the respiration process of plants is more evident during night time as there is no photosynthesis process during dark due to the absence of sunlight which is the most important requirement for the production of food in plants. In short, plants undergo respiration throughout the day as they need a usable form of energy the entire day to carry out various functions.

What are respiratory substrates?

Respiratory substrates are organic compounds of high energy which gets oxidized during respiration process in plants. These organic compounds can be anything from carbohydrates to proteins. Also, they can be high energy compounds belonging to the fats group. Usually, carbohydrates compounds such as glucose, sucrose, and fructose serve as respiratory substrates. Sometimes, in the absence of these carbohydrate compounds, proteins and fats get oxidized as respiratory substrates.

  • Fat as respiratory substrates-
    Fats due to their dense energy content are considered as better respiratory substrates than their carbohydrate counterparts. They are stored in adipose tissue as the triglycerides. They are not oxidized as it is. They get converted into glycerol and fatty acids in the cell cytoplasm before getting oxidized in the respiratory process of plants.

  • Protein as respiratory substrates-
    Proteins are generally oxidized when carbohydrate reserves in plants are exhausted. It is only then the stored protein in plants is converted into amino acids in the cell cytoplasm to get oxidized for the process. The amino acids than either through transamination or deamination follow the respiration process.


Respiratory mechanism

To understand plant respiration, one has to study the mechanism involved in it. Plants undergo respiration in two phases namely glycolysis and Krebs cycle. Both of them are discussed below.

  • Glycolysis-
    Plant respiration is an enzyme controlled process. During it, carbohydrate compounds get converted into pyruvic acid in the cytoplasm of the cell through various reactions which are altogether termed as glycolysis. Two pyruvic acid molecules get created from every glucose molecule. One glucose molecule releases four ATP molecules. Metabolic reactions linearly take place. Glycolysis takes place in both aerobic as well as anaerobic respiration. Oxygen not required and carbon dioxide is not evolved during the process.

  • Krebs cycle-
    Krebs cycle which is the second phase of plant respiration follows a cyclic pathway. Pyruvic acid produced during glycolysis enters the cell's mitochondria. There it gets converted into water and carbon dioxide as many enzymes catalyze the reactions. These reactions are termed as Krebs cycle. Unlike glycolysis, the Krebs cycle takes place only in aerobic respiration. Carbohydrate molecule gets completely oxidized. Every glucose molecule is involved in the production of two pyruvic acid molecules. Every pyruvic acid molecule after oxidation releases three carbon dioxide molecules. Thus, every glucose molecule at the end of Kreb's cycle releases six carbon dioxide molecules. In this process, carbon dioxide gas is evolved.


Importance of the Krebs cycle

During this cycle, intermediate compounds produced get utilized in the production of chlorophyll, fats, amino acids, cytochromes and other biomolecules. On the other hand, carbon skeletons formed get used up in the growth and maintenance of plant cell. In this cycle, ATP molecules are generated which serves as energy source required in performing many biochemical reactions. Krebs cycle is directly and intimately related to the metabolism of nitrogen and fat respectively.


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