Respiration in Plants Class 11 Notes

Respiration is an essential process for all the living being on the planet. The mere fact that taking in oxygen and releasing out carbon dioxide in fractions of seconds gives energy to living organisms, is wonderful. You must be familiar with the respiration process in human beings but what about plants? Do plants respire too? NCERT biology class 11 chapter on Respiration in Plants can answer your questions in detail. Let us have a look at the entire process of respiration in plants and discover how is it different from the human respiration system.

Breathing: An Essential Function of Living Beings

Breathing is one of the most vital and essential functions for a living organism– be it unicellular or multicellular. The importance of breathing can be understood better by answering the question –“Why do we need to eat food?” Surprised? Well, our eating food and breathing are related. We eat food for nutrients and energy to perform other bodily functions. We get energy by the oxidation of food macromolecules. This oxidation occurs because we take in oxygen during the process of respiration. 

What is Respiration?

The act of breathing in oxygen to metabolise food for the production of energy is called respiration. The process remains the same for all living organisms- plants as well as animals.

We often think of plants as the food manufacturers, and they truly are. However, not all the cells in a plant are responsible for photosynthesis. The process of photosynthesis occurs within the chlorophyll-containing cell organelles called chloroplasts. But, respiration occurs in the mitochondria and the cytoplasm. Before we get to respiration in plants, let us first understand what exactly is respiration? 

Respiration is defined as the process by which the complex carbon bonds between compounds are broken down by the process of oxidation within the cells to release energy. The compounds that get oxidised during this process are called respiratory substrates. The first set of compounds that are usually utilised as substrates are carbohydrates, followed by fats and proteins. Energy is released in the form of ATP.

Also Read: Class 11 Photosynthesis in Higher Plants

How do Plants Respire?

We have learnt in another NCERT biology class 11 chapters about how plants require CO₂ for the process of photosynthesis. But did you know that plants require O₂ for respiration? Yes, it is true! Plants take in oxygen and give out carbon dioxide via small pores called stomata. These stomata are found on the undersurface of their leaves and are organs for respiration in plants. It is her gaseous exchange takes place. 

Respiration in plants is performed by not just the stomata in the leaves but other living cells across the plant. Stems with barks also have a small number of living cells that line the outer layers and contain lenticels for gaseous exchange. Most cells in a plant have some part of their surfaces in contact with air, along with loose packing of parenchyma cells to provide air spaces. 

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What is the Difference Between Respiration and Photosynthesis?

Photosynthesis	Respiration
Only plants do this process	All living beings respire
Food is synthesized in this process	Food is oxidized in this process
Energy is created	Energy is released
Anabolic process	Catabolic process
Endothermal process	Exothermal process.
Water, oxygen and sugar are the end products	Carbon dioxide and hydrogen are the final products
Radiant energy is converted into potential energy	Potential energy is converted into kinetic energy
Occurs during daytime in the presence of sunlight only	Continuous process

Glycolysis

In normal respiration of plants, O₂ is utilised to breakdown carbohydrates into CO₂, H₂O and energy as by-products. This energy is used for other body processes. In cases where sufficient oxygen is not available, there is partial oxidation of glucose to form pyruvic acid. This process is called glycolysis. This is the only way respiration happens in anaerobic organisms.

In plants, glucose is derived from sucrose, an end product of photosynthesis. This sucrose is converted into glucose and fructose in the presence of the enzyme invertase. This glucose and fructose now enter the glycolic pathway.

This is the Glycolytic pathway, and you can see that pyruvic acid is the end product of this pathway. Based on how this pyruvic acid is handled, the fate of pyruvic acid gets decided. There are three pathways mentioned by biology class 11 syllabus that handle this pyruvic acid –lactic acid fermentation, alcoholic fermentation and aerobic respiration. While fermentation occurs under anaerobic conditions in prokaryotes and many unicellular eukaryotes, aerobic respiration occurs in organisms by the Krebs Cycle, and this requires oxygen.

Anaerobic Respiration

As mentioned earlier in the respiration of plants lesson, fermentation occurs in anaerobic condition and results in the incomplete oxidation of pyruvic acid to form CO₂ and ethanol. This reaction is brought about by enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyse in unicellular organisms like yeast. As compared to aerobic respiration, fermentation of both types releases less than 7%  of the energy, and not many ATPs are released. This process is dangerous to these unicellular organisms as higher quantities of alcohol production results in their death.

Aerobic Respiration 

In eukaryotic cells, specialised cell organelles called mitochondria perform aerobic respiration. Aerobic respiration is the process by which substrates are completely oxidised in the presence of oxygen to form CO₂, water and large amounts of energy. This is the form of respiration that predominantly occurs in higher organisms like plants and multicellular animals. Here’s what occurs in aerobic respiration:-

• The removal of hydrogen atoms completely oxidised pyruvic acid(formed in the cytosol enters the mitochondria) and in a stepwise manner
• CO₂ is formed
• The electrons released from the hydrogen atoms pass to molecular oxygen O_2, and ATP is simultaneously synthesised

The first two steps occur in the mitochondrial matrix, whereas the third occurs in the inner mitochondrial membrane. By these reactions, we can say that pyruvic acid undergoes oxidative decarboxylation in the presence of the enzyme pyruvic dehydrogenase. This process also requires many coenzymes like NAD+ and Coenzyme A. At the end of this process, two NADH molecules are formed by the metabolism of two molecules of pyruvic acid. Acetyl CoA formed in this process now enters the TCA cycle(also called the Krebs cycle).

Also Read: The Living World Class 11 Notes

Tricarboxylic Acid Cycle (TCA Cycle) and Electron Transport System(ETS)

Above is the diagrammatic representation of the Tricarboxylic acid Acid Cycle, similar to that given in the chapter on respiration in plants. The steps in the TCA cycle are:

• The cycle begins with the condensation of acetyl group with OAA(OxaloAcetic Acid) and water to give citric acid. This reaction is catalysed by enzyme citrate synthase and releases one molecule of CoA.
• Citrate gets isomerised into isocitrate.
• The next two steps are decarboxylation steps that result in the formation of  α-ketoglutaric acid, and succinyl-CoA
• The subsequent steps result in the continued oxidation of succinyl-CoA to form OAA, and the cycle continues.
• When succinyl-CoA is converted to succinic acid, one GTP is synthesised. This is called substrate-level phosphorylation.
• GTP is converted to GDP, and at the same time ATP is formed from ADP
• At three different points of the cycle, NAD+ is reduced to NADH and H+, whereas FAD+ gets reduced to FADH2.

The TCA cycle can be summarised in an equation format as:

While the substrate level reactions are occurring in the mitochondrial matrix, Electron Transport System (ETS) is occurring at the inner mitochondrial layer. The biologic pathway in which electrons pass from one carrier molecule to another is defined as the Electron Transport System(ETS). Below is the diagram that explains the ETS in a simple form.

Class 11 Biological Classification

• Electrons produced in the mitochondrial matrix during the Citric Acid Cycle are oxidised by the enzyme NADH dehydrogenase to form Complex 1. 
• These electrons are then transferred to ubiquinone present in the inner membrane of the mitochondria. 
• Via complex II, the ubiquinone now comes in contact with FADH2 that is reducing in nature.
• These equivalents bring about the reduction of ubiquinone, and it gets converted to ubiquinol.
• This ubiquinol again gets oxidised due to the transfer of electrons to cytochrome c via cytochrome bc1 complex, i.e. complex III. 
• Cytochrome c is a protein found on the other layer of the inner mitochondrial membrane. It is important as it helps in electron transfer between complexes III and IV. 
• Complex IV is made up of cytochrome c oxidase complex that comprises cytochromes a, a3 and two copper centres.

As derived from the topic respiration of plants, it is evident that during the process of ETS, electrons get passed from one carrier to the next via the complexes I to IV. In this process, they are coupled to ATP synthase (that forms complex V) to produce ATP from ADP and inorganic phosphate.

Respiratory Balance Sheet

Though one may think it is easy to make calculations on the number of ATP molecules synthesised during the oxidation of each glucose molecule, it is not practically applicable as all the reactions involved occur simultaneously rather than one after the other. However, for the purpose of knowledge, it has been calculated that one molecule of glucose, when oxidised completely, results in the formation of 36 ATP molecules. 

Respiration is an Amphibolic Pathway

The process of respiration is a catabolic one as it is responsible for the breakdown of substrates to release energy. However, the respiratory pathway is considered to be an amphibolic pathway because it is involved in both metabolisms as well as anabolism. The reason being that some substrates in the respiratory pathways are withdrawn and used for the synthesis of other substrates. 

Respiratory Quotient

The ratio of the volume of CO₂ released to the volume of O₂ consumed during respiration is called the Respiratory Quotient(RQ) or Respiratory Ratio.

RQ= Volume of  CO₂ evolved / Volume of O₂ consumed

The RQ is substrate-specific. For example, if the substrate is a carbohydrate, and it is surely going to be oxidised, RQ will be 1 as equal amounts of O₂ and CO₂ will be consumed and evolved. In the case of fats, RQ will be less than 1 as more O2 will be consumed as compared to the volume of CO₂ that will get released. Similarly, for proteins, the RQ is expected to be around  0.9.

If you have understood the lesson on respiration in plants, you will easily be able to solve the practice questions given below. For more questions and answers, follow class 11 biology NCERT solutions.

Practise Questions

Hence, we hope that this blog provided you with the chapter summary of Respiration of Plants. Want to pursue a career in Biology? Our Leverage Edu experts are here to guide you in selecting a suitable course and university as per your interests and career goals. Sign up for a free counselling session with us today and get all your queries answered.