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NCERT Class 11 Biology Chapter 12 Notes: Respiration in plants is an important biological process that yields energy through the degradation of organic molecules. While animals possess organs that are dedicated to respiration, plants depend on cellular systems for gas exchange. The process takes place at the cellular level where oxygen is taken up and carbon dioxide emitted via diffusion. The respiration rate in plants fluctuates according to environmental factors, including temperature, the availability of oxygen, and water content.
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The NCERT Notes for Class 11 Biology of this chapter discusses various forms of respiration, such as aerobic and anaerobic respiration, and important concepts like respiratory substrates, glycolysis, the Krebs cycle, and the electron transport system. Every step in the process is crucial in the generation of ATP, which is the currency of energy of the cell. Fermentation in plants, which is an alternate pathway when oxygen is lacking, is also explained in detail. It is important to understand these topics for NEET and board examinations, as they are the basics for higher studies in plant physiology and metabolism.
Also, students can refer,
Plant respiration is a metabolic process where organic compounds, mainly glucose (C₆H₁₂O₆), are decomposed to release energy in the form of Adenosine Triphosphate (ATP). This energy is used in different physiological processes like cell division, growth, repair, transportation of nutrients, and biosynthesis of vital compounds. The overall chemical equation for respiration is:
In contrast to animals, plants lack specialized respiratory organs. They depend on simple diffusion for gas exchange via stomata (in leaves), lenticels (in stems), and root hairs (in roots). Respiration in all living cells takes place constantly to supply their energy needs. The process depends on oxygen availability, temperature, and the respiratory substrate type.
Respiration in plants may be aerobic (oxygen involving) or anaerobic (oxygen absent). Aerobic respiration yields a large quantity of ATP, but anaerobic respiration yields less energy and usually results in ethanol or lactic acid production. Respiration is important as it directly affects the survival, productivity, and general metabolic equilibrium of plants.
Plants respire in different ways depending on the availability of oxygen. The two major types of respiration are aerobic respiration and anaerobic respiration, with fermentation being a specific type of anaerobic respiration.
Aerobic respiration is the process by which glucose is fully oxidized in the presence of oxygen (O₂) and yields carbon dioxide (CO₂), water (H₂O), and a lot of ATP (energy).
Glycolysis (in Cytoplasm) – Glucose is converted into two molecules of pyruvate, generating a little ATP and NADH.
Krebs Cycle (in Mitochondria) – Pyruvate is further converted into CO₂, yielding NADH and FADH₂ molecules.
Electron Transport Chain (in Mitochondria) – Electrons from NADH and FADH₂ are passed through protein complexes, generating ATP. Oxygen serves as the terminal electron acceptor, forming water (H₂O).
Aerobic respiration is very efficient, yielding 36-38 molecules of ATP per molecule of glucose, which is vital for different cellular processes in plants, including nutrient transportation, plant growth, and plant development.
Anaerobic respiration is carried out without oxygen. Glucose is partially broken down instead of completely oxidised, forming less energy and byproducts such as ethanol or lactic acid.
Anaerobic respiration is located in the cytoplasm because mitochondria need oxygen to work.
Anaerobic respiration is wasteful in comparison to aerobic respiration, as it yields much less ATP (only 2 molecules per molecule of glucose).
Example
Fermentation is a special type of anaerobic respiration where glucose is broken down into simpler compounds like ethanol or lactic acid, producing a small amount of ATP.
Alcoholic Fermentation (in Yeast & Some Plants)
Pyruvate is converted into ethanol (alcohol) and CO₂.
Used in the brewing and baking industries.
Lactic Acid Fermentation (in Some Microorganisms)
Pyruvate is converted into lactic acid instead of ethanol.
Occurs in certain bacteria and muscle cells (during heavy exercise in animals).
End Products
Alcoholic Fermentation → Ethanol (C₂H₅OH), CO₂, ATP
Lactic Acid Fermentation → Lactic acid (C₃H₆O₃), ATP
Produces only 2 ATP molecules per glucose molecule, similar to anaerobic respiration.
Used in the production of bread, beer, and wine.
Helps in preserving foods like yogurt and cheese.
Important in some plants and microorganisms for survival in low-oxygen environments.
Also Read:
Feature | Aerobic Respiration | Anaerobic Respiration | Fermentation |
---|---|---|---|
Oxygen Requirement | Present | Absent | Absent |
Location in Cell | Mitochondria & Cytoplasm | Cytoplasm | Cytoplasm |
End Products | CO₂, H₂O, ATP | Ethanol or Lactic Acid, CO₂, ATP | Ethanol/Lactic Acid, CO₂, ATP |
Energy Yield | High (36-38 ATP) | Low (2 ATP) | Low (2 ATP) |
Occurs In | Most plants and all aerobic organisms | Flooded plant roots, certain bacteria | Yeast, some plant cells, bacteria |
Importance | Essential for energy production and plant metabolism | Helps plants survive in oxygen-deficient conditions | Used in industries (brewing, baking, dairy) |
The main substrate for respiration is glucose (C₆H₁₂O₆).
Other molecules like fats and proteins can also be used as respiratory substrates.
Respiratory Quotient (RQ) is the ratio of CO₂ released to O₂ consumed:
For carbohydrates (Glucose): RQ = 1
Plant respiration is a process of successive biochemical reactions that gradually degrade glucose to produce ATP, which is used in cellular functions. It has three primary phases: Glycolysis, the Krebs Cycle (TCA Cycle), and the Electron Transport Chain (ETC) & Oxidative Phosphorylation.
Glycolysis is the initial process of respiration and is carried out in the cytoplasm of the cell. Glycolysis does not involve oxygen and occurs in aerobic as well as anaerobic respiration.
Glucose Activation – A glucose molecule of 6-carbon (C₆H₁₂O₆) is phosphorylated with the help of ATP and converted into fructose-1,6-bisphosphate.
Cleavage of Glucose – The molecule of 6-carbon is divided into two molecules of 3-carbons of glyceraldehyde-3-phosphate (G3P).
Energy Harvesting Phase – The G3P molecules go through a series of phosphorylation and oxidation reactions to yield:
The Krebs cycle, otherwise known as the Tricarboxylic Acid (TCA) Cycle or the Citric Acid Cycle, is the second process of aerobic respiration. It takes place within the mitochondrial matrix and is where pyruvate is fully oxidized.
Conversion of Pyruvate to Acetyl-CoA
Pyruvate from glycolysis is converted to Acetyl-CoA by the removal of one CO₂ molecule and the generation of one NADH.
Acetyl-CoA (2-carbon molecule) and oxaloacetate (4-carbon molecule) join together to create citrate (6-carbon molecule).
Citrate is transformed through a series of reactions, losing two CO₂ molecules and generating NADH, FADH₂, and ATP.
Electron Transport Chain (ETC) is the last stage of aerobic respiration, which takes place in the inner mitochondrial membrane. This process produces the maximum number of ATP via a process called oxidative phosphorylation.
NADH and FADH₂ (formed in previous stages) transfer electrons (e⁻) to the electron transport chain, a chain of protein complexes in the inner mitochondrial membrane.
As electrons travel through the complexes, protons (H⁺ ions) are pumped through the inner mitochondrial membrane into the intermembrane space, establishing a proton gradient.
The protons flow back to the mitochondrial matrix via an enzyme known as ATP synthase, which powers the reaction ADP + Pi → ATP.
Oxygen (O₂) accepts electrons at the end of the chain and combines with protons to make water (H₂O). This is the reason why oxygen is vital for aerobic respiration.
Subject Wise NCERT Exemplar Solutions
The essential points from this chapter are listed below for quick and easy revision:
Types of Respiration
Glycolysis and Its Steps
Krebs Cycle
Electron Transport Chain (ETC) and ATP Production
Factors Affecting Respiration in Plants
To help with better preparation, here are some of the key questions that students should focus on, and to increase their understanding and performance.
Question 1: Which one of the following enzymes is not a part of the pyruvate dehydrogenase enzyme complex in the glycolysis pathway?
A. Pyruvate dehydrogenase
B. Dihydrolipoyl transferase
C. Dihydrolipoyl dehydrogenase
D. Dihydrolipoyl oxidase
Answer:
The enzyme "Dihydrolipoyl oxidase" is not a part of the pyruvate dehydrogenase enzyme complex in the glycolysis pathway.
Hence, the correct answer is option D. Dihydrolipoyl oxidase
Question 2: What are the energy-rich fuel molecules produced in the TCA cycle?
Answer:
The energy-rich fuel molecules produced in the tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, are as follows:
Question 3: Which step in the tricarboxylic acid cycle does not involve the oxidation of the substrate?
Answer:
Most reactions in the TCA cycle are oxidation reactions, but the reaction of succinyl CoA being reduced to succinate is the exception, in which the acceptor molecule does not get oxidized but instead takes part in substrate-level phosphorylation for the release of energy.
Question 4: How many ATP molecules could maximally be generated from one molecule of glucose, if the complete oxidation of one mole of glucose to CO2 and H2O yields 686 kcal and the useful chemical energy available in the high energy phosphate bond of one mole of ATP is 12 kcal?
Answer:
When one mole of glucose undergoes complete oxidation, it releases 686 kcal of energy. If each mole of ATP releases 12 kcal of energy when hydrolyzed, we can calculate how many moles of ATP can be generated from the energy released by glucose. By dividing the total energy released by glucose (686 kcal) by the energy released from one mole of ATP (12 kcal), we find that 57.1 moles of ATP can be maximally generated.
Question 5: What is the effect of a sudden increase in the levels of ATP and Citrate on an erythrocyte undergoing glycolysis?
Answer:
A sudden increase in the levels of ATP and citrate in an erythrocyte can act as a feedback inhibitors of glycolysis. ATP is an indicator of high energy levels in the cell, and citrate is an intermediate of the citric acid cycle (also known as the Krebs cycle). When ATP levels are high, it signals that the cell has sufficient energy, and further, ATP production through glycolysis is not needed. Citrate, being an intermediate of the citric acid cycle, indicates that the cell has already produced ample energy through oxidative metabolism.
Subject Wise NCERT Solutions
To solve questions from this chapter, students should focus on understanding the process and different terms of respiration in plants, its types, and the various steps involved. They need to prepare the flow charts for different pathways, such as glycolysis, the tricarboxylic acid (TCA) cycle, and oxidative phosphorylation, for quick revision and better retention. Concentration on the topics that how energy is produced, the role of enzymes, and how ATP is synthesized, is to be given, as some questions appear in the exam from these concepts. Regular practice of previous years' questions and referring to diagrams will increase the understanding of the key concepts.
Here are the links to the notes for each chapter to help with your Biology revision.
Respiration in plants is a biochemical process where glucose is broken down to release energy in the form of ATP. Unlike animals, plants respire at the cellular level using mechanisms like glycolysis, the Krebs cycle, and the electron transport chain.
Plants exhibit aerobic respiration (in the presence of oxygen, producing CO₂, H₂O, and ATP) and anaerobic respiration (in the absence of oxygen, producing ethanol, CO₂, and less ATP). Fermentation is a special form of anaerobic respiration in some plants and microbes.
Glycolysis occurs in the cytoplasm, breaking down one glucose molecule into two pyruvate molecules, generating 2 ATP and 2 NADH. It is the first step in both aerobic and anaerobic respiration and does not require oxygen.
Cellular respiration consists of glycolysis (cytoplasm), the Krebs cycle (mitochondria), and the electron transport chain (ETC) (inner mitochondrial membrane). These steps collectively break down glucose into ATP, releasing CO₂ and H₂O in aerobic conditions.
Mitochondria are the powerhouse of the cell, where the Krebs cycle and ETC occur. They generate most ATP by oxidizing pyruvate, utilizing NADH and FADH₂ to produce energy via oxidative phosphorylation.
The respiratory quotient (RQ) is the ratio of CO₂ released to O₂ consumed during respiration. It varies based on the substrate:
Carbohydrates: RQ = 1
Fats: RQ < 1
Proteins: RQ ≈ 0.8
Respiration rate increases with temperature due to enzyme activation, peaking at an optimum (usually 25-35°C). Beyond this, high temperatures denature enzymes, reducing respiration, while low temperatures slow down metabolic reactions.
During the day, photosynthesis dominates, producing O₂ and storing energy. At night, photosynthesis stops, and plants rely on respiration to break down stored glucose, generating ATP for cellular functions, growth, and repair.
During the day, photosynthesis dominates, producing O₂ and storing energy. At night, photosynthesis stops, and plants rely on respiration to break down stored glucose, generating ATP for cellular functions, growth, and repair.
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