NCERT Solutions for Class 11 Biology Chapter 12 (2025-2026)

Biology explores how living organisms obtain and utilize energy to sustain life. Chapter 12 of Class 11 Biology, Respiration in Plants, focuses on this essential process that enables plants to release the energy stored during photosynthesis. Unlike animals, plants do not have specialized organs for respiration; instead, every living cell in a plant respires to meet its energy needs. This chapter explains the complete process of cellular respiration -from the breakdown of glucose in glycolysis to the energy release in the Krebs cycle and electron transport system -highlighting how plants generate ATP, the universal energy currency of cells. This chapter is part of the comprehensive NCERT Solutions Class 11 Biology series, which covers all chapters in detail.

The chapter also distinguishes between aerobic and anaerobic respiration, explores the energy yield from different pathways, and shows how the products of respiration are interconnected with other metabolic activities in plants. The NCERT Solutions for Respiration in Plants provided here offer detailed, step-by-step answers to all textbook questions, helping students build a strong conceptual foundation, clear doubts effectively, and prepare thoroughly for both school examinations and competitive exams like NEET.

NCERT Solutions for Class 11 Biology Chapter 12 - All Exercise Questions

Respiration in Plants
Medium
Q.
Differentiate between

(a) Respiration and Combustion

(b) Glycolysis and Krebs’ cycle

(c) Aerobic respiration and Fermentation F
Respiration in Plants
Medium
Q.
What are respiratory substrates? Name the most common respiratory substrate.
Respiration in Plants
Medium
Q.
Give the schematic representation of glycolysis?
Respiration in Plants
Medium
Q.
What are the main steps in aerobic respiration? Where does it take place?
Respiration in Plants
Medium
Q.
Give the schematic representation of an overall view of Krebs’ cycle.
Respiration in Plants
Medium
Q.
Explain ETS.
Respiration in Plants
Medium
Q.
Distinguish between the following:
 
(a) Aerobic respiration and Anaerobic respiration
 
(b) Glycolysis and Fermentation
 
(c) Glycolysis and Citric acid Cycle
Respiration in Plants
Medium
Q.
What are the assumptions made during the calculation of net gain of ATP?
Respiration in Plants
Medium
Q.
Discuss “The respiratory pathway is an amphibolic pathway.”
Respiration in Plants
Medium
Q.
Define RQ. What is its value for fats?
Respiration in Plants
Medium
Q.
What is oxidative phosphorylation?
Respiration in Plants
Medium
Q.
What is the significance of step-wise release of energy in respiration?

Download the PDF of NCERT Solutions for Class 11 Biology Chapter 12 – The Living World

Class 11 Chapter 12 Biology Questions & Answers  -Respiration in Plants

Q1. Differentiate between
(a) Respiration and Combustion
(b) Glycolysis and Krebs’ cycle
(c) Aerobic respiration and Fermentation 

Solution: (a)Respiration and Combustion

Respiration Combustion
1. It occurs within living cells only. It does not occur inside a living system.
2. It requires enzymes. It does not require enzymes.
3. It occurs in a highly regulated mode under controlled condition. It is non-regulated and uncontrolled.
4. It produces energy equivalents in the form of high energy ATP molecules. It produces energy in terms of heat and light only.

(b) Glycolysis and Krebs’ cycle

Glycolysis Krebs’ Cycle
1. It occurs in the cytoplasm. It occurs in the mitochondria.
2. It is a non-cyclic process. It is a cyclic process.
3. It is common in both aerobic and anaerobic respiration. It takes place only in aerobic respiration.
4. Less productive in terms of ATP and NADP generation. Produces 8 ATP molecules from one glucose molecule Produces 15 ATP molecules from one molecule of Acetyl CoA

(c) Aerobic respiration and Fermentation 

Aerobic respiration Fermentation
1. It occurs in the presence of molecular oxygen only. It does not require molecular oxygen.
2. It takes place in both cytoplasm and mitochondria. It takes place only in the cytoplasm.
3. It is highly efficient and produces 38 ATP molecules per molecule of glucose. It is non-economical, produces only 2 molecules of ATP per molecule of glucose
4. The final products formed from one glucose molecule are carbon dioxide and water. The final products formed from one glucose molecule are ethyl alcohol and carbon dioxide

 

Q2. What are respiratory substrates? Name the most common respiratory substrate.

Solution: The complex organic compound that gets oxidized in the cell during respiration to release large amounts of energy is called respiratory substrate. Under normal condition, glucose is the most common respiratory substrate which is a carbohydrate along with six carbon atoms.

 

Q3. Give the schematic representation of glycolysis?

Solution: Schematic representation of glycolysis:

Q4. What are the main steps in aerobic respiration? Where does it take place?

Solution:  The four main steps of aerobic respiration are as follows:

S.No Steps of aerobic respiration Site of occurrence in the cell
1. Glycolysis Cytoplasm
2. Krebs’ Cycle Mitochondrial matrix
3. Electron transport chain Inner membrane of mitochondria
4. Oxidative phosphorylation F0-F1 particle of  cristae present in the inner membrane of mitochondria

 

Q5. Give the schematic representation of an overall view of Krebs’ cycle.
Solution: Schematic representation of Krebs’ cycle:

 

Q6. Explain ETS.

Solution:  The electrons removed from the substrates of glycolysis and the Krebs’ cycle are stored in the reduction equivalents, namely NADH2 and FADH2. This energy is released when NADH2 and FADH2 are oxidized by passing their electrons to a chain of electrons carrier complex called Electron transport system, present in the inner membrane of mitochondria. These complexes transfer the electron through a series of redox reactions with high energy electrons entering the system and low-energy electrons leaving the system. The energy released through this process is utilized to pump out protons which develop a proton gradient (Proton motive force) across the inner membrane. This proton motive force is utilized by ATP synthase to generate high energy ATP molecules at 3 different sites.

Process: The NADH2 produced during the citric acid cycle are oxidized by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone which gets reduced. Ubiquinone also receives reducing equivalents via FADH2 (complex II). The reduced ubiquinone (ubiquinol) is then re-oxidized by transferring its electrons to cytochrome c via cytochrome bc1 complex (complex III). The electron is transferred from complex III to complex IV through cytochrome C which is a mobile carrier present in the inner membrane. Complex IV is called cytochrome C oxidase complex and consists of cytochromes a-a3, and two copper centers.

 

Q7. Distinguish between the following:

(a) Aerobic respiration and Anaerobic respiration
(b) Glycolysis and Fermentation
(c) Glycolysis and Citric acid Cycle
Solution:  (a) Aerobic respiration and Anaerobic respiration
Aerobic respiration Anaerobic respiration
1  It occurs only in the presence of molecular oxygen. It occurs in the absence of molecular oxygen.
2. It is highly efficient and produces 38 ATP molecules. It is less efficient and generates only 2 ATP molecules.
3. It takes place in both cytoplasm and mitochondria. It takes place in the cytoplasm only.
4. It is a multistep process having glycolysis, Krebs’ cycle, and ETS. It is not a multistep process.
5. It produces carbon dioxide and water as the by-product. It produces ethyl alcohol and carbon dioxideas a by-product.

(b) Glycolysis and Fermentation

Glycolysis Fermentation
1. It is a common step in both aerobic and anaerobic respiration. It is strictly an anaerobic mode of respiration.
2. It results in the production of pyruvic acid. It produces ethyl alcohol.
3. Net gain is 8 ATP molecules. Net gain is 2 ATP molecules.

 
4. The product of glycolysis is used as an intermediate in Krebs’ cycle. The product of fermentation (ethyl alcohol) is not used by cells further.

(c)Glycolysis and Citric acid Cycle

Glycolysis Citric acid Cycle
1. It occurs in the cytoplasm. It occurs in the mitochondria.
2. It is a non-cyclic process. It is a cyclic process.
3. It is common in both aerobic and anaerobic respiration. It takes place only in aerobic respiration.
4. It produces 8 ATP molecules from one molecule of glucose. It produces 15 ATP molecule from one molecule of Acetyl CoA.

 

 

Q8. What are the assumptions made during the calculation of net gain of ATP?

Solution:  Many assumptions have been made in order to calculate the net gain of ATP from one molecule of glucose. This is required as the cellular system is very complex where numerous biochemical reactions take place simultaneously. The assumptions are as follows:

  1. All the steps of aerobic respiration (glycolysis, TCA cycle, ETS and oxidative phosphorylation) take place in sequential order where the product of the first step enters the subsequent step as a substrate.
  2. The NADH synthesized in glycolysis enters in the mitochondria and produces ATP through oxidative phosphorylation.
  3. The intermediates formed in various steps of aerobic respiration are not utilized in any other metabolic pathways other than subsequent steps of respiration.
  4. The glucose molecule is the only substrate and no other molecule enters as an intermediate substrate in the process.

 

Q9. Discuss “The respiratory pathway is an amphibolic pathway.”

Solution: The process of metabolism involves both anabolic and catabolic reactions. Anabolism is the synthesis of complex macromolecules like lipids and proteins from simple molecules like glycerol and amino acid respectively. On the other hand, catabolism includes the breakdown of macromolecules into simple molecules so that they can enter in the respiratory pathway as a substrate for the release of energy. If fatty acids are used as a respiratory substrate they are broken down to glycerol and acetyl CoA. Glycerol gets converted to 3-phosphoglyceraldehyde (PGAL) and enters in glycolysis while Acetyl CoA directly enters in Krebs’ cycle. However, when an organism needs to synthesize fatty acids, acetyl CoA is withdrawn from the above-said pathway and is made available for catabolic reaction. Similarly, when proteins are used as a substrate, they are first broken down to amino acid, which in turn, depending on their structure, gets converted into different intermediates of Krebs’ cycle. At the time of need, the same molecules are withdrawn to synthesize new proteins. Most of these reactions are reversible and depending on the requirement, the cell uses the respiratory substrate in the process of anabolism or catabolism. Thus, the respiratory pathway is known as the amphibolic pathway rather than only a catabolic pathway.

 

Q10. Define RQ. What is its value for fats?

Solution: Complete oxidation of substrates during aerobic respiration requires oxygen and apart from energy, carbon dioxide is produced as the by-product. The ratio of the volume of CO2 released to the volume of O2 consumed during complete oxidation of one molecule of a substrate in a given period of time at standard temperature and pressure is called the respiratory quotient (RQ).

For example, during aerobic respiration of one molecule of glucose, 6 molecules of CO2 are released and 6 molecules of O2 are consumed. Thus RQ for glucose is 1.

RQ value for fats: Fats need more oxygen molecule than carbohydrate (glucose) for complete oxidation through aerobic respiration, due to which the value of RQ for fat is always less than 1.

For example, when fatty acid tripalmitin is used as a substrate, 145 molecules of O2 are consumed whereas 102 molecules of CO2 are produced, the RQ value is 0.7.

 

Q11. What is oxidative phosphorylation?

Solution: The metabolic pathway that uses the energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP) is called oxidative phosphorylation. Almost all the forms of life on earth use a range of different nutrients to carry out oxidative phosphorylation to produce the molecule that supplies energy to metabolism i.e. ATP. This is a very efficient process of energy generation.

This process requires the presence of oxygen in the system. Oxygen drives the whole process as it removes hydrogen from the system and acts as the final hydrogen acceptor. During oxidative phosphorylation, electrons are transferred from electron donors like NADH2 to electron acceptors such as oxygen. These redox reactions release energy, which is used to form ATP. In eukaryotes, these redox reactions are carried out by a series of protein complexes within mitochondria, whereas, in prokaryotes, these proteins are located in the cells' inner membranes. These linked sets of proteins are called electron transport chains. It is the energy of the oxidation-reduction process that is used for the production of proton gradient required for phosphorylation and thus, this process is called oxidative phosphorylation.

 

Q12. What is the significance of step-wise release of energy in respiration?

Solution:  Carbohydrates, proteins, fats and organic acids are used as respiratory substrates and oxidation of these compounds releases energy in the cell. However, the energy released is not dissipated freely in the cell. In other words, it does not occur in one step. Instead, it is released in a series of slow step-wise reactions controlled by enzymes and is trapped in the form of ATP. This prevents the sudden increase in the temperature and avoids wastage of energy. This holds a lot of significance as ATP which stores the energy can be broken down whenever and wherever it is needed in the various energy-requiring processes of the organisms.

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NCERT Solutions for Class 11 Biology Chapter 12  –FAQs

Q1. What are the main stages of aerobic respiration in plants?

Aerobic respiration occurs in three major stages:

  1. Glycolysis – in cytoplasm; glucose → 2 pyruvate + 2 ATP + 2 NADH
  2. Krebs (TCA) Cycle – in mitochondrial matrix; pyruvate → CO₂ + NADH + FADH₂ + ATP
  3. Electron Transport Chain (ETS) – in inner mitochondrial membrane; NADH/FADH₂ oxidized → H₂O + ~34 ATP

Total yield: ~36–38 ATP per glucose molecule.

Q2. What is the difference between aerobic and anaerobic respiration?

Feature Aerobic Anaerobic (Fermentation)
Oxygen required? Yes No
End products CO₂ + H₂O Ethanol + CO₂ (plants/yeast) or Lactic acid (animals)
ATP yield High (~36 ATP) Low (~2 ATP)
Site Cytoplasm + Mitochondria Cytoplasm only

Anaerobic respiration helps cells survive temporary lack of oxygen (e.g., in water-logged soils).

Q3. What is the Respiratory Quotient (RQ)?

Formula:

RQ=CO₂ evolvedO₂ consumed

 

RQ = \frac{\text{CO₂ evolved}}{\text{O₂ consumed}}

Substrate Example RQ Value
Carbohydrate Glucose 1.0
Fat Palmitic acid < 1.0
Protein Albumin ~ 0.9
Organic acid Malic acid > 1.0

RQ indicates the type of respiratory substrate being used by the plant.

Q4.What is glycolysis?

  • It is the first step of respiration, occurring in the cytoplasm.

  • One glucose (6C) molecule breaks down into two pyruvate (3C) molecules.

  • ATP formed: 4 ATP (gross), 2 ATP net gain.

  • Does not require oxygen, so it happens in both aerobic and anaerobic respiration.

It is a universal pathway — occurs in all living cells.

 

Q.1 ‘All elements that are present in a plant need not be essential to its survival’. Comment.

The statement that “all the elements that are present in a plant need not be essential to its survival” holds true as only the elements which fulfill the below-mentioned criteria are essential for the survival of the plant, rest are not essential.

  • The element must be absolutely necessary for supporting growth and reproduction. In the absence of these elements, plants cannot grow or produce seeds.
  • The element cannot be replaced with any other element which means that the requirement is very specific. Deficiency of that particular element cannot be met by replacing it with other elements.
  • The element must be directly involved in the metabolism of the plant.

There are cases where elements like gold, selenium, etc., have been found in plants but these are not essential.

Q.2 Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?

The purification of water and nutrient salts is very important in studies involving mineral nutrients using hydroponics because the presence of impurities in a mineral solution may alter the concentration of the solution therefore, altering the optimum growth conditions like pH required for Hydroponics. Due to such alterations, the standardised mineral solution might be rendered non-useful for the growth of the plant. These impurities could be in the form of inorganic elements or compounds that are detrimental to the plants. For example, Sodium (Na) if greater than 70 ppm, Boron (B) if greater than 2.0 ppm, anions of chloride (Cl–) if greater than 100 ppm and bicarbonate (HCO3–) if greater than 40 ppm can be very harmful to the growth of the plants. Also, in order to determine the exact nature and amount of mineral required, it is necessary that the nutrient is present in known amounts. Any kind of impurity from water or nutrient salt will hamper the validity of the experiment.

Q.3 Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.

Macronutrients: They have the following features:

  • Essential for the growth of the plant
  • Required in large amounts
  • Directly involved in the metabolism of the plant
  • Cannot be replaced with any other nutrients
  • Generally present in large amounts (in excess of 10 mmole Kg-1 of dry matter).

Example: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur etc.

Micronutrients: They have the following features:

  • Essential for the growth of the plant
  • Required in small amounts
  • Directly involved in the metabolism of the plant
  • Cannot be replaced with any other nutrient
  • Generally present in very small amounts (less than 10 mmole Kg-1 of dry matter).

Example: iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Beneficial Nutrients: The elements which are not essential but their presence enhances the growth of the plants are known as beneficial nutrients. In addition to the essential elements, beneficial elements are also required by some higher plants for better growth. These elements have been reported to enhance the resistance of the plant to biotic and abiotic stresses. Example: sodium, silicon, cobalt and selenium.

Essential Elements: Essential elements are elements that have the following features:

  • Absolutely necessary for supporting normal plant growth
  • Cannot be replaced by any other element
  • Directly involved in the metabolism of the plant
  • Can be macronutrients or micronutrients (depending upon the quantities in which they are required by the plant)
  • 17 essential elements which carry out essential functions in the plants
  • Classified into four subgroups based on the function.

Example: carbon, hydrogen, oxygen, nitrogen, iron, manganese, copper, molybdenum, zinc.

Q.4 Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.

The different deficiency symptoms in plants are as follows:

  • Chlorosis is the loss of chlorophyll leading to yellowing of leaves. This is caused by the deficiency of elements like N, K, Mg, S, Fe, Mn, Zn and Mo.
  • Necrosis is the death of tissues, particularly leaf tissue. This is caused by the deficiency of elements like Ca, Mg, Cu, K.
  • Delayed flowering is the result of low levels of elements like N, S or Mo.
  • Stunted plant growth is the result of deficiencies of Cu and K.
  • Inhibition of cell division is caused by lack or low levels of N, K, S or Mo.

[N-Nitrogen, K-Potassium, Mg-Magnesium, S-Sulphur, Fe-Iron, Mn-Manganese, Ca-Calcium, Cu-Copper, Zn-Zinc and Mo-Molybdenum]

Q.5 If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element?

If a plant shows a symptom which could develop due to deficiency of more than one nutrient, the real deficient mineral element can be found by following steps:

  • Observe the morphological changes in all the parts of the plant.
  • Compare the changes with the available standard tables.
  • Design an experiment where one can add the nutrient one by one and then look for the recovery of the plants after addition of the particular element.

This strategy helps in finding out the real deficient mineral element.

Q.6 Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?

The reason behind the appearance of deficiency symptoms first in younger parts of the plant as compared to that of mature organs is due to the difference in the mobility of the element under study. When the elements are relatively immobile and are not transported out of the mature organs, the deficiency symptoms first appear in younger growing parts. For example, elements like sulphur and calcium are part of the structural component of the cell and are not easily released from the older tissues making it deficient in the younger and newer organs. The elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms first appear in the older tissues. For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves. In the older leaves, biomolecules containing these elements are broken down, making it available for younger leaves.

Q.7 How are the minerals absorbed by the plants?

The main organ of the plant through which minerals are absorbed from the soil is root. The process of absorption has two well-demarcated phases.

  • In the initial phase, rapid uptake of ions occurs passively into the space outside the cell membrane of the cells (known as apoplast). This uptake occurs through ion-channels which are transmembrane proteins that function as selective pores.
  • In the second phase of uptake, the ions are taken in slowly into the ‘inner space’ or the space inside the plasma membrane (known as symplast) of the cell. This process is energy-driven and thus an active process. The movement of ions is called flux.

Q.8 What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in N2 -fixation?

Nitrogen is one of the most prevalent elements in living organisms. The atmospheric nitrogen cannot be absorbed by plants as such – it needs to be fixed as nitrates before plants can absorb and transport it to leaves. Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation and few prokaryotic species are capable of fixing nitrogen. These prokaryotic microbes, which have the enzyme nitrogenase, are capable of nitrogen reduction and are called N2 fixers. Rhizobium is one such N2 fixer. The conditions necessary for fixation of atmospheric nitrogen by Rhizobium are as follows:

  1. The most important condition required for fixation of atmospheric nitrogen by Rhizobium is the presence of the anaerobic condition. The enzyme nitrogenase, that is required for the conversion of atmospheric nitrogen to ammonia cannot function in the presence of oxygen. The nodule where the bacteria reside, provide the anaerobic condition.
  2. The presence of leguminous haemoglobin, an oxygen scavenger is required for the process of nitrogen fixation to occur.
  3. The process of ammonia synthesis by nitrogenase requires a high input of energy and this is obtained from the respiration of the host cells.

Role in N2-fixation: The free-living aerobic bacteria Rhizobium enters into a symbiotic relationship with leguminous plants and starts fixing free nitrogen of the atmosphere into ammonia which can be utilised by the host plant. The important features of Rhizobium are

  • It occurs inside the anaerobic nodule which carries all the necessary biochemical components such as the enzyme nitrogenase and leghaemoglobin, which is an oxygen scavenger.
  • The enzyme nitrogenase of Rhizobium is a Mo-Fe protein and catalyzes the below-written reaction. Ammonia is the first stable product of nitrogen fixation. The reaction is as follows-

N2+ 8e–+ 8H+ + 16ATP → 2NH3+ H2+ 16ADP+16Pi

  • The ATP required for the above reaction is provided by the host.
  • Ammonia formed following nitrogen fixation is incorporated into amino acids as the amino groups.

Q.9 What are the steps involved in formation of a root nodule?

The formation of nodule requires a series of interaction between the Rhizobium, which is free-living aerobic bacteria and the roots of the host leguminous plants. Rhizobium is capable of fixing free nitrogen of the atmosphere into ammonia which can be utilised by the host plant. This reaction occurs inside the anaerobic conditions maintained inside nodule which carries all the necessary biochemical components. Main stages in the formation of the nodule are described below:

  • Rhizobia multiply, colonise the root surroundings and get attached to epidermal and root hair cells.
  • Root hairs curl and bacteria invade the root hair.
  • Infection thread carries the bacteria into the root cortex, where nodule formation is initiated.
  • Bacteria are released from the infection thread into the cells – specialised nitrogen-fixing cells start differentiating.
  • The nodule is formed – it has a direct vascular connection with the host for exchange of nutrients.

Q.10 Which of the following statements are true? If false, correct them:

(a) Boron deficiency leads to stout axis.

(b) Every mineral element that is present in a cell is needed by the cell.

(c) Nitrogen as a nutrient element, is highly immobile in the plants.

(d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.

(a) True

(b) False. All the mineral elements that are present in a cell are not needed by the cell. For example, gold and selenium are found accumulated in plants but they are not used in any plant process.

(c) False. Nitrogen as a nutrient is highly mobile in the plants. Its deficiency results in deficiency symptoms to show up first in older tissues.

(d) False. It is very difficult to establish the essentiality of micronutrients because of the trace requirements

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