NCERT Class 11 Biology Chapter 14 Notes Respiration In Plants - Download PDF Notes

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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This Story also Contains

1. What is Respiration in Plants?
2. Types of Respiration in Plants
3. Comparison of Aerobic, Anaerobic Respiration, and Fermentation
4. Respiratory Substrates
5. Steps of Respiration in Plants
6. Krebs Cycle (Citric Acid Cycle / TCA Cycle)
7. Electron Transport Chain (ETC) & Oxidative Phosphorylation
8. Factors Affecting Respiration in Plants
9. Chapter-Wise NCERT Class 11 Notes Biology

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,

• NCERT Solutions for Class 11 Biology Chapter 12 Respiration in Plants
• NCERT Exemplar Class 11 Biology Chapter 12 Solutions Respiration in Plants

What is Respiration in Plants?

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:

$$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{Energy (ATP)}$$

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.

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Types of Respiration in 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

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).

Location

• The process occurs mainly in the plant cells' mitochondria.
• Glycolysis, the initial step, happens in the cytoplasm, whereas the subsequent stages (Krebs cycle and Electron Transport Chain) happen in the mitochondria.

Process

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).

End Products

• Carbon dioxide (CO₂) – Emitted into the air.
• Water (H₂O) – Used or emitted as gas.
• ATP (Adenosine Triphosphate) – The cell's energy currency.

Energy Yield

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.

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Anaerobic Respiration

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.

$$C_6{H}_{12}{O}_6\to 2C_2{H}_{5}OH+2C{O}_2+\text{Energy (ATP)}$$

Location

Anaerobic respiration is located in the cytoplasm because mitochondria need oxygen to work.

Process

• Glycolysis – Glucose is broken down to form pyruvate in the cytoplasm.
• Fermentation Pathway – Rather than going into the Krebs cycle, pyruvate is reduced to form ethanol and CO₂ (in plants) or lactic acid (in certain microorganisms such as bacteria).

End Products

• Ethanol (C₂H₅OH) or Lactic Acid (C₃H₆O₃) – Depending on the organism.
• Carbon dioxide (CO₂) – Released during the process of fermentation.
• ATP – Very poor energy yield (only 2 ATP molecules for every glucose molecule).

Energy Yield

Anaerobic respiration is wasteful in comparison to aerobic respiration, as it yields much less ATP (only 2 molecules per molecule of glucose).

Where Does It Take Place in Plants?

• Where plant roots are in low oxygen supply in waterlogged conditions.
• In some plant tissues, like seeds during germination, when there's a shortage of oxygen.
• In some microorganisms that are part of the plant fermentation processes.

Example

• Root cells under flooded conditions resort to anaerobic respiration owing to oxygen shortage.
• Yeast cells employ anaerobic respiration to produce alcohol.

Fermentation

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.

Types of Fermentation

Alcoholic Fermentation (in Yeast & Some Plants)

• Pyruvate is converted into ethanol (alcohol) and CO₂.

• Used in the brewing and baking industries.

• $$C_6{H}_{12}{O}_6\to 2C_2{H}_{5}OH+2C{O}_2+\text{Energy (ATP)}$$

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).

• $$C_6{H}_{12}{O}_6\to 2C_3{H}_6{O}_3+\text{Energy (ATP)}$$

End Products

• Alcoholic Fermentation → Ethanol (C₂H₅OH), CO₂, ATP

• Lactic Acid Fermentation → Lactic acid (C₃H₆O₃), ATP

Energy Yield

• Produces only 2 ATP molecules per glucose molecule, similar to anaerobic respiration.

Importance of Fermentation

• 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:

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Comparison of Aerobic, Anaerobic Respiration, and Fermentation

Respiratory Substrates

• 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

• $$RQ=\frac{O_2\text{ consumed}}{C{O}_2\text{ released}}$$
• For fats and proteins: RQ < 1

Steps of Respiration in Plants

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 (Cytoplasmic Pathway)

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.

Process

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:

• 2 molecules of Pyruvate (C₃H₄O₃)
• 2 molecules of ATP (Net gain)
• 2 molecules of NADH (utilized further in the ETC)

End Products of Glycolysis

• Pyruvate (moved to the next phase: Krebs cycle if oxygen is available, or fermentation if oxygen is not available).
• ATP (utilized directly for cellular functions).
• NADH (utilized in the electron transport chain for the production of ATP).

The Steps of Glycolysis

Krebs Cycle (Citric Acid Cycle / TCA Cycle)

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.

Process

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.

Entry into the Krebs Cycle

Acetyl-CoA (2-carbon molecule) and oxaloacetate (4-carbon molecule) join together to create citrate (6-carbon molecule).

Series of Oxidation Reactions

Citrate is transformed through a series of reactions, losing two CO₂ molecules and generating NADH, FADH₂, and ATP.

Regeneration of Oxaloacetate

• Oxaloacetate is regenerated at the end of the cycle so that the cycle can repeat.
• End Products per Glucose Molecule (Two Pyruvates)
• 6 NADH (utilized in the electron transport chain).
• 2 FADH₂ (utilized in the electron transport chain).
• 2 ATP (available for use in cellular processes).
• 4 CO₂ (excreted as a waste product).

Electron Transport Chain (ETC) & Oxidative Phosphorylation

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.

Electron Transfer

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.

Proton Gradient Formation

As electrons travel through the complexes, protons (H⁺ ions) are pumped through the inner mitochondrial membrane into the intermembrane space, establishing a proton gradient.

ATP Synthesis (Oxidative Phosphorylation)

The protons flow back to the mitochondrial matrix via an enzyme known as ATP synthase, which powers the reaction ADP + Pi → ATP.

Oxygen as the Final Electron Acceptor

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.

End Products

• 34 ATP molecules (principal ATP production location).
• Water (H₂O) (created when oxygen accepts electrons).

Factors Affecting Respiration in Plants

• Temperature – Higher temperatures increase enzyme activity up to an optimum level.
• Oxygen Availability – More oxygen increases aerobic respiration; limited oxygen promotes anaerobic respiration.
• Water Content – Dry conditions reduce respiration as enzymes need water to function.
• Type of Substrate – Carbohydrates are preferred; fats and proteins require more oxygen for breakdown.

Chapter-Wise NCERT Class 11 Notes Biology