NCERT Solutions for Class 11 Biology Chapter 12 Respiration in Plants
NCERT Solutions for Class 11 Biology Chapter 12 Respiration in Plants provides concise revision notes and step-by-step textbook answers covering cellular respiration, Glycolysis (EMP pathway), fermentation, the Krebs cycle (TCA cycle), Electron Transport System (ETS), Respiratory Balance Sheet, Amphibolic Pathway, and Respiratory Quotient (RQ).
Overview of Topics & Subtopics (Click to Jump)
1. Do Plants Breathe? & Glycolysis (EMP Pathway)
Short Note:
• Respiration: Mechanism of breaking down C-C bonds of complex organic compounds through oxidation within cells, releasing energy trapped as ATP.
• Gas Exchange in Plants: Plants lack specialized respiratory organs. They exchange gases using stomata (leaves/green stems) and lenticels (woody bark). Each plant part takes care of its own gas-exchange needs.
• Glycolysis (EMP Pathway): Discovered by Embden, Meyerhof, and Parnas.
- Occurs in the cytoplasm of all living organisms (both aerobic and anaerobic).
- Partial oxidation of 1 molecule of Glucose (6C) into 2 molecules of Pyruvic Acid (3C).
- Key Yield: Uses 2 ATPs, generates 4 ATPs directly (Net gain = 2 ATP) and 2 NADH + H+ (equivalent to 6 ATP via oxidative phosphorylation).
2. Fermentation (Anaerobic Respiration)
Short Note:
• Incomplete breakdown of glucose under anaerobic conditions without oxygen.
• Alcoholic Fermentation: Carried out by Yeast. Pyruvic acid is converted to Ethanol and CO2 in two steps catalyzed by Pyruvate decarboxylase and Alcohol dehydrogenase.
• Lactic Acid Fermentation: Occurs in certain bacteria and oxygen-starved muscle cells. Pyruvic acid is reduced to Lactic Acid by Lactate dehydrogenase using NADH + H+.
• Energy Efficiency: Less than 7% of the energy in glucose is released during fermentation. Yeasts poison themselves to death when alcohol concentration reaches about 13%.
3. Aerobic Respiration: TCA Cycle & Electron Transport System (ETS)
Short Note:
• Complete oxidation of pyruvic acid in the presence of oxygen within the mitochondria.
• Link Reaction (Mitochondrial Matrix): Oxidative decarboxylation of Pyruvate to Acetyl CoA catalyzed by Pyruvate dehydrogenase, generating 1 NADH + H+ and 1 CO2 per pyruvate.
• Tricarboxylic Acid Cycle (Krebs / TCA Cycle): Discovered by Hans Krebs in the mitochondrial matrix.
- First stable intermediate is Citric acid (6C) formed by condensation of Acetyl CoA with Oxaloacetic Acid (OAA, 4C).
- Yield per Acetyl CoA: 3 NADH + H+, 1 FADH2, 1 GTP (ATP), and 2 CO2.
• Electron Transport System (ETS) & Oxidative Phosphorylation: Located on the inner mitochondrial membrane.
- Electrons pass through Complexes I to IV (Complex I: NADH dehydrogenase, Complex II: Succinate dehydrogenase, Complex III: Cytochrome bc1, Complex IV: Cytochrome c oxidase).
- Oxygen acts as the final hydrogen/electron acceptor, forming water (H2O).
- 1 NADH yields 3 ATPs; 1 FADH2 yields 2 ATPs. Total theoretical net yield per glucose molecule = 36 to 38 ATPs.
4. Amphibolic Pathway & Respiratory Quotient (RQ)
Short Note:
• Amphibolic Pathway: Respiration involves both catabolism (breakdown of substrates for energy) and anabolism (synthesis of precursors for biomolecules, e.g., Acetyl CoA used for fatty acid synthesis). Hence, the respiratory pathway is amphibolic rather than purely catabolic.
• Respiratory Quotient (RQ): Ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration.
- Carbohydrates (Glucose): RQ = 1.0 (Equal CO2 evolved and O2 consumed).
- Fats (e.g., Tripalmitin): RQ = 0.7 (Less than 1, as fats require more O2 for oxidation).
- Proteins: RQ ≈ 0.9.
- Organic Acids: RQ > 1.0 (e.g., Malic acid RQ = 1.33).
Intext & Conceptual Questions
A pathway involved only in breakdown processes is catabolic, while one involved only in synthesis is anabolic. Respiratory pathways serve both purposes:
1. Respiratory substrates (carbohydrates, fats, proteins) are broken down to yield energy (Catabolism).
2. Intermediates of the pathway are withdrawn to synthesize other compounds when needed (Anabolism) — for instance, Acetyl CoA is used for fatty acid synthesis, and α-ketoglutaric acid is used to synthesize amino acids.
Because it plays a dual role in both catabolism and anabolism, respiration is correctly termed an amphibolic pathway.
• Definition: Respiratory Quotient (RQ) is the ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration in a given period.
• Formula:
RQ = (Volume of CO2 evolved) / (Volume of O2 consumed)• Values:
- Carbohydrates (Glucose): RQ = 1.0
- Tripalmitin (Fat): RQ = 0.7
Textbook Chapter End Exercise Questions
| Feature | Glycolysis | Fermentation |
|---|---|---|
| Site of Occurrence | Occurs in the cytoplasm of all cells. | Occurs in cytoplasm under anaerobic conditions. |
| End Product | 2 molecules of Pyruvic Acid. | Ethanol + CO2 or Lactic Acid. |
| NADH Utilization | NADH + H+ is produced. | NADH + H+ is oxidized back to NAD+. |
| Net Energy Yield | Net gain of 2 ATP molecules. | Very little energy (< 7% of glucose energy). |
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen Requirement | Requires molecular oxygen (O2). | Occurs in the absence of oxygen. |
| Location | Cytoplasm and Mitochondria. | Cytoplasm only. |
| End Products | CO2 and H2O (Complete oxidation). | Ethanol/Lactic acid + CO2 (Incomplete oxidation). |
| ATP Production | High yield (36 to 38 ATP per glucose). | Low yield (2 ATP per glucose). |
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.