Class 11 Biology Notes Chapter 14 (Chapter 14) – Examplar Problems (English) Book
Detailed Notes with MCQs of Chapter 14, Respiration in Plants, from your NCERT Exemplar. This is a crucial chapter, not just for understanding plant physiology but also because the fundamental processes like glycolysis and the Krebs cycle are universal. Pay close attention, as these concepts frequently appear in various government examinations.
Chapter 14: Respiration in Plants - Detailed Notes
1. Introduction to Respiration:
- Definition: Respiration is the biochemical process involving the oxidation of complex organic substances (like glucose) within living cells to release energy in the form of ATP (Adenosine Triphosphate), along with by-products.
- Overall Equation (for aerobic respiration of glucose):
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + Heat) - Purpose: To generate metabolic energy (ATP) required for various life processes (synthesis, transport, movement, etc.).
- Respiratory Substrates: Compounds oxidized during respiration. Most common is glucose. Others include fats, proteins, and organic acids.
2. Do Plants Breathe?
- Yes, plants respire constantly, day and night.
- Gas Exchange: Occurs primarily through stomata (leaves, young stems) and lenticels (older stems, roots). Loose parenchyma cells also facilitate gas diffusion.
- Reasons why plants don't have specialized respiratory organs:
- Every plant part manages its own gas exchange needs. Little transport of gases from one part to another.
- Respiratory demands are lower compared to active animals.
- Photosynthesis during the day provides oxygen directly within cells (though respiration still occurs).
- Large surface area and internal air spaces facilitate diffusion.
3. Glycolysis (EMP Pathway - Embden-Meyerhof-Parnas Pathway):
- Location: Cytoplasm of all living cells (prokaryotic and eukaryotic).
- Oxygen Requirement: Anaerobic process (does not directly use oxygen).
- Process: Partial oxidation of one molecule of glucose (6-carbon) into two molecules of pyruvic acid (3-carbon).
- Phases:
- Energy Investment Phase: 2 ATP molecules are consumed.
- Energy Payoff Phase: 4 ATP molecules are produced via substrate-level phosphorylation, and 2 molecules of NADH + H⁺ are produced.
- Net Gain per Glucose Molecule:
- 2 molecules of Pyruvic Acid
- 2 ATP
- 2 NADH + H⁺
4. Fate of Pyruvic Acid:
- Depends on the availability of oxygen and the organism.
- Aerobic Conditions: Pyruvic acid enters the mitochondria for complete oxidation (Link Reaction & Krebs Cycle).
- Anaerobic Conditions: Pyruvic acid undergoes fermentation in the cytoplasm.
5. Fermentation (Anaerobic Respiration):
- Purpose: Incomplete oxidation of glucose under anaerobic conditions; main goal is to regenerate NAD⁺ from NADH to allow glycolysis to continue.
- Location: Cytoplasm.
- Types:
- Alcoholic Fermentation: Occurs in yeast and some plants under anaerobic conditions.
Pyruvic Acid → Ethanol + CO₂ + NAD⁺ (regenerated)
Enzymes: Pyruvic acid decarboxylase, Alcohol dehydrogenase. - Lactic Acid Fermentation: Occurs in some bacteria, fungi, and animal muscle cells during strenuous exercise.
Pyruvic Acid → Lactic Acid + NAD⁺ (regenerated)
Enzyme: Lactate dehydrogenase.
- Alcoholic Fermentation: Occurs in yeast and some plants under anaerobic conditions.
- Energy Yield: Very low; only a net gain of 2 ATP (from glycolysis). Much energy remains locked in ethanol or lactic acid.
6. Aerobic Respiration:
- Complete oxidation of organic substrates in the presence of oxygen, releasing a large amount of energy.
- Location: Starts in the cytoplasm (glycolysis), continues in the mitochondria.
- Steps:
- a) Link Reaction (Oxidative Decarboxylation):
- Location: Mitochondrial Matrix.
- Process: Pyruvic acid (3C) is converted into Acetyl-CoA (2C).
- Products per Pyruvic Acid: 1 NADH + H⁺ + 1 CO₂ + Acetyl-CoA.
- Enzyme Complex: Pyruvate Dehydrogenase.
- (Note: Since 1 glucose yields 2 pyruvic acids, this step yields 2 NADH, 2 CO₂, and 2 Acetyl-CoA per glucose molecule).
- b) Krebs Cycle (Tricarboxylic Acid Cycle - TCA cycle or Citric Acid Cycle):
- Location: Mitochondrial Matrix.
- Process: Acetyl-CoA (2C) enters the cycle by combining with Oxaloacetic Acid (OAA - 4C) to form Citric Acid (6C). Through a series of oxidative steps, Citric Acid is broken down, regenerating OAA.
- Products per Acetyl-CoA (one turn of the cycle):
- 2 CO₂
- 3 NADH + H⁺
- 1 FADH₂ (Flavin Adenine Dinucleotide - reduced)
- 1 GTP (Guanosine Triphosphate - equivalent to ATP, via substrate-level phosphorylation)
- (Note: Since 1 glucose yields 2 Acetyl-CoA, the cycle turns twice per glucose molecule, doubling these products).
- c) Electron Transport System (ETS) and Oxidative Phosphorylation:
- Location: Inner Mitochondrial Membrane.
- Process:
- NADH and FADH₂ produced during glycolysis, link reaction, and Krebs cycle are oxidized, releasing electrons and protons (H⁺).
- Electrons pass through a series of electron carriers (Complex I to IV - cytochromes are key components). Energy is released at each step.
- This energy is used to pump protons (H⁺) from the mitochondrial matrix to the intermembrane space, creating a proton gradient (Proton Motive Force).
- Oxygen acts as the final/terminal electron acceptor, combining with electrons and protons to form water (H₂O).
- Protons flow back into the matrix down their concentration gradient through a channel protein called ATP Synthase (Complex V).
- The energy of this proton flow drives the synthesis of ATP from ADP and inorganic phosphate (Pi). This is Oxidative Phosphorylation.
- ATP Yield from ETS:
- 1 NADH → approx. 3 ATP (older convention) or 2.5 ATP (newer convention)
- 1 FADH₂ → approx. 2 ATP (older convention) or 1.5 ATP (newer convention)
- a) Link Reaction (Oxidative Decarboxylation):
7. The Respiratory Balance Sheet:
- Calculates the theoretical net gain of ATP for each glucose molecule oxidized aerobically.
- Assumptions: Sequential pathway, NADH transferred efficiently into mitochondria, no intermediates used elsewhere.
- Calculation (using older convention often tested):
- Glycolysis: 2 ATP (net) + 2 NADH (→ 6 ATP) = 8 ATP
- Link Reaction (2 Pyruvate → 2 Acetyl CoA): 2 NADH (→ 6 ATP) = 6 ATP
- Krebs Cycle (2 turns): 2 GTP (→ 2 ATP) + 6 NADH (→ 18 ATP) + 2 FADH₂ (→ 4 ATP) = 24 ATP
- Total: 8 + 6 + 24 = 38 ATP (Note: If NADH from glycolysis requires shuttle systems costing energy, the yield might be 36 ATP). Newer conventions often calculate closer to 30-32 ATP. Understand the basis of calculation.
8. Amphibolic Pathway:
- Respiration is primarily a catabolic (breakdown) process, but it involves both catabolism and anabolism (synthesis). Hence, it's better termed an amphibolic pathway.
- Catabolism: Breakdown of carbohydrates, fats (β-oxidation → Acetyl CoA), and proteins (deamination → pyruvate, Acetyl CoA, or Krebs cycle intermediates).
- Anabolism: Respiratory intermediates can be withdrawn from the pathway for synthesis.
- Acetyl-CoA → Fatty acids, Gibberellins, Carotenoids
- α-Ketoglutarate → Amino acids (e.g., Glutamate)
- Oxaloacetate → Amino acids (e.g., Aspartate), Alkaloids, Pyrimidines
- Succinyl-CoA → Chlorophyll, Cytochromes
9. Respiratory Quotient (RQ):
- Definition: The ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration.
RQ = Volume of CO₂ evolved / Volume of O₂ consumed - Significance: Depends on the type of respiratory substrate used.
- Values:
- Carbohydrates (e.g., Glucose): RQ = 1 (6 CO₂ / 6 O₂ = 1)
- Fats (e.g., Tripalmitin): RQ < 1 (approx. 0.7) - Require more O₂ for oxidation compared to CO₂ produced.
- Proteins: RQ ≈ 0.9
- Organic Acids (e.g., Malic acid): RQ > 1 - Rich in oxygen, require less external O₂ for oxidation.
- Anaerobic Respiration: RQ = Infinite (CO₂ evolved, but no O₂ consumed).
Multiple Choice Questions (MCQs):
-
In which part of the plant cell does glycolysis occur?
a) Mitochondria
b) Chloroplast
c) Cytoplasm
d) Nucleus -
What is the net gain of ATP molecules during glycolysis from one molecule of glucose?
a) 0
b) 2
c) 4
d) 38 -
During alcoholic fermentation by yeast, pyruvic acid is converted into:
a) Lactic acid only
b) Ethanol and Carbon dioxide
c) Acetic acid and Oxygen
d) Acetyl-CoA and Carbon dioxide -
The Krebs cycle (TCA cycle) takes place in the:
a) Cytoplasm
b) Inner mitochondrial membrane
c) Intermembrane space of mitochondria
d) Mitochondrial matrix -
Which molecule acts as the final electron acceptor in the Electron Transport System (ETS) during aerobic respiration?
a) NAD⁺
b) FAD
c) Oxygen (O₂)
d) Water (H₂O) -
How many molecules of NADH are produced from one molecule of Acetyl-CoA during a single turn of the Krebs cycle?
a) 1
b) 2
c) 3
d) 4 -
The synthesis of ATP using the energy released during the electron transport system, involving a proton gradient, is called:
a) Substrate-level phosphorylation
b) Photophosphorylation
c) Oxidative phosphorylation
d) Dephosphorylation -
If the respiratory substrate is a fat like Tripalmitin, the Respiratory Quotient (RQ) would be expected to be:
a) Equal to 1
b) Less than 1
c) Greater than 1
d) Infinite -
Which of the following statements best describes an amphibolic pathway?
a) A pathway that occurs only in amphibians.
b) A pathway strictly involved in the breakdown of molecules.
c) A pathway strictly involved in the synthesis of molecules.
d) A pathway that involves both catabolic and anabolic processes. -
The enzyme complex responsible for converting pyruvic acid to Acetyl-CoA in the mitochondrial matrix is:
a) Lactate dehydrogenase
b) ATP synthase
c) Pyruvate dehydrogenase
d) Alcohol dehydrogenase
Answer Key:
- c) Cytoplasm
- b) 2
- b) Ethanol and Carbon dioxide
- d) Mitochondrial matrix
- c) Oxygen (O₂)
- c) 3
- c) Oxidative phosphorylation
- b) Less than 1
- d) A pathway that involves both catabolic and anabolic processes.
- c) Pyruvate dehydrogenase
Make sure you understand the why behind each answer. Go through these notes thoroughly, compare them with your NCERT textbook and Exemplar questions. Focus on the locations, inputs, outputs, and energy yields of each stage. Good luck with your preparation!