Respiration in Plants Notes: Class 11 Biology Chapter 12
Class 11 Biology Chapter 12 Respiration in Plants covers glycolysis, fermentation, aerobic respiration (Krebs cycle and electron transport system), and the respiratory balance sheet.
Overview of Topics & Subtopics (Click to Jump)
1. Glycolysis
Short Note:
• Glycolysis (the common pathway in both aerobic and anaerobic respiration) occurs in the cytoplasm, breaking down one molecule of glucose into two molecules of pyruvate, with a net gain of 2 ATP and 2 NADH.
2. Fermentation (Anaerobic Respiration)
Short Note:
• In the absence of oxygen, pyruvate undergoes fermentation: converted to ethanol + CO2 (alcoholic fermentation, e.g. in yeast) or to lactic acid (lactic acid fermentation, e.g. in muscle cells), regenerating NAD+ needed to sustain glycolysis.
3. Aerobic Respiration
Short Note:
• In the presence of O2, pyruvate enters the mitochondria and is converted to Acetyl CoA, which enters the Krebs Cycle (Citric Acid Cycle) in the mitochondrial matrix, generating NADH, FADH2, ATP, and releasing CO2.
• NADH and FADH2 then feed electrons into the Electron Transport System (ETS) on the inner mitochondrial membrane, generating a large amount of ATP via oxidative phosphorylation, with O2 as the final electron acceptor (forming water).
4. Respiratory Balance Sheet
Short Note:
• The complete aerobic oxidation of one glucose molecule theoretically yields a maximum of about 36-38 ATP molecules (through glycolysis, Krebs cycle, and ETS combined), although actual yield varies due to factors like transport costs across membranes.
NCERT Textbook Exercise Questions
What are the main steps in aerobic respiration?
Answer: The main steps are: (i) Glycolysis — glucose broken down to pyruvate in the cytoplasm; (ii) Oxidative decarboxylation — pyruvate converted to Acetyl CoA; (iii) Krebs Cycle — Acetyl CoA fully oxidised, releasing CO2 and generating NADH/FADH2; (iv) Electron Transport Chain and Oxidative Phosphorylation — NADH/FADH2 used to generate large amounts of ATP, with O2 as the final electron acceptor.
Distinguish between aerobic and anaerobic respiration.
Answer: Aerobic respiration requires oxygen, occurs in the cytoplasm and mitochondria, completely oxidises glucose to CO2 and water, and yields a high amount of ATP (about 36-38). Anaerobic respiration (fermentation) does not require oxygen, occurs entirely in the cytoplasm, incompletely oxidises glucose to ethanol/CO2 or lactic acid, and yields a much lower amount of ATP (net 2).
What is the significance of the respiratory quotient (RQ)?
Answer: The Respiratory Quotient (RQ) is the ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration. Its value indicates the type of respiratory substrate being used — RQ = 1 for carbohydrates, RQ < 1 for fats and proteins (since they contain less oxygen relative to carbon and hydrogen and require more O2 for complete oxidation), and RQ > 1 for organic acids.
Why is the ATP molecule considered as an energy currency of the cell?
Answer: ATP (Adenosine Triphosphate) is called the energy currency because it stores energy in its high-energy phosphate bonds, which is released upon hydrolysis and readily used by the cell for various energy-requiring processes (muscle contraction, active transport, biosynthesis); it acts as a universal, immediately usable energy carrier linking energy-releasing (catabolic) and energy-requiring (anabolic) reactions.
