Respiration in Plants Class 11 NCERT Biology Complete Guide

Understanding Cellular Respiration in Plants

Unlike animals, plants do not possess specialized organs for gaseous exchange, such as lungs or gills. Instead, they rely on stomata and lenticels for diffusion. But why don't plants need complex respiratory organs? First, each plant part takes care of its own gas-exchange needs. Second, plants do not present a high demand for gas exchange except during active photosynthesis. Lastly, the distance that gases must diffuse in even a large, bulky plant is extremely short because living cells are located close to the surface.

At the cellular level, cellular respiration is the mechanism of breakdown of food materials within the cell to release energy, and the trapping of this energy for synthesis of ATP. The compounds that are oxidized during this process are known as respiratory substrates. Carbohydrates (mainly glucose) are the most common respiratory substrates, but proteins, fats, and organic acids can also be used under specific physiological conditions.

The Four Stages of Aerobic Respiration

1. Glycolysis (EMP Pathway) — Occurring in the cytoplasm, glycolysis is the breakdown of one glucose molecule (6C) into two molecules of pyruvic acid (3C). This anaerobic process does not require oxygen and yields a net of 2 ATP and 2 NADH molecules.
2. The Link Reaction (Oxidative Decarboxylation) — Pyruvate enters the mitochondrial matrix. Here, it undergoes oxidative decarboxylation catalyzed by pyruvate dehydrogenase, producing Acetyl CoA (2C), releasing CO2, and generating 1 NADH per pyruvate (2 NADH per glucose).
3. Krebs Cycle (TCA / Citric Acid Cycle) — Acetyl CoA enters a cyclic pathway in the mitochondrial matrix. It condenses with oxaloacetate (4C) to form citric acid (6C). For every turn of the cycle, 2 CO2 molecules are released, and 3 NADH, 1 FADH2, and 1 GTP (ATP) are formed.
4. Electron Transport System (ETS) & Oxidative Phosphorylation — Located in the inner mitochondrial membrane, the ETS utilizes energy stored in NADH and FADH2 to pump protons and establish an electrochemical gradient. Oxygen acts as the final hydrogen acceptor, forming water, while ATP synthase drives the synthesis of ATP.

The Respiratory Quotient (RQ) & Energy Balances

• Respiratory Quotient (RQ) is defined as the ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration.
• RQ for Carbohydrates is exactly 1.0 (e.g., glucose) because equal volumes of CO2 and O2 are exchanged: C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy.
• RQ for Fats (lipids) is less than 1 (approximately 0.7 for Tripalmitin) because fats are highly reduced molecules and require more oxygen for oxidation.
• RQ for Proteins is around 0.9, while organic acids (like malic acid) have an RQ greater than 1.0.
• Under anaerobic respiration (fermentation), since no oxygen is consumed, the RQ value theoretically becomes infinity (CO2 / 0 = infinity).

CBSE Board Exam Pitfalls & ATP Calculation Traps

Students often make mistakes when calculating the total ATP yield. Remember, 1 NADH produces 3 ATP (or 2.5 ATP under modern biochemistry standards, but stick to 3 ATP for CBSE) and 1 FADH2 yields 2 ATP during oxidative phosphorylation.

Trap Alert: Pay close attention to whether the question asks for the net ATP from glycolysis alone (which is 2 ATP via substrate-level phosphorylation), or the total ATP yielded from one glucose molecule during aerobic respiration (which is theoretically 36 or 38 ATP). Also, remember that glycolysis takes place in the cytoplasm, whereas the Krebs cycle and ETS occur within the mitochondria. Keep these compartments clear on your board exam sheets!

Practice Questions with Solutions

• Q: Explain why Glycolysis is also referred to as the EMP pathway, and state where it occurs in the cell. A: Step 1: Identify the discoverers of the pathway. Glycolysis was discovered by three German scientists: Gustav Embden, Otto Meyerhof, and J. Parnas. Thus, it is abbreviated as the EMP pathway. Step 2: Identify the cellular location. Glycolysis occurs entirely in the cytoplasm (cytosol) of the cell. Step 3: State its key characteristic. It is the common pathway for both aerobic and anaerobic respiration because it does not require molecular oxygen. Final answer: Glycolysis is called the EMP pathway after Embden, Meyerhof, and Parnas, and it occurs in the cytoplasm.
• Q: Calculate the total theoretical number of ATP molecules produced from the complete oxidation of one molecule of glucose in aerobic respiration. A: Step 1: Calculate Glycolysis yield: Substrate-level phosphorylation = 2 ATP; 2 NADH converted in ETS = 6 ATP (or 4 ATP depending on the shuttle system used). Step 2: Calculate Link Reaction yield: 2 Pyruvate to 2 Acetyl CoA produces 2 NADH = 6 ATP. Step 3: Calculate Krebs Cycle yield: 2 turns of the cycle produce 6 NADH (18 ATP), 2 FADH2 (4 ATP), and 2 GTP/ATP (2 ATP) directly. Step 4: Sum all values: 2 + 6 (or 4) + 6 + 18 + 4 + 2 = 38 (or 36) ATP. Final answer: The complete oxidation of one glucose molecule yields a theoretical maximum of 36 or 38 ATP molecules.
• Q: What is the Respiratory Quotient (RQ)? Write the reaction and calculate the RQ for Tripalmitin. A: Step 1: Define RQ. It is the ratio of the volume of CO2 eliminated to the volume of O2 absorbed during respiration. Step 2: Write the balanced chemical equation for the oxidation of Tripalmitin (a fat): 2(C51H98O6) + 145O2 -> 102CO2 + 98H2O + Energy. Step 3: Perform the calculation using the stoichiometric coefficients: RQ = Volume of CO2 evolved / Volume of O2 consumed = 102 CO2 / 145 O2 = 0.7. Final answer: The Respiratory Quotient of Tripalmitin is 0.7, which is characteristic of lipids being oxygen-poor compared to carbohydrates.
• Q: What is oxidative phosphorylation and where does it occur in the cell? A: Step 1: Define the process. Oxidative phosphorylation is the synthesis of energy-rich ATP molecules from ADP and inorganic phosphate (Pi) using the energy released during the oxidation of NADH and FADH2 in the Electron Transport System (ETS). Step 2: Specify the driving force. It is driven by the proton gradient (proton motive force) established across the inner mitochondrial membrane by the proton pumps of the electron transport chain. Step 3: Identify the exact cellular location. It occurs in the inner mitochondrial membrane, specifically at the F0-F1 particles (ATP synthase). Final answer: Oxidative phosphorylation is the ATP synthesis driven by electron transport energy, taking place across the inner mitochondrial membrane.

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