Respiration in Plants: Class 11 NCERT Biology Explained

What is Respiration in Plants?

Respiration in plants is the process by which plants break down glucose molecules to release energy in the form of ATP (adenosine triphosphate). Unlike photosynthesis, which stores energy, respiration releases energy necessary for various cellular activities such as growth, repair, and active transport.

Key points:

• Occurs in all living plant cells.
• Converts glucose into usable energy.
• Takes place day and night.
• Involves multiple biochemical pathways depending on oxygen availability.

Understanding respiration is crucial for Class 11 NCERT students as it links energy metabolism with plant physiology and growth.

Glycolysis: The First Step of Respiration in Plants

Glycolysis is the initial phase of respiration where one glucose molecule (C₆H₁₂O₆) is enzymatically broken down into two molecules of pyruvic acid (C₃H₄O₃). This process occurs in the cytoplasm and does not require oxygen, making it common to both aerobic and anaerobic respiration.

Steps in glycolysis:

1. Glucose Activation: Glucose is phosphorylated twice using 2 ATP molecules, forming fructose-1,6-bisphosphate. 2. Cleavage: The 6-carbon sugar splits into two 3-carbon molecules. 3. Energy Harvesting: These molecules are converted into pyruvic acid, producing 4 ATP molecules and 2 NADH molecules.

Energy balance:

NADH formed carries electrons to later stages for further ATP synthesis.

Example:

If a plant cell breaks down 1 glucose molecule via glycolysis, it nets 2 ATP and 2 NADH molecules, providing immediate energy for cellular functions.

Role of Sucrose and Glucose in Plant Respiration

Plants primarily store carbohydrates as sucrose, which is transported from photosynthetic cells to other parts for respiration. Before glycolysis, sucrose is hydrolyzed by the enzyme invertase into glucose and fructose.

• Sucrose → Glucose + Fructose (via invertase)
• Both monosaccharides are phosphorylated by hexokinase to glucose-6-phosphate and fructose-6-phosphate.

This conversion ensures that the sugars enter the glycolytic pathway efficiently.

Importance:

• Provides a steady glucose supply for respiration.
• Connects photosynthesis products to energy release.

This step is essential for Class 11 students to understand the link between carbohydrate metabolism and respiration.

Aerobic vs Anaerobic Respiration in Plants

After glycolysis, the fate of pyruvic acid depends on oxygen availability:

Aerobic respiration: Pyruvate enters mitochondria, undergoes Krebs cycle and electron transport chain, producing large ATP amounts.

Anaerobic respiration: Pyruvate is converted into ethanol and CO₂ (fermentation), releasing energy but much less efficiently.

Understanding these pathways helps Class 11 students grasp how plants adapt their energy production based on oxygen availability.

ATP Production and Its Importance in Plant Cells

ATP (adenosine triphosphate) is the energy currency of the cell. During respiration in plants, ATP is produced mainly by:

• Glycolysis: Produces a net 2 ATP molecules per glucose.
• Krebs Cycle and Electron Transport Chain: Yield up to 34 ATP molecules in aerobic respiration.

Why ATP is important:

• Powers active transport across membranes.
• Drives biosynthetic reactions.
• Supports cell division and growth.

Worked example:

If a plant cell metabolizes 1 mole of glucose aerobically:

$$ \text{Total ATP} = 2 (glycolysis) + 2 (Krebs cycle) + 34 (electron transport) = 38 \text{ ATP} $$

This energy sustains vital processes and explains why respiration is critical for plant survival.

Summary Table: Key Differences in Plant Respiration Stages

This table helps Class 11 students visualize respiration stages and their characteristics.