Respiration in Plants – Class 11 Biology Notes
Introduction to Respiration in Plants
Respiration in plants is the process by which they release energy from the stored food in the form of glucose, using oxygen in the presence of enzymes. This energy is used for various metabolic activities, such as growth, repair, and maintenance of cell structures. It is a catabolic process, meaning it breaks down larger molecules into smaller ones, releasing energy.
Unlike photosynthesis, which occurs in the presence of light, respiration in plants occurs continuously, both day and night, in all living cells. The primary purpose of respiration is to produce energy (ATP) required for cellular processes.
1. Types of Respiration in Plants
There are two main types of respiration in plants:
- Aerobic Respiration (with oxygen)
- Anaerobic Respiration (without oxygen)
1.1. Aerobic Respiration
Aerobic respiration occurs in the presence of oxygen and is the most common and efficient form of respiration in plants. It involves the complete breakdown of glucose to produce a large amount of energy (ATP), carbon dioxide (CO₂), and water (H₂O).
Steps of Aerobic Respiration:
- Glycolysis (in the cytoplasm):
- Glucose (a 6-carbon sugar) is broken down into two molecules of pyruvate (a 3-carbon compound).
- This process releases 2 ATP molecules and produces 2 NADH molecules.
- Krebs Cycle (in the mitochondria):
- Each pyruvate is further broken down into acetyl-CoA, which enters the Krebs cycle.
- In the Krebs cycle, acetyl-CoA reacts with oxaloacetate to form citric acid (6-carbon compound), which is then oxidized, releasing CO₂, high-energy electrons, and ATP.
- Electron Transport Chain (ETC) (in the inner mitochondrial membrane):
- The high-energy electrons from NADH and FADH₂ (produced in the Krebs cycle) are transferred through a series of proteins in the mitochondrial membrane.
- As electrons pass through the chain, their energy is used to pump protons (H⁺) across the membrane, creating an electrochemical gradient.
- The flow of protons back through ATP synthase drives the production of ATP.
- Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.
Overall Equation of Aerobic Respiration:
C6H12O6+6O2→6CO2+6H2O+38ATP
This means that one molecule of glucose, when completely oxidized, produces carbon dioxide, water, and a large amount of energy (ATP).
1.2. Anaerobic Respiration
Anaerobic respiration occurs in the absence of oxygen. It is less efficient than aerobic respiration and results in the partial breakdown of glucose to produce energy.
Types of Anaerobic Respiration in Plants:
- Alcoholic Fermentation: In the absence of oxygen, pyruvate is converted into ethanol (ethyl alcohol) and carbon dioxide by the action of enzymes like pyruvate decarboxylase and alcohol dehydrogenase. This process occurs in yeast cells, but in plants, it can happen in root cells when oxygen is scarce (e.g., waterlogged soils).
- Lactic Acid Fermentation: This occurs in some plant tissues under stress, like in anaerobic conditions, where pyruvate is converted into lactic acid (lactate). However, this is more common in animal cells, not typically in plants.
Overall Equation of Alcoholic Fermentation:
C6H12O6→2C2H5OH+2CO2+2ATP
Alcoholic fermentation results in the production of ethanol, carbon dioxide, and a small amount of energy (ATP).
2. Differences Between Aerobic and Anaerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Presence of Oxygen | Requires oxygen | Does not require oxygen |
| End Products | CO₂, H₂O, ATP | Ethanol, CO₂ (or Lactic Acid in some cases), ATP |
| ATP Yield | High (around 38 ATP per glucose molecule) | Low (2 ATP per glucose molecule) |
| Location | Mitochondria | Cytoplasm (for fermentation processes) |
| Efficiency | Very efficient | Less efficient |
3. Respiration in Different Plant Parts
- Leaves: While photosynthesis occurs in the leaves, respiration also takes place in the leaf cells, especially in the mitochondria. This ensures that the plant has a constant energy supply.
- Roots: Roots generally carry out more anaerobic respiration, especially when the soil is waterlogged and oxygen is limited.
- Stems and Flowers: These parts of the plant also respire, ensuring the supply of energy required for growth and development.
4. Importance of Respiration in Plants
- Energy Production: Respiration provides the energy required for various metabolic processes in the plant, such as growth, repair, and reproduction.
- Carbon Dioxide Recycling: Respiration in plants releases CO₂, which is essential for photosynthesis, creating a balance in the ecosystem.
- Support for Growth: ATP produced during respiration is used to build new cells, proteins, and other important molecules.
- Regulation of Metabolism: Respiration helps in maintaining the cellular balance of substances like sugars and proteins.
5. Respiratory Quotient (RQ)
The respiratory quotient (RQ) is the ratio of the volume of CO₂ released to the volume of O₂ consumed during respiration. The RQ value differs depending on the type of substrate used.
- RQ for Carbohydrates: RQ = 1 (because equal amounts of CO₂ and O₂ are involved)
- RQ for Fats: RQ < 1 (as more oxygen is required to oxidize fats)
- RQ for Proteins: RQ ≈ 0.8 (depends on amino acid composition)
The RQ value can provide information about the metabolic pathways used by the plant for energy production.
MCQs – Respiration in Plants
1. Where does aerobic respiration take place in plant cells?
a) Cytoplasm
b) Nucleus
c) Mitochondria
d) Chloroplast
Answer: c) Mitochondria
2. What is the final electron acceptor in the electron transport chain during aerobic respiration?
a) Oxygen
b) Carbon dioxide
c) Glucose
d) NADH
Answer: a) Oxygen
3. Which of the following is produced during alcoholic fermentation in plants?
a) Lactic acid
b) Ethanol
c) Oxygen
d) Water
Answer: b) Ethanol
4. What is the main product of glycolysis in plants?
a) Pyruvate
b) Oxygen
c) Glucose
d) Lactic acid
Answer: a) Pyruvate
5. The ATP yield in anaerobic respiration is:
a) 38 ATP per glucose molecule
b) 2 ATP per glucose molecule
c) 1 ATP per glucose molecule
d) 6 ATP per glucose molecule
Answer: b) 2 ATP per glucose molecule
6. In which part of the plant does anaerobic respiration occur most frequently?
a) Leaves
b) Flowers
c) Roots
d) Stem
Answer: c) Roots
7. Which process generates the most ATP in plants?
a) Glycolysis
b) Krebs Cycle
c) Electron Transport Chain
d) Alcoholic fermentation
Answer: c) Electron Transport Chain
8. During alcoholic fermentation in plants, what is the by-product produced along with ethanol?
a) Carbon dioxide
b) Oxygen
c) Water
d) Glucose
Answer: a) Carbon dioxide
9. What is the RQ value for the oxidation of carbohydrates?
a) 0.8
b) 1
c) 0.5
d) 0
Answer: b) 1
10. The primary function of respiration in plants is to:
a) Produce oxygen
b) Convert sunlight into energy
c) Release energy stored in glucose
d) Absorb water
Answer: c) Release energy stored in glucose