Respiration in Plants: The Energy of Life

Introduction: Why Do Plants Respire?

Just like animals, plants are living organisms that need a constant supply of energy to carry out their life processes, such as growth, absorption of minerals, and transport of substances. Respiration is the biochemical process of breaking down organic food (primarily glucose) to release this energy in a usable form, as ATP (Adenosine Triphosphate). This process occurs in every living cell of the plant, 24 hours a day.

It is important not to confuse respiration with photosynthesis. They are, in many ways, opposite processes:

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Gaseous Exchange in Plants

Plants need to take in oxygen and release carbon dioxide during respiration. Unlike animals, plants do not have a specialized respiratory system. Instead, gaseous exchange occurs through different structures in different parts of the plant.

• In Leaves: Gaseous exchange occurs through tiny pores called stomata on the leaf surface. Oxygen diffuses into the leaf cells, and carbon dioxide diffuses out.
• In Stems: In woody stems, the bark has special openings called lenticels, which are porous tissues that allow for the exchange of gases between the internal tissues and the atmosphere.
• In Roots: The root cells take in oxygen from the air spaces present in the soil. The exchange occurs across the moist surface of the root hairs by diffusion.

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Outline of the Respiration Process

The complete breakdown of one molecule of glucose to release energy is a complex, multi-step process. It can be broadly divided into two main phases: Glycolysis and the Krebs Cycle.

1. Glycolysis (The First Step)

• Location: Cytoplasm of the cell.
• Process: Glycolysis means "splitting of sugar." In this stage, one molecule of glucose (a 6-carbon compound) is broken down into two molecules of pyruvic acid (a 3-carbon compound).
• Oxygen Requirement: This process does not require oxygen and is common to both aerobic and anaerobic respiration.
• Significance: It is the initial stage of respiration that begins the breakdown of glucose and produces a small amount of energy (2 ATP molecules) and pyruvic acid, which can then enter the next stage if oxygen is available.

2. Krebs Cycle (The Citric Acid Cycle)

• Location: Mitochondria of the cell.
• Process: This stage only occurs if oxygen is present. The pyruvic acid produced during glycolysis enters the mitochondria and is completely broken down in a series of cyclic reactions. This process releases carbon dioxide, a large amount of chemical energy (captured in molecules like NADH and FADH2), and a small amount of ATP.
• Significance: The Krebs cycle is the main energy-releasing phase of aerobic respiration. It completely oxidizes the glucose molecule, releasing the stored energy and producing carbon dioxide as a waste product. The high-energy molecules produced here go on to a final step (the Electron Transport Chain) to produce the majority of the cell's ATP.

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Types of Respiration

1. Aerobic Respiration

This is the most common form of respiration and occurs in the presence of oxygen. It involves the complete breakdown of glucose into carbon dioxide and water, releasing a large amount of energy (approximately 38 ATP molecules from one glucose molecule).

Chemical Equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP (Energy) (Glucose + Oxygen → Carbon Dioxide + Water + Energy)

2. Anaerobic Respiration (Fermentation)

This type of respiration occurs in the absence of oxygen. It is an incomplete breakdown of glucose. In plants and yeast, this process is called alcoholic fermentation.

Here, the pyruvic acid from glycolysis is further broken down into ethanol (ethyl alcohol) and carbon dioxide. It releases a much smaller amount of energy (only 2 ATP molecules from one glucose molecule).

Chemical Equation (in Plants/Yeast):

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP (Energy) (Glucose → Ethanol + Carbon Dioxide + Energy)

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Experiments on Respiration

1. Experiment to Show that Carbon Dioxide is Produced During Respiration

• Apparatus: Two flasks (A and B), germinating bean seeds, boiled/dead bean seeds, a beaker of limewater (calcium hydroxide solution).
• Procedure:
  1. In Flask A (experimental setup), place a handful of live, germinating bean seeds.
  2. In Flask B (control setup), place an equal amount of boiled (and cooled) bean seeds. Adding a few drops of an antiseptic like carbolic acid prevents decay.
  3. Cork both flasks and leave them for a few hours.
  4. After some time, pass the gas from each flask through a test tube containing limewater.
• Observation: The gas from Flask A turns the limewater milky (a white precipitate of calcium carbonate is formed). The gas from Flask B shows no change.
• Conclusion: This proves that germinating seeds (which are actively respiring) produce carbon dioxide.

2. Experiment to Show that Heat is Produced During Respiration

• Apparatus: Two thermos flasks (A and B), germinating seeds, boiled/dead seeds, two thermometers.
• Procedure:
  1. Take a handful of germinating seeds and place them in Thermos Flask A. Insert a thermometer so its bulb is surrounded by the seeds. Seal the mouth of the flask with cotton wool.
  2. In Thermos Flask B (control), take an equal quantity of boiled (dead) seeds. Also, insert a thermometer and seal the mouth.
  3. Record the initial temperature in both flasks.
  4. Observe the temperature readings after a few hours.
• Observation: The thermometer in Flask A shows a significant rise in temperature. The thermometer in Flask B shows no change in temperature.
• Conclusion: This demonstrates that germinating seeds produce heat during the process of respiration.