Plant Nutrition

Photosynthesis is the process by which green plants create food (glucose) from Carbon (IV) Oxide and water using light energy and chlorophyll. The term "photosynthesis" comes from two Greek words: "photo" meaning light, and "synthesis" meaning building up.

Symbol Equation:

6CO2 + 6H2O → C6H12O6 + 6O2

Word Equation:

Carbon (IV) Oxide + water → Glucose + oxygen

Requirements for Photosynthesis

  1. Carbon (IV) Oxide: This gas enters the plant through the stomata on the leaves and moves into the mesophyll cells where chloroplasts are located.
  2. Water and Mineral Salts: Absorbed by the roots from the soil, water is transported through the xylem to various parts of the plant.
  3. Sunlight: Captured by the chlorophyll in the leaves, sunlight provides the energy needed for photosynthesis.
  4. Optimum Temperature: Enzymes that help photosynthesis work best within a specific temperature range.
  5. Chlorophyll: A green pigment in the mesophyll cells that is essential for photosynthesis.

The main product of photosynthesis is glucose (sugar), and oxygen is released as a by-product. Photosynthesis is an endothermic reaction and consists of two main stages:

  1. Light-dependent reaction
  2. Light-independent reaction

Light-dependent Reaction

This stage of photosynthesis requires light.

Steps:

  1. Chlorophyll Activation: Chlorophyll absorbs light energy, becoming energized.
  2. Water Splitting (Photolysis): Light energy splits water molecules into hydrogen ions (H+) and hydroxide ions (OH-).
    4H2O → 4H+ + 4OH-
  3. The hydroxide ions recombine to form water, releasing oxygen as a by-product.
    4OH- → 2H2O + O2
  4. Hydrogen ions reduce coenzyme NADP, and ATP is produced to store energy for the next stage.

Light-independent Reaction

This phase happens in the chloroplast stroma and does not require light. In this stage, hydrogen ions and carbon dioxide combine to form simple sugars, which are the foundation for carbohydrates, lipids, and proteins.
4H+ + CO2 → CH2O + 2H2O

Photosynthesis Experiments

Testing for Starch in a Green Leaf

Materials: Fresh green leaf, beaker, burner, boiling tubes, white tile, iodine solution.

Method:

  1. Boil the leaf for 4–6 minutes to kill cells and release starch.
  2. Dip the leaf in hot alcohol to remove its green pigment.
  3. Place the leaf in hot water to soften it.
  4. Place the leaf on a white tile and add iodine solution.

Observation: The leaf turns blue-black, indicating starch is present.

Conclusion: Starch is produced in the leaf through photosynthesis.

Experiment to Show Light is Required for Photosynthesis

Materials: Potted plant, black paper, iodine solution.

Method:

  1. Place the plant in darkness for 24–48 hours to remove starch.
  2. Cover part of a leaf with black paper, then place the plant in sunlight for 1–3 hours.
  3. Remove the paper and test the leaf with iodine solution.

Observation: Only the part exposed to light turns blue-black, proving light is necessary for photosynthesis.

Experiment to Prove Oxygen is Produced During Photosynthesis

Materials: Water plant, funnel, beaker, test tube, glowing splinter.

Method:

  1. Place the water plant in a beaker filled with water.
  2. Invert a test tube over the funnel and expose the setup to sunlight.
  3. Collect the gas and test with a glowing splinter.

Observation: The splinter relights, confirming oxygen is produced during photosynthesis.

Experiment to Show Chlorophyll is Necessary for Photosynthesis

Materials: Variegated plant.

Method:

  1. Expose the plant to sunlight for a few hours.
  2. Test the leaf for starch using iodine solution.

Observation: Only the green parts of the leaf turn blue-black, showing that chlorophyll is essential for photosynthesis.

Significance of Photosynthesis

  1. Provides energy for plant metabolism.
  2. Produces important organic compounds, including carbohydrates, lipids, and proteins.
  3. Removes Carbon (IV) Oxide from the atmosphere, helping to purify the air.
  4. Supports life by providing food and oxygen for all living organisms.

Chemosynthesis

Chemosynthesis is the process by which non-green organisms, such as Nitrosomonas, Nitrobacter, and iron bacteria, produce organic compounds from inorganic substances like Carbon (IV) Oxide and water. This process uses energy derived from the oxidation of compounds like ammonia and hydrogen sulfide.