Photosynthesis, Plant Nutrition & Transport

• Photosynthesis Definition: Photosynthesis is the biochemical process by which green plants, algae, and certain photosynthetic bacteria synthesize organic food (glucose/starch) from carbon dioxide and water using light energy absorbed by chlorophyll, releasing oxygen as a byproduct. • Word Equation: Carbon dioxide + Water --[Sunlight / Chlorophyll]--> Glucose + Oxygen • Balanced Chemical Equation: 6CO₂ + 6H₂O --[Light energy / Chlorophyll]--> C₆H₁₂O₆ + 6O₂ • The Raw Materials and Sources: 1. Carbon Dioxide (CO₂): Absorbed from atmospheric air through microscopic pores called STOMATA on the underside of leaves by diffusion. 2. Water (H₂O): Absorbed from soil by root hairs via osmosis, then transported upwards through XYLEM vessels to the leaves. • The Necessary Factors: 1. Sunlight: Provides the radiant energy required to split water molecules (photolysis) and power the endothermic chemical synthesis. 2. Chlorophyll: The green magnesium-containing pigment located in chloroplasts that absorbs blue and red wavelengths of solar light energy. • Fate of Photosynthetic Products: - Glucose: Used immediately in cellular respiration for plant energy; converted into Cellulose for building cell walls; stored as insoluble STARCH in leaves, tubers, and seeds. - Oxygen: Released into the atmosphere through stomata for animal and plant respiration, or used internally by the plant for respiration.

Because glucose is quickly converted to starch for storage in leaves, testing for starch is the standard laboratory method to prove that photosynthesis occurred: • Step-by-Step Practical Procedure: 1. Pluck a green leaf from a plant that has been exposed to bright sunlight for at least 4 to 6 hours. 2. Boil the leaf in a beaker of water for 1 minute: - Purpose: Kills the leaf cells, breaks cell membranes, and denatures enzymes, stopping all chemical reactions. 3. Turn OFF the Bunsen burner flame. Place the boiled leaf into a boiling tube containing ethanol (methylated spirit), and place the tube into the hot water bath for 5–10 minutes: - Purpose: Chlorophyll dissolves in hot ethanol, DECOLORIZING the leaf to a pale white/cream color so color changes can be seen clearly. - CRITICAL SAFETY RULE: Ethanol is highly flammable! Never boil ethanol over an open flame; always use a hot water bath with the burner turned off! 4. Remove the leaf and dip it into a beaker of warm water: - Purpose: Boiling in alcohol makes the leaf brittle and hard; warm water softens the leaf. 5. Spread the softened leaf flat on a clean white ceramic tile and add several drops of Iodine solution: - Observation: The leaf turns BLUE-BLACK. - Conclusion: Blue-black coloration confirms the presence of STARCH, proving photosynthesis occurred. - If the leaf turns yellow-brown (iodine color), no starch is present.

A green leaf is anatomically optimized as a solar food factory: • Leaf Layers (Top to Bottom): 1. Upper Cuticle & Epidermis: Transparent waxy layer preventing excessive water evaporation while allowing light to penetrate. 2. Palisade Mesophyll: Densely packed vertical cylindrical cells packed with abundant chloroplasts. This is the PRIMARY site of photosynthesis. 3. Spongy Mesophyll: Loosely arranged cells with large intercellular air spaces facilitating rapid diffusion of CO₂ and O₂. 4. Lower Epidermis & Stomata: Contains tiny pores called STOMATA, flanked by kidney-shaped GUARD CELLS that open and close to regulate gas exchange and transpiration. • Plant Vascular Bundles (Transport System): - Xylem Vessels: Thick, hollow, dead lignified tubes that conduct water and dissolved mineral salts UPWARDS from the roots through the stem to the leaves in a single direction (Transpiration stream). - Phloem Vessels: Living sieve tube elements with companion cells that transport manufactured soluble food (sucrose and amino acids) from the leaves DOWNWARDS and UPWARDS to storage organs, roots, and growing tips (TRANSLOCATION).