Formation of organic substances in leaves
Photosynthesis is the formation of organic substances from inorganic ones in green plants, using sunlight and chlorophyll, with the release of oxygen into the air. The word comes from Greek “photos” (light) and “synthesis” (putting together). The root absorbs water and mineral salts that reach the leaf through the stem; carbon dioxide enters from the air through the stomata. In the chloroplasts of mesophyll cells, using light energy, water and carbon dioxide first make sugar and then starch, while oxygen leaves through the stomata. In short: carbon dioxide + water — (light, chlorophyll) → organic substance (sugar) + oxygen; in chemical notation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Sugar dissolves in water and is carried through the sieve tubes to all organs; the surplus is stored in seeds, fruits, roots and tubers. Green plants are autotrophs: they turn light energy into organic matter, and other living things use this matter and the oxygen. With enough light, warmth, water, salts and CO₂ much organic matter is made — that is why conditions are created artificially in greenhouses. The Russian scientist K. Timiryazev studied photosynthesis in depth.
Experiment (with the teacher): cover part of a leaf with black paper and keep it in the light for 1–2 days; then pick it, decolorize it in alcohol in a hot-water bath (teacher only, no open flame!), rinse with water and add iodine. Draw the result. Extra: watch elodea in an aquarium give off bubbles (oxygen) in the light. Safety: alcohol is flammable — keep it away from flames, only the teacher handles it, do not breathe the vapour; iodine must not be swallowed and stains clothes; never focus sunlight with a magnifier.