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Photosynthesis is a physico-chemical process by which green plants use light energy (solar energy) to synthesize organic compounds, making them autotrophs.
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It is the basis of life on earth, as all living forms ultimately depend on sunlight for energy.
Importance of Photosynthesis
- It is the primary source of all food on earth.
- It releases oxygen into the atmosphere.
Experiments Related to Photosynthesis
1. Variegated Leaf Experiment
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Take a variegated leaf (or a leaf partially covered with black paper) exposed to light.
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Test the leaf for starch. The results show that photosynthesis occurs only in the green parts of the leaves in the presence of light.
2. Half-Leaf Experiment
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Enclose part of a leaf in a test tube containing KOH-soaked cotton (which absorbs CO2).
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Expose the other half of the leaf to air and place the setup in light for some time.
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Test the leaf for starch. The exposed part tests positive for starch, while the portion in the tube tests negative, proving that CO2 is required for photosynthesis.
Early Experiments
Experiments by Joseph Priestley (1770)
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Priestley performed experiments to prove the role of air in the growth of green plants and discovered oxygen in 1774.
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He observed that a candle burning in a closed bell jar gets extinguished, and a mouse suffocates in a closed jar, concluding that a burning candle or breathing animal damages the air.
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By placing a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn. He hypothesized that plants restore air damaged by animals or candles.
Experiments by Jan Ingenhousz (1730–1799)
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Ingenhousz conducted Priestley’s experiment in darkness and sunlight, showing that sunlight is essential for plants to purify air fouled by candles or animals.
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Using an aquatic plant, he observed that in bright sunlight, small bubbles formed around green parts, but not in the dark. He identified these bubbles as oxygen, proving that only green parts of plants release O2.
Experiments by Julius von Sachs (1854)
Sachs proved that:
- Glucose is produced during plant growth and usually stored as starch.
- Chlorophyll is located in chloroplasts.
- Glucose is made in the green parts of plants.
Experiments by T.W. Engelmann (1843–1909)
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Engelmann split light using a prism and illuminated a green alga Cladophora) in a suspension of aerobic bacteria to detect O₂ evolution sites.
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Bacteria accumulated mainly in the blue and red light regions of the spectrum, describing the first action spectrum of photosynthesis, resembling the absorption spectra of chlorophyll a and b.
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By the mid-19th century, it was known that plants use light energy to make carbohydrates from CO₂ and H₂O. The empirical equation is:
Where [CH₂O] represents a carbohydrate.
Experiments by Cornelius van Niel
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Van Niel studied purple and green bacteria, demonstrating that photosynthesis is a light-dependent reaction where hydrogen from an oxidizable compound reduces CO₂ to carbohydrates.
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In plants, H₂O is the hydrogen donor, oxidized to O₂. In purple and green sulfur bacteria, H₂S is the hydrogen donor, producing sulfur or sulfate, not O₂.
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Van Niel inferred that O₂ evolved by green plants comes from H₂O, not CO₂, later confirmed by radioisotopic techniques. The correct equation for photosynthesis is:
