Historical Experiments in Photosynthesis
Key terms before we begin
Role of air and the discovery that plants restore air "damaged" by burning candles or breathing animals.
Role of sunlight and green plant parts in purifying air (releasing oxygen).
Production of glucose (stored as starch) in green parts of plants.
Action spectrum of photosynthesis using Cladophora and aerobic bacteria.
Understanding the concept
Scientists spent many years figuring out how plants make food. They used simple experiments to understand the process we now call photosynthesis (a process where plants use light energy to make food).
- Joseph Priestley (1770): He placed a burning candle and a mouse in a closed glass jar (a bell jar). The candle went out and the mouse suffocated because they "damaged" the air. But when he added a mint plant, the candle stayed lit and the mouse lived! He concluded that plants "restore" the air that animals and candles use up.
- Jan Ingenhousz: He repeated Priestley’s experiment but tested it in the dark and in sunlight. He found that plants only purify the air in sunlight. He also used an aquatic plant and saw bubbles forming around green parts in sunlight, which he later identified as oxygen. This proved that sunlight and green parts are essential.
- Julius von Sachs (1854): He proved that when plants grow, they produce glucose (a simple sugar), which is stored as starch. He also confirmed that this happens specifically in the green parts of the plant.
- T.W. Engelmann: He wanted to see which colors of light are best for photosynthesis. He used a prism to split light into a rainbow (spectrum) and shone it on a green alga called Cladophora. He added bacteria that love oxygen to see where they would gather. The bacteria crowded around the blue and red light regions, showing that these colors are the most effective for photosynthesis. This created the first "action spectrum" (a graph showing the rate of photosynthesis at different light wavelengths).
The historical development of our understanding of photosynthesis was driven by key experiments:
- Priestley’s Experiment: Demonstrated that plants have the ability to restore air fouled by burning candles or breathing animals. He hypothesized that plants "restore to the air whatever breathing animals and burning candles remove."
- Ingenhousz’s Experiment: Utilized an aquatic plant to demonstrate that sunlight is essential for the purification of air. He observed the formation of oxygen bubbles specifically around the green parts of the plant in the presence of bright sunlight.
- Sachs’ Evidence: Provided evidence that plants produce glucose during growth, which is stored as starch. He confirmed that the green substance (chlorophyll) is located in specialized bodies (chloroplasts) within the cells.
- Engelmann’s Action Spectrum: Illuminated the green alga Cladophora with light split by a prism into its spectral components. Using aerobic bacteria as a biological indicator for oxygen evolution, he observed that bacteria accumulated primarily in the blue and red regions of the spectrum. This established the first action spectrum of photosynthesis, which roughly corresponds to the absorption spectra of chlorophyll a and b.
Practice questions
Which scientist used aerobic bacteria to detect the sites of oxygen evolution and describe the first action spectrum of photosynthesis?
Jan Ingenhousz showed that sunlight is essential for the plant process that purifies the air.
Ingenhousz observed that bubbles were formed around the green parts of an aquatic plant in the dark.
Which of the following statements correctly describes the findings of Julius von Sachs?
Lock it in
| Scientist | Key Contribution |
|---|---|
| Priestley | Plants restore air "damaged" by candles/animals. |
| Ingenhousz | Sunlight is essential; green parts release oxygen. |
| Sachs | Plants produce glucose, stored as starch in green parts. |
| Engelmann | First action spectrum; photosynthesis peaks in blue/red light. |
- Oxygen source: Initially identified by Ingenhousz as bubbles from green parts.
- Action Spectrum: Engelmann used Cladophora (alga) and aerobic bacteria.
- Starch: The form in which glucose is typically stored in plants.
Images Required
1. Engelmann’s Experiment Setup
Type: diagram
Purpose: To help the student visualize how Engelmann used a prism to split light into a spectrum and used aerobic bacteria as an indicator to identify the regions of maximum oxygen evolution (blue and red light). This is a classic NCERT figure that is essential for understanding the action spectrum.
2. Summary of Historical Experiments
Type: table
Purpose: To provide a quick, comparative overview of the four key scientists and their specific contributions, allowing the student to distinguish between their findings at a glance.