Biology · Photosynthesis in Higher Plants · NEET
Joseph Priestley (in 1770) is credited with the first key experiment. He put a burning candle in a closed bell jar and it went out; a mouse in the jar suffocated. He said the air was 'damaged'. But when he kept a mint plant in the same jar, the candle kept burning and the mouse stayed alive. His conclusion: plants restore to the air whatever burning candles and breathing animals remove. He did NOT know sunlight was needed. Priestley also discovered oxygen later, in 1774.
Priestley only showed that plants 'purify' air, but he did not know light was needed. Jan Ingenhousz used a similar setup once in the dark and once in sunlight, and proved that sunlight is essential for this air-purifying process. Using an aquatic plant he showed small oxygen bubbles form around the GREEN parts in bright light, but not in the dark. So Ingenhousz added two facts: light is essential, and only green parts release oxygen.
In 1854 Julius von Sachs gave evidence that glucose is produced when plants grow, and that glucose is usually stored as starch. He also showed the green substance (chlorophyll) sits in special bodies inside cells (later called chloroplasts), and that glucose is made in the green parts. So von Sachs = the food (glucose/starch) step.
T.W. Engelmann split light with a prism into its colours and shone it on the green alga Cladophora placed in a suspension of aerobic bacteria. The bacteria moved to where oxygen was released (they detect O2). Bacteria collected mainly in the BLUE and RED regions. This gave the first action spectrum of photosynthesis, which roughly matches the absorption spectra of chlorophyll a and b.
F.F. Blackman gave the Law of Limiting Factors in 1905. It states: if a process is affected by more than one factor, the rate is set by the factor nearest its minimum value. Example: even with a green leaf, good light and enough CO2, a plant will not photosynthesise if temperature is very low. Fixing that lowest factor (temperature) restarts the process.
The order tested in NEET is: role of air (Priestley, 1770) -> release of oxygen needing light (Ingenhousz) -> production of glucose/starch (von Sachs, 1854) -> absorption/action spectrum of chlorophyll a and b (Engelmann) -> Law of Limiting Factors (Blackman, 1905). Remember it as Air, Oxygen, Glucose, Spectrum, Factors.
One scientist cultured Cladophora in a suspension of aerobic bacteria and illuminated the culture by splitting light through a prism. He observed that bacteria accumulated mainly in the region of:
Select the correct sequence of experiments that led to a gradual understanding of photosynthesis in green plants.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Priestley only showed that plants restore 'damaged' air. It was Jan Ingenhousz who later proved that sunlight is essential, by comparing the same setup in the dark and in sunlight.
Jan Ingenhousz. With an aquatic plant he saw oxygen bubbles form around the green parts in bright sunlight, but not in the dark.
It gave the first action spectrum of photosynthesis, showing that blue and red light drive the most O2 release. This roughly matches the absorption spectrum of chlorophyll a and b.
When a process depends on several factors, its rate is set by the factor that is nearest to its minimum (lowest) value.
Van Niel (a microbiologist) later showed photosynthesis is a light-dependent reaction where hydrogen from an oxidisable compound reduces CO2. He came after these five, and his work hinted that the O2 released comes from water, not CO2.
