Photosynthesis – Early Experiments: NEET MCQs with Answers

Explanation: Green sulphur bacteria use H₂S rather than H₂O as the hydrogen donor, so they deposit sulphur instead of liberating oxygen. Nostoc (a cyanobacterium), Chara (an alga) and Cycas (a gymnosperm) all split water and evolve O₂.

Explanation: Rhodospirillum is a purple non-sulphur photosynthetic bacterium that uses hydrogen donors other than water, so no oxygen is evolved — the photosynthesis is anoxygenic. Spirogyra, Chlamydomonas and Ulva are green algae and carry out oxygenic photosynthesis.

Explanation: The oxygen released in photosynthesis comes entirely from the photolysis of water, not from carbon dioxide. Therefore, if the evolved O₂ is enriched in the heavy isotope ¹⁸O, the water supplied to the plant must have contained ¹⁸O. This was demonstrated by Ruben, Hassid and Kamen (1941) using heavy-isotope-labelled water, confirming water as the source of photosynthetic oxygen.

Explanation: Oxygenic photosynthesis occurs in cyanobacteria, algae and green plants, all of which possess both PS-I and PS-II and use water as the electron donor. Oscillatoria is a cyanobacterium (blue-green alga) and therefore evolves oxygen. Chromatium (purple sulphur bacterium), Rhodospirillum (purple non-sulphur bacterium) and Chlorobium (green sulphur bacterium) are all anoxygenic photosynthetic bacteria.

Explanation: Rhodospirillum is a purple non-sulphur bacterium and a free-living nitrogen fixer. Like other purple and green photosynthetic bacteria it possesses only a single photosystem and uses hydrogen, organic acids or reduced sulphur compounds — not water — as the electron donor. Consequently no oxygen is released, i.e. its photosynthesis is anoxygenic. Spirogyra, Chlamydomonas and Ulva are green algae and are oxygenic.

Explanation: Green sulphur bacteria perform anoxygenic photosynthesis. They cannot use water as an electron donor because they lack the oxygen-evolving complex (and PS-II) needed to oxidise water; instead they use H₂S or other reduced sulphur compounds, depositing elemental sulphur rather than releasing O₂. Nostoc (a cyanobacterium), Cycas (a gymnosperm) and Chara (a green alga) all split water and therefore liberate oxygen.

Explanation: This describes Engelmann's classic experiment. He used a prism to split white light into its spectral components and illuminated the filamentous green alga Cladophora suspended in a culture of aerobic bacteria (Azotobacter). Since the bacteria move towards regions of higher oxygen concentration, they act as living oxygen detectors. They accumulated mainly in the blue and red regions of the split spectrum, showing that these wavelengths drive maximum oxygen evolution. This gave the first action spectrum of photosynthesis, which closely matches the absorption spectrum of chlorophyll-a.

Explanation: Hatch and Slack worked out the four-carbon dicarboxylic acid pathway in sugarcane and maize, and it is named after them. Calvin described the C₃ cycle, Hill demonstrated oxygen evolution by isolated chloroplasts, and Arnon discovered photophosphorylation.

Explanation: Calvin worked with the unicellular green alga Chlorella, which could be grown in uniform suspension and killed instantly, making very short exposures possible. Within seconds of supplying ¹⁴CO₂ the label appeared in a three-carbon compound, 3-phosphoglyceric acid, which gave the C₃ pathway its name.

Explanation: Calvin exposed the alga Chlorella to radioactive ¹⁴CO₂ for progressively shorter periods, killed the cells and separated the labelled compounds by paper chromatography followed by autoradiography. By identifying which compound became labelled first he was able to trace the sequence of the carbon fixation pathway.

Explanation: Ruben and Kamen used the heavy isotope to settle the origin of photosynthetic oxygen. When water labelled with ¹⁸O was supplied the oxygen evolved carried the label, whereas labelled carbon dioxide did not produce labelled oxygen. This established that the oxygen comes from the photolysis of water and not from carbon dioxide.

Explanation: Diatoms are the chief (primary) producers in the oceans, contributing the largest share of marine photosynthesis.

Explanation: Green sulphur bacteria carry out anoxygenic photosynthesis (using H₂S, releasing sulphur, not O₂). Cycas, Nostoc, and Chara are all oxygenic.

Explanation: The red drop and Emerson enhancement effect revealed that two photosystems (PS-I and PS-II) operate simultaneously and cooperatively in photosynthesis.

Explanation: Rhodospirillum (a purple non-sulphur bacterium) performs anoxygenic photosynthesis (no O₂ released). Spirogyra, Chlamydomonas, and Ulva are oxygenic.