Photosynthesis-1 - NEET Biology Class 11 Free MCQ Test with solutions

T. W. Engelmann was the first to describe the action spectrum of photosynthesis. He conducted an experiment using a prism to split light into its different wavelengths and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria accumulated in regions where oxygen was released in greater amounts — primarily in the blue and red regions of the spectrum. This demonstrated that these wavelengths are the most effective for photosynthesis.

Topic in NCERT: Engelmann's Experiments on Photosynthesis

Line in NCERT: "Using a prism he split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. A first action spectrum of photosynthesis was thus described."

In the half-leaf experiment, part of a leaf is enclosed in a bottle containing KOH (Potassium Hydroxide) soaked cotton, while the other part remains exposed to air. KOH absorbs carbon dioxide from the air inside the bottle. After testing the leaf with iodine, only the portion exposed to normal air turns blue-black, showing starch formation. The enclosed part remains unstained, proving that CO₂ is essential for photosynthesis.

Topic in NCERT: EARLY EXPERIMENTS

Line in NCERT: "Another experiment you may have carried out where a part of a leaf is enclosed in a test tube containing some KOH soaked cotton (which absorbs CO2), while the other half is exposed to air."

Jan Ingenhousz demonstrated that sunlight is essential for photosynthesis. He placed a green aquatic plant in sunlight and observed the formation of oxygen bubbles around the green parts, but no bubbles appeared when the plant was kept in darkness. This proved that light is necessary for oxygen evolution and, hence, for photosynthesis.

Topic in NCERT: EARLY EXPERIMENTS

Line in NCERT: "Ingenhousz in an elegant experiment with an aquatic plant showed that in bright sunlight, small bubbles were formed around the green parts while in the dark they did not."

Here,

• Total molecules utilized = 6 CO₂ + 12 H₂O = 18
• Total molecules produced = 1 glucose + 6 H₂O + 6 O₂ = 13
• Therefore, total number of molecules involved = 18 + 13 = 31

The chemiosmotic hypothesis proposed by Peter Mitchell explains ATP synthesis through the movement of protons (H⁺ ions) across a membrane, creating a proton gradient.

• In photosynthesis, this occurs across the thylakoid membrane during the light reactions.
• In respiration, it happens across the inner mitochondrial membrane during oxidative phosphorylation.
• Thus, ATP synthesis by the chemiosmotic mechanism occurs in both photosynthesis and respiration.

Topic in NCERT: Chemiosmotic Hypothesis

Line in NCERT: "Like in respiration, in photosynthesis too, ATP synthesis is linked to development of a proton gradient across a membrane."