You want to confirm the movement of oxygen atoms through the

You want to confirm the movement of oxygen atoms through the photosynthetic reactions. You can radiolabel with O^18 isotopes, and provide the plants with 1) radiolabeled H_2O_18 or 2) radiolabeled C(O^18)_2. For these two conditions, where would you find the radioactive atoms after photosynthesis has proceeded? Some plants have gene variants that allow them to be atrazine resistant. What protein(s) would have altered amino acid sequences as a result of these genetic variations? Why? Consider looking at a purple flower (and referring to Figure 10-7): Approximately which wavelengths of light are absorbed by the flower? Which wavelengths of light are reflected/transmitted to your eye? In lab you will measure the volume of gas produced by plants during photosynthesis. If the plants are sealed inside a respirometer tube, how will they continue to obtain carbon dioxide? Increased CO_2 in our planet\'s atmosphere is leading to ocean acidification. Write the chemical equation showing why this happens.

Solution

Ans. 1. Radiolabeled O¹⁸ of H₂O is present in O₂ gas released during photosynthesis. It is because water undergoes photolysis at photosystem II to produce free electrons (that further carries energy through rest of photosynthesis pathways) and O₂ (released as byproduct of photosynthesis).

Radiolabeled O¹⁸ of CO₂ is present in carbohydrates (say, phosphoglycerate in C3 cycle; or glucose, etc.) because CO₂ is incorporated into C₃ ,C₄ cycles, etc. to synthesize carbohydrates.

Ans. 2. Atrazine resistant plants have altered amino acid sequence of D1 protein, a product of gene psbA gene. D1 protein, a part of PSII, is the target site for atrazine. Binding of atrazine to D1 inhibits electron transfer through PS II (Q_A to Q_B), thus also inhibits photosynthesis, killing the plant.

A resistant plant has altered D1, that no longer bind to atrazine. SO, such plants remain unaffected to the herbicide.

Ans. 3. A purple flower appears so because it reflects off ‘purple light’. Purple light is a mixture of red and blue light. So, the reflected wavelengths are for blue (450.-495 nm) and red (620-750 nm) color.

The remaining wavelengths of the visible spectrum violet (380-450 nm), green (495-570 nm), yellow (570-590 nm), and orange (590-620) are absorbed by the petals surface.   

Ans. 4. The plant can continue to obtain CO₂ from the air already present in the tube before sealing it. Since, the tube in evacuated, some air remains in it even after placing the study plant. That air has sufficient CO₂ for plant to carry out photosynthesis for some minutes.

Ans. 5. CO₂ + H₂O ------> H₂CO₃ + H⁺ + HCO₃^-

Carbon dioxide and water form carbonic acid (H₂CO₃). Many organisms have carbonic anhydrase that catalyze this reaction. Moreover, this reaction occurs spontaneously without enzyme requirement, too. The resultant carbonic acid dissociates into proton (H⁺) and bicarbonate ion (HCO₃^-). The proton donated to the aqueous medium lowers its pH or make the solution acidic. Therefore, the process lads to ocean acidification.