Question

Lab 7 7. In this experiment, DCPIP was used to indirectly measure photosynthetic rate. If you were asked to design an experiment so that photosynthesis could be measured directly, what factors might you measure? 8. Compare your results for the absorption spectrum and the action spectrum. Explain any discrepancies between the wavelengths of light that are absorbed maximally and those wavelengths that cause the greatest photosynthetic activity. (Note that the wavelengths of light transmitted by each coloured filter can be found in Table 7-1).

Answer 1

Ans. 7)To directly measure the photosynthesis, we can calculate following:

• CO₂ consumption
• O₂ production
• Carbohydrates production
• Increased in dry weight

As all the above event takes place during photosynthesis we can design an experiment in which we calculate any of these perimeter. For example we can calculate the CO₂ consumption by means of an IRGA (Infra-Red Gas Analyser) which can compare the CO2 concentration in gas passing into a chamber surrounding a leaf and the CO2 leaving the chamber.

Similarly we can design experiments to measure the Oxygen production. To detect carbohydrate presence by doing iodine test, iodine measures the presence of starch so if starch is produced then leaf will turn blue-black color.

Ans. 8) when we compare the absorption spectrum and action spectrum, it is clear from the experiment that in blue light absorbption is max than red light and leat in green light. It means plant absorbs blue light wavelength for photosynthesis. Similarly according to the DCPIP experiment it is clear that in red light the action i.e, photosynthesis occur. DCPIP loss it's color as it get reduced by the photosynthesis reducing agents so the absoption will decrease.

The reason behind having different wavelengths for absorption and action is, in choloplast to start photosynthesis first the centermost electron of chlorophyll should be exited, this is known as reaction centre. As plant need red light for exitation, when blue light are absorbed by leaves this light have to pass through many diffrent organelles and reaches the reaction center, so its wavelengths increase or it's frequency decreases. If we directly supply red light then it may convert into far red light when it reaches the reaction centre. Therefore action spectrum is red light and absorption spectrum is blue light.