Question

1. Light reactions:
   • what's the relationship of the nature of sunlight and photosynthetic pigments
   • Understand the relationship between an action spectrum and an absorption spectrum (including Englemann’s experiment)
   • Describe how light reactions convert solar energy to the chemical energy of ATP and NADPH
   • Explain the role of photosystems and their reaction center

Answer 1

Sunlight radiates high energy in the form of photons which is absorbed by photosynthetic pigment molecules. These photons are captured by different pigments absorbing light of different wavelength of electromagnetic spectrum. The energy absorbed by pigments such as chlorophyll, carotenoids, xanthophyll, pheophytin is then used for photosynthesis process.

Action spectrum is the pattern of action in terms of physiological activity conducted by different wavelength of light whereas absorption spectrum is the rate of measurement of light absorption at different wavelengths. Action spectrum an absorption spectrum are almost similar in pattern. Engleman experiments also approved this hypothesis. In order to prove his hypothesis he used green algae under microscope irradiating with different wavelength of ljght. He observed congregation of aerobic bacteria at oxygen releasing place of algal filaments.

Light reaction convert solar energy into chemical energy in the form of ATP and NADPH which at the energy carriers for dark reaction. The energy of photons are absorbed by pigment system PS1 and PS2 having reaction centers of 700 nm and 680 nm respectively.Furthur, sequence of several redox reactions involving many intermediate proteins generates ATP and NAHPH by photo phosphorylation process with oxygen as final electron acceptor.

Photosystems are the group of pigment molecules

or photosynthetic units having light harvesting complexes with a reaction centre. The function of accessory pigments is to absorb high energy radiation and transfer it to the reaction centers to conduct downhill reactions for the synthesis of ATP and NADPH.