Question

Chemistry 430: Metabolism Spring 2020 4. How many photons must be absorbed by photosynthetic complexes to generate the energy needed to operate a full Calvin cycle, i.e. the fixation of 3 CO2 molecules? Show your reasoning. Assume that the H+ gradient is transduced at a rate of 4 H/ATP. Calvin Cycle costs 6 NADPH & 9 ATP 5. Do the photons above all have to impact the chlorophyll "special pair" directly? Briefly explain. 6. Diuron is an algicide that prevents plastoquinone from binding to photosystem II. Predict the effects of this inhibitor on the following processes. Process Effect of Diuron increase, decrease, no effect) ATP synthesis Oz production NADP reduction Decrease Increase No effect

Answer 1

4.To fix one Co2 molecule ,8 photons are needed .To make a glucose molecule you need 6 Co2 molecule i,e. atleast 48 photons will be needed .As photosynthesis is in two step:- electron transport.and carbon fixation.The calvin cycle as the carbon fixation is called requires 9 ATP to transfer dihydroxy acetone into the cytosol. Two processes are required to make one glucose,so this is overall cost of 18 ATP and 12 NADPH which is equivalent of 36 ATP. In the photosystem II and I , 8 photons of light are use to produce approximately 3 ATP and 2 NADPH which is again equivalent to 6 ATP. So, thus can say a regular C3 pathway require 48 photons to complete a glucose molecule.

5. No not all photon impact 'special pair ' directly because it transfer light energy to a dimer of chlorophyll pigment molecules near the periplasmic side of the membrane.This dimer is called a special pair because of its fundamental role in photosynthesis.This special pair is slightly different  in PSI and PSII. This pair of chlorophyll molecules absorbs photon at 870 nm or 960 nm depending on the species and thus is called P870( for Rhodobacter sphareiodes ) or P960 ( for Blastochloris viridis) with P standing for pigment.