Biochemistry Online: An Approach Based on Chemical Logic

CHAPTER 8 - OXIDATION/PHOSPHORYLATION

D: THE LIGHT REACTION OF PHOTOSYNTHESIS

BIOCHEMISTRY - DR. JAKUBOWSKI

04/15/16

Learning Goals/Objectives for Chapter 8D:

After class and this reading, students will be able to

described the mechanistic similarities between mitochondrial oxidative/phosophorylation in which NADH and FADH₂ are regenerated on reduction of O₂ and the light reaction of photosynthesis in which O₂ and a reducing agent, NADPH are produced;
describe similarities in fluorescence resonance energy transfer and exciton transfer;
describe the difference in properties between chlorophylls acting as antennae and chlorophylls at the reaction center;
describe how sunlight driven excitation of chlorophyll molecules at the reaction center produces an oxidzing agent strong enough to oxide water and form O₂, itself a powerful oxidizing agent;
explain the general flow of electrons from dioxgen to NADP⁺ through a series of mobile and membrane protein bound electron carriers in the Z scheme of electron transport in the chloroplast thylacoid membranes;
explain with picture diagrams how oxidation of H₂O and phosphorylation reactions (to produce ATP) are coupled in in the Z scheme;

D1. Introduction

"Of all the biochemical inventions in the history of life, the machinery to oxidize water — photosystem II — using sunlight is surely one of the grandest." (Sessions, A. et al, 2009)

We have just seen how we can transduce the chemical potential energy stored in carbohydrates into chemical potential energy of ATP - namely through coupling the energy released during the thermodynamically favored oxidation of carbon molecules through intermediaries (high energy mixed anhydride in glycolysis or a proton gradient in aerobic metabolism) to the thermodynamically uphill synthesis of ATP. There is a situation that occurs when we wish to actually reverse the entire process and take CO₂ + H₂O to carbohydrate + O₂. This process is of course photosynthesis which occurs in plants and certain photosynthetic bacteria and algae. Given that this process must by nature be an uphill thermodynamic battle, let us consider the major requirements that must be in place for this process to occur:

A strong oxidizing agent must be formed which can take water and oxidize it to dioxygen. We know that redox reactions occur in the direction of stronger to weaker oxidizing agent (just as acid base reactions are thermodynamically favored in the direction of strong to weak acid). Somehow we must generate a stronger oxidizing agent than dioxygen, which often has the most positive standard reduction potential in tables.
Plants must have high concentrations of a reducing agent for the reductive biosynthesis of glucose from CO₂. The reducing agent used for most biosynthetic reactions in nature is NADPH, which differs from NADH only by the addition of a phosphate to the ribose ring. This phosphate differentiates the pool of nucleotides in the cells used for reductive biosynthesis (NADPH/NADP⁺) from those used for oxidative catabolism (NADH/NAD⁺)
Finally, plants need an abundant source of ATP which will be required for reductive biosynthesis.

We will discuss only the light reaction of photosynthesis which produces these three types of molecules. The dark reaction , which as the name implies can occur in the dark, involves that actual fixation of carbon dioxide into carbohydrate using the ATP and NADPH produced in the light reaction.

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