Lecture 37 April 30 2003 R.Jones Chapter 7
1. The Dutch physician Jan Ingenhousz (1730-1809) was the first to show that light was involved in this process of air purification when he reported that light, and bright light-bright sunlight after dawn and before dusk and not in shade, was needed for plants to purify air and he further showed that leaves and green stems but not other parts of the plant were involved in air purification. CO2 and O2 were still not identified as being involved although CO2 was discovered by the English chemist Joseph Black in 1755.
2. The French chemist Antoine Lavoisier (1743-94) was the first to correctly identify CO2 and O2 as the gaseous components involved in photosynthesis and by the beginning of the 19th century it was accepted that the equation
CO2 + H2O-----> CH2O + O2 (CH2O being a generic formula for a simple sugar).
3. Englemann in the mid 1800s did an experiment with Spirogyra and motile bacteria that require oxygen. He illuminated the Spirogyra filament with white light that was resolved into the component wavelengths of the visible spectrum (red, orange, yellow, green, blue, indigo and violet) and showed that light in the red and blue regions of the spectrum were effective in evolving oxygen.
4. The next important experiment was carried out by an English plant biologist Robin Hill who showed in 1937 that isolated chloroplasts could carry out photosynthesis in the test tube when illuminated and they could be made to reduce a chemical electron acceptor. Later at Cal in 1954 Daniel Arnon and his group showed that this property of electron transport in the chloroplast was confined to the membranes, and Arnon's group showed that carbon fixation was confined to the stroma of the organelle.
5. The experiments of Hill and Arnon showed that light energy could be converted to chemical energy and that one of the forms of chemical energy that is formed in chloroplasts in the light is ATP. It is now known that the so-called light reactions of photosynthesis produce ATP and one of the reduced co-enzymes NADPH (being reduced NADPH now has a higher energy level). The reactions of photosynthesis like most of the other reactions in cells dealing with energy transfer belong to the oxidation/reduction class of reactions. In these reactions one partner becomes reduced (i.e. gains an electron and has a higher energy level) while the other partner becomes oxidized (loses an electron and is at a lower energy level).
6. ATP and NADPH formation occur on the thylakoid membranes of the chloroplast. These membranes act like a battery to produce chemical energy. Light causes the splitting of water into O2, e- & H+. The O2 is lost from the chloroplast by diffusion. The H+ accummulate on the inside of the thylakoid membrane whereas the electrons are transported across the membrane into the stroma of the chloroplast . As electrons move across the thylakoid membrane more H+ are transportd into the thylakoid. The result is that the thylakoids accumulate H+ and the stroma accumulates electrons. This difference in H+ concentration and electrical charge is used to synthesize ATP and form NADPH.
7. To summarize, light splits H2O into O2, H+ and e-. A difference in the concentration of H+ and e- across the thylakoid membrane is used to generate ATP and NADPH. O2 diffuses out of the chloroplast and ATP and NADPH accumulate in the stroma.