• Plastoquinone uses the released energy to transport protons across the thylakoid membrane, building up a proton electrochemical gradient. • ATP synthase uses this force to phosphorylate ADP, producing ATP.
The Z Scheme • At the end of photosystem II’s ETC, the electron is passed to a protein called plastocyanin . • Plastocyanin carries the electron back across the thylakoid membrane and donates it to photosystem I, thus physically linking the two photosystems. • Electrons from PC replace electrons from the P700 pair of chlorophyll molecules in the photosystem I reaction centre. • These electrons enter an ETC, then are eventually passed to ferredoxin and used to reduce NADP + to NADPH.
Cyclic Photophosphorylation • No NADP+ • Provides extra ATP required by Calvin cycle
Oxygenic and Anoxygenic Photosynthesis • Photosystem II “splits” water to replace its lost electrons and in the process produces oxygen: 2 H 2 O 4 H + + 4 e – + O 2 • Photosystem II is the only known protein complex able to oxidize water in this way. • Purple non-sulphur and purple sulphur bacteria, with their single photosystem, cannot oxidize water and thus perform anoxygenic photosynthesis .
Purple sulfur bacteria
The Importance of Oxygenic Photosynthesis • The oxygen released from oxygenic photosynthesis was critical to the evolution of life as we know it. – O 2 was almost nonexistent on Earth before enzymes evolved that could catalyze the oxidation of water.
Photosynthesis consists of 2 linked sets of reactions: the light-dependent reactions, and the light-independent reactions of the Calvin cycle. In eukaryotic cells, both processes take place in chloroplasts. The light-dependent reactions transform the energy in sunlight to chemical energy in the form of electrons with high potential energy. These electrons are used to produce ATP and the electron carrier NADPH. Then the reactions of the Calvin cycle use the ATP and NADPH to reduce carbon dioxide to carbohydrate. Summary
THE CARBON-FIXATION REACTION Next Lecture:
You've reached the end of your free preview.
Want to read all 35 pages?
- Fall '17
- Photosynthesis, Dr. L. Campbell