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PHOTOSYNTHESIS Class Notes PHOTOSYNTHESIS Photosynthesis is ability of a plant to turn sunlight, air, and water into sugar (energy). The overall reaction is: light chlorophyll H2O -------> CH2O CO2 + + O2 Photosynthesis is vital to life for two reasons –1. The oxygen in the air comes from photosynthesis. The plants continue to replenish the oxygen in the air. –2. All of our food comes directly or indirectly from photosynthesis. Parts of a leaf Stomata palisade layer spongy layer veins chloroplast thylakoids grana stroma The Chloroplast -double membrane, stroma, thylakoid, grana Light Photosynthesis uses visible light. – Only visible light with intermediate wavelengths has enough energy to cause chemical change without destroying biological molecules. What are the colors of the visible spectrum? Why does a leaf look green? . Light is captured by pigments. The main photosynthetic pigment is chlorophyll. There is chlorophyll a, b and c. Chlorophyll a is the major photosynthetic pigment and is found in all photosynthetic plants, protist, and cyanobacteria Carotenoids Carotenoids are accessory pigments found in all green plants. They absorb blue and green wavelengths and give a plant a yellow or orange color. In the fall when chlorophyll breaks down, it is the accessory pigments which give colors of fall. The red color of some autumn leaves is due to the anthocyanin pigments. These are not photosynthetic. Stages of Photosynthesis LIGHT REACTIONS DARK REACTIONS Light absorption Carbon Fixation Electron transport Oxygen production Chemiosmotic ATP synthesis The Energy-Capturing Reactions Light absorption – Chlorophyll a and b are antennae pigments Electron transport – Electrons lost by chlorophyll a end up at NADP – Produce oxygen Chemiosmotic ATP synthesis – Powered by H ion reservoir The Dark reactions use more ATP then NADPH. Each turn of the cycle uses 9 ATP and only 6 NADPH. How are more ATP acquired than NAPH since one ATP is produced in photosystem II and one NADPH is produced in photosystem I? Cyclic and noncyclic electron flow Noncyclic Electron Flow Cyclic Electron Flow The Dark Reactions The dark reactions take place outside the thylakoid membrane in the stroma. The Calvin Cycle converts CO2 to sugar in a three phase process. 1. Carbon fixation - The carbon dioxide is incorporated into an organic molecule (1C + 5C = 6C) called RuBP by the enzyme Rubisco. This 6 carbon structure is unstable and immediately splits into 2, 3 carbon molecules. This is the same molecule that glucose is split into. 2. 3. Reduction – Each 3 carbon molecule is phosphorylated by ATP and NAPH to create G3P. The cycle must turn 3 times for a net gain of one molecule of G3P. Regeneration of CO2 acceptor – The rearrangement of 5 molecules of G3P into 3 molecules of RuBP required 3 more ATP. ( 9 ATP and 6 NADPH yields 1 G3P) What controls photosynthesis? Intensity of light Scarcity of water Ecological Aspects C4 PLANTS - Use more ATP but at high temperatures they can photosynthesize faster -allowing plants to grow and reproduce faster. These include many weeds and important crops such as corn. A special variation of C4 plants are the CAM plants. These are mostly desert plants. These plants keep the stomata closed during the day and open at night. They must conserve water! Vocabulary Glossary – blade – stipules – petiole – vein – midrib of terms Vocabulary carotenoids chlorophyll electromagnetic spectrum granum NADP+ NADPH photon photosystem photosystem I photosystem II pigment stroma thylakoid