Photosynthesis cont'd Calvin Cycle (non-light requiring) Energy stored in ATP and NADPH from light reaction now used to make sugars from CO2 & H2O Carbon Fixation Starts with a type of sugar that has 5 carbon atoms and a phosphorus ribulose phosphate ATP from the light reaction reacts to add a 2nd P to the molecule to make ribulose bisphosphate (RuBP to its friends) CO2 and H2O add the 6th carbon -- molecule immediately splits into 2 - 3 carbon molecules - phosphoglycerate (PGA - NOT to be confused with a pro golf organization) More (2) ATPs from the dark reaction (rxn.) is used to add a P to (2) PGAs, making (2) diPGAs NADPH from dark rxn. Makes phosphoglyceraldehyde (PGAl). At the same time P and H2O are released from the molecule For every 12 PGALs formed, 2 return to the cycle and combine with other molecules to make more ribulose phosphate which combine with CO2 and H2O The other 10 PGAls join to make 5 6 carbon sugars with 2 Ps on it - fructose 1,6-phosphate (meaning that the Ps are on the 1st and 6th carbons. One of the 5 molecules loses the two Ps forming glucose. Glucose is the sugar product that leaves the chloroplast and is considered to be the primary product of photosynthesis. The other 4 fructose molecules lose one of their Ps and undergo several chemical changes creating 2, 3, 4, and 5 phosphorylated sugar molecules that ultimately become more ribulose phosphate It takes 6 turns of the the Calvin Cycle for a molecule of new sugar to be formed. Each step of the reaction is catalyzed by a specific enzyme. The process is regulated by how much of any enzyme is present and if it is in an active or inactive state. The key enzyme is the one that fixes the carbon molecule onto RuBP: RuBP carboxylase-oxygenase (Rubisco) is one of the most important and abundant proteins in the world. RuBP has a dual role, when CO2 is limiting, Rubisco can oxygenate RuBP in a process called photorespiration This process is thought to serve as a means of conserving some energy compounds when CO2 is limited in a leaf but light is high and temperatures are favorable for photosynthesis. NO CO2 is fixed, NO storing of energy, O2 is used, not given off. Basically a wasteful process. Two types of plants can compensate for this. Most plants are C3 i.e. carbon is attached to a 3 carbon compound C4 (corn, sugarcane, sorghum are this type) which uses a special anatomy Kranz Anatomy. CO2 enters the leaf , energy from the light rxn is used to temporarily attach it to a 3 carbon compound phosphenolpyruvate (PEP) to form malic or aspartic acid, PEP is transported to special cells deep inside the leaf where it is released as CO2 and fixed via Calvin Cycle rxns. The advantage is that deep in the leaf, O2 is less concentrated, therefore, CO2 can favorably compete for rubisco. Photosyn. favored over photoresp. Not cheap, costs energy to have C4 anatomy. 2nd type of anatomy is Crassula Acid Metabolism (CAM) anatomy. Succulent plants (right) such as cacti, orchids, lilies, euphorbias are often CAMs. CAMs open leaf pores at night when cool to transpire and take in air. CO2 temporarily attached to PEP to form malic acid. During the day when it is hot and the leaf pores are closed to conserve H2O, malate releases CO2 and photosynthesis can take place without supply of air to the plant.