 Determine how the plant utilizes the energy from the light dependent reactions to build sugars from CO 2.  Identify some common adaptations to this metabolic pathway which allow plants to thrive in various environments.

 L6MI L6MI

 The following reactions all take place in the Stroma of the chloroplast.

 ATP and NADPH produced from the light dependent reactions are used to provide energy for synthesis of sugar from molecules of CO 2. This is termed carbon fixation.  This occurs in a cycle, whereby the primary reactant, a 5- carbon molecule termed ribulose bisphosphate (RuBP), is regenerated constantly.  For every three CO 2 molecules that are fixed, one 3-carbon sugar is produced that can leave the cycle.

 Rubisco catalyzes the addition of CO 2 to RuBP, and a 6-Carbon intermediate breaks into two three carbon molecules called 3-Phosphoglycerate (3PG).  Think of these molecules as being low energy; they need to be energized to be useful to the plant.

RuBisCO (Ribulose Bisphosphate Carboxylase/Oxygenase) is an enzyme that catalyzes the addition of CO 2 to RuBP, the primary reactant of the Calvin Benson Cycle.

 Each of the 3PG molecules is phosphorylated by ATP, and then reduced by NADPH into a sugar called Glyceraldehyde 3-phosphate (G3P).  If RuBisCO catalyzes three carboxylations, how many G3P molecules would we have?  fun fact: RuBisCO is slow, and can only catalyze about 3-10 molecules of CO 2 per second per enzyme.

 If the cell used all 6 molecules of G3P produced from 3 CO 2 molecules, it would run out of RuBP very quickly.  Rather, 5 of the G3P molecules are used to regenerate the 3 molecules of RuBP necessary for the reaction.  This means, for every three CO 2 molecules fixed, only one G3P molecule is available to be used for other purposes.

 RuBisCO not only catalyzes the addition of CO 2 to RuBP, it can also catalyze the addition of oxygen. No useable sugar results from this reaction, though energy is still used.  This is termed photorespiration.  In this way, CO 2 and O 2 are competing for the active sites of RuBisCO.  What conditions do you think could affect the rate of photorespiration?

 The main factor that affects the rate at which photorespiration occurs is O 2 concentration vs CO 2 concentration.  O 2 wins out in high temperatures, or in periods of water stress for the plant.  Plants that undergo too much photorespiration are not productive, and will die if they can’t synthesize sugars.

 The “normal” form of photosynthesis we just learned is called C3 photosynthesis (the first step produces a 3 Carbon molecule).  C2 is another term for the photorespiratory pathway (The first step produces a 2 Carbon molecule).  Some plants have evolved other adaptations in order to reduce photorespiration. Two commonly studied adaptations are:  C4 Photosynthesis  CAM Photosynthesis some plants are constantly exposed to high temperatures, and minimal H 2 O. They require adaptations to survive.

 C4 Plants have a different leaf anatomy, with no spongy mesophyll.

 In C3 plants, Mesophyll cells are responsible for complete photosynthesis, whereas in C4 plants, Mesophyll cells sequester Carbon in a 4 Carbon molecule via a different metabolic pathway.  This Carbon is then released into Bundle Sheath Cells where O 2 concentrations are low and the normal Calvin-Benson Cycle can occur without much photorespiration. EP carboxylase is more efficient at carboxylation in high temperatures

 Amazingly, C4 Photosynthesis has evolved on up to 40 independent occasions, acting as a prime example of convergent evolution.  Over 7500 species of plants utilize C4 Photosynthesis (~3% of terrestrial plant species). Corn is an example, as are many grasses and other plants exposed to large amounts of sunlight and higher temperatures.  The C4 Rice Project The C4 Rice Project

 Crassulacean Acid Metabolism (CAM) is a mechanism of storing Carbon Dioxide in a four carbon acid in Vacuoles overnight, so the plant can have its stomata remain shut during the day, and only open it at night.  Plants that have evolved this mechanism exist in arid environments, and can avoid rapid evaporation of H 2 O from stomata. Cactus are examples of CAM Plant

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