Lecture 7- photosynthesis 3

Carboxlyase the incorporation of a co2 molecule into

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Unformatted text preview: of O2 and much higher levels of CO2 - An early form of Rubisco that had an active site that could bind O2 as well as Co3 wold not have been detrimental to an organisms - Photorespiration became a problem as the levels of oxygen in the atmosphere increased - Over time, Rubisco slowly evolved to be more speci5ic for CO2, but the inhibition of O2 remains - O2 can directly compete with CO2 for the active site of Rubisco (competitive inhibitor) - When oxygen binds to the active site of Rubisco, the enzyme acts as an oxgenase instead of a carboxlyase - The incorporation of a CO2 molecule into RuBP leads to a new increase in carbon by producing two molecules of the three- carbon compound 3- phosphoglycerate - - > Gain 1 carbon - The incorporation of O2 into RuBP in the oxygenation reaction produces a single molecule of 3- phosphoglycerate and one molecule of the two- carbon compounds phosphoglycerate - - > There is NO carbon gain - Carboxlyation reaction leads to carbon gain, oxygenation reaction results in losing carbon - Carbozylation reactions will dominate because the active site of Rubisco has a greater af5inity for CO2 than O2 - Carboxylation reaction will occur 80 times as quickly as the oxygenation reaction The Calvin cycle uses ATP and NADPH to convert CO2 to sugar • Calvin cycle regenerates its starting material after molecules enter and leave the cycle. • CO2 enters the cycle and leaves as sugar. • Cycle spends the energy of ATP and the reducing power of electrons carried by NADPH to make the sugar. • The sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). 08:55 Gardiner 19 - G3P molecule formed by three turns of the Calvin Cycle Gardiner 1202B 08:55 BIOLOGY • Each turn of the Calvin cycle fixes one carbon. • For the net synthesis of one G3P molecule, the cycle must take place three times, fixing three molecules of CO2. • To make one glucose molecule would require six cycles and the fixation of six CO2 molecules. is the staring point for the production of a wide variety of organic molecules - More complex carbohydrates such as glucose and other monosaccharides are made form G3P by reactions that reverse the 5irst half of glycolysis 20 http://www.science.smith.edu/departments/ Biology/Bio231/calvin.html 08:55 Gardiner 1202B – This is catalyzed by RuBP carboxylase / oxygenase or rubisco. – The six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate per CO2. Phase 1: Fixation - Incorporation (5ixing) of a carbon atom from CO2 BIOLOGY • The Calvin cycle has three...
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