Photosynthesis Problem Set 2
Problem 6 Tutorial: Dark reactions
Which of the following does NOT occur during the dark reaction of photosynthesis?
A. utilization of NADPHB. utilization of ATP
C. reduction of oxygen to produce water
D. synthesis of glucose
E. utilization of carbon dioxide
Dark reactions of photosynthesis
| The "dark reactions" of photosynthesis refer to the carbohydrate biosynthesis reactions that occur in the chloroplast stroma. These reactions are catalyzed by a series of soluble enzymes, known collectively as "The Calvin-Benson Cycle". Carbohydrate biosynthesis can occur in the absence of photosynthetic electron transport. A summary of the events that occur during the dark reactions of photosynthesis is given by the summary equation: |
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Major components of this equation
| This overall equation can be broken down into three major components: |
What does not occur during the dark reaction? The reduction of oxygen to produce water, which is actually an event that occurs in the mitochondrial respiratory pathway.
1. Utilization of the NADPH produced during photosynthetic electron transport. Carbohydrate biosynthesis requires a source of electrons to ultimately reduce carbon dioxide to carbohydrate. The source is the oxidation of the NADPH produced during photosynthetic electron transport. 
2. Utilization of ATP produced during photosynthetic electron transport. Carbohydrate biosynthesis from carbon dioxide is an endergonic process. It is coupled to the hydrolysis of ATP, a highly exergonic reaction. ATPs produced during photosynthetic electron transport are hydrolyzed during carbohydrate biosynthesis. 
3. Utilization of CO2 for the synthesis of glucose. Atmospheric carbon dioxide is converted to glucose during the Calvin-Benson cycle. This requires the overall reduction of CO2, using the electrons available from the oxidation of NADPH. Thus the dark reactions represent a redox pathway. NADPH is oxidized to NADP+ and CO2 is reduced to glucose. 
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Thursday, October 3, 1996
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