Objectives
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Explain how the Calvin cycle makes sugar.
Summarize the overall process of photosynthesis.
It would be unfortunate for humans and many other living things if photosynthesis
stopped after the light reactions. The process so far has released one important final
product, oxygen. But as you have read, organisms depend on the sugars and other organic
compounds produced by plants as fuel for cellular respiration and as building materials.
The Calvin cycle is responsible for producing the raw materials for these compounds.
A Trip Around the Calvin Cycle
You can think of the Calvin cycle as being somewhat like a sugar factory within a
chloroplast. It is called a cycle because, like the Krebs cycle in cellular respiration, the
starting material is regenerated each time the process occurs. In this case, the starting
material that gets regenerated is a compound called RuBP, a sugar with five carbons.
With each turn of the Calvin cycle, there are chemical inputs and outputs. The inputs are
carbon dioxide from the air and the ATP and NADPH produced by the light reactions.
The Calvin cycle uses carbon from the carbon dioxide, energy from the ATP, and highenergy electrons and hydrogen ions from the NADPH. The cycle's output is an energyrich sugar molecule. That sugar is not yet glucose, but a smaller sugar named G3P. The
plant cell uses G3P as the raw material to make glucose and other organic molecules it
needs. You can follow the process of the Calvin cycle in Figure 8-13.
Figure 8-13
Follow the fate of three carbon dioxide molecules through the Calvin cycle. This diagram
shows simplified representations of some of the molecules formed during the reactions.
Each gray ball represents a carbon atom.
Summary of Photosynthesis
Now that you've read about the details of photosynthesis, take a step back and look at the
overall process again. Recall that the overall equation for photosynthesis is:
6 CO2 + 6 H2O
C6H12O6 + 6 O2
The light reactions, which take place in the thylakoid membranes, convert light energy to
the chemical energy of ATP and NADPH. The light reactions use the reactant water from
the equation and release the product oxygen. The Calvin cycle, which takes place in the
stroma, uses ATP and NADPH to convert carbon dioxide to sugar (Figure 8-14).
Figure 8-14
The light reactions and the Calvin cycle together convert light energy
to the stored chemical energy of sugar. The plant can use the sugar to
build other organic molecules.
By converting light energy to chemical energy, photosynthesis is the first step in the flow
of energy through an ecosystem. Some of that chemical energy then passes from
producers to consumers. Even when people eat meat, you can trace its stored energy back
to photosynthesis. For example, the beef in a hamburger came from cattle that ate plants.
Photosynthesis is the ultimate source of all the food you eat and all the oxygen you
breathe.
Concept Check 8.3
1. What are the inputs and outputs of the Calvin cycle?
2. Which stage of photosynthesis uses each reactant from the overall photosynthesis
equation? Which stage generates each product from the overall photosynthesis equation?
3. Why is the Calvin cycle called a cycle?
4. What molecule is the direct product of photosynthesis? How is that molecule then used
by plant cells?
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