Objectives
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 high-energy electrons and
hydrogen ions from the NADPH. The cycle's output is an energy-rich
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.
Staff
Wednesday, October 19, 2011 9:20:50 AM CT
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).
Staff
Wednesday, October 19, 2011 9:20:50 AM CT
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?
Copyright © 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights
reserved.
Staff
Wednesday, October 19, 2011 9:20:50 AM CT
Staff
Wednesday, October 19, 2011 9:20:50 AM CT