Biology (Holt)

C

CCCCC

CCC

BIO

graphic

Six

3-carbon

compounds

Three

5-carbon

compounds

Three

carbon dioxide

molecules

Six

3-carbon

sugars

3 CO 2

P

P

P

3 ATP

3 ADP

6 ATP

6 ADP

6 NADPH

6 NADP+

Organic

compounds

6 CCC P

6 CCC P

One

3-carbon

sugar

1

The Calvin cycle is a common method of carbon dioxide fixation.

Calvin Cycle

The other five three-carbon

sugars regenerate the

five-carbon compound that

began the cycle.

4

A CO 2 molecule is

added to a five-carbon

compound.

1

The three resulting

six-carbon compounds

split, forming a total of six

three-carbon compounds.

2

One three-carbon sugar is used

(^3) to make organic compounds.
Figure 9
Stage Three: Storage of Energy
In the first and second stages of photosynthesis, light energy is used
to make ATP and NADPH, which temporarily store chemical energy.
These stages are therefore considered light-dependent. In the third
(final) stage of photosynthesis, however, carbon atoms from carbon
dioxide in the atmosphere are used to make organic compounds in
which chemical energy is stored. The transfer of carbon dioxide to
organic compounds is called .The reactions
that “fix” carbon dioxide are sometimes called “dark reactions,” or
light-independent reactions. Among photosynthetic organisms,
there are several ways in which carbon dioxide is fixed.
Calvin Cycle
The most common method of carbon dioxide fixation is the Calvin
cycle. The is a series of enzyme-assisted chemical
reactions that produces a three-carbon sugar. The Calvin cycle is
summarized in Figure 9.

Step In carbon dioxide fixation, each molecule of carbon dioxide,

CO 2 , is added to a five-carbon compound by an enzyme.

Calvin cycle

carbon dioxide fixation

102 CHAPTER 5Photosynthesis and Cellular Respiration