Biology (Holt)

Step The resulting six-carbon compound splits into two three-

carbon compounds. Phosphate groups from ATP and

electrons from NADPH are added to the three-carbon com-

pounds, forming three-carbon sugars.

Step One of the resulting three-carbon sugars is used to make

organic compounds—including starch and sucrose—in

which energy is stored for later use by the organism.

Step The other three-carbon sugars are used to regenerate the

initial five-carbon compound, thereby completing the cycle.
The Calvin cycle is named for Melvin Calvin, the American bio-
chemist who worked out the chemical reactions in the cycle. The
reactions are cyclic—they recycle the five-carbon compound needed
to begin the cycle again. A total of three carbon dioxide molecules
must enter the Calvin cycle to produce each three-carbon sugar that
will be used to make other organic compounds. These organic
compounds provide the organism with energy for growth and
metabolism. The energy used in the Calvin cycle is supplied by ATP
and NADPH made during the second stage of photosynthesis.

Factors that Affect Photosynthesis

Photosynthesis is directly affected by various environmental factors.
The most obvious of these factors is light. In general, the rate of
photosynthesis increases as light intensity increases until all the
pigments are being used. At this saturation point, the rate of photo-
synthesis levels off because pigments cannot absorb any more light.
The carbon dioxide concentration affects the rate of photosynthesis
in a similar manner. Once a certain concentration of carbon dioxide
is present, photosynthesis cannot proceed any faster.
Photosynthesis is most efficient within a certain range of tem-
peratures. Like all metabolic processes, photosynthesis involves
many enzyme-assisted chemical reactions. Recall that unfavorable
temperatures may inactivate certain enzymes.

SECTION 2 Photosynthesis 103

Real Life

Some houseplants thrive

in dim light.

A plant inside a home may

receive 100 times less

bright light than it would if

it were grown outdoors.

Recognizing Patterns

Examine several species

of houseplants in a

store or nursery.

What features

are common

among

house-

plants?

http://www.scilinks.org

Topic:Factors Affecting

Photosynthesis

Keyword:HX4079

Section 2 Review

Summarizehow photosynthetic organisms

capture the energy in sunlight. 4B

Comparethe roles of water molecules and

hydrogen ions in electron transport chains.

Describethe role of the Calvin cycle in the third

stage of photosynthesis.

Critical Thinking Organizing Information

Make a table in which you identify the role of each

of the following in photosynthesis: light, water,

pigments, ATP, NADPH, and carbon dioxide.

Critical ThinkingInferring Relationships

What combination of environmental factors

affects the rate of photosynthesis? 9D

During photosynthesis,

plants store energy in

AADP. C3-carbon sugars.

Bcarbon dioxide. Dwater.

TAKS Test PrepTAKS Test Prep

4B