SECTION
4.3
Photosynthesis in Detail
KEY CONCEPT
Student
text pages
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108–112
SC.912.L.18.7
Photosynthesis requires a series of chemical reactions.
The first stage of photosynthesis captures and
transfers energy.
In Section 4.2 you read a summary of photosynthesis. Now, we will look
at the process more closely. During the light-dependent reactions, energy
is captured from sunlight and moved along the thylakoid membrane.
This process involves two groups of molecules called photosystems. The
two photosystems are called photosystem I* and photosystem II*. Both
photosystems absorb energy from sunlight.
Photosystem II and Electron Transport
NOTE:
The photosystems are
In photosystem II, several things happen.
named I and II because
• Chlorophyll and other light-absorbing molecules absorb energy from
they were discovered in
that order. The numbers
sunlight. The energy is transferred to electrons (e–) that leave the
do not refer to the order
chlorophyll. These high-energy electrons enter the electron transport
of the process. In other
chain, which is made of proteins in the thylakoid membrane.
words, photosystem I
does not happen first.
• Water molecules are split apart into oxygen, hydrogen ions, and
electrons. The oxygen is released—the same oxygen you breathe.
The electrons from water take the place of the electrons that left
the chlorophyll.
thylakoid
• Electrons in the electron transport chain move from protein to
protein. Their energy pumps hydrogen ions (H+) across the
thylakoid membrane, from outside the thylakoid to inside. The
transport of H+ ions across the membrane makes the inside of
the thylakoid have a higher concentration of H+ ions than on
the outside of the thylakoid membrane. When there is a difference in the concentration of a substance—like there is here with
Light-dependent reactions take
H+ ions—it is called a concentration gradient.
Photosystem I and Energy-Carrying Molecules
place in and across the thylakoid membrane.
In photosystem I, several things happen.
• The electrons from photosystem II move on to photosystem I. In
addition, chlorophyll absorbs energy from sunlight, which results in
even more high-energy electrons.
• The electrons are added to NADP+, a molecule that is similar to ADP.
This makes NADPH, a molecule that acts a lot like ATP.
* ACADEMIC VOCABULARY
I and II part of a number system called Roman numerals, I = 1 and II = 2
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• The concentration gradient in the thylokoid, described previously,
provides the energy to make ATP. The H+ ions will diffuse, or flow,
back out of the thylakoid through a channel* in the membrane. This
channel is part of an enzyme called ATP synthase. As the H+ ions
flow through ATP synthase, phosphate groups are added to ADP to
make ATP. The energy from both ATP and NADPH is used later to
make sugars.
LIGHT-DEPENDENT REACTIONS
Photosystem II and electron transport
1
Energy is absorbed
from sunlight.
Photosystem I and energy-carrying molecules
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NADPH is produced
when electrons are
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ADP is changed into
ATP when hydrogen
ions flow through
ATP synthase.
Circle the part of the figure above that shows ADP turning into
ATP when H+ ions flow through ATP synthase.
The second stage of photosynthesis uses energy
from the first stage to make sugars.
The second stage of photosynthesis is the light-independent reactions.
This stage also happens in the chloroplast—but in the stroma, not in
the thylakoids. This stage is light-independent because it does not need
light. These reactions use energy from ATP and NADPH to run the
chemical reactions that make up the Calvin cycle. The chemical reactions of the Calvin cycle use carbon dioxide (CO2) and energy from
ATP and NADPH to make sugars.
* ACADEMIC VOCABULARY
channel a passage, or a space that can be moved through
60
McDougal Littell Biology
stroma
(fluid)
Light-independent reactions
take place in the stroma.
THE CALVIN CYCLE
1
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Carbon dioxide
(CO2) molecules
enter the cycle +6IE
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and are added to
five-carbon mol( 8 8 8 8 8 8
ecules. Six-carbon molecules
are formed.
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Energy is added. The six-carbon molecules
split to form three-carbon molecules. More
energy is added and the molecules are rearranged into higher-energy molecules.
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A high-energy threecarbon molecule exits for
every 3 CO2 molecules
that enter. After 2 threecarbon molecules have
exited, they bond to form
1 six-carbon sugar.
Three-carbon molecules are changed back to
five-carbon molecules by energy from ATP.
Photosynthesis is important for many reasons. Plants produce food
for themselves and for other organisms. They use the sugars from photosynthesis to build carbohydrates necessary for plant growth and development. Photosynthesis also removes carbon dioxide from Earth’s
atmosphere and produces the oxygen that you breathe.
theof
product
of the
Calvin
cycle
in theabove.
diagram
Circle theCircle
product
the Calvin
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in the
diagram
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above.
4.3
Mark It Up
Vocabulary Check
photosystem
ATP synthase
electron transport chain
Calvin cycle
Go back and highlight
each sentence that
has a vocabulary
word in bold.
1. Which part of photosynthesis produces
sugars?
2. What molecule adds a phosphate group to ADP to make ATP?
3. What groups of molecules are involved in photosynthesis?
4. What is the name for the set of proteins in the thylakoid membrane
that electrons move through?
4.3
The Big Picture
5. In which stage of photosynthesis does electron transport begin? In
which stage is sunlight needed?
6. What is the main product of the Calvin cycle?
7. What are the reactants of photosynthesis? What are the products?
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