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Chapter Presentation
Transparencies
Visual Concepts
Standardized Test Prep
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Chapter 6
Photosynthesis
Table of Contents
Section 1 The Light Reactions
Section 2 The Calvin Cycle
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Chapter 6
Section 1 The Light Reactions
Objectives
• Explain why almost all organisms depend on
photosynthesis.
• Describe the role of chlorophylls and other pigments
in photosynthesis.
• Summarize the main events of the light reactions.
• Explain how ATP is made during the light reactions.
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Chapter 6
Section 1 The Light Reactions
Obtaining Energy
• Photosynthesis converts light energy from the sun
into chemical energy in the form of organic
compounds through a series of reactions known as
biochemical pathways.
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Chapter 6
Section 1 The Light Reactions
Obtaining Energy, continued
• Autotrophs use energy from sunlight or from
chemical bonds in inorganic substances to make
organic compounds.
• Animals and other organisms that must get energy
from food instead of directly from sunlight or
inorganic substances are called heterotrophs.
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Chapter 6
Section 1 The Light Reactions
Comparing Autotrophs and Heterotrophs
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Visual Concept
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Chapter 6
Section 1 The Light Reactions
Overview of Photosynthesis
• The oxygen (O2) and
some of the organic
compounds produced
by photosynthesis are
used by cells in a
process called cellular
respiration.
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Chapter 6
Section 1 The Light Reactions
Linking Photosynthesis and Respiration
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Visual Concept
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Chapter 6
Section 1 The Light Reactions
Overview of Photosynthesis, continued
• Photosynthesis can be divided into two stages:
Light Reactions and Calvin Cycle
– In the light reactions, light energy is converted to
chemical energy, which is temporarily stored in
ATP and the energy carrier molecule NADPH.
– In the Calvin Cycle, organic compounds are
formed using CO2 and the chemical energy stored
in ATP and NADPH.
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Chapter 6
Section 1 The Light Reactions
Overview of Photosynthesis, continued
• Equation of Photosynthesis:
6CO2 + 6H2O light energy C6H12O6 + 6O2
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Chapter 6
Section 1 The Light Reactions
Capturing Light Energy
• The light reactions begin with the absorption of light in
chloroplasts, organelles found in the cells of plants,
some bacteria, and algae.
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Chapter 6
Section 1 The Light Reactions
Parts of a Chloroplast
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Chapter 6
Section 1 The Light Reactions
Capturing Light Energy, continued
• Light and Pigments
– White light from the sun is composed of an array of
colors called the visible spectrum.
– Pigments absorb certain colors of light and reflect
or transmit the other colors.
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Chapter 6
Section 1 The Light Reactions
Capturing Light Energy, continued
• Chloroplast Pigments
– Located in the membrane of the thylakoids of
chloroplasts are several pigments, including
chlorophylls (such as chlorophyll a and
chlorophyll b) and carotenoids.
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Chapter 6
Section 1 The Light Reactions
Spectrum of Light and Plant Pigments
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Visual Concept
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Chapter 6
Section 1 The Light Reactions
Converting Light Energy To Chemical Energy
• The pigments are grouped in clusters of a few
hundred molecules in the thylakoid membrane. Each
cluster and the proteins that the pigment molecules
are embedded in are referred to collectively as a
photosystem.
• By absorbing light, pigment molecules in photosystem
I and photosystem II acquire some of the energy
carried by the light.
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Chapter 6
Section 1 The Light Reactions
Converting Light Energy To Chemical Energy,
continued
• In each photosystem, the acquired energy is passed
quickly to other pigment molecules until it reaches a
specific pair of chlorophyll a molecules.
• The acquired energy forces electrons to enter a
higher energy level in the two chlorophyll a
molecules of photosystem II. These energized
electrons are said to be “excited.” The excited
electrons have enough energy to leave the
chlorophyll a molecules.
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Chapter 6
Section 1 The Light Reactions
Converting Light Energy To Chemical Energy,
continued
• The acceptor of these electrons from photosystem II
is a molecule called the primary electron acceptor,
which donates the electrons to the electron transport
chain.
• As the electrons move from molecule to molecule in
this chain, they lose most of the acquired energy. The
energy they lose is used to move protons into the
thylakoid.
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Chapter 6
Section 1 The Light Reactions
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Chapter 6
Section 1 The Light Reactions
Converting Light Energy To Chemical Energy,
continued
• Light is absorbed by photosystem I at the same time it
is absorbed by photosystem II. Electrons move from
chlorophyll a molecules to another primary electron
acceptor.
• The electrons lost from photosystem I are replaced by
electrons that have passed through the electron
transport chain from photosystem II.
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Chapter 6
Section 1 The Light Reactions
Converting Light Energy To Chemical Energy,
continued
• These electrons are then donated to another electron
transport chain, which brings the electrons to the side
of the thylakoid membrane that faces the stroma.
• In the stroma, the electrons combine with a proton
and NADP+. This causes NADP+ to be reduced to
NADPH.
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Chapter 6
Section 1 The Light Reactions
Converting Light Energy To Chemical Energy,
continued
•
Replacing Electrons in Light
Reactions
– Electrons from photosystem
II replace electrons that leave
photosystem I. Replacement
electrons for photosystem II
are provided by the splitting
of water molecules.
– Oxygen produced when
water molecules are split
diffuses out of the chloroplast
and then
leaves the plant.
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Chapter 6
Section 1 The Light Reactions
Converting Light Energy To Chemical Energy,
continued
• Making ATP in Light
Reactions
– An important part of the
light reactions is the
synthesis of ATP. During
chemiosmosis, the
movement of protons
through ATP synthase into
the stroma releases energy,
which is used to produce
ATP.
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Chapter 6
Section 1 The Light Reactions
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Chapter 6
Section 2 The Calvin Cycle
Objectives
• Summarize the main events of the Calvin cycle.
• Describe what happens to the compounds that are
made in the Calvin cycle.
• Distinguish between C3, C4, and CAM plants.
• Summarize how the light reactions and the Calvin
cycle work together to create the continuous cycle of
photosynthesis.
• Explain how environmental factors influence
photosynthesis.
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Chapter 6
Section 2 The Calvin Cycle
Carbon Fixation
• The ATP and NADPH produced in the light reactions
drive the second stage of photosynthesis, the Calvin
cycle.
• In the Calvin cycle, CO2 is incorporated into organic
compounds, a process called carbon fixation.
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Chapter 6
Section 2 The Calvin Cycle
Carbon Fixation, continued
• The Calvin cycle, which occurs in the stroma of the
chloroplast, is a series of enzyme-assisted chemical
reactions that produces a three-carbon sugar.
• Most of the three-carbon sugars (G3P) generated in
the Calvin cycle are converted to a five-carbon sugar
(RuBP) to keep the Calvin cycle operating. But some
of the three-carbon sugars leave the Calvin cycle and
are used to make organic compounds, in which energy
is stored for later use.
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Chapter 6
Section 2 The Calvin Cycle
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Chapter 6
Section 2 The Calvin Cycle
The Calvin Cycle
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Chapter 6
Section 2 The Calvin Cycle
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Chapter 6
Section 2 The Calvin Cycle
Factors That Affect Photosynthesis
• Light Intensity
– The rate of photosynthesis increases as light
intensity increases, because more electrons are
excited in both photosystems.
– However, at some point all of the available electrons
are excited, and the maximum rate of
photosynthesis is reached. The rate then stays level
regardless of further increases in light intensity.
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Chapter 6
Section 2 The Calvin Cycle
Factors That Affect Photosynthesis, continued
• Carbon Dioxide Levels
– As with increasing light intensity, increasing levels of
carbon dioxide also stimulate photosynthesis until
the rate levels off.
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Chapter 6
Section 2 The Calvin Cycle
Factors That Affect Photosynthesis, continued
• Temperature
– As temperature increases, the rate of
photosynthesis increases to a maximum and then
decreases with further rises in temperature.
– The rate peaks at a certain temperature, at which
many of the enzymes that catalyze the reactions
become ineffective. Also, the stomata begin to
close, limiting water loss and entry of carbon
dioxide.
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Chapter 6
Section 2 The Calvin Cycle
Environmental Influences on Photosynthesis
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Chapter 6
Standardized Test Prep
Multiple Choice
1. Which of the following is a reactant in the Calvin
cycle?
A. O2
B. CO2
C. H2O
D. C6H12O6
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
1. Which of the following is a reactant in the Calvin
cycle?
A. O2
B. CO2
C. H2O
D. C6H12O6
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
2. Which of the following statements is correct?
F. Accessory pigments are not involved in
photosynthesis.
G. Accessory pigments add color to plants but do not
absorb light energy.
H. Accessory pigments absorb colors of light that
chlorophyll a cannot absorb.
J. Accessory pigments receive electrons from the
electron transport chain of photosystem I.
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
2. Which of the following statements is correct?
F. Accessory pigments are not involved in
photosynthesis.
G. Accessory pigments add color to plants but do not
absorb light energy.
H. Accessory pigments absorb colors of light that
chlorophyll a cannot absorb.
J. Accessory pigments receive electrons from the
electron transport chain of photosystem I.
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
3. Oxygen is produced at what point during
photosynthesis?
A. when CO2 is fixed
B. when water is split
C. when ATP is converted into ADP
D. when 3-PGA is converted into G3P
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
3. Oxygen is produced at what point during
photosynthesis?
A. when CO2 is fixed
B. when water is split
C. when ATP is converted into ADP
D. when 3-PGA is converted into G3P
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
The diagram below shows
a portion of a chloroplast.
Use the diagram to answer
the question that follows.
4. Which of the following
correctly identifies the
structure marked X and the
activities that take place
there?
F. stroma—Calvin cycle
G. stroma—light reactions
H. thylakoid—Calvin cycle
J. thylakoid—light reactions
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
The diagram below shows
a portion of a chloroplast.
Use the diagram to answer
the question that follows.
4. Which of the following
correctly identifies the
structure marked X and the
activities that take place
there?
F. stroma—Calvin cycle
G. stroma—light reactions
H. thylakoid—Calvin cycle
J. thylakoid—light reactions
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
5. light reactions : ATP :: Calvin cycle :
A. H+
B. O2
C. G3P
D. H2O
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
5. light reactions : ATP :: Calvin cycle :
A. H+
B. O2
C. G3P
D. H2O
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
The diagram below shows a
step in the process of
chemiosmosis. Use the
diagram to answer the question
that follows.
6. What is the substance
identified as Y in the
image?
F. H+
G. NAD+
H. NADPH
J. ADP synthase
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Chapter 6
Standardized Test Prep
Multiple Choice, continued
The diagram below shows a
step in the process of
chemiosmosis. Use the
diagram to answer the question
that follows.
6. What is the substance
identified as Y in the
image?
F. H+
G. NAD+
H. NADPH
J. ADP synthase
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Chapter 6
Standardized Test Prep
Short Response
Chloroplasts are organelles with areas that conduct
different specialized activities.
Where in the chloroplast do the light reactions and
the Calvin cycle occur?
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Chapter 6
Standardized Test Prep
Short Response, continued
Chloroplasts are organelles with areas that conduct
different specialized activities.
Where in the chloroplast do the light reactions and
the Calvin cycle occur?
Answer:
The light reactions of photosynthesis occur along the
thylakoid membrane. The Calvin cycle occurs in the
stroma, surrounding the thylakoids.
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Chapter 6
Standardized Test Prep
Extended Response
The reactions of photosynthesis make up a
biochemical pathway.
Part A What are the reactants and products for both
the light reactions and the Calvin cycle?
Part B Explain how the biochemical pathway of
photosynthesis recycles many of its own
reactants, and identify the recycled reactants.
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Chapter 6
Standardized Test Prep
Extended Response, continued
Answer:
Part A The reactants for the light reactions of
photosynthesis are sunlight, water, NADP+, and ADP.
The products are oxygen, ATP, and NADPH. The
reactants for the Calvin cycle are ATP, NADPH, CO2,
and RuBP. The products are NADP+, ADP, and
organic compounds.
Part B ADP/ATP, NADP+/NADPH, and electrons are
recycled during photosynthesis. RuBP, which reacts
with CO2 in the Calvin cycle, is regenerated at each
turn of the cycle.
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