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Chapter 7
 Capturing Solar Energy: Photosynthesis
Biology: Life on Earth, 9e
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What Is Photosynthesis?
 For most organisms, energy is derived from
sunlight, either directly or indirectly
 Those organisms that can directly trap sunlight do
so by photosynthesis
 Photosynthesis is the process by which solar
energy is trapped and stored as chemical energy in
the bonds of a sugar
– In water – protists and certain bacteria
– On land – plants
Biology: Life on Earth, 9e
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An Overview of Photosynthetic Structures
cuticle
upper
epidermis
mesophyll
cells
(a) Leaves
stoma
outer membrane
inner membrane
thylakoid
stroma
(d) Chloroplast
Biology: Life on Earth, 9e
lower
epidermis
stoma chloroplasts
bundle sheath cells
vascular bundle
(vein)
(b) Internal leaf structure
Fig. 7-1
channel
interconnecting
thylakoids
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Education Inc.
(c) Mesophyll
cell containing
chloroplasts
What Is Photosynthesis?
 Leaves and chloroplasts are adaptations for
photosynthesis in plants
– Leaves are flat and thin for best light penetration
– Takes place in chloroplasts contained within leaf cells
– Both the upper and lower surfaces of a leaf consist of a
layer of transparent cells, the epidermis
Biology: Life on Earth, 9e
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What Is Photosynthesis?
 Leaves and chloroplasts are adaptations for
photosynthesis in plants
– The outer surface of both epidermal layers is covered by
the cuticle, a transparent, waxy, waterproof covering that
reduces the evaporation of water from the leaf
– Leaves obtain CO2 for photosynthesis from the air
through pores in the epidermis called stomata (singular,
stoma)
Biology: Life on Earth, 9e
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What Is Photosynthesis?
 Leaves and chloroplasts are adaptations for
photosynthesis
– Inside the leaf are layers of cells called the mesophyll,
where the chloroplasts are located and where
photosynthesis occurs
– Bundle sheath cells surround the vascular bundles,
which form veins in the leaf and supply water and
minerals to the mesophyll
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
What Is Photosynthesis?
 Leaves and chloroplasts are adaptations for
photosynthesis
– Chloroplasts are organelles with a double membrane
enclosing a fluid called the stroma
– Embedded in the stroma are disk-shaped membranous
sacs called thylakoids
– Reactions that depend on light take place in the
thylakoids
– Reactions of the Calvin cycle that capture carbon dioxide
and produce sugar occur in the stroma
Biology: Life on Earth, 9e
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What Is Photosynthesis?
 Photosynthesis consists of the light reactions and
the Calvin cycle
– Starting with carbon dioxide (CO2) and water (H2O),
photosynthesis converts sunlight energy into chemical
energy stored in bonds of glucose and releases oxygen
(O2) as a by-product
6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2
carbon
water
sunlight
glucose
oxygen
dioxide
(sugar)
Biology: Life on Earth, 9e
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An Overview of the Relationship Between the
Light Reactions and the Calvin Cycle
Fig. 7-3
Biology: Life on Earth, 9e
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What Is Photosynthesis?
 Photosynthesis consists of the light reactions and
the Calvin cycle
– In the light reactions, chlorophyll captures light energy
and converts some into energy-carrier molecules ATP
and NADPH. Water is split releasing O2
– In the reactions of the Calvin cycle, enzymes in the
stroma use CO2 from the air and chemical energy from
the energy-carrier molecules to synthesize a threecarbon sugar that will be used to make glucose
Biology: Life on Earth, 9e
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Light Reactions: Light Energy Converted
to Chemical Energy
 Light is captured by pigments in chloroplasts
– The sun emits energy within a broad spectrum of
electromagnetic radiation
– This electromagnetic spectrum ranges from shortwavelength gamma rays, through ultraviolet, visible,
and infrared light, to long-wavelength radio waves
Biology: Life on Earth, 9e
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Light Reactions: Light Energy Converted
to Chemical Energy
 Light is captured by pigments in chloroplasts
– Light is composed of individual packets of energy called
photons
– Visible light has wavelengths with energies strong
enough to alter biological pigment molecules such as
chlorophyll a
– Chlorophyll a is a key light-capturing pigment molecule in
chloroplasts, absorbing violet, blue, and red light
– Green light is reflected, which is why leaves appear
green
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Light Reactions: Light Energy Converted
to Chemical Energy
 Light is captured by pigments in chloroplasts
– Accessory pigments, that absorb additional
wavelengths of light energy and transfer them to
chlorophyll a
– Chlorophyll b - absorbs blue and red-orange light,
and appear yellow-green
– Carotenoids - absorb blue and green light, and appear
yellow or orange
– In autumn, more-abundant, green chlorophyll breaks
down before the carotenoids do, revealing their yellow
color, which in summer is masked
Biology: Life on Earth, 9e
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Light Reactions: Light Energy Converted
to Chemical Energy
 The light reactions occur in association with the
thylakoid membranes
– Contain many photosystems
– each consists of a cluster of chlorophyll and accessory
pigment molecules surrounded by various proteins
– These electron transport chains (ETC) each consist of
a series of electron carrier molecules embedded in the
thylakoid membrane
Biology: Life on Earth, 9e
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Light Reactions: Light Energy Converted
to Chemical Energy
 The hydrogen ion gradient generates ATP by
chemiosmosis
– The energy of electron movement through the thylakoid
membrane creates an H+ gradient that drives ATP
synthesis in a process called chemiosmosis
– The generation of ATP  ADP + phosphate resembles
the electrical energy obtained from water flowing downhill
and driving an electrical turbine
Biology: Life on Earth, 9e
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Energy Stored in a Water “Gradient” Can
Be Used to Generate Electricity
1 Energy is released as
water flows downhill
2 Energy is harnessed to
rotate a turbine
3 The energy of the
rotating turbine is used
to generate electricity
Fig. 7-8
Biology: Life on Earth, 9e
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thylakoid
Events of the Light Reactions
chloroplast
Fig. 7-7
(stroma)
light
energy
1 H+ are pumped to
the thylakoid space
H+
electron
transport
NADP+
chain I

H+
electron transport chain II
e–
e–
e–
photosystem I
2 H+
1/
2
O2
H+
(thylakoid space)
Biology: Life on Earth, 9e
sugar
C6H12O6
ADP
+
P
H+
H+
NADPH
ATP
synthase
e– photosystem II
H+
Calvin
cycle
e–
e–
H2O
CO2
H+
ATP
H+
H+
2 High
concentration
is created
H+
H+
thylakoid
membrane
3 Flow of H+ down their
concentration gradient
powers ATP synthesis
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Review
1. Why is photosynthesis important?
2. What is the basic equation for photosynthesis?
3. What is the main light-capturing molecule in chloroplasts?
4. What are the two main end products from the light
reactions?
5. How and where are they created?
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
The Calvin Cycle: Chemical Energy Stored
in Sugar Molecules
 The Calvin cycle captures carbon dioxide
– ATP and NADPH synthesized from light reactions are
used to power the synthesis of a simple sugar
(gyceraldehyde-3-phosphate, or G3P)
– A series of reactions occurring in the stroma
– In reactions that occur outside the Calvin cycle, two G3P
molecules can be combined to form one six-carbon
glucose molecule
– Glucose may then be converted to the disaccharide
sucrose or linked to form starch (a storage molecule) or
cellulose (a major component of plant cell walls)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
The Calvin Cycle Fixes Carbon from CO2
and Produces G3P
3 C
CO2
H2O
ATP
light
reactions
NADPH
Calvin
cycle
Calvin
cycle
ADP
NADP
sugar
3
ATP
6
ADP
6 NADPH
ATP
6 NADP+
C6H12O6
5 C C C
G3P
3 Using the energy
from ATP, five of the
six molecules of G3P
are converted to three
molecules of RuBP
6 C C C
G3P
1 C C C
G3P
2 Energy from ATP
and NADPH is used
to convert the six
molecules of PGA to
six molecules of G3P
4 One molecule of
G3P leaves the cycle
1 C C C
G3P
Biology: Life on Earth, 9e
6
ADP
3
O2
6 C C C
PGA
3 C C C C C
RuBP
CO2
Fig. 7-9
1 Carbon fixation
combines three CO2
with three RuBP using
the enzyme rubisco
+ 1 C C C
G3P
1 C C C C C C
glucose
5 Two molecules of G3P combine to
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form glucose and other molecules