Transcript Slide 1

Chapter 7
Photosynthesis: Using Light to
Make Food
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Figure 7.0_1
Chapter 7: Big Ideas
An Overview of
Photosynthesis
The Calvin Cycle:
Reducing CO2 to Sugar
The Light Reactions:
Converting Solar Energy to
Chemical Energy
Photosynthesis Reviewed
and Extended
AN OVERVIEW
OF PHOTOSYNTHESIS
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7.1 Autotrophs are the producers of the biosphere
 Autotrophs
– make their own food through the process of
photosynthesis,
– sustain themselves, and
– do not usually consume organic molecules derived from
other organisms.
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7.1 Autotrophs are the producers of the biosphere
 Photoautotrophs use the energy of light to produce
organic molecules.
 Chemoautotrophs are prokaryotes that use
inorganic chemicals as their energy source.
 Heterotrophs are consumers that feed on
– plants or
– animals, or
– decompose organic material.
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Figure 7.1A-D
7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Chloroplasts are the major sites of photosynthesis
in green plants.
 Chlorophyll
– is an important light-absorbing pigment in chloroplasts,
– is responsible for the green color of plants, and
– plays a central role in converting solar energy to
chemical energy.
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7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Chloroplasts are concentrated in the cells of the
mesophyll, the green tissue in the interior of the
leaf.
 Stomata are tiny pores in the leaf that allow
– carbon dioxide to enter and
– oxygen to exit.
 Veins in the leaf deliver water absorbed by roots.
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Figure 7.2
Leaf
Leaf Cross Section
Mesophyll
Vein
CO2
O2
Stoma
Mesophyll Cell
Chloroplast
Inner and outer
membranes
Granum
Thylakoid
Thylakoid space
Stroma
Figure 7.2_1
Leaf Cross
Section
Leaf
Mesophyll
Vein
Mesophyll Cell
CO2
O2
Stoma
Chloroplast
7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Chloroplasts consist of an envelope of two
membranes, which
– enclose an inner compartment filled with a thick fluid
called stroma and
– contain a system of interconnected membranous sacs
called thylakoids.
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7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Thylakoids
– are often concentrated in stacks called grana and
– have an internal compartment called the thylakoid space,
which has functions analogous to the intermembrane
space of a mitochondrion.
– Thylakoid membranes also house much of the machinery
that converts light energy to chemical energy.
 Chlorophyll molecules
– are built into the thylakoid membrane and
– capture light energy.
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Figure 7.2_2
Chloroplast
Inner and outer
membranes
Granum
Thylakoid
Thylakoid space
Stroma
7.4 Photosynthesis is a redox process, as is
cellular respiration
 Photosynthesis, like respiration, is a redox
(oxidation-reduction) process.
– CO2 becomes reduced to sugar as electrons along with
hydrogen ions from water are added to it.
– Water molecules are oxidized when they lose electrons
along with hydrogen ions.
Becomes reduced
Becomes oxidized
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7.5 Overview: The two stages of photosynthesis
are linked by ATP and NADPH
 Photosynthesis occurs in two metabolic stages.
1. The light reactions occur in the thylakoid membranes.
In these reactions
– water is split, providing a source of electrons and giving off
oxygen as a by-product,
– ATP is generated from ADP and a phosphate group, and
– light energy is absorbed by the chlorophyll molecules to drive
the transfer of electrons and H+ from water to the electron
acceptor NADP+ reducing it to NADPH.
– NADPH produced by the light reactions provides the
electrons for reducing carbon in the Calvin cycle.
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7.5 Overview: The two stages of photosynthesis
are linked by ATP and NADPH
2. The second stage is the Calvin cycle, which occurs in
the stroma of the chloroplast.
– The Calvin cycle is a cyclic series of reactions that
assembles sugar molecules using CO2 and the energy-rich
products of the light reactions.
– During the Calvin cycle, CO2 is incorporated into organic
compounds in a process called carbon fixation.
– After carbon fixation, enzymes of the cycle make sugars by
further reducing the carbon compounds.
– The Calvin cycle is often called the dark reactions or lightindependent reactions, because none of the steps requires
light directly.
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Figure 7.5_s3
H2O
CO2
Light
NADP+
ADP
P
Calvin
Cycle
(in stroma)
Light
Reactions
(in thylakoids)
ATP
NADPH
Chloroplast
O2
Sugar
THE LIGHT REACTIONS:
CONVERTING SOLAR ENERGY
TO CHEMICAL ENERGY
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7.6 Visible radiation absorbed by pigments
drives the light reactions
 Sunlight contains energy called electromagnetic
energy or electromagnetic radiation.
– Visible light is only a small part of the electromagnetic
spectrum, the full range of electromagnetic
wavelengths.
– Electromagnetic energy travels in waves, and the
wavelength is the distance between the crests of two
adjacent waves.
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Figure 7.6A
Increasing energy
105 nm 103 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
103 m
1m
Microwaves
Radio
waves
Visible light
380 400
500
600
Wavelength (nm)
700
650
nm
750
7.6 Visible radiation absorbed by pigments
drives the light reactions
 Pigments
– absorb light and
– are built into the thylakoid membrane.
 Plant pigments
– absorb some wavelengths of light and
– reflect or transmit other wavelengths.
 We see the color of the wavelengths that are
transmitted. For example, chlorophyll transmits
green wavelengths.
Animation: Light and Pigments
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7.7 Photosystems capture solar energy
 Pigments in chloroplasts absorb photons (capturing
solar power), which
– increases the potential energy of the pigment’s
electrons and
– sends the electrons into an unstable state.
– These unstable electrons
– drop back down to their “ground state,” and as they do,
– release their excess energy as heat.
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Figure 7.7A_2
Excited state
Photon
of light
Heat
Photon
(fluorescence)
Ground state
Chlorophyll
molecule
7.7 Photosystems capture solar energy
 Within a thylakoid membrane, chlorophyll and other
pigment molecules
– absorb photons and
– transfer the energy to other pigment molecules.
 In the thylakoid membrane, chlorophyll molecules
are organized along with other pigments and
proteins into photosystems.
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Figure 7.7B
Photosystem
Light
Light-harvesting Reaction-center
complexes
complex
Thylakoid membrane
Primary electron
acceptor
Transfer
of energy
Pair of
chlorophyll a molecules
Pigment
molecules
7.8 Two photosystems connected by an electron
transport chain generate ATP and NADPH
 In the light reactions, light energy is transformed
into the chemical energy of ATP and NADPH.
 To accomplish this, electrons are
– removed from water,
– passed from photosystem II to photosystem I, and
– accepted by NADP+, reducing it to NADPH.
 Between the two photosystems, the electrons
– move down an electron transport chain and
– provide energy for the synthesis of ATP.
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Figure 7.8A
Light
Photosystem II
Stroma
Electron transport chain
Provides energy for
synthesis of ATP
by chemiosmosis
NADP  H
Light
Photosystem I
1
Primary
acceptor
Thylakoid membrane
Primary
acceptor
2
4
P700
P680
Thylakoid
space
3
H2O
1
2
5
O2  2 H
6
NADPH
Figure 7.8B
ATP
NADPH
Mill
makes
ATP
Photosystem II
Photosystem I
7.8 Two photosystems connected by an electron
transport chain generate ATP and NADPH
 The products of the light reactions are
– NADPH,
– ATP, and
– oxygen.
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7.9 Chemiosmosis powers ATP synthesis in the
light reactions
 Interestingly, chemiosmosis is the mechanism that
– is involved in oxidative phosphorylation in
mitochondria and
– generates ATP in chloroplasts.
 ATP is generated because the electron transport
chain produces a concentration gradient of
hydrogen ions across a membrane.
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7.9 Chemiosmosis powers ATP synthesis in the
light reactions
 In photophosphorylation, using the initial energy
input from light,
– the electron transport chain pumps H+ into the thylakoid
space, and
– the resulting concentration gradient drives H+ back
through ATP synthase, producing ATP.
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Figure 7.9
Chloroplast
To Calvin
Cycle
Light
Light
Stroma
(low H+
concentration)
H+
ADP
H+
NADP+
P
NADPH
H+
H+
H+
Thylakoid
membrane
H2O
Thylakoid space
(high H+
concentration)
1 O + 2 +
H
2 2
Photosystem II
H+
+
H
H+
Electron
transport chain
H+
H+
H+
H+
H+
H+
Photosystem I
H+
H+
H+
H+
ATP synthase
ATP
THE CALVIN CYCLE:
REDUCING CO2TO SUGAR
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7.10 ATP and NADPH power sugar synthesis in
the Calvin cycle
 The Calvin cycle makes sugar within a chloroplast.
 To produce sugar, the necessary ingredients are
– atmospheric CO2 and
– ATP and NADPH generated by the light reactions.
 The Calvin cycle uses these three ingredients to
produce an energy-rich, three-carbon sugar called
glyceraldehyde-3-phosphate (G3P).
 A plant cell may then use G3P to make glucose
and other organic molecules.
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Figure 7.10A
Input
CO2
ATP
NADPH
Calvin
Cycle
Output:
G3P
7.10 ATP and NADPH power sugar synthesis in
the Calvin cycle
 The steps of the Calvin cycle include
– carbon fixation,
– reduction,
– release of G3P, and
– regeneration of the starting molecule ribulose
bisphosphate (RuBP).
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Figure 7.10B_s4
Step
1
Carbon fixation
Input:
3
CO2
Rubisco
1
3 P
6
P
RuBP
Step 2 Reduction
P
3-PGA
6
ATP
3 ADP
6 ADP
Step 3 Release of one
molecule of G3P
Calvin
Cycle
4
3 ATP
4
Regeneration of RuBP
6 NADPH
6 NADP
5
P
6
P
G3P
Step
2
G3P
3
Output: 1
P
G3P
Glucose
and other
compounds
P
7.11 EVOLUTION CONNECTION: Other
methods of carbon fixation have evolved in
hot, dry climates
 Most plants use CO2 directly from the air, and
carbon fixation occurs when the enzyme rubisco
adds CO2 to RuBP.
 Such plants are called C3 plants because the first
product of carbon fixation is a three-carbon
compound, 3-PGA.
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7.11 EVOLUTION CONNECTION: Other
methods of carbon fixation have evolved in
hot, dry climates
 C4 plants have evolved a means of
– carbon fixation that saves water during photosynthesis
while
– optimizing the Calvin cycle.
 C4 plants are so named because they first fix CO2
into a four-carbon compound.
 When the weather is hot and dry, C4 plants keep
their stomata mostly closed, thus conserving water.
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7.11 EVOLUTION CONNECTION: Other
methods of carbon fixation have evolved in
hot, dry climates
 Another adaptation to hot and dry environments
has evolved in the CAM plants, such as
pineapples and cacti.
 CAM plants conserve water by opening their
stomata and admitting CO2 only at night.
 CO2 is fixed into a four-carbon compound,
– which banks CO2 at night and
– releases it to the Calvin cycle during the day.
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Figure 7.11
Mesophyll
cell
Bundlesheath
cell
CO2
4-C compound
4-C compound
CO2
CO2
Calvin
Cycle
Calvin
Cycle
3-C sugar
C4 plant
Sugarcane
CO2
Night
3-C sugar
Day
CAM plant
Pineapple
7.12 Review: Photosynthesis uses light energy, carbon
dioxide, and water to make organic molecules
 About half of the carbohydrates made by
photosynthesis are consumed as fuel for cellular
respiration in the mitochondria of plant cells.
 Sugars also serve as the starting material for making
other organic molecules, such as proteins, lipids, and
cellulose.
 Excess food made by plants is stockpiled as starch
in roots, tubers, seeds, and fruits.
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Figure 7.12
H2O
CO2
Chloroplast
Light
NADP
Light
Reactions
ADP
P
RuBP
Calvin
Cycle 3-PGA
(in stroma)
Photosystem II
Electron
transport chain
Thylakoids
Photosystem I
ATP
NADPH
O2
Stroma
G3P
Sugars
Cellular
respiration
Cellulose
Starch
Other organic
compounds
Figure 7.UN04
Photosynthesis
converts
includes both
(a)
(c)
(b)
to
in which
chemical
energy
light-excited
electrons of
chlorophyll
H2O is split
in which
CO2 is fixed to
RuBP
and then
and
are passed
down
(d)
reduce
NADP to
(h)
using
(f)
to produce
(e)
producing
(g)
by
chemiosmosis
sugar
(G3P)