Biology
A Guide to the Natural World
Chapter 8 • Lecture Outline
The Green World’s Gift: Photosynthesis
Fifth Edition
David Krogh
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8.1 Photosynthesis and Energy
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Photosynthesis and Energy
• Photosynthesis has made possible life as we
know it on Earth because the organic
material produced in photosynthesis (a
sugar) is the source of food for most of
Earth’s living things.
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Photosynthesis and Energy
• Photosynthesis also is responsible for the
atmospheric oxygen used by many living
things in cellular respiration.
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Three Types of Photosynthesis
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Figure 8.1
8.2 The Components of
Photosynthesis
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The Components of Photosynthesis
• In plants and algae, photosynthesis takes
place in organelles called chloroplasts,
which can exist in great abundance in the
mesophyll cells of plant leaves.
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Photosynthesis
• The energy for photosynthesis comes
mostly from various blue and red
wavelengths of visible sunlight that are
absorbed by pigments in the chloroplasts.
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The Electromagnetic Spectrum
Sunlight
The process of cellular
respiration converts the energy
stored in carbohydrates to ATP,
the most important energytransfer molecule in living things.
Energy comes
from the sun and
then, in photosynthesis,
is stored in plants in the
complex molecules we
call carbohydrates
ATP
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Powers many
chemical reactions
Figure 8.2
Stomata
• Plant leaves contain microscopic pores
called stomata that can open and close,
letting carbon dioxide in and water vapor
out.
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1. Leaf
The primary site of photosynthesis in plants, leaves have a
two-part structure: a petiole (or stalk) and a blade (normally
thought of as the leaf).
petiole
blade
2. Leaf cross section
In cross section, leaves have a sandwich-like
structure, with epidermal layers at top and
bottom and mesophyll cells in between. Most
photosynthesis is performed within mesophyll
cells. Leaf epidermis is pocked with a large
number of microscopic openings, called stomata,
that allow carbon dioxide to pass in and water
vapor to pass out.
epidermis
mesophyll cells
epidermis
stomata
nucleus
3. Mesophyll cell
A single mesophyll cell within a leaf contains all
the component parts of plant cells in general,
including the organelles—called chloroplasts—
that are the actual sites of photosynthesis.
chloroplast
cell wall
vacuole
4. Chloroplast
Each chloroplast has an outer
membrane at its periphery; then an
inner membrane; then a liquid material,
called the stroma, that has immersed
within it a network of membranes, the
thylakoids. These thylakoids sometimes
stack on one another to create a granum.
thylakoids
stroma
granum
inner membrane
outer membrane
Energy from sunlight
is absorbed by
pigments in the
thylakoid membrane.
5. Granum
Electrons used in photosynthesis will come
from water contained in the thylakoid
compartment, and all the steps of
photosynthesis will take place either within
the thylakoid membrane, or in the stroma
that surrounds the thylakoids.
thylakoid
thylakoid membrane
thylakoid compartment
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Figure 8.4
Stages of Photosynthesis
• There are two primary stages to
photosynthesis.
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The Light Reactions
• In the first stage, called the light reactions,
electrons derived from water are
energetically boosted by the power of
sunlight.
• These electrons physically move in this
process: they are passed along through a
series of electron carriers, ending up as part
of the electron carrier NADPH, which
carries them to the second stage of
photosynthesis.
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The Calvin Cycle
• In this second stage, the Calvin cycle, the
electrons are brought together with carbon
dioxide and a sugar.
• The product is a high-energy sugar in a
process powered by ATP that is produced in
the light reactions.
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8.3 Stage 1: The Light Reactions
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Stage 1: The Light Reactions
• In its first stage, photosynthesis works
through a pair of molecular complexes,
photosystems II and I.
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Photosystems
• Photosystems II and I are composed partly
of antennae molecules—chlorophyll and
some accessory molecules—that absorb and
transmit solar energy.
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primary
electron
acceptor
e–
sunlight
reaction
center
antennae
pigments
photosystem
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Figure 8.5
What Makes the Light Reactions
So Important?
• Two actions of great consequence take
place in the light reactions.
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Two Key Actions in Light
Reaction
1. Water is split, yielding both electrons and
oxygen.
• The electrons move through the light
reactions.
• The oxygen is what organisms such as
ourselves breathe in.
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Two Key Actions in Light
Reaction
2. The electrons that are derived from the
water, and then given an energy boost by
the sun’s rays, are transferred to a different
molecule: the initial electron acceptor.
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The Light Reactions
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Figure 8.7
Importance of Light Reactions
• This is the means through which the sun’s
energy is transferred into the living world.
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Importance of Light Reactions
• The energetic fall of electrons through the
electron transport chain between
photosystems II and I also yields energy
that produces ATP, which is used to power
the second stage of photosynthesis.
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Collecting Solar Energy
primary
electron
acceptor
primary
electron
acceptor
Energy scale
e–
e–
sunlight
sunlight
to Calvin
cycle
electrons
photosystem I
photosystem II
electron fall supplies
ATP
energy that will
lead to ATP synthesis
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Figure 8.6
Other Photosystem Components
• Other photosystem components include
reaction center molecules, which accept
both this energy and electrons derived from
water; and primary electron acceptors
(a part of the reaction centers), to which the
electrons move after being energetically
boosted.
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8.4 Stage 2: The Calvin Cycle
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Stage 2: The Calvin Cycle
• The Calvin Cycle is the second stage of
photosynthesis.
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The Calvin Cycle
• In the Calvin cycle, carbon dioxide from the
atmosphere is brought together with a sugar,
RuBP.
• The resulting compound is energized with
addition of electrons supplied by the first
stage of photosynthesis.
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Photosynthesis
Suggested Media Enhancement:
Photosynthesis
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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The Calvin Cycle
• The result of the Calvin cycle is the highenergy sugar G3P, which is the product of
photosynthesis.
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The Calvin Cycle
• All these steps are powered by ATP
produced in the light reactions.
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1. Carbon fixation. An enzyme called
rubisco brings together three
molecules of CO2 with three
molecules of the sugar RuBP. In
this reaction, one carbon from
each CO2 molecule is being added
to the five-carbon RuBP, and this
is being done three times. The three
resulting six-carbon molecules are
immediately split into six
three-carbon molecules named
3-PGA (3-phosphoglyceric acid).
2. Energizing the sugar. In two separate
reactions, six ATP molecules react
with six 3-PGA, in each case
transferring a phosphate onto the
3-PGA. The six 3-PGA derivatives
oxidize (gain electrons from) six
NADPH molecules; in so doing, they
are transformed into the energy-rich
sugar G3P (glyceraldehyde
3-phosphate).
3. Exit of product. One molecule of G3P
exits as the output of the Calvin cycle.
This molecule, the product of
photosynthesis, can be used for
energy or transformed into materials
that make up the plant.
4. Regeneration of RuBP. In several
reactions, five molecules of G3P are
transformed into three molecules of
RuBP, which enter the cycle.
6
Calvin
cycle
ATP
3
ATP
6
sugar
3 molecules
3 molecules
Rubisco
3 molecules
of RuBP
6 molecules of
3-PGA
1. Carbon
fixation
from light
reactions
3 ADP
6
3
ATP
4. Regeneration
of RuBP
ATP
6 ADP
from light
reactions
2. Energizing
the sugar
5 molecules
of G3P
6 molecules of
3-PGA
derivative
3. Exit of
product
1 molecule
of G3P
6
6 molecules
of G3P
from
light
reactions
glucose and
other derivatives
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Figure 8.8
The Calvin Experiments
Animation 8.3: The Calvin Experiments
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G3P
• G3P can be used for energy or for plant
growth.
• Everything in the plant ultimately is derived
from this sugar, in association with minerals
and water that the plant absorbs through its
roots.
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8.5 Photorespiration and the
C4 Pathway
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Photorespiration and the C4 Pathway
• In plants, the enzyme rubisco frequently
binds with oxygen rather than with carbon
dioxide—a process called photorespiration
that undercuts photosynthesis.
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Photorespiration
• This problem increases as the temperature
rises because as plants close their stomata to
keep in moisture, they also keep out carbon
dioxide, thus increasing the likelihood that
rubisco will bind with oxygen.
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Photorespiration
• Some warm-climate plants have evolved a
means of dealing with photorespiration:
C4 photosynthesis.
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C4 Photosynthesis
• C4 photosynthesis employs an enzyme that
binds with carbon dioxide but not with
oxygen.
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C4 Photosynthesis
• The carbon dioxide is then shuttled to
special bundle-sheath cells in the plant and
released, after which it moves into the
Calvin cycle.
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CO2
O2
C4
pathway
CO2
Calvin
cycle
sugar
mesophyll cells
bundle-sheath cells vein cells
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Figure 8.10
C4 Photosynthesis
• With high levels of carbon dioxide in the
bundle-sheath cells, rubisco binds with
carbon dioxide (and not oxygen), thus
greatly reducing photorespiration.
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8.6 CAM Photosynthesis
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CAM Photosynthesis
• In CAM photosynthesis, the plant’s stomata
open only at night, letting in and fixing
carbon dioxide.
• Carbon dioxide is then “banked” until
sunrise, when the sun’s rays will supply the
energy needed to power the Calvin cycle.
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Another Photosynthetic Variation:
CAM Plants
• Dry-weather plants (such as cacti) employ
another form of photosynthesis, CAM
photosynthesis.
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