Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 6
Where It Starts – Photosynthesis
(Sections 6.5 - 6.8)
Albia Dugger • Miami Dade College
6.5 Light-Dependent Reactions
• The light-dependent reactions of the first stage of
photosynthesis convert the energy of light to the energy of
chemical bonds
• There are two different sets of light-dependent reactions : a
noncyclic pathway and a cyclic pathway
Capturing Light for Photosynthesis
• Light-harvesting complexes in the thylakoid membrane
absorb photons and pass the energy to photosystems,
which then release electrons
• photosystem
• A cluster of hundreds of chlorophylls, accessory pigments,
and other molecules that converts light energy to chemical
energy in photosynthesis
The Thylakoid Membrane
• Some components
of the thylakoid
membrane as seen
from the stroma
The Thylakoid Membrane
light-harvesting complex
photosystem
Fig. 6.6, p. 98
The Noncyclic Pathway
• Electrons released from photosystem II flow through an
electron transfer chain, then to photosystem I
• Photon energy causes photosystem I to release electrons,
which end up in NADPH
Replacing Lost Electrons
• Photosystem II replaces lost electrons by pulling them from
water, which then dissociates into H+ and O2 (photolysis)
• photolysis
• Process by which light energy breaks down a molecule
Harvesting Electron Energy
• The process by which the flow of electrons through electron
transfer chains drives ATP formation is called electron
transfer phosphorylation
• electron transfer phosphorylation
• Electron flow through electron transfer chains sets up a
hydrogen ion gradient that drives ATP formation
Steps in Noncyclic Reactions
1. Light energy ejects electrons from photosystem II
2. Photosystem II pulls replacement electrons from water
molecules, which break apart into oxygen and hydrogen ions;
the oxygen leaves the cell as O2
3. Electrons enter transfer chains in the thylakoid membrane
4. Energy from electrons in the transfer chain pumps hydrogen
ions from the stroma into the thylakoid compartment; a
hydrogen ion gradient forms across the thylakoid membrane
Steps in Noncyclic Reactions (cont.)
5. Light energy ejects electrons from photosystem I;
replacement electrons come from an electron transfer chain
6. The electrons move through a second electron transfer chain,
then combine with NADP+ and H+ to form NADPH
7. Hydrogen ions in the thylakoid compartment diffuse through
the interior of ATP synthases and across the thylakoid
membrane; hydrogen ion flow causes ATP synthases to
attach phosphate to ADP, forming ATP in the stroma
Noncyclic Light-Dependent Reactions
Noncyclic Light-Dependent Reactions
to light-independent
reactions
light energy
light energy
4
1
5
3
7
6
2
The Light-Dependent Reactions of Photosynthesis
Fig. 6.7, p. 99
ANIMATION: Sites of photosynthesis
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The Cyclic Pathway
• Electrons released from photosystem I enter an electron
transfer chain, then cycle back to photosystem I
• NADPH does not form – ATP forms by electron transfer
phosphorylation
• Electrons flowing through electron transfer chains cause H+ to
accumulate in the thylakoid compartment
• H+ follows its gradient back across the membrane through
ATP synthases, driving ATP synthesis
3D ANIMATION:
Photophosphorylation
Animation: Noncyclic pathway of electron
flow
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ANIMATION: Harvesting photo
energy
ANIMATION: Photosynthesis - Light system
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ANIMATION: Light-dependent
reactions
6.6 Energy Flow in Photosynthesis
• Energy flow in light-dependent reactions is an example of how
organisms use energy harvested from the environment to
drive cellular processes
• The simpler cyclic pathway evolved first, and operates in
nearly all photosynthesizers
• Some organisms became modified to add photosystem II,
beginning a sequence of reactions that removes electrons
from water molecules, releasing hydrogen ions and oxygen
Making ATP and NADPH
• Having alternate pathways is efficient because cells can
produce NADPH and ATP, or produce ATP alone
• NADPH accumulates when it is not being used, which backs
up the noncyclic pathway, so the cyclic pathway
predominates – the cell makes ATP, but not NADPH
• When sugar production is high, NADPH is used quickly, and
does not accumulate – the noncyclic pathway predominates
The Cyclic Pathway
The Cyclic
Pathway
energy
Excited
P700
P700
(photosystem I)
light energy
Energy flow in the cyclic reactions of
photosynthesis
B In the cyclic pathway,
electrons ejected from
photosystem I are returned
to it. As long as electrons
continue to pass through
its electron transfer chain,
H+ continues to be carried
across the thylakoid
membrane, and ATP
continues to form. Light
provides the energy boost
that keeps the cycle going.
Fig. 6.8b, p. 100
energy
Excited
P700
The Cyclic
Pathway
P700
(photosystem I)
light energy
Energy flow in the cyclic reactions of
photosynthesis
Stepped Art
Fig. 6.8b, p. 100
The Noncyclic Pathway
Excited
P700
energy
Excited
P680
The
Noncyclic
Pathway
P700
(photosystem I)
P680
(photosystem II)
light energy
light energy
Energy flow in the noncyclic reactions of photosynthesis
A The noncyclic pathway is a one-way flow of electrons from water, to
photosystem II, to photosystem I, to NADPH. As long as electrons continue to
flow through the two electron transfer chains, H+ continues to be carried across
the thylakoid membrane, and ATP and NADPH keep forming. Light provides the
energy boosts that keep the pathway going.
Fig. 6.8a, p. 100
The Noncyclic Pathway
Excited
P700
energy
Excited
P680
P700
(photosystem I)
P680
(photosystem II)
light energy
light energy
Energy flow in the noncyclic reactions of photosynthesis
Stepped Art
Fig. 6.8a, p. 100
Key Concepts
• Making ATP and NADPH
• ATP forms in the first stage of photosynthesis, which is
light-dependent because the reactions run on the energy
of light
• The coenzyme NADPH forms in a noncyclic pathway that
also releases oxygen
• ATP also forms in a cyclic pathway that does not release
oxygen
ANIMATION: Energy Changes in
Photosynthesis
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6.7 Light-Independent Reactions:
The Sugar Factory
• The cyclic, light-independent reactions of the Calvin–Benson
cycle are the “synthesis” part of photosynthesis
• Carbon fixation occurs, and sugars are synthesized
• Inside the stroma, the enzyme rubisco attaches a carbon
from CO2 to RuBP to start the Calvin–Benson cycle
Key Terms
• Calvin–Benson cycle
• Light-independent reactions of photosynthesis; cyclic
carbon-fixing pathway that forms sugars from CO2
• carbon fixation
• Process by which carbon from an inorganic source such
as CO2 gets incorporated into an organic molecule
• rubisco (ribulose bisphosphate carboxylase)
• Carbon-fixing enzyme of the Calvin–Benson cycle
Energy for Sugar Synthesis
• Photo:
• ATP and NADPH are produced by the light-dependent
reactions using light energy
• Synthesis:
• Light-independent reactions use energy from ATP, and
hydrogen and electrons from NADPH, to synthesize
sugars from CO2
Steps of the Calvin–Benson Cycle
1. 6 CO2 enter a chloroplast; rubisco attaches each to a RuBP
molecule – resulting intermediates split –12 PGA form
2. Each PGA gets a phosphate group from ATP, plus hydrogen
and electrons from NADPH – 12 PGAL form
3. 2 PGAL combine to form 1 glucose molecule
4. Remaining 10 PGAL receive phosphate groups from ATP –
endergonic reactions regenerate 6 RuBP
Steps of the Calvin–Benson Cycle
Steps of the Calvin–Benson Cycle
1
4
Calvin–
Benson
Cycle
2
3
glucose
other molecules
Fig. 6.9, p. 101
Steps of the
Calvin–Benson
Cycle
1
4
Calvin–
Benson
Cycle
2
other molecules
3
glucose
Stepped Art
Fig. 6.9, p. 101
ANIMATION: Calvin-Benson cycle
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Key Concepts
• Making Sugars
• The second stage is the “synthesis” part of photosynthesis
• Sugars are assembled with carbon and oxygen atoms
from CO2
• The reactions run on the chemical bond energy of ATP,
and electrons donated by NADPH—molecules that formed
in the first stage of photosynthesis
6.8 Adaptations:
Carbon-Fixing Pathways
• When environments differ, so do details of light-independent
reactions
• Three pathways of sugar synthesis:
• C3 plants
• C4 plants
• CAM plants
Key Terms
• C3 plant
• Type of plant that uses only the Calvin–Benson cycle to fix
carbon
• C4 plant
• Type of plant that minimizes photorespiration by fixing
carbon twice, in two cell types
• CAM plant
• Type of C4 plant that conserves water by fixing carbon
twice, at different times of day
Photorespiration
• On dry days, plants conserve water by closing their stomata
• When stomata are closed, O2 from photosynthesis can’t
escape, and CO2 for photosynthesis can’t enter
• In C3 plants, high O2 levels cause rubisco to attach O2 to
RuBP instead of CO2
• This pathway (photorespiration) reduces the efficiency of
sugar production on dry days
Key Terms
• stomata
• Openings through plant surfaces
• Allow water vapor and gases to diffuse across the
epidermis (through the cuticle)
• photorespiration
• Reaction in which rubisco attaches oxygen instead of
carbon dioxide to ribulose bisphosphate
Photorespiration
CO2
O2
glycolate
RuBP
PGA
ATP
Calvin–
Benson
Cycle
NADPH
sugars
Fig. 6.10b, p. 102
ANIMATION: C3-C4 comparison
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C3 Plants
• C3 plants use only the
Calvin–Benson cycle
• Most plants, including
basswood (Tilia
americana), are C3
plants
C4 Plants
• In C4 plants, carbon
fixation occurs twice
• The first reactions
release CO2 near
rubisco, which limits
photorespiration when
stomata are closed
• Example: corn (Zea
mays)
CAM Plants
• CAM plants minimize
photorespiration by
opening stomata and
fixing carbon at night
• Example: Jade plants
(Crassula argentea)
Key Concepts
• Alternate Pathways
• Details of light-independent reactions that vary among
organisms are evolutionary adaptations to different
environmental conditions
Green Energy (revisited)
• Photosynthesis removes carbon dioxide from the atmosphere,
and locks its carbon atoms inside organic compounds
• When aerobic organisms break down the organic compounds
for energy, carbon atoms are released in the form of CO2
• Since photosynthesis evolved, these two processes have
constituted a balanced cycle of the biosphere
• Burning fossil fuels for energy has put Earth’s atmospheric
cycle of carbon dioxide out of balance
Fossil Fuel Emissions
• The sky over New York City on a sunny day
ANIMATION: Photosynthesis - Carbon
Fixing
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