Chapter 6
Where It Starts –
Photosynthesis
Electromagnetic Spectrum
of Radiant Energy
shortest wavelengths
(highest energy)
gamma
rays
x-rays
range of heat
range of most radiation escaping from
reaching Earth’s surface Earth’s surface
visible light
near-infrared
ultraviolet
radiation
radiation
400 nm
500 nm
longest wavelengths
(lowest energy)
radiation
microwaves radio waves
infrared
600 nm
700 nm
Absorption Spectra
chlorophyll b
phycoerythrobilin
phycocyanobilin
Light absorption
β-carotene
400 nm
chlorophyll a
500 nm
600 nm
Wavelength
700 nm
Take-Home Message: Why do cells use more
than one photosynthetic pigment?
 A combination of pigments allows a photosynthetic organism to most
efficiently capture the particular range of light wavelengths that reaches
the habitat in which it evolved
5
two outer membranes
of chloroplast
stroma
part of thylakoid
membrane system:
thylakoid
compartment,
cutaway view
Figure 6-5b p105
6
Figure 6-7 p106
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The Noncyclic Pathway
 Photosystems (type II and type I) contain “special pairs” of chlorophyll a molecules that eject
electrons
 Electrons lost from photosystem II are replaced by photolysis of water molecules – the process
by which light energy breaks down a water molecule into hydrogen and oxygen
 Electrons lost from a photosystem enter an electron transfer chain (ETC) in the thylakoid
membrane
 In the ETC, electron energy is used to build up a H+ gradient across the membrane
 H+ flows through ATP synthase, which attaches a phosphate group to ADP
 ATP is formed in the stroma by chemiosmosis, or electron transfer phosphorylation
 Electrons from the first electron transfer chain (from photosystem II) are accepted by
photosystem I
 Electrons ejected from photosystem I enter a different electron transfer chain in which the
coenzyme NADP+ accepts the electrons and H+, forming NADPH
 ATP and NADPH are the energy products of light-dependent reactions in the noncyclic
pathway
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Noncyclic Pathway of Photosynthesis
H+
light energy
electron transfer chain
light energy
to second stage
of reactions
ADP + Pi
ATP
synthase
photosystem II
photosystem I
thylakoid
compartment
stroma
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The Cyclic Pathway
 When NADPH accumulates in the stroma, the noncyclic pathway stalls
 A cyclic pathway runs in type I photosystems to make ATP; electrons are cycled back to
photosystem I and NADPH does not form
Photophosphorylation
• Photophosphorylation is a light-driven reaction that attaches a
phosphate group to a molecule
• In noncyclic photophosphorylation, electrons move from water
to photosystem II, to photosystem I, to NADPH
• In cyclic photophosphorylation, electrons cycle within
photosystem I
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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
Figure 6-9a p108
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energy
Excited
P700
P700
(photosystem I)
light energy
Energy flow in the cyclic reactions of
photosynthesis
Stepped Art
Figure 6-9b p108
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What happens during the
light-dependent reactions of
photosynthesis?
Take-Home Message:
 In light-dependent reactions, chlorophylls and other pigments in thylakoid
membrane transfer light energy to photosystems
 Photosystems eject electrons that enter electron transfer chains in the
membrane; electron flow through ETCs sets up hydrogen ion gradients that
drive ATP formation
 In the noncyclic pathway, oxygen is released and electrons end up in NADPH
 A cyclic pathway involving only photosystem I allows the cell to continue
making ATP when the noncyclic pathway is not running; NADPH does not form;
O2 is not released
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How does energy flow
during the reactions of photosynthesis?
Take-Home Message:
 Light provides energy inputs that keep electrons flowing through electron
transfer chains
 Energy lost by electrons as they flow through the chains sets up a hydrogen
ion gradient that drives the synthesis of ATP alone, or ATP and NADPH
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Light-Independent Reactions
 The cyclic, light-independent reactions of the Calvin-Benson cycle are the
“synthesis” part of photosynthesis
 Calvin-Benson cycle
 Enzyme-mediated reactions that build sugars in the stroma of chloroplasts
 Carbon fixation
 Extraction of carbon atoms from inorganic sources (atmosphere) and
incorporating them into an organic molecule
 Builds glucose from CO2
 Uses bond energy of molecules formed in light-dependent reactions (ATP,
NADPH)
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The Calvin-Benson Cycle
 The enzyme rubisco attaches CO2 to RuBP
 Forms two 3-carbon PGA molecules
 PGAL is formed
 PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH
 Two PGAL combine to form a 6-carbon sugar
 Rubisco is regenerated
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1
4
Calvin–
Benson
Cycle
2
other molecules
3
glucose
Stepped Art
Figure 6-10 p109
18
19
Take-Home Message: What happens in light-
independent reactions of photosynthesis?
 Light-independent reactions of photosynthesis run on the bond energy of
ATP and energy of electrons donated by NADPH; both formed in the lightdependent reactions
 Collectively called the Calvin–Benson cycle, these carbon-fixing reaction
use hydrogen (from NADPH), and carbon and oxygen (from CO2) to build
sugars
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21
22
Adaptations:
Different Carbon-Fixing Pathways
 Environments differ, and so do details of photosynthesis:
 C3 plants
 C4 plants
 CAM plants
 Stomata
 Small openings through the waxy cuticle covering epidermal surfaces of leaves
and green stems
 Allow CO2 in and O2 out
 Close on dry days to minimize water loss
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C3 Plants
palisade
mesophyll cell
 C3 plants
 Plants that use only the Calvin–Benson cycle to fix carbon
 Forms 3-carbon PGA in mesophyll cells
spongy
mesophyll cell
 Used by most plants, but inefficient in dry weather when stomata are closed
 Example: barley
 When stomata are closed, CO2 needed for light-independent reactions can’t
enter, O2 produced by light-dependent reactions can’t leave
 Photorespiration
 At high O2 levels, rubisco attaches to oxygen instead of carbon
 CO2 is produced rather than fixed
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mesophyll cell
CO2
O2
glycolate
RuBP
Calvin–
Benson
PGA
Cycle
ATP
NADPH
B On dry days, stomata close and oxygen accumulates
inside leaves. The excess causes rubisco to attach oxygen
instead of carbon to RuBP. This is photorespiration, and it
makes sugar production inefficient in C3 plants.
sugars
Figure 6-11b p110
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C4 Plants
 C4 plants
 Plants that have an additional set of reactions for sugar production on dry days
when stomata are closed; compensates for inefficiency of rubisco
 Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make
sugar
 Examples: Corn, switchgrass, bamboo
 C4 plants. Oxygen also builds up inside leaves when stomata close during
photosynthesis.
 An additional pathway in these plants keeps the CO2 concentration high enough in
bundle-sheath cells to prevent photorespiration.
mesophyll cell
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mesophyll cell
B C4 plants.
Oxygen also builds
up inside leaves
oxaloacetate C4
when stomata
Cycle
close during
photosynthesis.
bundle-sheath cell
bundle-sheath cell
CO2 from inside plant
An additional
pathway in these
plants keeps the
CO2 concentration
high enough in
bundle-sheath
cells to prevent
photorespiration.
CO2
PGA
RuBP
Calvin–
Benson
Cycle
sugars
Figure 6-12b p110
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CAM Plants
A CAM Plant: Jade
Plant
 CAM plants (Crassulacean Acid Metabolism)
 Plants with an alternative carbon-fixing pathway that allows them to conserve
water in climates where days are hot
 Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for
sugar production
 Example: succulents, cactuses
mesophyll cell
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CO2 from outside plant
oxaloacetate C4
Cycle
night
day
CO2
PGA
RuBP
Calvin–
Benson
Cycle
sugars
Figure 6-13a p111
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Take-Home Message:
How do carbon-fixing reactions vary?
 When stomata are closed, oxygen builds up inside leaves of C3 plants;
rubisco then can attach oxygen (instead of carbon dioxide) to RuBP;
photorespiration reduces the efficiency of sugar production, so it can limit
the plant’s growth
 Plants adapted to dry conditions limit photorespiration by fixing carbon
twice: C4 plants separate the two sets of reactions in space; CAM plants
separate them in time