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Where It Starts – Photosynthesis
Chapter 7 Part 2
7.6 Light-Independent Reactions:
The Sugar Factory
 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
 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 lightdependent reactions (ATP, NADPH)
The Calvin-Benson Cycle
 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
Inputs and Outputs of
the Calvin-Benson Cycle
The Calvin-Benson Cycle
A Six CO2 in air spaces inside of
a leaf diffuse into a photosynthetic
cell. Rubisco attaches each to a
RuBP molecule. The resulting
intermediates split, so twelve
molecules of PGA form.
A
12 ATP
B Each PGA molecule gets a
phosphate group from ATP,
plus hydrogen and electrons
from NADPH. Twelve
intermediate molecules
(PGAL) form.
6CO2
B
12 PGA
6 RuBP
6 ADP
12 ADP + 12 Pi
Calvin–Benson
Cycle
6
ATP
12 NADPH
4 Pi
12 NADP+
12 PGAL
C
D
10 PGAL
other molecules
glucose
C Two of the PGAL
combine and form one
molecule of glucose.
The glucose may enter
reactions that form
other carbohydrates,
such as sucrose and
starch.
D The remaining ten
PGAL get phosphate
groups from ATP.
The transfer primes
them for endergonic
reactions that
regenerate the 6
RuBP.
Fig. 7-11, p. 115
A Six CO2 in air spaces inside of
a leaf diffuse into a photosynthetic
cell. Rubisco attaches each to a
RuBP molecule. The resulting
intermediates split, so twelve
molecules of PGA form.
B Each PGA molecule gets a
phosphate group from ATP,
plus hydrogen and electrons
from NADPH. Twelve
intermediate molecules
(PGAL) form.
A
B
12
ATP
12 PGA
6 RuBP
6 ADP
12 ADP + 12 Pi
C Two of the PGAL
combine and form one
molecule of glucose.
The glucose may enter
reactions that form
other carbohydrates,
such as sucrose and
starch.
6CO2
Calvin–Benson
Cycle
6
ATP
12 NADPH
4 Pi
12 NADP+
12 PGAL
C
D
10 PGAL
glucose
other
molecules
D The
remaining ten
PGAL get
phosphate
groups from
ATP. The
transfer primes
them for
endergonic
reactions that
regenerate the
6 RuBP.
Stepped Art
Fig. 7-11, p. 115
Animation: Calvin-Benson cycle
7.7 Adaptations:
Different Carbon-Fixing Pathways
 Environments differ, and so do details of
photosynthesis
• C3 plants
• C4 plants
• CAM plants
Stomata
 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
C3 Plants
 C3 plants
• Plants that use only the Calvin–Benson cycle to
fix carbon
• Forms 3-carbon PGA in mesophyll cells
• Used by most plants, but inefficient in dry weather
when stomata are closed
Photorespiration
 When stomata are closed, CO2 needed for lightindependent 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
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
C3 and C4 Plant Leaves
Fig. 7-12a, p. 116
palisade mesophyll
cell
spongy mesophyll
cell
A C3 plant leaves. Chloroplasts are distributed evenly among two
kinds of mesophyll cells in leaves of C3 plants such as basswood
(Tilia americana). The light-dependent and light-independent
reactions occur in both cell types.
Fig. 7-12a, p. 116
Fig. 7-12b, p. 116
bundle-sheath cell
mesophyll cell
B C4 plant leaves. In C4 plants such as corn (Zea mays), carbon is fixed
the first time in mesophyll cells, which are near the air spaces in the leaf,
but have few chloroplasts. Specialized bundle-sheath cells ringing the
leaf veins closely associate with mesophyll cells. Carbon fixation occurs
for the second time in bundle-sheath cells, which are stuffed with
rubisco-containing chloroplasts.
Fig. 7-12b, p. 116
CAM Plants
 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
A CAM Plant
 Jade plant (Crassula argentea)
C3, C4, and CAM Reactions
Fig. 7-13a, p. 117
mesophyll cell
CO2
O2
glycolate
RuBP
Calvin–
Benson
Cycle
sugar
PGA
ATP
NADPH
A C3 plants. On dry days, stomata close and oxygen
accumulates to high concentration inside leaves. The
excess causes rubisco to attach oxygen instead of
carbon to RuBP. Cells lose carbon and energy as they
make sugars.
Fig. 7-13a, p. 117
Fig. 7-13b, p. 117
mesophyll cell
CO2
from
inside
plant
bundle-sheath cell
C4 oxaloacetate
Cycle
CO2
RuBP
Calvin–
Benson
PGA
Cycle
sugar
B 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 to prevent rubisco from using oxygen.
Fig. 7-13b, p. 117
Fig. 7-13c, p. 117
mesophyll cell
CO2
from
outsid
e plant
C4 oxaloacetate
Cycle
night
day
CO2
RuBP
Calvin–
Benson
PGA
Cycle
sugar
C CAM plants open stomata and fix carbon
using a C4 pathway at night. When stomata
are closed during the day, the organic
compounds made during the night are
converted to CO2 that enters the Calvin–
Benson cycle.
Fig. 7-13c, p. 117
7.6-7.7 Key Concepts:
Making Sugars
 The second stage is the “synthesis” part of
photosynthesis, in which sugars are assembled
from CO2
 The reactions use ATP and NADPH that form in
the first stage of photosynthesis
 Details of the reactions vary among organisms
7.8 Photosynthesis and the Atmosphere
 The evolution of photosynthesis dramatically
and permanently changed Earth’s atmosphere
Different Food Sources
 Autotrophs
• Organisms that make their own food using energy
from the environment and inorganic carbon
 Heterotrophs
• Organisms that get energy and carbon from
organic molecules assembled by other organisms
Two Kinds of Autotrophs
 Chemoautotrophs
• Extract energy and carbon from simple molecules
in the environment (hydrogen sulfide, methane)
• Used before the atmosphere contained oxygen
 Photoautotrophs
• Use photosynthesis to make food from CO2 and
water, releasing O2
• Allowed oxygen to accumulate in the atmosphere
Earth With and Without
Oxygen Atmosphere
Fig. 7-15a, p. 118
Fig. 7-15b, p. 118
Effects of Atmospheric Oxygen
 Selection pressure on evolution of life
• Oxygen radicals
 Development of ATP-forming reactions
• Aerobic respiration
 Formation of ozone (O3) layer
• Protection from UV radiation
7.8 Key Concepts:
Evolution and Photosynthesis
 The evolution of photosynthesis changed the
composition of Earth’s atmosphere
 New pathways that detoxified the oxygen byproduct of photosynthesis evolved
7.9 A Burning Concern
 Earth’s natural atmospheric cycle of carbon
dioxide is out of balance, mainly as a result of
human activity
The Carbon Cycle
 Photosynthesis locks CO2 from the atmosphere
in organic molecules; aerobic respiration returns
CO2 to the atmosphere
• A balanced cycle of the biosphere
 Humans burn wood and fossil fuels for energy,
releasing locked carbon into the atmosphere
• Contributes to global warming, disrupting
biological systems
Fossil Fuel Emissions
7.9 Key Concepts:
Photosynthesis, CO2 & Global Warming
 Photosynthesis by autotrophs removes CO2
from the atmosphere; metabolism by all
organisms puts it back in
 Human activities have disrupted this balance,
and contribute to global warming
Animation: C3-C4 comparison
Animation: Harvesting photo energy
Animation: Light-dependent reactions
Animation: Photosynthesis overview
Animation: Structure of a chloroplast
Animation: Wavelengths of light
ABC video: Solar Power
Video: Biofuels