Download Chapter 7 Notes

• Chapter 7~
Photosynthesis
Photosynthesis in nature
• Autotrophs:
• biotic producers;
photoautotrophs;
chemoautotrophs; obtains
organic food without eating
other organisms
• Heterotrophs:
biotic consumers; obtains
organic food by eating other
organisms or their byproducts (includes
decomposers)
The chloroplast
•
•
•
•
•
•
Sites of photosynthesis
Pigment: chlorophyll
Plant cell: mesophyll
Gas exchange: stomata
Double membrane
Thylakoids, grana,
stroma
Leaves and Photosynthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
cuticle
upper
epidermis
Leaf cross section
mesophyll
lower
epidermis
CO2
O2
leaf vein
outer membrane
stoma
inner membrane
stroma
stroma
granum
Chloroplast
37,000
thylakoid space
thylakoid membrane
Grana
independent thylakoid
in a granum
overlapping thylakoid
in a granum
© Dr. George Chapman/Visuals Unlimited
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7.2 The Process of Photosynthesis
• Light Reactions – take place only in the presence
of light
–
–
–
–
Energy-capturing reactions
Chlorophyll absorbs solar energy
This energizes electrons
Electrons move down an electron transport chain
• Pumps H+ into thylakoids
• Used to make ATP out of ADP and NADPH out of NADP
• Calvin Cycle Reactions – take place in the stroma
– CO2 is reduced to a carbohydrate
– Use ATP and NADPH to produce carbohydrate
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Photosynthetic Pigments and
Photosynthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Increasing wavelength
chlorophyll a
chlorophyll b
carotenoids
Gamma
rays
X rays
UV
Infrared
Micro- Radio
waves waves
visible light
500
600
Wavelengths (nm)
a. The electromagnetic spectrum includes visible light.
750
Relative Absorption
Increasing energy
380
500
600
750
Wavelengths (nm)
b. Absorption spectrum of photosynthetic pigments.
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Electromagnetic Spectrum
• Visible Light – only a small portion of the
spectrum, but it is the radiation that drives
photosynthesis.
• Photons – fixed amounts of energy, nontangible objects. Amount of energy is
inversely related to wavelength
Photons and Energy
• When pigments (ex. Chlorophyll) absorb
photons, the energy has to go somewhere.
The electrons of a pigment jump to an
excited state and absorb the photon. The
electrons will then have potential energy,
but will be very unstable so will fall back to
the ground state and when they fall they
release energy as heat.
Plants as Solar Energy Converters
• The light reactions consist of two alternate
electron pathways:
– Noncyclic pathway
– Cyclic pathway
• Capture light energy with photosystems
– Pigment complex helps collect solar energy like an
antenna
– Occur in the thylakoid membranes
• Both pathways produce ATP
• The noncyclic pathway also produces NADPH
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Photosystems
• Light harvesting units of the
thylakoid membrane
• Composed mainly of protein
and pigment antenna complexes
• Antenna pigment molecules are
struck by photons
• Energy is passed to reaction
centers (redox location)
• Excited e- from chlorophyll is
trapped by a primary eacceptor
Figure 10.6 Why leaves are green: interaction of light with chloroplasts
Photosystem I & II
• Photosystem I – the reaction center is
known as P700 because it best absorbs light
at 700nm
• Photosystem II – the reaction center is
known as P680 because it absorbs light best
at 680nm
• The two pigments are actually the same, but
are associated with different proteins so
work differently
Noncyclic electron flow
• Photosystem II (P680):
– photons excite chlorophyll e- to an acceptor
– e- are replaced by splitting of H2O (release of O2)
– e-’s travel to Photosystem I down an electron transport chain
– as e- fall, ADP  ATP (noncyclic photophosphorylation)
• Photosystem I (P700):
– ‘fallen’ e- replace excited e- to primary e- acceptor
– 2nd ETC ( Fd~NADP+ reductase) transfers e- to NADP+ 
NADPH (...to Calvin cycle…)
• These photosystems produce equal amounts of ATP and NADPH
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 1)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 2)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 3)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 4)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 5)
Chemiosmosis
• Chloroplasts and Mitochondria generate ATP through
chemiosmosis. Just like we learned in Chapter 9 in the electron
transport chain and oxidative phosphorylation, the chloroplasts
do this much the same way except for:
• In cellular respiration the e- dropped down the etc came from
food molecules. In chloroplasts the e- came from captured light.
Meaning the mitochondria transforms chemical energy from
food molecules to ATP, and chloroplasts transform light energy
into chemical energy.
• In mitochondria the protons were pumped into the intercellular
matrix then back into the mitochondrial matrix. In chloroplasts
the protons are pumped into the thylakoid space and then back
into the stroma, so ATP is made in the stroma where it is used to
drive sugar synthesis during the Calvin cycle.
D:\ImageLibrary1-17\10Photosynthesis\10-17LightReactions.mov
Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts
Figure 10.16 The light reactions and chemiosmosis: the organization of the thylakoid
membrane
Organization of a Thylakoid
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O
CO2
solar
energy
ADP +
P
NADP +
Light
reactions
Calvin
cycle
reactions
NADPH
ATP
thylakoid membrane
thylakoid
membrane
thylakoid
thylakoid space
CH2O
O2
granum
photosystem II
electron transport
chain
H+
stroma
photosystem I
H+
NADP
reductase
Pq
ee-
e-
NADP+
NADPH
e-
eH+
H+
H2 O
2
H+
+
1
2
H+
H+
H+
H+
H+
H+
H+
O2
ATP synthase
H+
H+
H+
H+
Thylakoid
space
H+
ATP
H+
H+
chemiosmosis
P
Stroma
+ ADP
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Animation
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The Calvin cycle
• 3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3phosphate (G3P)
• Phases:
– 1- Carbon fixation~ each CO2 is attached to RuBP
(rubisco enzyme)
– 2- Reduction~ electrons from NADPH reduces to G3P;
ATP used up
– 3- Regeneration~ G3P rearranged to RuBP; ATP used;
cycle continues
D:\ImageLibrary
1-17\10Photosynthesis\10
-16CalvinCycle.mov
Animation
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
26
Calvin cycle
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Calvin Cycle, net synthesis
• For each G3P (and for 3 CO2)…….
Consumption of 9 ATP’s & 6 NADPH
(light reactions regenerate these
molecules)
• G3P can then be used by the plant to make
glucose and other organic compounds
Cyclic electron flow
• Alternative cycle when
ATP is deficient
• Photosystem I used but
not II; produces ATP but
no NADPH
• Why? The Calvin cycle
consumes more ATP than
NADPH…….
• Cyclic
photophosphorylation
Fate of G3P
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
G3P
fatty acid
synthesis
glucose
phosphate
amino acid
synthesis
+
fructose
phosphate
Sucrose (in leaves,
fruits, and seeds)
Starch (in roots
and seeds)
Cellulose (in trunks,
roots, and branches)
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© Herman Eisenbeiss/Photo Researchers, Inc.
7.5 Other Types of
Photosynthesis
• In hot, dry climates
–
–
–
–
Stomata must close to avoid wilting
CO2 decreases and O2 increases
O2 starts combining with RuBP, leading to the production of CO2
This is called photorespiration
• C4 plants solve the problem of photorespiration
– Fix CO2 to PEP (a C3 molecule)
– The result is oxaloacetate, a C4 molecule
– In hot & dry climates
• C4 plants avoid photorespiration
• Net productivity is about 2-3 times greater than C3 plants
– In cool, moist environments, C4 plants can’t compete with C3
plants
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Chloroplast Distribution in C4 vs. C3 Plants
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
C3 Plant
C4 Plant
mesophyll
cells
bundle sheath
cell
vein
stoma
bundle sheath
cell
vein
stoma
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CO2 Fixation in C3 and C4 Plants
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CO2
RuBP
Calvin
cycle
3PG
G3P
mesophyll cell
a. CO2 fixation in a C3 plant, wildflowers
CO2
mesophyll C
4
cell
bundle
sheath
cell
CO2
Calvin
cycle
G3P
b. CO2 fixation in a C4 plant, corn, Zea mays
a: © Brand X Pictures/PunchStock RF; b: Courtesy USDA/Doug Wilson, photographer
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Other Types of Photosynthesis
• CAM Photosynthesis
– Crassulacean-Acid Metabolism
– CAM plants partition carbon fixation by time
• During the night
– CAM plants fix CO2
– Form C4 molecules, which are
– Stored in large vacuoles
• During daylight
– NADPH and ATP are available
– Stomata are closed for water conservation
– C4 molecules release CO2 to Calvin cycle
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CO2 Fixation in a CAM Plant
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
night
CO2
C4
day
CO2
Calvin
cycle
G3P
CO2 fixation in a CAM plant, pineapple, Ananas comosus
© S. Alden/PhotoLink/Getty Images.
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Other Types of Photosynthesis
• Each method of photosynthesis has advantages and
disadvantages
– Depends on the climate
• C4 plants most adapted to:
– High light intensities
– High temperatures
– Limited rainfall
• C3 plants better adapted to
– Cold (below 25°C)
– High moisture
• CAM plants are better adapted to extreme aridity
– CAM occurs in 23 families of flowering plants
– Also found among nonflowering plants
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A review of photosynthesis