Download Essentials of Biology

2/6/2011
Investigating Photosynthesis
Essentials of
Biology
Sylvia S. Mader
One of the first questions….
• When a tiny seedling grows
into a tall tree with a mass of
several tons, where does all
that mass come from?
Chapter 6
Photosynthesis
Lecture Outline
Chandelier Tree
•
•
315 ft. high and 21 ft. in diameter
Giant Redwood tree at Drive Thru
Tree Part, Leggett, CA
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2. ___________
Experiment (1771)
Priestly’s
Van Helmont’s Experiment (1643)
1. ______________

Put soil in pot and took mass

Took a seedling and took mass

Put seed in soil...watered...waited five years...
the seedling became a tree. Mass of soil barely changed.

He concluded that the mass came from water

He determined the
the ―hydrate‖ in the carbohydrate
portion of photosynthesis
Jan Ingenhousz
3. ________________Experiment
(1779)

Put aquatic plants in light... produced oxygen

Put aquatic plants in dark... No oxygen
Put a lit candle in a bell jar and… The flame died out.

Placed a mint plant in the jar with the candle and …
Flame lasted longer

Concluded…
plants release a substance needed for a
candle to burn
 He determined that plants release… oxygen
6.1 Overview of Photosynthesis
•
•
 He determined:
Light is needed to produce oxygen

•
•
PSN occurs in plants, algae, some
protists and some prokaryotes
Why is PSN the most important
chemical process on earth?
Provides food nearly all organisms
Transforms solar energy into
chemical energy of carbohydrates.
Melvin Calvin
4. _______________
(1948)

He determines carbon’s path to make glucose

Known as the Calvin’s cycle
Figure 6.1
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Flowering plants
Photosynthesis Overview
• Green portions carry on
photosynthesis.
• CO2 enters & O2 exits
leaves through stomata.
• Roots absorb water.
• Autotrophs (“self feeders”)
 PSN is the process by which autotrophs use light energy to make sugar and
oxygen gas from carbon dioxide and water
• CO2 and H2O diffuse into
mesophyll cells and then
into chloroplasts.
Figure 6.2a
• An overview of photosynthesis
H 2O
Figure 6.3
CO2
Chloroplast
NADP+
ADP
+ P
CALVIN
CYCLE
(in stroma)
Two sets of reactions
• Light reactions
 ATP, NADPH & O2 produced
 H and O2 come from H2O
• Calvin cycle reactions
 Occur in stroma
ATP
 CO2 taken up
 ATP and NADPH used to
reduce CO2 to produce
carbohydrate
NADPH
O2
Figure 6.3
 Occur in thylakoid membrane
 Chlorophyll absorbs solar
Light
LIGHT
REACTIONS
(in grana)
Photosynthesis Overview
Sugar
BioFlix: Photosynthesis
Photosynthetic process
 Begins with the end products of cellular
respiration: CO2 and H2O
 Hydrogen atoms removed from water are added to
carbon dioxide.
• Solar energy is required.
 O2 is a byproduct of the oxidation of water.
 End product is CH2O or glucose C6H12O6.
6.2 Light Reactions
• Leaf pigments
absorb solar energy.
• Light travels as
waves
 Shorter wavelengths
contain more
energy.
 Longer wavelengths
contain less energy.
• Vision and
photosynthesis are
adapted to use
visible light
Figure 6.4
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2/6/2011
Figure 6.5
What determines the color of a leaf?
• Electron pathway of light reactions
 Capture sun’s energy and stores in the
form of a hydrogen ion (H+) gradient
• Gradient used to produce ATP
 NADPH is also produced.
• What colors of light does a green leaf absorb? Color(s) reflected?
• Why do leaves change color?
• Chlorophyll covers up other pigments that are always present
 e.g. Carotenoids (orange); xanthophylls (red)
The Interdependence of the Light Reactions & the Calvin Cycle
H2O
CO2
ADP +
P
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NADP+
Calvin
cycle
reactions
Light
reactions
NADPH
ATP
Figure 6.6
O2
CH2O
(carbohydrate)
A mechanical analogy for the light reactions
The Steps of the Light Reactions
e–
ATP
Electron acceptor
e–
e–
Electron acceptor
NADPH
e–
e–
Mill
makes
ATP
Light Energy
Light
Energy
e–
Energy for
synthesis of
e–
PHOTOSYSTEM I
PHOTOSYSTEM II
Figure 6.6
Photosystem 2
Photosystem 1
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chloroplast
Making ATP, NADPH & O2 with Sunlight
(Low H+)
Light
Light
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Figure 6.7
(High H+)
Steps of the Light Reactions
• Photosystem II
6.3
The Calvin Cycle
 Solar energy energizes electrons.
 Electrons escape to electron acceptor molecule.
• Sent through electrons transport chain
 Replacement electrons obtained by splitting water
• Releases oxygen gas as waste product
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operating systems, some animations
will not appear until the presentation is
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• Electron Transport Chain
 Series of carriers pass electrons along releasing energy.
 Energy stored in form of H+ gradient
 Will be used to make ATP
• Photosystem I




Solar energy energizes electrons.
Electrons captured by another electron acceptor molecule
Electrons and a hydrogen passed to NADP+ to become NADPH
Replacement electrons come from electron transport chain.
6.3
Calvin Cycle Reactions
H2O
CO2
ADP +
1.
•
•
•
•
Powered by __???__ and __???__ produced by the light
reactions
Occurs in the __???__ of the chloroplast
End product is __???__
3 steps
1.
2.
3.
Carbon Dioxide Fixation
P

NADP+
Calvin
cycle
reactions
Ligh
reactionst
NADPH
ATP
3 CO2
O2
Unstable C6
Intermediate
CH2O
stroma
(carbohydrate)

3 C6
3 RuBP
Rubisco
CO2
fixation
Calvin
cycle
reactions
CO2 fixation
CO2 reduction
Regeneration of first substrate , RuBP
CO2 from the atmosphere
attached to RuBP by RuBP
carboxylase (rubisco)
6 carbon molecule split into
two 3 carbon molecules.
6 3PG
CO2
reduction
Regeneration
of RuBP
Key molecules of the Calvin Cycle
RuBP ribulose 1,5-bisphosphate
Figure 6.8
3PG
3-phosphoglycerate
BPG
1,3-bisphosphoglycerate
G3P
glyceraldehyde-3-phosphate
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H2O
2.
CO2
ADP +
Reduction of Carbon dioxide

P
NADP+
Uses NADPH (for H) and some
ATP (for energy) from light
reactions
G3P reacts to produce glucose &
other organic molecules
Calvin
cycle
reactions
Ligh
reactionst
NADPH
ATP
3 CO2
O2
intermediate

stroma
CH2O
(carbohydrate)
3 C6
H2O
CO2
3.

NADP+
Calvin
cycle
reactions
Ligh
reactionst
NADPH
ATP
3 CO2
O2
intermediate
stroma
CH2O
(carbohydrate)

3 C6
3 RuBP
3 RuBP
6 3PG
CO2
fixation
Calvin
cycle
reactions
CO2
reduction
6 3PG
CO2
fixation
6 ATP
From light
reactions
3 ADP + 3 P
6 ADP + 6 P
Regeneration
of RuBP
Calvin
cycle
reactions
From light
reactions
3 ATP
6 BPG
6 ATP
CO2
reduction
Regeneration
of RuBP
6 NADP+
Glucose and other
organic molecules
6 BPG
From light
reactions
5 G3P
Key molecules of the Calvin Cycle
6 G3P
6 NADP+
3-phosphoglycerate
BPG
1,3-bisphosphoglycerate
G3P
glyceraldehyde-3-phosphate
• Plants and algae can make any
molecule they need from G3P!
Figure 6.8
amino acids
glycerol
fatty acids
G3P
glucose
phosphate
cellulose
 Nucleotides for DNA and RNA
 Sucrose for transport through plant
 Starch for storage
Key molecules of the Calvin Cycle
RuBP ribulose 1,5-bisphosphate
Net gain of one G3P
3PG
starch
Glucose and other
organic molecules
3PG
3-phosphoglycerate
BPG
1,3-bisphosphoglycerate
G3P
glyceraldehyde-3-phosphate
6.4 Other types of Photosynthesis
Why is G3P possibly the most important molecule for life?
 Amino acids
 Fatty acid and glycerol
 Glucose for energy needs
Uses ATP from light
reactions
6 ADP + 6 P
RuBP ribulose 1,5-bisphosphate
Net gain of one G3P
One G3P made into
glucose or other organic
molecules.
5 G3P used to reform
RuBP (5 carbon
molecule)
6 NADPH
From light
reactions
6 G3P

From light
reactions
6 NADPH
Figure 6.8
Regeneration of RuBP
P
ADP +
• Plants are adapted to their climate
• When temperature and rainfall are moderate
 Use C3 photosynthesis
 C3 plants
• C3 compound formed first after CO2 fixation
sucrose
 Cellulose for cell walls
Figure 6.9 The fate of G3P
C3 plants in Hot and Dry Weather
Figure 6.10 Carbon fixation in C3 plants
day
• Water loss can be deadly!
• Close stomata to prevent water loss
CO2
mesophyll
cell
RuBP
Calvin
cycle
G3P
CO2 fixation in a C3 plant,
blue columbine
C3
 Limits water loss, but…
 CO2 intake stops
 O2 builds up; results in photorespiration
• Photorespiration—a farmer’s worst enemy!




Rubisco uses O2 instead of CO2
Destroys RuBP
Limits growth in C3 plants
Common C3 Plants
 wheat, rice, barley, potatoes, sugar beet, soybeans, many grasses
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Figure 6.11
Comparison of C3 and C4 plant anatomy
Carbon dioxide fixation in C4 plants
Daytime
• C4 compound formed after 1st CO2
fixation
• Partitioning of pathways in space
• Mesophyll cells shield bundle sheath
cells from buildup of O2.
• Chloroplasts in bundle sheath cells
carry out Calvin cycle only.
• Common C4 Plants
 Corn, sorghum, millet, and sugarcane
C4 plant
C3 plant
• Calvin cycle and light reactions both
in Mesophyll cells
 An advantage in cool, wet
weather
• C4 plants partition reactions
• Avoids O2 exposure to Rubisco
•
Allows stomata to stay closed
(conserving water)
•
Advantage in hot dry weather
Figure 6.12
CAM photosynthesis





Crassulacean acid metabolism
Most succulents in a desert environment
Partitioning in time
CAM plants open stomata at night when it is cooler.
Use C3 molecules to fix CO2 forming C4 molecules
Figure 6.13
night
Carbon dioxide fixation in a CAM plant
CO2
C4
CO2
• Store C4 molecules in vacuoles
Calvin
cycle
 Close stomata during the day to avoid water loss
 Release stored CO2 when NADPH and ATP available
from light reaction
G3P
day
CO2 fixation in a CAM Plant,
Pineapple
• Evolutionary trends
 C4 plants most likely evolved in, and are adapted
to, areas of high light, high temperature, and
limited rainfall.
• More sensitive to cold – C3 plants do better than C4
plants below 25°C
 CAM plants compete well with C3 or C4 when the
environment is extremely arid.
• CAM is quite widespread
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