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Plant Metabolism
Chapter 10
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Outline
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Photosynthesis
 Major Steps of Photosynthesis
 Light-Dependent Reactions
 Light-Independent Reactions
 C4 Photosynthesis
 CAM Photosynthesis
Respiration
 Glycolysis
 Electron Transport Chain
Enzymes and Energy Transfer
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Enzymes regulate most metabolic activities.
 Anabolism - Storing Energy.
- Photosynthesis reactions
 Catabolism - Consuming Stored Energy.
- Respiration reactions
Oxidation-Reduction Reactions
 Oxidation - Loss of electron(s).
 Reduction - Gain of electron(s)
- Usually coupled
Photosynthesis
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Energy for most cellular activity involves
adenosine triphosphate (ATP).
 Plants make ATP using light as an energy
source.
- Take place in cholorpolasts and other
green parts of the organisms.
6CO2+12H2O + light  C6H12O6+6O2+6H2O
Carbon Dioxide
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Carbon dioxide (0.035% of air) reaches
cholorplasts in the mesophyll cells by diffusing
through the stomata into the leaf interior.
 If not replenished, CO2 would be used up in 22
years. Use of fossil fuels, deforestation, and
other human activities have added excess
carbon dioxide to the atmosphere.
- May enhance photosynthesis.
 Plants may counter-balance by developing
fewer stomata.
Water
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Less than 1% of all the water absorbed by
plants is used in photosynthesis.
 Most of the remainder is transpired or
incorporated into plant materials.
If water is in short supply, stomata usually
close and thus reduce the supply of carbon
dioxide available for photosynthesis.
Light
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About 40% of the radiant energy received on
earth is in the form of visible light.
 Leaves commonly absorb about 80% of
the visible light reaching them.
- Light intensity varies with time of day,
season, altitude, latitude, and
atmospheric composition.
 Considerable variation in the light
intensities necessary for optimal
photosynthetic rates.
Light Wavelengths
Effects of Light and Temperature
on Photosynthesis
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Chlorophyll
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Several different types of chlorophyll.
 Most plants contain both chlorophyll a
(blue-green) and chlorophyll b (yellowgreen).
- Other pigments include carotenoids
(yellow and orange) phycobilins (blue or
red), and several other types of
chlorophyll.
- About 250-400 pigment molecules group
as a photosynthetic unit.
Grass green in color: C55H72MgN4O5
Chlorophyll b: bluish green in color : C55H70MgN4O6
3a:1b
Major Steps of Photosynthesis
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Light Dependent Reactions
 Water molecules split apart.
 Electrons passed along electron transport.
 ATP produced.
 NADPH produced.
Major Steps of Photosynthesis
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Light Independent Reactions
 Calvin Cycle
- Carbon dioxide combined with RuBP
and then combined molecules are
converted to sugars (Glucose).
 Energy furnished by ATP and NADPH
from Light-Dependent Reactions.
Light Dependent Reactions - In Depth
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Each pigment has its own distinctive pattern
of light absorption.
When pigment absorbs light
: electrons are excited. When
this occurs, energy may be
immediately emmitted as light
:flourescence or emmitted after
a delay : phosphorescence or
may be converted to heat.
Light Dependent Reactions - In Depth
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Two types of photosynthetic units present in
most chloroplasts make up photosystems.
 Photosystems I and II
- Both can produce ATP.
- Only organisms with both photosystem I
and photosystem II can produce NADPH
and oxygen as a consequence of
electron flow.
Photosystems
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Mitchell Theory or Chemiosmosis
Light Independent Reactions - In Depth
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Calvin Cycle
 Six molecules of CO2 combine with six
molecules of RuBP with the aid of rubisco.
 Resulting complexes split into twelve
3PGA molecules.
 NADPH and ATP supply energy and
electrons that reduce 3PGA to 12 GA3P.
 Ten of the twelve GA3P molecules are
restructured into six RuBP molecules.
The Calvin Cycle
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Photorespiration
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Stomata usually close on hot, dry days.
 Closed stomata prevent carbon dioxide
from entering the leaf. Then CO2 decrease
as relative O2 increase.
- When carbon dioxide levels drop below
about 50 parts per million,
photorespiration is initiated.
 Rubisco fixes oxygen instead of
carbon dioxide.
Light Independent Reactions - In Depth
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4-Carbon Pathway
 Sugar cane, corn, sorghum and at least
1000 species of plants have Kranz
Anatomy. Two forms of chloroplasts:
- Large chloroplast with few to no grana in
the bundle sheath cells surrounding the
veins.
- Smaller chloroplasts with well-developed
grana in the mesophyll cells.
Corn (Zea Mays) Cross-Section
Fig. 10.2a
4-Carbon Pathway
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Plants with Kranz
Anatomy produce
oxaloacetic acid (4carbon compound).
 Phosphoenolpyruvat
e (PEP) and carbon
dioxide combined in
mesophyll cells with
the aid of PEP
carboxylase.
- Provides a major
reduction in
photorespiration.
CAM Photosynthesis
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Seen in plants of about 30 families including
cacti orchids etc.
Similar to C4 photosynthesis in that 4-carbon
compounds are produced during the lightindependent reactions.
 However, in CAM, the organic acids
accumulate at night and break down during
the day, releasing carbon dioxide.
- Allows plants to function well under
limited water supplies, as well as high
light intensity.
CAM Photosynthesis
Fig. 10.22
Comparision
C3
C4
CAM
Leaf Anatomy
No bundle
sheath
Krantz
Anatomy
No palliside cells,
large vauoles
Carboxylation
Enzymes
RuBisco
PEP +RuBisco
Drakness PEP
+Light RuBisco
Transpiration ratio 450-950
(gm H2O/ gm dry
weight increase)
250-350
50-55
Optimum P.S.
Temperature
15-25 oC
30-40 oC
35 oC
CO2
Compensation
point
30-70 ppm
0-10 ppm
0-5 ppm in dark
Dry matter prod.
22 tons/ha/year 39 tons/ha/year Low; highly variable
C4 vs. CAM
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C4
 Stores CO2 in 4
carbon acid
 Separates CO2
fixation from sugar
synthesis in
SPACE(Bundle
sheath vs mesophyll
space)
 Adaptations to
TROPICAL
environments
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CAM
 Stores CO2 in 4
carbon acid
 Separates CO2
fixation from sugar
synthesis in TIME
 Adaptations to ARID
environments
Other Processes in Chloroplasts
1) Sulphates : reduced to sulphides
these then used for biosynthesis of
S-containing amino acids
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2) Nitrites converted to ammonia. This is then
converted to amino groups which are integral
part of several important amino acids such
as glutamine and aspartic acid.
Respiration
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Respiration is essentially the release of
energy from glucose molecules that are
broken down to individual carbon dioxide
molecules.
C6H12O6 + 6O2 6CO2 + 6H2O + energy
Respiration
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Fermentation
C6H12O6 2C2H5OH + 2CO2 + ATP
C6H12O6 2C3H6O3 + ATP
Factors Affecting the Rate of Respiration
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Temperature
Water
Oxygen
Major Steps of Respiration
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Glycolysis
 Glucose molecule becomes a fructose
molecule carrying two phosphates.
 Fructose molecule is split into two GA3P
molecules.
 Some hydrogen, energy, and water are
removed, leaving pyruvic acid.
Major Steps of Respiration
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Aerobic Respiration
 Citric Acid (Krebs) Cycle
O.A. + acetyl CoA + ADP+P+3NAD + FAD

O.A. + CoA+ATP+3NADH+H+ + FADH2+2CO2
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Electron Transport
 Oxidative Phosphorylation
 Chemiosmosis
Assimilation and Digestion
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Assimilation is the process of using organic
matter produced through photosynthesis to
build protoplasm and cell walls.
Digestion is the conversion of starch and
other insoluble carbohydrates to soluble
forms.
 Nearly always hydrolysis.
Endosperm starch grains
of rice
Starch
Soluble forms
DIGESTION
Conversion of PSic
products to structural forms
such as protoplasm or cell
wall is called ASSIMILATION
Ps
•Stores energy in sugar
•Uses carbon dioxide and
water
•Increases weight
•Occurs only in light
•Occurs only in cells
containing chlorphyll
•Produces oxygen in green
organisms
•Produces ATP with light
energy
Rs
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Releases energy from
sugar molecules
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Releases carbon dioxide
and water
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Decreases weight
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Occurs in either light or
darkness
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Occurs in all living cells
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Utilizes oxygen (aerobic
respiration)
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Produces ATP with energy
released from sugar
Review
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Photosynthesis
 Major Steps of Photosynthesis
 Light-Dependent Reactions
 Light-Independent Reactions
 C4 Photosynthesis
 CAM Photosynthesis
Respiration
 Glycolysis
 Electron Transport Chain
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Required for Reproduction or Display