Transcript Understanding Our Environment

Plant Metabolism
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 in cells
in which chlorophyll is embedded.
6CO2+12H2O + light  C6H12O6+6O2+6H2O
Carbon Dioxide
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Carbon dioxide (0.037% of air) reaches
cholorplasts in the mesophyll cells by diffusing
through the stomata into the leaf interior.
 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 cytoplasm, vacuoles, and
other 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
Chlorophyll
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Several different types of chlorophyll.
 Most plants contain both chlorophyll a (bluegreen) and chlorophyll b (yellow-green).
- Other pigments include carotenoids (yellow
and orange) phycobilins (blue or red found in
cyanobacteria and red algae), and several
other types of chlorophyll (chlorophylls c, d,
and e in algae and photosynthetic bacteria).
- About 250-400 pigment molecules group as a
photosynthetic unit.
Fig. 10.4
Major Steps of Photosynthesis
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Light Dependent Reactions
 Water molecules split apart releasing
electrons and hydrogen ions (H+), and O2.
 Electrons from splitting water passed along
an electron transport chain.
 ATP produced.
 NADPH is produced.
Major Steps of Photosynthesis
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Light Independent Reactions
 Calvin Cycle
- Takes place in the stroma of chloroplasts.
- Carbon dioxide combines with RuBP and
then the resulting molecule enters the
Calvin cycle and is converted to sugars
(Glucose) .
 Energy is furnished by ATP and
NADPH produced from the LightDependent Reactions.
Fig. 10.5
Fig. 10.6
Light Dependent Reactions - In Depth
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Each pigment has its own distinctive pattern of light
absorption.
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
Fig. 10.8
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Noncyclic photophosphorylation: The unidirectional
flow of electrons from water through photosystem I
(PS I) then photosystem II (PS II) and finally to
NADP to produce NADPH. H+ are used to produce
ATP via ATP synthase. Oxygen gas, produced from
splitting water, is a byproduct of noncyclic
photophosphorylation.
Cyclic photophosphorylation: Only PS I is involved.
Electrons boosted from PS I are shunted back into
the reaction center via the electron transport
system. ATP is produced from ADP and P, but no
NADPH or oxygen is produced.
Fig. 10.9
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 (RuBP
carboxylase/oxygenase).
 Resulting complexes split into twelve 3PGA molecules.
 NADPH and ATP (from light dependent reactions) supply
energy and electrons that reduce the twelve 3PGA to 12
GA3P.
 Ten of the twelve GA3P molecules are restructured into
six RuBP molecules using another 6 ATPs. The
remaining two GA3P are used in making glucose, starch
and other cellular products (lipids & amino acids).
 This is known as the C3 pathway (C3 plants).
The Calvin Cycle
Photorespiration
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Stomata usually close on hot, dry days.
 Closed stomata prevent carbon dioxide from
entering the leaf.
- When carbon dioxide levels drop below about
50 parts per million, photorespiration is
initiated.
 Rubisco fixes oxygen instead of carbon
dioxide allowing C3 plants to survive under
hot, dry conditions. This helps to dissipate
ATP and accumulated electrons from the
light reactions thereby preventing
photooxidative damage.
Light Independent Reactions - In Depth
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4-Carbon Pathway
 Plants have Kranz Anatomy.
- 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
4-Carbon Pathway
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Plants w (has high affinity to CO2 and is not sensitive to oxygen).ith
Kranz Anatomy produce oxaloacetic acid (4-carbon compound).
 Phosphoenolpyruvate (PEP) and carbon dioxide are combined in
mesophyll cells with the aid of PEP carboxylase.
 PEP carboxylase has high affinity to CO2 and is not sensitive to
oxygen thus provides a major reduction in photorespiration.
 Plants with Kranz anatomy are called C4 plants
 C4 plants have two advantages:
- high concentration of PEP carboxylase (with high affinity to CO2) in
mesophyll cells thus converts CO2 at a lower concentration than
Rubisco.
- Optimum temperatures for C4 photosynthesis are much higher than
C3 photosynthesis allowing C4 plants to thrive under temperatures
that would adversely affect C3 plants.
CAM Photosynthesis
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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
Respiration
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Respiration is essentially the release of
energy from glucose molecules that are
broken down to individual carbon dioxide
molecules.
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Aerobic respiration:
C6H12O6 + 6O2 6CO2 + 6H2O + energy
Respiration
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Fermentation
C6H12O6 2C2H5OH + 2CO2 + ATP
C6H12O6 2C3H6O3 + ATP
Anaerobic respiration and fermentation
Release less than 6% of the energy released
from a molecule of glucose by aerobic
Respiration.
Factors Affecting the Rate of Respiration
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Temperature
Water
Oxygen
Major Steps of Respiration
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Glycolysis
 Occurs in the cytoplasm.
 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
Occurs in the mitochondria
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
Energy (ATP) resulting from aerobic respiration of a
glucose molecule
Glycolysis (cytoplasm)
2 NADH (ea = 2 ATP when moved into mitoch.)
Pyruvic acid to acetyl CoA (mitoch. matrix)
2 NADH (ea = 3 ATP)
Citric Acid Cycle (mitoch. matrix)
6 NADH (ea = 3 ATP)
2 FADH2 (ea = 2 ATP)
TOTAL
ATP
-2
+4
+4
+6
+18
+4
+2
+36
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Efficiency of aerobic respiration:
The 36 ATP molecules produced represent
39% of the energy harnessed in the glucose
molecule. The remaining 61% of the energy
in glucose is lost as heat or is unavailable.
Aerobic respiration is still 18 times more
efficient than anaerobic respiration.
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.
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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