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Plant Metabolism
Chapter 10
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Outline
• Photosynthesis Major Steps of Photosynthesis Light-Dependent Reactions Light-Independent Reactions C4 Photosynthesis CAM Photosynthesis
• Respiration Glycolysis Electron Transport Chain
Enzymes and Energy Transfer
• 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
• 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
• 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
• 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
• 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
• Several different types of chlorophyll. Most plants contain both chlorophyll a
(blue-green) and chlorophyll b (yellow-green).
- 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
• Light Dependent Reactions Water molecules split apart. Electrons passed along electron transport. ATP produced. NADPH produced.
Major Steps of Photosynthesis
• 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
• Each pigment has its own distinctive pattern of light absorption.
When pigment absorbs light: electrons are excited. When this occurs, energy may beimmediately emmitted as light:flourescence or emmitted after a delay : phosphorescence ormay be converted to heat.
Light Dependent Reactions - In Depth
• 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
• 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
• 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
• 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
• Plants with Kranz Anatomy produce oxaloacetic acid (4-carbon 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
• Seen in plants of about 30 families including cacti orchids etc.
• Similar to C4 photosynthesis in that 4-carbon compounds are produced during the light-independent 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
Leaf Anatomy No bundle sheath
Krantz Anatomy
No palliside cells, large vauoles
Carboxylation Enzymes
RuBisco PEP +RuBisco Drakness PEP +Light RuBisco
Transpiration ratio (gm H2O/ gm dry weight increase)
450-950 250-350 50-55
Optimum P.S. Temperature
15-25 oC 30-40 oC 35 oC
C3 C4CAM
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
• C4 Stores CO2 in 4
carbon acid Separates CO2
fixation from sugar synthesis in SPACE(Bundle sheath vs mesophyll space)
Adaptations to TROPICAL environments
• 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
• 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
• 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
• Fermentation
C6H12O6 2C2H5OH + 2CO2 + ATP
C6H12O6 2C3H6O3 + ATP
Factors Affecting the Rate of Respiration
• Temperature• Water• Oxygen
Major Steps of Respiration
• 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
• Aerobic Respiration Citric Acid (Krebs) Cycle
O.A. + acetyl CoA + ADP+P+3NAD + FAD
O.A. + CoA+ATP+3NADH+H+ + FADH2+2CO2
• Electron Transport Oxidative Phosphorylation Chemiosmosis
Assimilation and Digestion
• 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 grainsof rice
Starch Soluble forms
DIGESTION
Conversion of PSic products to structural formssuch as protoplasm or cellwall is called ASSIMILATION
Rs• Releases energy from
sugar molecules• Releases carbon dioxide
and water• Decreases weight• Occurs in either light or
darkness• Occurs in all living cells• Utilizes oxygen (aerobic
respiration)• Produces ATP with energy
released from sugar
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
Review
• Photosynthesis Major Steps of Photosynthesis Light-Dependent Reactions Light-Independent Reactions C4 Photosynthesis CAM Photosynthesis
• Respiration Glycolysis Electron Transport Chain
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