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Chapter 6 Concepts
Energy comes in three forms: light, organic molecules and inorganic molecules
Energy intake is a limiting factor for organisms
Organisms are adapted to optimize energy intake
Challenges
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Heterotrophs, Photoautotrophs, Chemoautotrophs (Fig 6.2)
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Consumers
Producers (Autotrophs)
– Input Quality
– Balancing energy fixation and water loss
– Balancing nutrient and energy demands
– Carbon/Nitrogen ratios (10:1 – 5:1)
• Herbivores
• Detritivores
– Prey characteristics
• Carnivores
Light Quality and Availability
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Light has a range of wavelengths
Wavelengths striking a surface are variable (infrared to UV and beyond)
Visible spectrum responsible for energy input in photosynthetic systems
Also known as Photosynthetically Active Radiation (PAR)
PAR
PAR Levels in the Forest (Fig 6.3)
Light Intensity and Photosynthesis
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At low light intensities:
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As light intensity increases, compensation point is reached
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– increase in photosynthetic rate is proportional to increase in light intentsity
– below compensation point, energy used is greater than energy produced
– above compensation point, surplus energy is produced
Light Intensity and Photosynthesis
At high light intensities:
– light saturation point is reached
– at saturation point, increased intensity does not result in increased photosynthetic
rate
At highest light intensities photoinhibition may occur
P-syn and Available Light
Shade Tolerance and Intolerance
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Fixing Carbon
Challenge is:
– Maximizing influx of a gas
– Minimizing loss of water
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Challenge because:
– Gradient differences
• Water vapor density in plants high
• Water vapor density in external atmosphere is low
• Carbon dioxide density gradient is less steep
– Water diffuses out faster than carbon dioxide diffuses in
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C3 Photosynthesis (Fig 6.4)
Moderate to high moisture availability
Carbon fixation and light reaction occur in the same cells
Stomates remain open during the day
“Free” flow of carbon dioxide and water vapor
Stomates close during periods of water stress
C4 Photosynthesis (Fig 6.5)
Low to moderate water availability
Initial carbon fixation occurs in mesophyll cells
Enzyme has high affinity for carbon dioxide, increases concentration gradient
Product is transferred to bundle sheath cells where remainder of reaction occurs
Effect is to increase carbon dioxide gradient
Stomates can remain closed for longer periods, fewer stomates needed
CAM Photosynthesis (Fig. 6.6)
Arid environments
All reactions occur in mesophyll cells
Carbon fixation occurs at night when temperature (and water loss) is low
Products stored until daylight when stomates close and light reaction begins
Advantages
C3 plants lose 400-800 g of water for every g of tissue produced
C4 plants lose 250-350 g of water for every g of tissue produced
CAM plants lose 50 g of water for every g of tissue produced
Herbivores
Challenge
– Low nutritive value
• High C:N ratios (25:1)
• Large volume of undigestible material
– Plant defenses
• Toxins
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C:N ratios of Woody Plants (Fig. 6.8, 6.9)
Toxicity and Selectivity (Fig 6.10, 6.11)
Detritivores (Fig 6.12)
Challenge
– Low nutritive value of dead leaves
– Even higher C:N ratios (100:1)
Carnivores
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Challenge
– Prey identification
• Mimicry
• Camouflage
– Prey
capture
• Size-selective predation
• Diet variability
Mullerian and Batesian Mimicry (Fig 6.13)
Camouflage and Natural Selection (Fig 6.14)
Diet breadth (Fig 6.15)
Size Selectivity (Fig 6.16)