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Energy and Nutrient Relations

Chapter 6

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Energy Sources

• Organisms can be classified by trophic levels. Autotrophs use inorganic sources of carbon

and energy. Photosynthetic: Use CO2 as carbon

source, and sunlight as energy. Chemosynthetic: Use inorganic

molecules as source of carbon and energy.

Heterotrophs use organic molecules as sources of carbon and energy.

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Oxygenic Photosynthesis(There are anoxygenic photosynthetic bacteria!)

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Photosynthetic Pathways

• C3 Photosynthesis Used by most plants and algae. CO2 + ribulose bisphosphate (RuBP;

5 carbon sugar) = 2x phosphoglyceric acid (3PGA; 3 carbon acid)

To fix carbon, plants must open stomata to let in CO2 .

Water gradient may allow water to escape.

http://ntri.tamuk.edu/bio/photo/

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C3 Plant Photorespiration:“Too much of a good thing.”

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Photosynthetic Pathways

• C4 Photosynthesis

First CO2 fixed to phosphoenolpyruvate (3 carbons) to form 4 carbon acid.

Reduces internal CO2 concentrations.

Increases rate of CO2 diffusion inward. Need fewer stomata open.

Conserves water

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C4 Photosynthesise.g. grasses, corn, sugar cane

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Photosynthetic Pathways

• CAM Photosynthesis (Crassulacean Acid Metabolism) Limited to succulent plants in arid and

semi-arid environments. Carbon fixation takes place at night.

Reduces water loss. Low rates of photosynthesis. Extremely high rates of water use

efficiency.

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CAM Photosynthesis

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Energy Limitation

• Liebig’s Law of the Minimum: The resource in least supply relative to an

organism’s needs will control growth. It is the growth limiting resource.

• Limits on the potential rate of energy intake by plants (hence growth) have been demonstrated by studying response of photosynthetic rate (P) to photon flux density (light intensity or irradiance; I).

• The P versus I curve.

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P versus I curve

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Photon Flux and Photosynthetic Response Curves

• Rate of photosynthesis increases linearly with photon flux density at low light intensities, rises more slowly with intermediate light intensities, and tends to level off at high light intensities. Response curves for different species may:

level off at different maximum P. respond differently at lower levels of I.

13http://www.marietta.edu/~spilatrs/biol103/photolab/sunexpl.gif

Which P vs I curve would favor a sunny as opposed to shady habitat?

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Using Organic Molecules for Energy and Growth

• Three Feeding Methods of Heterotrophs: Herbivores: Feed on plants. Carnivores: Feed on animal flesh. Detritivores: Feed on non-living organic

matter that becomes colonized by microbes (detritus)

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Herbivores• Substantial nutritional chemistry problems.

Low N:C ratio compared to plants. Carbon mostly as cellulose & NO

cellulases of their own.• Must overcome plant physical and chemical

defenses. Physical: structures, woody parts, silica Chemical: toxins, inhibitors of digestion.

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Detritivores

• Consume food rich in carbon and energy, but very poor in nitrogen. Dead leaves may have half nitrogen

content of living leaves.• Fresh detritus may still have considerable

chemical defenses present.• Microbial colonization of plant derived

detritus increases nutritional value.

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Carnivores• Consume nutritionally-rich prey.• Carnivore C:N is equivalent to prey C:N.

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Energy Limitation

• Limits on potential rate of energy intake by animals (hence growth) have been demonstrated by studying relationships between feeding rate and food availability.

• Holling described (3) basic functional responses to food density: 1. Feeding rate increases linearly as food

density increases - levels off at maximum. Consumers require little or no search

and handling time.

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2. Feeding rate rises in proportion to food density.

Feeding rate partially limited by search/handling time.

3. Feeding rate increases most rapidly at intermediate densities.

S-shaped curve.

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http://www.tnstate.edu/ganter/Functional.Response.JPG

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06_22.jpgWhich type of response?

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06_23.jpg

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Optimal Foraging Theory• All other things being equal, more abundant

prey yields larger energy return. • What about different prey types and sizes as

well as densities (real world complexity)?• Must consider energy expended during:

Search for prey Handling time

• Consumers choose prey that maximize rate of energy intake per unit of foraging energy expenditure.

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Optimal Foraging in Bluegill Sunfish

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Optimal Foraging By Plants

• Limited supplies of energy for allocation to leaves, stems and roots.

• Bloom suggested plants adjust allocation in such a manner that all resources are equally limited. Appear to allocate growth in a manner that

increases rate of acquisition of resources in shortest supply.

E.g. shading of some plants causes increased stem growth (spindly looking).

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06_25.jpgWhich stand of birch trees grows on infertile (nutrient poor) soil?

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Using Reduced Inorganic Molecules

• 1977 - Organisms found living on sea floor. Near nutrients discharged from volcanic

activity through oceanic rift. Autotrophs depend on chemosynthetic

bacteria that oxidize reduced inorganics. Free-living forms. Living within tissue of invertebrates.