Energy and Nutrient Relations Chapter 6
Energy Sources • Organisms can be classified by trophic levels. • Autotrophsuse 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.
Oxygenic Photosynthesis (There are anoxygenic photosynthetic bacteria!)
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/
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
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.
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.
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.
Which P vs I curve would favor a sunny as opposed to shady habitat? http://www.marietta.edu/~spilatrs/biol103/photolab/sunexpl.gif
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)
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.
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.
Carnivores • Consume nutritionally-rich prey. • Carnivore C:N is equivalent to prey C:N.
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.
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.
Which type of response? 06_22.jpg
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.
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).
Which stand of birch trees grows on infertile (nutrient poor) soil? 06_25.jpg
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.