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Lecture 5: "Coping With Environmental Variation (Energy)"

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Western University
Biology 2483A
Hugh Henry

Ecology Lecture No. 5: Coping With Environmental Variation (Energy) Tuesday September 25 , 2012 Autotrophs & Heterotrophs: -Autotrophs are organisms that assimilate radiant energy from sunlight (photosynthesis), or from inorganic compounds (chemosynthesis). This energy is then converted into chemical energy stored in the bonds of organic molecules. Heterotrophs obtain their energy by consuming organic compounds from other organisms. This energy originated with organic compounds synthesized by autotrophs. -Some heterotrophs consume non-living organic matter and are known as detritivores or decomposers. Parasites and herbivores consume live hosts, but do not necessarily kill them. Predators capture and consume live prey animals. Miscellaneous Forms Of Heterotrophy: -Some plants are holoparasites –they have no photosynthetic pigments and get energy from other plants (heterotrophs).The Dodder plant is a holoparasite that is an agricultural pest and can significantly reduce biomass in the host plant. -Mistletoe is an example of a hemiparasite— it is photosynthetic, but obtains nutrients, water, and some of its energy from the host plant. Certain sea slugs have functional chloroplasts that are taken up from the algae that they eat and enable them to capture and store energy from the sun. Chemosynthesis: -As there is no light penetrating the bottom of the ocean, organisms practise chemosynthesis (chemolithotrophy) by obtaining energy from inorganic compounds and using them to produce carbohydrates. -Chemosynthesis is important in nutrient cycling bacteria, and in some ecosystems such as hydrothermal vent communities. We often rely on chemosynthetic bacteria to liberate nutrients (such as nitrogen) from the soil in a form that plants uptake easily. Photosynthesis: -Photosynthesis is the process by which most autotrophs obtain their energy. Here, sunlight provides the energy to take up CO and synthesize organic compounds. Most of the biologically available energy on 2 Earth is derived from photosynthesis. Photosynthetic organisms include some archaea, bacteria, and protists, and most algae and plants. -Photosynthesis has two major steps: The Light-dependent reaction (where light is harvested and used to split water and provide electrons to make ATP and NADPH) and the light-independent reaction (where CO i2 fixed in the Calvin cycle, and carbohydrates are synthesized for the plant’s benefit). Photosynthetic Rate & Terminology: -Photosynthetic rate determines the supply of energy, which in turn influences growth and reproduction. Environmental controls on photosynthetic rate are an important topic in physiological ecology. -Light response curves show the influence of light levels on photosynthetic rate. The light compensation point is where CO u2take is balanced by the CO loss2by cellular respiration. Saturation point is when photosynthesis no longer increases as light increases. Acclimatization In Plants: -Plants can acclimatize to changing light intensities with shifts in light response curves. Shifts in light saturation point involve morphological and physiological changes. For example, leaves at high light intensity, may have thicker leaves and more chloroplasts. -This is because as light penetrates through the leaf, it will be absorbed by chlorophylls in mesophyll cells. Here, the extra tissue is worth the energy cost because intense light would otherwise penetrate through the leaf and not be gained as energy. A thinner leaf would be more prevalent in low light conditions where investing in extra tissue would be a useless undertaking. Factors Affecting Photosynthetic Rate: -Water availability influences CO s2pply in terrestrial plants. Low water availability causes stomata to close, restricting CO2uptake. This is an example of an evolutionary trade-off between water conservation and energy gain. - Closing stomata increases the chance of light damage. If the Calvin cycle isn’t operating, energy accumulates in the light-harvesting arrays and can damage membranes. Plants have various mechanisms to dissipate this energy, including carotenoids. - Plants from different climate zones have enzyme forms with different optimal temperatures that allow them to operate in that climate. Thus, temperature plays a key role in the functioning of photosynthesis. -Nutrients can also affect photosynthesis. Most of the nitrogen in plants is associated with Rubisco and other photosynthetic enzymes (chlorophyll also has nitrogen in its structure). Thus, higher nitrogen levels in a leaf are correlated with higher photosynthetic rates. -But the supply of readily available nitrogen is generally low, relative to the demand for growth and metabolism. There is also a trade-off as increasing the nitrogen content of leaves increases the risk that herbivores will eat them (more tasty), as plant-eating animals are also nitrogen-starved. The Benefits & Disadvantages Of Photorespiration: - Some metabolic processes decrease photosynthetic efficiency. For instance, Rubisco can catalyze tw
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