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Ecology 5.docx

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Western University
Biology 2483A
Mark Moscicki

Ecology-Lecture 5 Sept 26 2013 Energy  Energy is a basic requirement for all organisms  We require it for physiological maintenance, growth, and reproduction  If energy input stops, so does biological functioning. Enzymes fail if replacement proteins can not be made and cell membranes degrade and organelles cease to operate without energy to maintain and repair them  Sources of energy include radiant energy which comes from the sun (chemical energy is derived from radiant energy) and chemical energy which is stored in the food that is being consumed (plants converted radiant energy into chemical energy). These are the types of energy that organisms use to meet the demands of growth and maintenance.  Kinetic energy is associated with motion of molecules that make them up objects. This is important for controlling the rate of activity and metabolic energy demand of organisms (done through influence on rate of chemical reactions & temp) Autotrophs  Convert a form of energy like solar energy or chemical energy into the chemicals stored in organic compounds. (chemical energy stored in C-C bonds of organic molecules)  Plants can assimilate radiant energy from sunlight (photosynthesis), or from inorganic compounds (chemosynthetic archaea and bacteria)  Vast majority of autotrophic production occurs though photosynthesis, a process using sunlight to provide the energy needed to take up CO2 and synthesize organic compounds (carbohydrates)  The remainder is produced via chemosynthesis (chemolithotrophy) which is a process that uses energy from inorganic compounds to produce carbs (important to some key bacteria involved in nutrient cycling and in some ecosystems such as hydrothermal vent communities). The earliest autotrophs were likely chemosynthetic as the atmosphere was largely composed of inorganic elements. Chemosynthesis involves organisms obtaining electrons from the inorganic substrate (oxidation). These electrons are used to generate ATP and NADPH for the uptake of carbon from CO2 which is then used to synthesize the carbs used for energy  Carbon is used as a measure of energy because the energy derived from photosynthesis and chemosynthesis is stored in the C-C double bonds. Heterotrophs  Obtain energy by consuming organic compounds from other organisms. They then convert them into usable chemical energy (ATP) via glycolysis which breaks down carbohydrates  This energy originated with organic compounds synthesized by autotrophs. Heterotrophs eat the plants holding the energy from photosynthesis  Includes organisms that consume non living organic matter (detritovores-eat nutrients and vitamins in the soil) as well as consumers that capture and kill their prey and organisms that consume living organisms but do not necessarily kill them (Parasites and herbivores)  It would seem that all plants are autotrophs, all animals and fungi are heterotrophs and archaea and bacteria can be both autotrophs and heterotrophs. However, it is not always so simple!  Some plants have lost photosynthetic function and obtain their energy by parasitizing other plants. These are called holoparasites. They have no photosynthetic pigments and get energy from other plants (heterotrophs). Dodder is a holoparasite that is an agricultural pest and can significantly reduce biomass in the host plant. It attaches to the host plant by growing in spirals around the stem and penetrates the tissue of the host, using modified roots to take up carbs.  Mistletoe is a hemiparasite. It is photosynthetic, but obtains nutrients, water and some of its energy from the host plant.  Animals can also act as autotrophs (rare). Their photosynthetic capacity is acquired by consuming photosynthetic organisms or by living with them in a close relationship (symbiosis). an example is a type of sea slug which has functional chloroplasts that are taken up from the algae that the slug eats.  Energy gain depends on the chemistry of the food, and how much effort is needed to find and ingest the food. For example, microorganisms that feed on detritus invest little energy in obtaining the food but the energy content is low. A cheetah hunting a gazelle invests a lot of energy to find, chase and kill its prey but it gets an energy-rich meal. Food chemistry depends on cell types from which it is derived (animal cells are more energy rich and have organelles that are difficult to digest). Most heterotrophs must break down their food before it can be used as an energy source.  Feeding methods are accordingly very diverse among heterotrophs. Prokaryotes typically absorb food directly through their cell membranes. They excrete enzymes which break down organic matter. Multicellular animals have evolved specialized tissues and organs for absorption, digestion, transport, and excretion. They have tremendous diversity in morphological and physiological feeding adaptations. Photosynthesis  Most of the biologically available energy on earth is derived from photosynthesis  Photosynthetic organisms include some archae, bacteria and protists, and most algae and plants.  Leaves are the principal photosynthetic tissue in plants, but photosynthesis may occur in stem and reproductive tissues as well  Involves conversion of CO2 into carboydrates used for energy storage and biosynthesis Light and Dark Reactions 1. Light reaction-light is harvested from sunlight and used to split water and provide electrons to make ATP and NADPH. Sunlight harvesting is accomplished by several pigments (chlorophyll and carotenoids) Chlorophyll gives photosynthetic organisms their green appearance. These photosynthetic pigments are embedded in a membrane which lies within chloroplast. Pigments absorb energy from photons and that energy is used to split water. The electrons produced are used to synthesize ATP and NADPH. Oxygen is also produced. 2. Dark reaction-CO2 is fixed in the calvin cycle, and carbohydrates are synthesized (despite daytime occurrence in most plants) ATP and NADPH are used to fix carbon. CO2 is taken up from the atmospher through the stomates. Rubisco catalyzes the uptake of CO2 and the synthesis of PGA. PGA is eventually converted to glucose. 6CO2 + 6H2O -> C6H12O6 + 6O2 Environmental Constraints/Controls  Photosynthetic rate determines the supply of energy, which in turn influences growth and reproduction (ecological success)  Environmental controls on photosynthetic rate are an important topic in physiological ecology.  Light is an important determinant of rates of photosynthesis in terrestrial and aquatic habitats.  Light response curves show the influence of light levels on photosynthetic rate.  Light compensation point: Where CO2 uptake is balanced by CO2 loss by respiration. Photosynthesis is limited by light availability below the saturation point.  Saturation point: When photosynthesis no longer increases as light increases (typically reached at a level below full sunlight) Coping with Light Variation  Plants can acclimatize to changing light intensities with shifts in light response curves. Acclimatization to different light levels involves a shift in the light saturation point.  Shifts in light saturation point involve morphological and physiological changes. These include alterations in the thickness of leaves and variation in the number of chloroplasts available to harvest light.  Photosynthetic organisms can also change their pigments to absorb more or less sunlight and the amounts of photosynthetic enzymes available for the dark reactions.  Water availability influences CO2 supply in terrestrial plants  Low water availability causes stomates to close, restricting CO2 uptake. This is a trade off (water conservation vs energy Leaves at high light intensity may have thicker gain)Keeping stomates open while the tissues lose leaves and more chloroplasts. water can permanently impair physiological processes  Closing stomates not only decreases CO2 uptake, but it increases the chance of light damage. If the Calvin cycle isn't operating, energy accumulates in the light harvesting arrays, and can damage photosynthetic membranes. Plants have various mechanism to dissipate this energy, including the use of carotenoids to release it as heat.  Temperature influences photosynthesis by its effects on the rates of chemical reactions and by influencing the structural integrity of membranes/enzymes  Plants from different climate zones have enzyme forms with different optimal temperatures that allow them to operate in that climate  Plants can acclimatize by synthesizing different enzyme forms.  Organisms from alpine environments may photosynthesize at temperatures close to freezing while desert plants may have their highest photosynthetic rate at temperatures hot enough to denature other enzymes.  Temperature also affects membrane composition. Cold sensitivity results in loss of membrane fluidity which inhibits the function of light harvesting molecules embedded in the membrane Figure 5.9 Photosynthetic Responses to Temperature (Part 2) Plants can acclimatize by synthesizing different enzyme forms.  Nutrients can also affect photosynthesis.  Most nitrogen in plants is associated with Rubisco and other photosynthetic enzymes. Thus, higher nitrogen levels in a leaf are correlated with higher photosynthetic rates  Why don't all plants allocate more nitrogen to their leaves to increase their photosynthetic
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