Chapter 5

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Biological Sciences
Marc Cadotte

Chapter 5: Coping with Environmental Variation: Energy TOOL MAKING CROWS: A CASE STUDY  Humans employ a multitude of tools to enhance our ability to gather food to meet our energy needs  We use a highly mechanized system of planting, fertilizing, and harvesting crops to feed ourselves or the livestock that we consume. For thousands of years, we have used specialized tools for hunting prey, including spears, bows and arrows, and rifles  We view our tool making capacity as something that differentiates us from other animals. However, humans are not alone in using tools to enhance their food acquisition ability  Psychologist studying the behavior of chimpanzees, observed that chimps in captivity made tools to retrieve bananas stashed in areas that were difficult to reach - chimpanzees in the wild using grass blades and plant stems to ―fish‖ for termites in holes in the ground and in decaying wood - crows use food-collecting tools manufactured from plants - Hunt found that two types of tools were being used by the crows 1. Hooked twig, fashioned from a shoot stripped of its leaves and bark 2. ―stepped-cut‖ serrated edge clipped from a Pandanus tree - The presence of hooks on both types of tools suggested an innovative element that might increase the birds’ efficiency in extracting prey from their refuges in the trees - The tools also appeared to be uniform in their construction; Hunt examined 55 tools manufactured by different birds and found that they differed little - Tow could birds have achieved a similar level of sophistication in their tool construction? o The high numbers of New Caledonian crows using tools, and the consistency in the construction of the tools, indicate a cultural phenomenon; that is, a skill learned socially within a population of animals—a phenomenon never before observed in birds. INTRODUCTION  Energy is the most basic requirement for all organisms  Physiological maintenance, growth, and reproduction all depend on energy acquisition  Organisms are complex systems, so if energy input stops, so does biological functioning Concept 5.1 Organisms Obtain Energy From Sunlight, from Inorganic Chemical Compounds, or Through the Consumption of Organic Compounds SOURCES OF ENERGY  We sense energy in our environment in a variety of forms - Light from the sun, a form of radiant energy, illuminates our world and warms our bodies - Objects that are cold or warm to our touch have different amounts of kinetic energy, which s associated with the motion of the molecules that make p the objects - Radiant and chemical energy are the forms organisms use to meet the demands of growth and maintenance, while kinetic energy, through its influence on the rate of chemical reactions and temperature, is important for controlling the rate of activity and metabolic energy demand of organisms. - Even the energy used to support industrial development fuel our cars, and heat our homes originated ultimately with photosynthesis, which produced the organisms that became the oil we pump out of the ground  Autotrophs – are organisms that assimilate energy from sunlight (photosynthetic organisms) or from inorganic chemical compounds in their environment - Autotrophs convert the energy of sunlight or inorganic compounds into chemical energy stored in the carbon-carbon bonds of organic compounds typically carbohydrates  Heterotrophs – are organisms that obtain their energy by consuming energy-rich organic compounds made by other organisms—energy that ultimately originated with organic compounds synthesized by autotrophs - Detritovores – consume nonliving organic matter - Parasites and Herbivores - consume living organisms, but do not necessarily kill them - Predators – consumers - Prey  He distinction between autotrophs and heterotrophs is not clear - Plants, known as holoparasites have no photosynthetic pigments, and are heterotrophs  Animals can act as autotrophs, although this phenomenon is relatively rare  Their photosynthetic capacity is acquired by consuming photosynthetic organisms or by living with them in a close relationship known as a symbiosis Concept 5.2 Radiant and Chemical Energy Captured by Autotrophs is Converted into Stored Energy in Carbon-Carbon Bonds AUTOTROPHY  Photosynthesis - a process that uses sunlight to provide the energy needed to take up CO2 and synthesize organic compounds, principally carbohydrates  Chemosynthesis (chemolithotrophy) – process that uses energy from inorganic compounds to produce carbohydrates, is important to some key bacteria involved in nutrient cycling and in some unique ecosystems, such as hydrothermal vent communities  Ecologists often use carbon as a measure of energy. CHEMOSYNTHESIS HARVESTS ENERGY FROM INORGANIC COMPOUNDS  The earliest autotrophs on Earth were probably chemosynthetic bacteria or archaea that evolved when the composition of the atmosphere was markedly different than it is today: low in oxygen, but rich in hydrogen, with significant amounts of carbon dioxide and methane (CH4)  Chemosynthetic bacteria are often named according to the inorganic substrate they use for energy  During chemosynthesis, organisms obtain electrons by oxidizing the inorganic substrate  They use the electrons to generate two high-energy compounds: ATP and NADPH  They then use energy from ATP and NADPH for the uptake of carbon from gaseous CO2 (known as fixation of CO2)  They use that carbon to synthesize carbohydrates or other organic molecules, which are then used for energy storage or biosynthesis (manufacture of chemical compounds, membranes, organelles, and tissues)  Alternatively, some bacteria can use electrons from the inorganic substrate directly to fix carbon  The biochemical pathway most commonly used to fix carbon is the Calvin cycle, named for Melvin Calvin, the biochemist who first described it  The Calvin cycle is catalyzed by several enzymes, and it occurs in both chemosynthetic and photosynthetic  One of the most widespread and ecologically important groups of chemosynthetic organisms is the nitrifying bacteria (e.g., Nitrosomonas, Nitrobacter), which are found in both aquatic and terrestrial ecosystem  In a two-step process, these bacteria convert ammonium (NH4) into nitrite (NO2), then oxidize it to nitrate (NO3)  These chemical conversions of nitrogen compounds are an important component of nitrogen cycling and plant nutrition  Another important chemosynthetic group is the sulfur bacteria associated with volcanic deposits, sulfur hot springs, and acidic mine waste  Sulfur bacteria initially use the higher-energy forms of sulfur, H2S and HS, producing elemental sulfur, which is insoluble and highly visible in the environment  Once the H2S and HS are exhausted, the bacteria use elemental S as an electron donor, producing SO4 (sulfate) PHOTOSYNTHESIS IS THE POWERHOUSE FOR LIFE ON EARTH  van Helmont’s experiment established the basis for the later discovery that it was photosynthetic uptake of CO2 from the air—not material Prom the soil—that was the source of the tree’s weight gain  The vast majority of biologically available energy on Earth is derived from the conversion of sunlight into energy-rich carbon compounds by photosynthesis  Leaves are the principal photosynthetic tissue in plants, but photosynthesis may also occur in stem and reproductive tissues  Like chemosynthesis, photosynthesis involves the conversion of CO2 into carbohydrates used for energy storage and biosynthesis  Photosynthesis is between Earth and the atmosphere, and is critically between Earth and the atmosphere, and is critically important to the global climate system  Light and Dark Reactions – Photosynthesis has two major steps. 1. Harvesting of energy from sunlight, which is used to split water to provide electrons for generating ATP and NADPH = light reaction 2. The fixation of carbon and the synthesis of carbohydrates = dark reaction - Chlorophyll gives photosynthetic organisms their green appearance because it absorbs red and blue light and reflects green wavelength - Plants and photosynthetic bacteria have similar chlorophyll pigments, but they absorb light at slightly different wavelengths - Additional pigments associated with photosynthesis, called accessory pigments, include the carotenoids, which are characteristically red, yellow. Or orange in appearance - These photosynthetic pigments involved in the light reaction. In plants, this membrane involved in the light reaction - The pigments absorb energy from discrete water and provide electrons - The electrons are passed on to molecular complexes on the membranes, where they are used to synthesize ATP and NADPH - The splitting of water to provide electrons for the light reaction generates oxygen - Atmospheric oxygen led to the creation of a layer of ozone (O3) high in the atmosphere that shields organisms from high-energy ultraviolet radiation - The evolution of aerobic respiration. N which O2 is used as an electron acceptor, facilitated great evolutionary advances for life on Earth - Energy from the high-energy compounds ATP and NADPH is used in the Calvin cycle to fix carbon - Carbon dioxide is taken up from the atmosphere through the stomates of vascular plants, or diffuses across the cell membranes in nonvascular plants, algae, and photosynthetic bacteria and archaea - Rubisco, the most abundant enzyme on Earth, catalyzes the uptake of CO2 and the synthesis of a three-carbon compound: PGA. o PGA is eventually converted into a six-carbon sugar glucose (C6H12O6 in most plants) - The net reaction of photosynthesis is: o 6 CO2 + 6 H2O --- C6H12O6 +6 O2  Environmental Constraints and Solutions – the rate of photosynthesis available to photosynthetic organisms, for biosynthesis available to photosynthetic organisms, which in turn influences their growth and reproduction, often equated with their ecological success (abundance and geographic range) - Let energy (carbon) gain is also influenced by CO2 losses associated with cellular respiration - Light is clearly an important determinant of rates of photosynthesis in terrestrial and aquatic habitats - When there is enough light that the plant’s photosynthetic CO2 uptake is balanced by its CO2 loss by respiration, the plant is said to have reached the light compensation point - As the light level increases above the light compensation point, the photosynthetic rate also increases; in other words, photosynthesis is limited by the availability of light - The photosynthetic rate levels off at a light saturation point, which is typically reached at a level below full sunlight - Bjorkman demonstrated that acclimatization to different light levels involves a shift in the light saturation point o Morphological changes in the thickness of leaves and variation in the number in the thickness of leaves and variation in the number of chloroplasts available to harvest light o Photosynthetic organisms may also alter the density of their light-harvesting pigments—a strategy analogous to changing the size of the antenna on a radio—and the dark reaction o Typically, the average light level a plant experiences, integrated over the course of the day, is near the transition point between light limitation and light saturation - Some specialized bacteria are especially well adapted to photosynthesis at low light levels, which allows them to thrive in dimly lit environments such as relatively deep ocean water - Chlorophyll f may be an adaptation that allows the cyanobacteria possessing it to grow underneath other photosynthetic organisms that use light in the blue and red wavelengths, as it lets them harvest energy at wavelengths that pass through those other photosynthetic organisms - Water availability is an important control on the supply of CO2 for photosynthesis in terrestrial plants - Low water availability results in closure of the stomates, restricting the entry of CO2 into leaves - keeping stomates open while tissues lose water can permanently impair physiological processes in the leaf - Closing stomates, however, not only limits photosynthetic CO2 uptake, but also increases the chances of light damage to the leaf - Plants have evolved a number of ways of dissipating this energy safely, including the use of carotenoids to release it as heat - Temperature influences photosynthesis in two main ways: 1. Through its effects on the rates of chemical reactions 2. By influencing the structural integrity of membranes and enzymes - Different photosynthetic organisms have different forms of the same photosynthetic enzymes that operate best under the environmental temperatures where they occur - These differences result in markedly different temperature ranges for photosynthesis in organisms from different climates - Temperature also influences the fluidity of the cell and organelle membranes - Bid sensitivity in plants of tropical and subtropical biomes is associated with loss of membrane fluidity, which inhibits the functioning of the light-harvesting molecules embedded in the chloroplast membranes - High temperatures, particularly in combination with intense sunlight, can damage photosynthetic membranes - Lighter amounts of nitrogen in leaves are correlated with higher photosynthetic rates - Why then, don’t all plants allocate more nitrogen to their leaves to increase their photosynthetic capacity? 1. He supply of nitrogen is low relative to the demand, and nitrogen is needed for growth and other metabolic functions in addition to photosynthesis 2. Increasing the nitrogen concentration of a leaf increases the risk that herbivores will consume the leaf, as plant-eating animals are also nitrogen-starved - Plants must balance the competing demands of photosynthesis, growth, and protection from herbivores Concept 5.3 Environmental Constraints Have Resulted in the Evolution of Biochemical Pathways that Improve the Efficiency of Photosynthesis PHOTOSYTHESIS  Anything that influences energy gain by photosynthesis has the potential to affect the survival, growth, and reproduction of the organism  Rates of photosynthesis are influenced by environmental conditions, particularly temperature and water availability  An apparent biochemical inefficiency in the initial step of the Calvin cycle limits energy gain by photosynthetic organisms  Two specialized photosynthetic pathways that make photosynthesis more efficient under particular potentially stressful environmental conditions: 1. the C4 pathway 2. Crassulacean acid metabolism (CAM)  Plants that lack these specialized pathways use the C3 photosynthetic pathway PHOTORESPIRATION LOWERS THE EFFICIENCY OF PHOTOSYNTHESIS  Rubisco - note that the ―o‖ in the abbreviation stands for ―oxygenase‖  Rubisco can catalyze two competing reactions 1. Carboxylase reaction – CO. Is taken up, leading to the synthesis of sugars and the release of O2 (photosynthesis) 2. Oxygenase reaction - in which O2 is taken up. Leading to the breakdown of carbon compounds and the release of CO2  This oxygenase reaction is part of a process called photorespiration, which results in a net loss of energy  The balance between photosynthesis and photorespiration is related to two main factors: 1. the ratio of O2 to CO2 in the atmosphere and 2. temperature  As the atmospheric concentration of CO2 decreases relative to that of O2, the rate of photorespiration increases relative to the rate of photosynthesis  Shifts in atmospheric CO2 concentrations would have influenced the balance between photosynthesis and photorespiration  As temperatures increase, the rate of O2 uptake catalyzed by rubisco increases relative to the rate of CO2 uptake, and the solubility of CO2 in the cytoplasm decreases more than that of O2  As a result of these two processes, photorespiration increases more rapidly at high temperatures than photosynthesis does  Thus, energy loss due to photorespiration is particularly acute at high temperatures and low atmospheric CO2 concentrations  If photorespiration is detrimental to the functioning of photosynthetic organisms, why hasn’t a new form of rubisco evolved that minimizes uptake of O2? - Arabidopsis thaliana: plants with a genetic mutation that knocks out photorespiration die under normal light and CO2 conditions - Plants with higher rates of photorespiration showed less damage than control plants with normal rates of photorespiration or plants with depressed rates of photorespiration  Despite this possibility that photorespiration plays a role in protecting plants from damage at high light leve
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