The largest source of energy for an ecosystem is the sun. Energy that is not used in an ecosystem is eventually lost as heat. Energy and nutrients are passed around through the food chain, when one organism eats another organism. Any energy remaining in a dead organism is consumed by decomposers. Nutrients can be cycled through an ecosystem but energy is simply lost over time. An example of energy flow in an ecosystem would begin with the autotrophs that take energy from the sun. Herbivores then feed on the autotrophs and change the energy from the plant into energy that they can use.
Carnivores subsequently feed on the herbivores and, finally, other carnivores prey on the carnivores. In each case, energy is passed on from one trophic level to the next trophic level and each time some energy is lost as heat into the environment. This is due to the fact that each organism must use some energy that they received from other organisms in order to survive. The top consumer of a food chain will be the organism that receives the least amount of energy. Specifically, they state that it is trophic structure, rather than energetics that controls the amount of energy consumed at each trophic level and that "ecological efficiencies" are the product of a trophic structure, not a determining factor.
Further, they state that trophic structure is instead the result of competition and predator-prey interactions.
Activation energy
It is important to remember that many species may occupy each trophic level and are so subject to interspecific competition. Energy is the ability to do work. Life manifests itself in energy changes, subject to the laws of thermodynamics. Ecosystems exist and operate by virtue of a flow of energy through the components of the system and thermodynamics the movement of energy forms the very basis of the biosphere organizing principles introduced in Chapter 2. Before proceeding into the relationship between ecology and thermodynamics, it is necessary to build a basic understanding of the physics of energetics, simply a further demonstration of the fact that ecology is multidisciplinary, requiring of its students a broad knowledge in all sciences.
Although several sources of energy are available for exploitation on earth e. Light and other radiation streaming out from the sun strikes the earth 93 million miles distant, providing energy to the atmosphere, the seas, and the land, warming objects that absorb this energy; that is, radiant energy is converted to heat energy molecular motion.
Differential heating causes winds and currents in the air and water, the heat energy becoming kinetic energy of motion. Warming results in evaporation of water into the atmosphere, setting up the hydrologic cycle Chapter 4 , the lifting of water into the atmosphere becoming potential energy that will convert to kinetic energy when the water begins to flow back downhill. However, the most significant solar energy driven process with respect to living systems is that of photosynthesis.
Light energy is converted by photosynthesizing cells into a form of potential energy held in the chemical bonds of organic compounds. Organisms require both the substance and the stored energy of chemical compounds to function and grow, and eventually reproduce. Substance to provide the building blocks of cellular and extracellular components that comprise structure, and energy to move substances around, affect chemical reactions, and carry out all manner of intracellular and organismal processes.
The Solar Constant is the average amount of radiant energy from the sun that reaches the Earth's atmosphere. This value is calculated at 2 calories per minute on each square centimeter of the Earth's upper atmosphere. This value can change because of Earth's elliptical path effecting seasonal changes and differences in the northern or southern slope which effect magnitude. The net radiation is what is left after some of the energy is reflected by the Earth's surface.
Calculations for the solar constant are done by using the astronomical unit AU which is the mean distance between the Earth and Sun; One AU is equivalent to 92,, miles ,, km. Movement of the air and evaporation are important factors that regulate the temperature of the Earth from the sun's energy. The movements of air allow energy to be given off into space and without this reflection of energy the Earth would rapidly overheat and life would extinguish.
This interaction is also a very important to the maintenance of the Earth's polar ice caps. Of course, as has been shown, the increasing use of industry by man, can increase earths temperature. If it were enough to cause the ice caps to melt, there would be an increase of approximately 7 degrees C. This reflection of the solar radiation is an essential part of the maintenance of the current climate that the Earth maintains. When it comes to the flow of energy in ecosystems there are two types of organisms: Plants are a common example of producers in all populations.
They are able to convert carbon dioxide into oxygen and glucose, a common sugar consumed by most organisms. They do this through a process called photosynthesis which allows the plants to use sunlight as a source of energy. Producers convert energy from the environment into chemical energy in the form of carbon to carbon bonds. A classic example is the one previously mentioned where the plants convert CO 2 to O 2 and glucose.
The second type of organism is the consumer. Consumers are unable to make chemical energy the way plants do and have to use metabolic processes to get energy from carbon to carbon bonds, which is called respiration. Respiration breaks the carbon to carbon bonds and combines them with oxygen to make carbon dioxide.
The energy released is used to help organisms move their muscles or as heat. Energy cannot be reused once it has been lost. Enzyme-catalyzed reactions are usually connected in series, so that the product of one reaction becomes the starting material, or substrate , for the next Figure These long linear reaction pathways are in turn linked to one another, forming a maze of interconnected reactions that enable the cell to survive, grow, and reproduce Figure How a set of enzyme-catalyzed reactions generates a metabolic pathway. Each enzyme catalyzes a particular chemical reaction, leaving the enzyme unchanged.
In this example, a set of enzymes acting in series converts molecule A to molecule F, forming a more Some of the metabolic pathways and their interconnections in a typical cell. About common metabolic reactions are shown diagrammatically, with each molecule in a metabolic pathway represented by a filled circle, as in the yellow box in Figure Two opposing streams of chemical reactions occur in cells: Together these two sets of reactions constitute the metabolism of the cell Figure Schematic representation of the relationship between catabolic and anabolic pathways in metabolism.
As suggested here, since a major portion of the energy stored in the chemical bonds of food molecules is dissipated as heat, the mass of food required more Many of the details of cell metabolism form the traditional subject of biochemistry and need not concern us here. But the general principles by which cells obtain energy from their environment and use it to create order are central to cell biology.
We begin with a discussion of why a constant input of energy is needed to sustain living organisms. The universal tendency of things to become disordered is expressed in a fundamental law of physics—the second law of thermodynamics —which states that in the universe, or in any isolated system a collection of matter that is completely isolated from the rest of the universe , the degree of disorder can only increase. This law has such profound implications for all living things that it is worth restating in several ways.
For example, we can present the second law in terms of probability and state that systems will change spontaneously toward those arrangements that have the greatest probability. If we consider, for example, a box of coins all lying heads up, a series of accidents that disturbs the box will tend to move the arrangement toward a mixture of 50 heads and 50 tails.
The reason is simple: An everyday illustration of the spontaneous drive toward disorder. Reversing this tendency toward disorder requires an intentional effort and an input of energy: In fact, from the second law of thermodynamics, we can be certain more The amount of disorder in a system can be quantified. The quantity that we use to measure this disorder is called the entropy of the system: Thus, a third way to express the second law of thermodynamics is to say that systems will change spontaneously toward arrangements with greater entropy. Living cells—by surviving, growing, and forming complex organisms—are generating order and thus might appear to defy the second law of thermodynamics.
How is this possible? The answer is that a cell is not an isolated system: In the course of the chemical reactions that generate order, part of the energy that the cell uses is converted into heat. The heat is discharged into the cell's environment and disorders it, so that the total entropy —that of the cell plus its surroundings—increases, as demanded by the laws of physics. To understand the principles governing these energy conversions, think of a cell as sitting in a sea of matter representing the rest of the universe.
As the cell lives and grows, it creates internal order. But it releases heat energy as it synthesizes molecules and assembles them into cell structures. Heat is energy in its most disordered form—the random jostling of molecules. When the cell releases heat to the sea, it increases the intensity of molecular motions there thermal motion —thereby increasing the randomness, or disorder, of the sea.
The second law of thermodynamics is satisfied because the increase in the amount of order inside the cell is more than compensated by a greater decrease in order increase in entropy in the surrounding sea of matter Figure A simple thermodynamic analysis of a living cell. In the diagram on the left the molecules of both the cell and the rest of the universe the sea of matter are depicted in a relatively disordered state. In the diagram on the right the cell has taken more Where does the heat that the cell releases come from? Here we encounter another important law of thermodynamics.
The first law of thermodynamics states that energy can be converted from one form to another, but that it cannot be created or destroyed. Some forms of energy are illustrated in Figure The amount of energy in different forms will change as a result of the chemical reactions inside the cell, but the first law tells us that the total amount of energy must always be the same. For example, an animal cell takes in foodstuffs and converts some of the energy present in the chemical bonds between the atoms of these food molecules chemical bond energy into the random thermal motion of molecules heat energy.
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This conversion of chemical energy into heat energy is essential if the reactions inside the cell are to cause the universe as a whole to become more disordered—as required by the second law. Some interconversions between different forms of energy. All energy forms are, in principle, interconvertible. In all these processes the total amount of energy is conserved; thus, for example, from the height and weight of the brick in the first example, more The cell cannot derive any benefit from the heat energy it releases unless the heat-generating reactions inside the cell are directly linked to the processes that generate molecular order.
It is the tight coupling of heat production to an increase in order that distinguishes the metabolism of a cell from the wasteful burning of fuel in a fire. Later in this chapter, we shall illustrate how this coupling occurs. All animals live on energy stored in the chemical bonds of organic molecules made by other organisms, which they take in as food.
The molecules in food also provide the atoms that animals need to construct new living matter. Some animals obtain their food by eating other animals. But at the bottom of the animal food chain are animals that eat plants. The plants, in turn, trap energy directly from sunlight. As a result, all of the energy used by animal cells is derived ultimately from the sun.
Solar energy enters the living world through photosynthesis in plants and photosynthetic bacteria. Photosynthesis allows the electromagnetic energy in sunlight to be converted into chemical bond energy in the cell. Plants are able to obtain all the atoms they need from inorganic sources: They use the energy they derive from sunlight to build these atoms into sugars, amino acids, nucleotides, and fatty acids.
These small molecules in turn are converted into the proteins, nucleic acids, polysaccharides, and lipids that form the plant. All of these substances serve as food molecules for animals, if the plants are later eaten. The reactions of photosynthesis take place in two stages Figure In the first stage, energy from sunlight is captured and transiently stored as chemical bond energy in specialized small molecules that act as carriers of energy and reactive chemical groups.
We discuss these activated carrier molecules later. Molecular oxygen O 2 gas derived from the splitting of water by light is released as a waste product of this first stage. The two stages of photosynthesis. In the second stage, the molecules that serve as energy carriers are used to help drive a carbon fixation process in which sugars are manufactured from carbon dioxide gas CO 2 and water H 2 O , thereby providing a useful source of stored chemical bond energy and materials—both for the plant itself and for any animals that eat it. We describe the elegant mechanisms that underlie these two stages of photosynthesis in Chapter The net result of the entire process of photosynthesis , so far as the green plant is concerned, can be summarized simply in the equation.
The sugars produced are then used both as a source of chemical bond energy and as a source of materials to make the many other small and large organic molecules that are essential to the plant cell. All animal and plant cells are powered by energy stored in the chemical bonds of organic molecules, whether these be sugars that a plant has photosynthesized as food for itself or the mixture of large and small molecules that an animal has eaten.
In order to use this energy to live, grow, and reproduce, organisms must extract it in a usable form. In both plants and animals, energy is extracted from food molecules by a process of gradual oxidation, or controlled burning. The Earth's atmosphere contains a great deal of oxygen, and in the presence of oxygen the most energetically stable form of carbon is as CO 2 and that of hydrogen is as H 2 O. A cell is therefore able to obtain energy from sugars or other organic molecules by allowing their carbon and hydrogen atoms to combine with oxygen to produce CO 2 and H 2 O, respectively—a process called respiration.
Photosynthesis and respiration are complementary processes Figure This means that the transactions between plants and animals are not all one way. Plants, animals, and microorganisms have existed together on this planet for so long that many of them have become an essential part of the others' environments. The oxygen released by photosynthesis is consumed in the combustion of organic molecules by nearly all organisms.
And some of the CO 2 molecules that are fixed today into organic molecules by photosynthesis in a green leaf were yesterday released into the atmosphere by the respiration of an animal—or by that of a fungus or bacterium decomposing dead organic matter. We therefore see that carbon utilization forms a huge cycle that involves the biosphere all of the living organisms on Earth as a whole, crossing boundaries between individual organisms Figure Similarly, atoms of nitrogen, phosphorus, and sulfur move between the living and nonliving worlds in cycles that involve plants, animals, fungi, and bacteria.
Photosynthesis and respiration as complementary processes in the living world. Photosynthesis uses the energy of sunlight to produce sugars and other organic molecules. These molecules in turn serve as food for other organisms. Many of these organisms more Individual carbon atoms are incorporated into organic molecules of the living world by the photosynthetic activity of plants, bacteria, and marine algae.
They pass to animals, microorganisms, and organic material in soil and oceans in more The cell does not oxidize organic molecules in one step, as occurs when organic material is burned in a fire. Through the use of enzyme catalysts, metabolism takes the molecules through a large number of reactions that only rarely involve the direct addition of oxygen. Before we consider some of these reactions and the purpose behind them, we need to discuss what is meant by the process of oxidation.
Oxidation , in the sense used above, does not mean only the addition of oxygen atoms; rather, it applies more generally to any reaction in which electrons are transferred from one atom to another.
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Oxidation in this sense refers to the removal of electrons, and reduction —the converse of oxidation—means the addition of electrons. Since the number of electrons is conserved no loss or gain in a chemical reaction, oxidation and reduction always occur simultaneously: When a sugar molecule is oxidized to CO 2 and H 2 O, for example, the O 2 molecules involved in forming H 2 O gain electrons and thus are said to have been reduced. When a carbon atom becomes covalently bonded to an atom with a strong affinity for electrons, such as oxygen, chlorine, or sulfur, for example, it gives up more than its equal share of electrons and forms a polar covalent bond: Conversely, a carbon atom in a C-H linkage has slightly more than its share of electrons, and so it is said to be reduced see Figure A When two atoms form a polar covalent bond see p.
The net effect in this case is to add a hydrogen atom to the molecule. Even though a proton plus an electron is involved instead of just an electron , such hydrogenation reactions are reductions, and the reverse, dehydrogenation reactions, are oxidations. It is especially easy to tell whether an organic molecule is being oxidized or reduced: Cells use enzymes to catalyze the oxidation of organic molecules in small steps, through a sequence of reactions that allows useful energy to be harvested. We now need to explain how enzymes work and some of the constraints under which they operate.
The paper burns readily, releasing to the atmosphere both energy as heat and water and carbon dioxide as gases, but the smoke and ashes never spontaneously retrieve these entities from the heated atmosphere and reconstitute themselves into paper.
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When the paper burns, its chemical energy is dissipated as heat—not lost from the universe, since energy can never be created or destroyed, but irretrievably dispersed in the chaotic random thermal motions of molecules. At the same time, the atoms and molecules of the paper become dispersed and disordered.
In the language of thermodynamics, there has been a loss of free energy , that is, of energy that can be harnessed to do work or drive chemical reactions. This loss reflects a loss of orderliness in the way the energy and molecules were stored in the paper. We shall discuss free energy in more detail shortly, but the general principle is clear enough intuitively: Although the most energetically favorable form of carbon under ordinary conditions is as CO 2 , and that of hydrogen is as H 2 O, a living organism does not disappear in a puff of smoke, and the book in your hands does not burst into flames.
This is because the molecules both in the living organism and in the book are in a relatively stable state, and they cannot be changed to a state of lower energy without an input of energy: In the case of a burning book, the activation energy is provided by the heat of a lighted match. For the molecules in the watery solution inside a cell, the kick is delivered by an unusually energetic random collision with surrounding molecules—collisions that become more violent as the temperature is raised.
The important principle of activation energy. Compound X is in a stable state, and energy is required to convert it to compound Y, even though Y is at a lower overall energy level than X. This conversion will not take place, therefore, unless compound more In a living cell, the kick over the energy barrier is greatly aided by a specialized class of proteins—the enzymes. Each enzyme binds tightly to one or two molecules, called substrates , and holds them in a way that greatly reduces the activation energy of a particular chemical reaction that the bound substrates can undergo.
A substance that can lower the activation energy of a reaction is termed a catalyst ; catalysts increase the rate of chemical reactions because they allow a much larger proportion of the random collisions with surrounding molecules to kick the substrates over the energy barrier, as illustrated in Figure Enzymes are among the most effective catalysts known, speeding up reactions by a factor of as much as 10 14 , and they thereby allow reactions that would not otherwise occur to proceed rapidly at normal temperatures.
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Lowering the activation energy greatly increases the probability of reaction. A population of identical substrate molecules will have a range of energies that is distributed as shown on the graph at any one instant. The varying energies come from collisions more Enzymes are also highly selective. Each enzyme usually catalyzes only one particular reaction: In this way, enzymes direct each of the many different molecules in a cell along specific reaction pathways Figure Multicellular life - the last hurdle? Norman Cheetham has extensive science teaching experience at Secondary School, Teachers' College and University level.
He has contributed chapters to three science text books used in Australian Secondary Schools, and developed the chemistry sections of a set of overhead projector-based teaching materials. Until retirement, he was an Associate Professor in the School of Chemistry, the University of NSW, Sydney, where he taught courses in food chemistry, polymer chemistry, carbohydrates, drug analysis, and instrumental techniques. He is now an Adjunct Professor at the University of the Sunshine Coast, Queensland, Australia, and is still involved in teaching and research.
It is enjoyable to read, and represents a highly successful effort on the part of the author to incorporate such an abundance of ideas and facts into a flowing, eminently readable narrative. As a scientifically literate person who would have studied the areas he discusses in my bachelor's degree program over 50 years ago which was before some of the discoveries which inform this book , I found reading it to be challenging, fascinating, and worthwhile. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.
Academic Skip to main content. Choose your country or region Close. Ebook This title is available as an ebook. To purchase, visit your preferred ebook provider. Cheetham Adopts a truly interdisciplinary approach to the study of living systems, centered on chemical thermodynamics as applied to biological energy processes Explores the connections between biology and the physical sciences, providing a clear explanation of unifying themes Concise and accessible writing style encourages a multidisciplinary readership Integrated approach provides each student group biologist, medic, or physical scientist with useful insights into the others' disciplines.