Chapter 4: Bioenergetics- Cells and Cell Processes Lesson 4.3: What Is Bioenergetics? This team of ants is breaking down a dead tree; a classic example of teamwork. And all that work takes energy. In fact, each chemical reaction - the chemical reactions that allow the cells in those ants to do the work - needs energy to get started. And all that energy comes from the food the ants eat. Whatever eats the ants gets their energy from the ants. Energy passes through an ecosystem in one direction only. Lesson Objectives • Learn the chemistry behind the energy of life. • Be able to differentiate between potential and kinetic energy. • Understand that an organism’s metabolism transforms matter and energy, following the laws of thermodynamics. • Define the first and second law of thermodynamics. Explain how energy coupling works. Understand the concept of Gibbs Free Energy. Know that ATP powers cellular work through exergonic and endergonic reactions. Vocabulary anabolic pathway catabolic pathway endergonic reactions energy energy coupling enthalpy entropy exergonic reactions Gibbs Free Energy kinetic energy • potential energy Introduction Chemical reactions always involve energy. Energy is a property of matter that is defined as the ability to do work. When methane burns, for example, it releases energy in the form of heat and light. Other chemical reactions absorb energy rather than release it. A chemical reaction that releases energy (as heat) is called an exothermic reaction. This type of reaction can be represented by a general chemical equation: Reactants → Products + Heat. In addition to methane burning, another example of 129 an exothermic reaction is chlorine combining with sodium to form table salt. This reaction also releases energy. A chemical reaction that absorbs energy is called an endothermic reaction. This type of reaction can also be represented by a general chemical equation: Reactants + Heat → Products. Did you ever use a chemical cold pack? The pack cools down because of an endothermic reaction. When a tube inside the pack is broken, it releases a chemical that reacts with water inside the pack. This reaction absorbs heat energy and quickly cools down the pack. All chemical reactions need energy to get started. Even reactions that release energy need a boost of energy in order to begin. The energy needed to start a chemical reaction is called activation energy. Activation energy is like the push a child needs to start going down a playground slide. The push gives the child enough energy to start moving, but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in Figure 4.33 below. Why do all chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must bump into each other, so they must be moving, and movement requires energy. When reactant molecules bump together, they may repel each other because of intermolecular forces pushing them apart. Overcoming these forces so the molecules can come together and react also takes energy. Figure 4.33 Activation energy provides the “push” needed to start a chemical reaction. Is the chemical reaction in this figure an exothermic or endothermic reaction? Chemistry of the Energy of Life The living cell generates thousands of different reactions during bioenergetics. Bioenergetics is the study of how organisms manage their energy resources through metabolic pathways. Biological energetics is the branch of biology and biochemistry that studies how organisms extract energy from their environment and how that energy is used to fuel life endergonic processes (processes by which organisms absorb free energy from their surroundings). Organisms may be usefully divided into two broad groups with respect to how they satisfy their need for energy. Autotrophic organisms convert energy from nonorganic sources such as light to chemical energy. As heterotrophic organisms, animals must ingest and break down complex organic molecules to provide the energy for life. Metabolism is the totality of an organism’s reactions that arise from interactions between molecules. An organism’s metabolism transforms matter and energy according to the laws of thermodynamics. Metabolic pathways take place in specific regions of cells and are a sequence of chemical reactions, where the product of one reaction serves as a substrate for the next reaction. Two different metabolic pathways form the organizational units of metabolism, there are catabolic pathways and anabolic pathways. Each of these pathways begins when a specific molecule is catalyzed by an enzyme to form an end product. Catabolic pathway reactions break down large molecules into smaller subunits and release energy. Anabolic pathways join small molecules together to form larger molecules that store energy. 130 Forms of Energy Energy is the capacity to do work or the ability to cause change. Biologically energy is used for chemical reactions, movement, and cellular transport. All changes in the universe require energy. Energy for these changes comes in two forms: potential energy and kinetic energy. Potential energy is stored energy, where no change is currently taking place. Potential energy includes the energy that is stored in molecular structures. Kinetic energy is change, this always involves some type of motion. Energy can be converted from potential energy to kinetic energy and from kinetic energy to potential energy, see Figure 4.34 below. Figure 4.34 Noticed that while the diver is on the platform he has potential energy but as he dives that energy is converted to kinetic energy. Interconversions of forms of energy are commonplace in the biological world. In photosynthesis, the electromagnetic energy of light is converted to chemical energy, largely in the form of carbohydrates, with high overall efficiency. The energy of light is used to drive reactions that could not take place in the dark. Light energy also powers the generation of a proton electrochemical potential across the green photosynthetic membrane. Thus, electrical work is an integral part of photosynthesis. Chemical energy is used in all organisms to drive the synthesis of large and small molecules, motility at the microscopic and macroscopic levels, the generation of electrochemical potentials of ions across cellular membranes, and even light emission as in fireflies. Given the diversity in the forms of life, it might be expected that organisms have evolved many mechanisms to deal with their need for energy. To some extent this expectation is the case, especially for organisms that live in extreme environments. However, the similarities among organisms in their bioenergetic mechanisms are as, or even more, striking than the differences. For example, the sugar glucose is catabolized (broken down) by a pathway that is the same in the enteric bacterium Escherichia coli as it is in higher organisms. All organisms use adenosine triphosphate (ATP) as an energy source in energy metabolism. ATP acts in a way as a currency of free energy. The anabolic pathway that synthesizes ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi) by dehydration synthesis is a strongly endergonic reaction (non-spontaneous reaction that absorb free energy from its surroundings) that is coupled to exergonic reactions (spontaneous reactions with a net release of free energy) such as the breakdown of glucose. Photosynthesis is another example of an endergonic reaction, it requires the input of light energy from the sun in order for its chemical reactions to take place. Cellular respiration follows a catabolic pathway and is an exergonic reaction that releases energy from ATP through hydrolysis to power many of life’s processes. The central role of ATP in bioenergetics is illustrated in Figure 4.35 on the next page. 131 Figure 4.35 Central role of adenosine triphosphate (ATP) in metabolism. Catabolic metabolism is exergonic and provides the energy needed for the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi). The exergonic hydrolysis of ATP in turn powers the endergonic processes of organisms. The Laws of Energy Transfer and Transformation The laws of thermodynamics govern all energy changes in the universe and are known as the first law of thermodynamics and the second law of thermodynamics. The first law of thermodynamics states that energy cannot be created or destroyed, it can only be transferred or transformed. The first law of thermodynamics is sometimes referred to as the principle of conservation of energy. The second law of thermodynamics states that the disorder, or entropy, of the universe is constantly increasing. Simply this means that when energy is transformed or transferred to matter from a more ordered , less stable form to a less ordered, more stable form spontaneously without the input of outside energy entropy, or disorder, increases in the universe, as well as the release of “free energy”. Each time energy is transformed some of it is lost as heat to the environment, see Figure 4.36. Heat is defined as the measure of the random motion of molecules. Figure 4.36 During the bioenergetic processes of photosynthesis and cellular respiration a small amount of heat energy is lost to the environment as shown in the illustration above. 132 Energy Coupling In order to increase energetic efficiency, cells often couple reactions together. Endergonic reactions are those that require an input of energy. Exergonic reactions are those that release energy. By coupling these two reactions together, the overall chemical process is made exergonic so it can occur spontaneously due to a negative free-energy change. Using this process, unfavorable chemical reactions can be made to proceed. The basis of reaction coupling is a shared chemical intermediate. After one reaction produces one product, another can use it as a reactant to drive the production of an essential compound. Most coupling reactions use the breakdown of adenosine triphosphate (ATP) as the intermediate process to drive chemical synthesis. ATP is used as an energy-storing compound.