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Chapter 4: Bioenergetics- Cells and 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 . In fact, each - 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 behind the energy of . • Be able to differentiate between potential and kinetic energy. • Understand that an ’s 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 .  Know that ATP powers cellular work through exergonic and endergonic reactions.

Vocabulary

 anabolic pathway  catabolic pathway  endergonic reactions  energy  energy coupling   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 and light. Other chemical reactions absorb energy rather than release it. A chemical reaction that releases energy (as heat) is called an . 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 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 manage their energy resources through metabolic pathways. Biological energetics is the branch of and 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 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 is catalyzed by an 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.

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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 , the electromagnetic energy of light is converted to chemical energy, largely in the form of , 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 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 (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 (ADP) and inorganic phosphate (Pi) by dehydration synthesis is a strongly (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. follows a catabolic pathway and is an exergonic reaction that releases energy from ATP through 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. The phosphoanhydride bond between phosphate groups found in ATP stores a significant amount of energy due to the negative charges carried by the phosphate groups. This bond stored energy that is not currently use, but available later for running reactions is called potential energy. This energy is required to keep the negatively charged groups close to each other in the ATP molecule because they, as do all like-charged groups, repel each other. This energy can be released (exothermic reaction) for use in the cell to do work, move things and build things by hydrolysis and breakage of the bond. ATP  ADP + P + Energy When carbohydrates and other foods are consumed, they are broken down by to release the energy within them. The exothermic energy released is used to reattach a phosphate to ADP through Endothermic reactions which will regenerate ATP formation. ADP + P + Energy  ATP Then, the process of bond breaking and bond forming will repeat over and over within human cells to provide energy for all the chemical reactions.

Gibbs Free Energy or Available Energy

Gibbs free energy is a thermodynamic property that was defined in 1876 by Josiah Willard Gibbs to predict whether a process will occur spontaneously at constant temperature and pressure. Gibbs free energy G is defined as ΔG = ΔH - TΔS where H, T and S are the enthalpy (H), temperature (T), and entropy. Enthalpy is the total energy of a system and entropy is measure or disorder of a system. Changes in the Gibbs free energy G correspond to changes in free energy for processes at constant temperature and pressure. The change in Gibbs free energy change is the maximum non- expansion work obtainable under these conditions. ΔG is negative for spontaneous processes, positive for nonspontaneous processes and zero for processes at equilibrium. In order to understand this concept think of ΔG as available energy; energy that can be used for work. This available energy arises from spontaneous reactions like cellular respiration that release energy to their surroundings. Dr. Anderson through his Bozeman Science videos explains the concepts very well through an example analogy which is summarized for you below:

“So the three reactions that I'm going to talk about here are, number one a ball rolling down a slide. Number two, diffusion. And then number three, a cherry bomb that's exploding. And so we're going to apply Gibbs free energy to each of these. So let's start with a ball at the top of a slide. And the ball at the top of the slide is going to roll down to the bottom. And so the first thing that I want to give you is what's called enthalpy or total energy of a system. And so if we say this is a system. What happens to the total energy of the system as the ball rolls down the slide? Well if you know anything about potential energy, the amount of potential energy that we have up here is going to be greater than the amount of potential energy that we have down here. And so the total energy of a system, we'll call that H or enthalpy. The total energy of the system has gone from a big H to a little H. In other words the total energy of the system has decreased when the ball rolls down. Now if I had to push it up 133 again, then we would add energy to the system. But this is a spontaneous reaction. And the enthalpy or the total energy is decreasing. Now in biology we don't move around because of potential energy or mechanical potential energy. In biology our energy or our potential energy is actually in these bonds. In other words there's a huge amount of potential energy in this bond between the carbon and the hydrogen. And so this right here is glucose. And if we can release some of that energy, we can do it to do work. And so again, what is H? H stands for the total energy of the system and it looks like in a spontaneous reaction that's actually going to get smaller. Or decrease. Okay. Next one. Let's talk about diffusion. So in diffusion imagine it right here that we've got a bunch of molecules in this container and they're bouncing around. And I remove this wall. So if I remove that wall, what's going to happen to the molecules? The molecules are going to spread out to fill that area. We call that diffusion. Now entropy is, we use the symbol S for that, entropy is a measure or the disorder of a system. Or sometimes we call that the randomness of the system. And so let's compare this. Right here we've got a bunch of molecules on this side. And then we've got a lot of space over here. So what happens to the disorder of that system as I do diffusion? Well it's becoming more disordered. In other words the entropy is increasing. What's an example of that? Let's say I go into your room and your room's a mess. It looks like this. If I say clean up your room, then you could go like that. And so what happens through diffusion or in the spontaneous reaction, well let's say remove this wall. What's going to happen now? We're going to get even more disorder. So we're going to even get a bigger S value. And so in this spontaneous reaction it looks like the S value is increasing. Okay. Last one is that cherry bomb. So let's say we have a cherry bomb. We put it on the desk. Does it explode? No. And one of the reasons it doesn't is the temperature is really low. And so let's say I add a bonfire to the situation. So I increase the temperature. Does that make it a more spontaneous reaction or is the reaction more likely to happen or less? Well it's more likely to happen if I increase the temperature. More likely to get an explosion. Okay. So those three things, total energy or enthalpy, entropy or S, and temperature can effect spontaneous reactions. And so now let's apply that to Gibbs free energy. And so before we actually get to the equation, let's do a little algebra here. So let's say I wrote this equation. X = Y - AB. Okay. So let's say we had this equation right here and I were to decrease this value, the Y value. What would that do to the X value? It would decrease it, right? Let's say I were to increase the A value and increase the B value, what would that do to the X value? Well since we're subtracting right here that would decrease it as well or make it go even farther down. And so let's go back and summarize those three spontaneous reactions. In the first one the ball rolled down. So what happened to our H value? Well our enthalpy of the system decreased. What should that do to our Gibbs free energy or our available energy? It should decrease that value. What happened here when we increased the entropy of the system or increased the delta S? Well if we increase the delta S that should also decrease the Gibbs free energy. So now we have two things decreasing that. And what happened here? Well if we increase the temperature that made it more spontaneous. So if we increase that, we also decrease the Gibbs free energy. So what's the moral of the story? Moral of the story is that if the delta G ever decreases or if it's ever less than zero, that's a spontaneous reaction. Likewise, if the delta G is greater than zero, let's turn to the next slide, that's going to not be a non-spontaneous reaction. Okay. So in summary. If the delta G is less than zero that's a spontaneous reaction or we call that an exergonic reaction or an energy releasing reaction. If it's greater than zero that's an endergonic reaction. And then finally if nothing happens to the available free energy then it's just at equilibrium. The quintessential example of a spontaneous reaction in life is going to be cellular respiration. Photosynthesis is an example of an endergonic reaction.”

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You can watch the Bozeman Science video on Gibbs Free Energy at: http://www.bozemanscience.com/gibbs-free-energy/.

Lesson Summary

Bioenergetics is the part of biochemistry concerned with the energy involved in making and breaking of chemical bonds in the molecules found in biological organisms. It can also be defined as the study of energy relationships and energy transformations in living organisms. Growth, development and metabolism are some of the central phenomena in the study of biological organisms. The role of energy is fundamental to such biological processes. The ability to harness energy from a variety of metabolic pathways is a property of all living organisms. Life is dependent on energy transformations; living organisms survive because of exchange of energy within and without. In a living organism, chemical bonds are broken and made as part of the exchange and transformation of energy. Energy is available for work (such as mechanical work) or for other processes (such as chemical synthesis and anabolic processes in growth), when weak bonds are broken and stronger bonds are made. The production of stronger bonds allows release of usable energy. Living organisms obtain energy from organic and inorganic materials. For example, can use minerals such as nitrates or forms of , such as elemental sulfur, sulfites, and to produce ATP. In photosynthesis, can produce ATP using light energy. must consume organic compounds. These are mostly carbohydrates, , and . The amount of energy actually obtained by the organism is lower than the amount present in the food; there are losses in digestion, and metabolism. Living organisms produce ATP from energy sources via bioenergetic. An organism's stockpile of ATP is used as a battery to store energy in cells, for metabolism. Catabolic reactions involve the breakdown of chemical molecules, while anabolic reactions involve the synthesis of compounds. Utilization of chemical energy from such molecular bond rearrangement powers biological processes in every biological organism.

References/ Multimedia Resources

"Bioenergetics." Whatwhenhow RSS. N.p., n.d. Web. 1 Aug. 2014.

Campbell, Neil A. "The Energy of Life." Biology. Student Ed. S.l.: Pearson/Benjamin Cummings, 2008. 142- 45. Print.

"Energy and Metabolism - PowerPoint." Docstoc.com. N.p., n.d. Web. 1 Aug. 2014.

"Gibbs Free Energy." Bozemanscience. N.p., n.d. Web. 1 Aug. 2014.

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