## Matter and Energy in Biology

### Matter and Energy

The concepts of matter and energy are essential to all scientific disciplines. Yet, while ubiquitous and fundamental, these concepts are often amongst the most confounding for students. Take the concept of energy. The term is used in a variety of contexts in everyday life - “Can we move the couch tomorrow? I don’t have the energy.” “Hey dude! Turn the light off. We need to conserve energy.” “This is a great energy drink.” In many sciences classes, students are told that energy comes in different forms (i.e. kinetic, thermal, electrical, potential, etc.), making it difficult to understand exactly what energy “is”. In class, the concept of energy is also associated with myriad different equations, each with different variables, but that somehow all seem to end up having units of work. Hold on! Work? I thought we were talking about energy?! Given all the different contexts and sometimes seemingly contradictory treatments and definitions, it’s not hard to understand why these topics seem challenging for many students and in some cases end up turning them off of the fields that make heavy use of these ideas. While the concepts of matter and energy are most often associated with chemistry and physics, they are nevertheless central ideas in biology and we don’t shy away from this in BIS2A. Our instructional goals, however, are to help students develop a conceptual framework that will help them use the concepts of matter and energy to:

• successfully describe biological reactions and transformations;
• create models and hypotheses for “how things work” in biology that explicitly include matter and energy and;
• be scientifically correct and transferable to new problems and to other disciplines.

While there may be a couple of energy-related equations to learn and use in BIS2A, the main focus of the course will be on the robust development of the concepts of energy and matter and their use in the interpretation of biological phenomena

### Motivation for Learning About Matter and Energy

Discussions about matter and energy make many, but not all, BIS2A students a little apprehensive. After all, aren’t these topics that belong in chemistry or physics? However, the transformations of matter and energy transfer are not phenomena reserved for the chemists and physicists or even scientists and engineers more generally. Understanding, conceptualizing, and doing some basic accounting of transformations of matter and transfers of energy are fundamental skills regardless of occupation or academic training. The scientist may need more rigorous and systematic descriptions of these transformations than the artist but both make use of these skills at various points of their personal and or professional lives. Take the following examples:

Example 1: Matter and Energy Transformation in Global Warming

Let us for a moment consider a topic that affects us all, global warming. At its core lies a relatively simple model that is based on our understanding of energy in solar radiation, the transfer of this energy with matter on the Earth, and the role and cycling of key carbon containing gases in the Earth's atmosphere. In simple terms solar energy hits the earth and transfers energy to its surface, heating it. Some of this energy is transferred back into space. However, depending on the concentration of carbon dioxide (and other so-called greenhouse gases) different amounts of this energy may become “trapped” in the Earth’s atmosphere. Too little carbon dioxide and relatively little energy/heat is trapped - the Earth freezes and becomes inhospitable for life. Too much carbon dioxide and too much heat is trapped - the Earth overheats and becomes inhospitable for life. It stands to reason, therefore, that mechanisms (biological or other) that influence the levels of carbon dioxide in the atmosphere may be important to consider in the story of global warming and that developing a good understanding of global warming phenomena requires one to trace the flow of the carbon and oxygen (matter) through their different forms and the mechanisms by which energy is transferred to and from different components of the system.

Example 2: Muscle Contraction

Let us now consider a more personal example, the flexing of an arm starting from an extended position and ending in a flexed position. Like most processes, this one can be described and understood at various levels of detail: from the anatomical point of view where the system consists of muscles, skin, and bones to the molecular where the system is composed of individual interacting biomolecules. At whatever level of detail, if we want to create a story describing this process we know that: (a) the description must include an accounting for what happened to the matter in the system (this includes the change in position of the molecules making up the various parts of the arm and the fuel “burned” to move it) and (b) that some fuel was burned to initiate the movement and therefore, that any description of the process must also include an accounting change in the energy of the system. In simpler terms, this is really just saying that if you want to describe a process where something has happened, you need to describe what happened to the “stuff” in the system and what happened to the energy in the system to make the process happen.

We can't possibly cover all examples of matter and energy transfer in BIS2A. But, we will explore these issues often and practice describing transformations that happen in Nature with a structured and explicit attention to what is happening to the matter and energy in a system as it changes. We will do this exercise across different structural levels in biology, from the molecular level (like a single chemical reaction) to more large-scale and abstracted models like nutrient cycling in the environment. We will practice this skill by using a pedagogical tool we call “The Energy Story”. Be prepared to participate!

## The Energy Story

### Overview of the Energy Story

Whether we know it or not, we tell stories that involve matter and energy everyday, we just don’t often use terminology associated with scientific discussions of matter and energy.

Example 1

The setup: a simple statement with implicit details
You tell your roommate a story about how you got to campus by saying, "I biked to campus today". In this simple statement are several assumptions that are instructive to unpack, even if they may not seem very critical to include explicitly in a casual conversation between friends over transportation choices.

An outsider's reinterpretation of the process
To illustrate this, imagine an external observer, for instance an alien being watching the comings and goings of humans on earth. Without the benefit of knowing much of the implied meanings and reasonable assumptions that are buried in our language, the alien's description of the morning cycling trip would be quite different than your own. What you described efficiently as "biking to campus" might be more specifically described by the alien as a change in location of a human body and its bicycle from one location (the apartment, termed position A) to a different location (the university, termed position B). The alien might be even more abstract and describe the bike trip as the movement of matter (the human body and its bike) between an initial state (at location A) to a final state (at location B). Furthermore, from the alien's standpoint what you'd call "biking" might be more specifically described as the use of a two-wheeled tool that couples the transfer of energy from the electric fields in chemical compounds to the acceleration of the two-wheeled tool-person combo and heat in its environment. Finally, buried within the simple statement describing how we got to work is also the tacit understanding that the mass of the body and bike were conserved in the process (with some important caveats we’ll look at in future lectures) and that some energy was converted to enable the movement of the body from position A to position B.

Note: Possible discussion:

Details are important. What if you owned a fully electric bike and the person you were talking with didn’t know that? What important details might this change about the “everyday” story you told that the more detailed description would have cleared up? How would the alien’s story have changed? In what scenarios might these changes be relevant?

As this simple story illustrates, irrespective of many factors, the act of creating a full description of a process includes some accounting of what happened to the matter, what happened to the energy and almost always some description of a mechanism that describes how changes in matter and energy of a system were brought about.

To practice this skill, in BIS2A we will make use of something we like to call "The Energy Story". You may be asked to tell an "energy story" in class, to practice telling energy stories on your lecture study guides, and to use the concept on your exams. In this section, we focus primarily on introducing the concept of an energy story and explaining how to tell one. It is worth noting that the term "energy story" is used almost exclusively in BIS2A (and has a specific meaning in this class). This precise term will not appear in other courses at UC Davis (at least in the short term) or if it appears, is not likely be used in the same manner. You can think of “The Energy Story” as a systematic approach creating a statement or story describing a biological process or event. Your BIS2A instructors have given this approach a short name "energy story," so that we can all associate it with the common exercise. That way when the instructor asks the class to tell or construct an energy story everyone knows what is meant.

### Definition 1: Energy Story

An energy story is a narrative describing a process or event. The critical elements of this narrative are:

1. Identifying at least two states (e.g. start and end) in the process.
2. Identifying and listing the matter in the system and its state at the start and end of the process.
3. Describing the transformation of the matter that occurs during the process.
4. Accounting for the “location” of energy in the system at the start and end of the process.
5. Describing the transfer of energy that happens during the process.
6. Identifying and describing mechanism(s) responsible for mediating the transformation of matter and transfer of energy.

A complete energy story will include a description of the initial reactants and their energetic states as well as a description of the final products and their energetic states after the process or reaction is completed.

Note: Possible discussion:

We argue that the energy story can be used to communicate all of the useful details that are required to describe nearly any process. Can you think of a process that cannot be adequately described by an energy story? If so, describe such a process.

Example 2: Energy Story Example

Let us suppose that we are talking about the process of driving a car from "Point A" to "Point B" (see the figure).

Figure 1: A schematic of a car moving at the start from position "Point A" to position "Point B" at the end. The blue rectangle depicted in the back of the car represents the level of gasoline, the purple squiggly line near the exhaust pipe represents the exhaust, squiggly blue lines on top of the car represent sound vibrations and the red shading represents areas that are hotter that at the start.

Source: Created by Marc T. Facciotti (Own work) A Car Moves from Point A to Point B

Let's step through the Energy Story rubric:

1. Identifying at least two states (e.g. start and end) in the process.
In this example we can easily identify two states. The first state is the non-moving car at "Point A", the start of the trip. The second state, after the process is done, is the non-moving car at "Point B".

2. Identifying and listing the matter in the system and its state at the start and end of the process.
In this case we first note that the "system" includes everything in the figure - the car, the road, the air around the car etc.

It is important to understand the we are going to apply the physical law of conservation of matter. That is, in any of the processes that we will discuss, matter is neither created or destroyed. It might change form, but one should be able to account for everything at the end of a process that was there at the beginning.

At the beginning of the process, the matter in the system consists of:
1. The car and all the stuff in it
2. The fuel in the car (a special thing in the car)
3. The air (including oxygen) around the car.
5. The driver

At the end of the process, the matter in the system is distributed as follows:
1. The car and all the stuff in it is in a new place (lets assume, aside from the fuel and position, that nothing else changed)
2. There is less fuel in the car and it too is in a new place
3. The air has changed - it now has less molecular oxygen, more carbon dioxide and more water vapor.
4. The road (let's assume it didn't change - other than a few pebbles moved around)
5. The driver (let's assume she didn't change - though we'll see by the end of the term that she did (at least a little). But, the driver is now in a different place.

3. Describing the transformation of the matter that occurs during the process.

What happened to the matter in this process? Thanks to a lot of simplifying assumptions, we see that two big things happened. First, the car and its driver changed positions - they went from "Point A" to "Point B". Second, we note that some of the molecules in the fuel, which used to be in the car as a liquid have changed forms and are now mostly in the form of carbon dioxide and water vapor (purple blob coming out the tailpipe). Some of the oxygen molecules that used to be in the air are now also in a new place as part of the carbon dioxide and water that left the car.

4. Accounting for the “location” of energy in the system at the start and end of the process.
It is again important to understand that we are going to invoke the physical law of conservation of energy. That is, we stipulate that the energy in the system cannot be created or destroyed and therefore the energy that is in the system at the start of the process must still be there at the end of the process. It may have been redistributed but you should be able to account for all the energy.

At the beginning of the process, the energy in the system is distributed as follows:
1. The energy tied up in the associations between atoms that make up the matter of the car.
2. The energy tied up in the associations between atoms that make up the fuel.
3. The energy tied up in the associations between atoms that make up the air.
4. The energy tied up in the associations between atoms that make up the road.
5. The energy tied up in the associations between atoms that make up the driver.
6. For all things above we can also say that there is energy in the molecular motions of the atoms that make up the stuff.

At the end of the process, the energy in the system is distributed as follows:
1. The energy tied up in the associations between atoms that make up the matter of the car.
2. The energy tied up in the associations between atoms that make up the fuel.
3. The energy tied up in the associations between atoms that make up the air.
4. The energy tied up in the associations between atoms that make up the road.
5. The energy tied up in the associations between atoms that make up the driver.
6. For all things above we can also say that there is energy in the molecular motions of the atoms that make up the stuff.

This is interesting in some sense because the lists are about the same. We know that the amount of energy stored in the car has decreased because there is less fuel. Something must have happened.

5. Describing the transfer of energy that happens during the process.
In the particular example it is the transfer of energy about the components of the system that is most interesting. As we mentioned, there is less energy stored in the gas tank of the car at the end of the trip because there is now less fuel. We also know intuitively (from our real life experience) that the transfer of energy from the fuel to something else was instrumental in moving the car from "Point A" to "Point B". So, where did this energy go? Remember, it didn't just disappear. It must have moved somewhere else in the system.

Well we know that there is more carbon dioxide and water vapor in the system after the process. There is energy in the associations between those atoms (atoms that used to be in the fuel and air). So some of the energy that was in the fuel is now in the exhaust. Let's also draw from our real life experience again and state that we know that parts of our car have gotten hot by the end of the trip (e.g. the engine, transmission, wheels/tires, exhaust etc.). For the moment we'll just tap our intuition and say that we understand that making something hot involves some transfer of energy. So we can reasonably postulate that some of the energy in the fuel went (directly or indirectly) into heating the car, parts of the road, the exhaust and thus the environment around the car. An amount of energy also went into accelerating the car from zero velocity to whatever speed it traveled, but most of that eventually went into heat when the car came to a stop.

This is a bit hand wavy of an explanation and we'll learn how to do a better job throughout the quarter. The main point is that we should be able to add all the energy at of the system at the beginning of the process (in all the places it is found) and at the end of the process (in all the places it is found) and those two values should be the same.

6. Identifying and describing mechanism(s) responsible for mediating the transformation of matter and transfer of energy.

Finally, it is useful to try understanding how those transformations of matter and transfers of energy might have been facilitated. For the sake of brevity, in this example we might just say that there was a complicated mechanical device (the engine) that helped facilitate the conversion of matter and transfer of energy about the system and coupled this to the change in position of the car. Someone interested in engines would, of course, give a more detailed explanation.

In this example we made a bunch of simplifying assumptions to highlight the process and to focus on the transformation of the fuel. But that's fine. The more you understand about the processes the finer details you can add. Note that you can use the Energy Story rubric for describing your understanding (or looking for holes in your understanding) of nearly any process (certainly in biology). In BIS2A we'll use the Energy Story to get an understanding of processes as varied as biochemical reactions, DNA replication, the function of molecular motors, etc.

Important:

First: We will be working many examples of the energy story throughout the course - do not feel that you need to have mastery over this topic today.

Second: Nevertheless, while it is tempting to think all this is superfluous or not germane to your study of biology in BIS2a, let this serve as a reminder that your instructors (those creating the course midterm and final assessments) view it as core material. We will revisit this topic often throughout the course but need you to get familiar with some of the basic concepts now.

This is important material and an important skill to develop - do not put off studying it because it doesn't "look" like "biology" to you today. The academic term moves VERY quickly and it will be difficult to catch up later if you don't give this some thought now.

## Energy

Energy is a central concept in all sciences. Energy is a property of a system. While it can be neither created nor destroyed understanding the transfer of energy around physical systems is a key component of understanding how and why things change. In this following sections we are going to explore some basic concepts associated with common transformations in biology and chemistry - the solubility of various biomolecules, the making and breaking of chemical bonds, the transfer of electrons, the transfer of energy to and from light, and the transfer of energy as heat. In class, many of the discussions will happen in the context of the Energy Story rubric, so when we consider a reaction of transformation, we will be interested in precisely defining the system in question and trying to account for all the various transfers of energy that occur within the system, making sure that we abide by the Law of conservation of energy.

There are plenty of examples where we use the concept of energy in our everyday lives to describe processes. A bicyclist can bike to get to campus for class. The act of moving herself and her bicycle from point A to point B can be explained to some degree by examining the transfers energy that take place. We can look at this example through a variety of lenses, but as biologists we more than likely want to understand the series of events that explain how energy is transferred from molecules of food, to the connected coordinated activity of biomolecules in her flexing muscle, and how this can finally be connected to moving the bike to get her from point A to point B. To do this we need to be able to talk about various ways in which energy can be transferred between parts of a system and where it is stored or transferred out of the system. In the next section, we will also see the need to consider how that energy is distributed among the many microstates (molecular states) of the system and its surroundings.

### How We will Approach Conceptualizing Energy

In BIS2A we will think about energy with a "stuff" metaphor. Note, however, that energy is NOT a substance, it is rather a property of a system. But we will think of it in some sense as property that can be stored in a part of a physical system and transferred or "moved" from one storage place to another. The idea, is to reinforce the ideas that energy maintains its identity when transferred - it is not changing forms per se. This in turn also encourages us to make sure that energy always has a home and that we account for all of the energy in a system before and after a transformation, it does not just "made" or get "lost" (both ideas in contradiction of the Law of Conservation of Energy). When energy is being transferred we therefore must identify where it is coming from and where it is going - all of it! Again, we can't just have some getting lost. When energy is transferred, there must be some mechanism associated with that transfer. Let's think about that to help us explain some of the phenomena we're interested in. That mechanism is part of the "how" that we are often interested in understanding. Finally, if we talk about transfer, we must realize that both components, the part of the physical system that gave up energy and the part of the system that received that energy are changed from their initial states. We should make sure that we are looking at all of the components of a system for changes in energy when examining a transformation.

### Energy Sources

Ultimately, the source of energy for many processes occurring on the earth's surface comes from solar radiation. But as we will see biology has been very clever at tapping a variety of forms of energy to construct and maintain living beings. As we move through this course we will explore a variety of energy sources and the ways in which biology has devised to transfer energy from these fuels.

## Energy in Chemical Reactions

Chemical reactions involve a redistribution of energy within the reacting chemicals and with their environment. So, like it or not, we need to develop some models that can help us to describe where energy is in a system (perhaps how it is "stored"/distrbuted) and how it can be moved around in a reaction. The models we develop will not be overly detailed - in the sense that they would not satisfy a hard-core chemist or physicist with technical detail - but we expect that they should still be technically correct and not form incorrect mental models that will make getting the "refinements" down later.

In this respect, one of the key concepts to understand is that we are going to think about energy being transferred between parts of a system. We'll try not to think about it as being transformed. Transfer vs. transform - that's important. The latter gives the impression that energy is a property which exists in different forms, that it gets reshaped somehow. One problem with the "transform" language is that it is difficult to reconcile with the idea that energy is being conserved (according to the first law of thermodynamics) if it constantly changing form. How can the entity be conserved if it is no longer the same thing? Moreover, the second law of thermodynamics tells us that no transformation conserves all energy in a system. If energy is getting "transformed" how can it be conserved?

So, instead, we are going to approach this issue by transferring and storing energy between different parts of a system and thus think about energy as a property that can get redistributed. That'll hopefully make the accounting easier.

CAUTION:

If we are going to think about transferring energy from one part of a system to another, we also need to be careful about NOT treating energy like a substance that moves like a fluid or "thing". Rather, we need to appreciate energy simply as a property of a system that can be measured and reorganized but that is neither a "thing" nor something that is at one time in one form then later in another.

Since we are will often be dealing with transformations of biomolecules we can start by thinking about where energy can be found/stored in these systems. We'll start with a couple of ideas and add more to them later.

Let us propose that one place that energy can be stored is in the motion of matter. For brevity we'll give the energy stored in motion a name: kinetic energy. Molecules in biology are in constant motion and therefore have a certain amount of kinetic energy (energy stored in motion) associated with them.

Let us also propose that there is a certain amount of energy stored in the biomolecules themselves and that the amount of energy stored in those molecules is associated with the types and numbers of atoms in the molecules and the their organization (the number and types of bonds between them). The discussion of exactly where the energy is stored in the molecules is beyond the scope of this class but we can approximate it by suggesting that a good proxy is in the bonds. Different types of bonds may be associated with storing different amounts of energy. In some contexts this type of energy storage could be labeled potential energy or chemical energy. With this view, one of the things that happens during the making and breaking of bonds in a chemical reaction is that the energy is transferred about the system into different types of bonds. In the context of an Energy Story one could theoretically count the amount of energy stored in the bonds and motion of the reactants and the energy stored in the bonds and energy of the products.

In some cases you might find that when you add up the energy stored in the products and the energy stored in the reactants that these sums are not equal. If the energy in the reactants is greater than the products where did this energy go? It had to get transferred to something else. Some will certainly have moved into other parts of the system, stored in the motion of other molecules (warming the environment) or perhaps in the energy associated with photons of light. One good real life example is the chemical reaction between wood and oxygen (reactants) and it's conversion to carbon dioxide and water (products). At the beginning, the energy in the system is largely in the molecular bonds of oxygen and the wood (reactants). There is still energy left in the carbon dioxide and water (products), but less than at the beginning. We all appreciate that some of that energy was transferred to the energy in light and heat. This reaction where energy is transferred to the environment is termed exothermic. By contrast, in some reactions energy will transfer in from the environment. These reactions are called endothermic.

The transfer of energy in or out of the reaction from the environment is NOT the only thing that determines whether a reaction will be spontaneous or not. We'll discuss that soon. For the moment, it is important to get comfortable with the idea that energy can be transferred between different components of a system during a reaction and that you should be able to envision tracking it.

## Thermodynamics

Thermodynamics is concerned with describing the changes in systems before and after a change. This usually involves a discussion about the energy transfers and its dispersion within the system. In nearly all practical cases, these analyses require that the system and its surroundings be completely described. For instance, when discussing the heating of a pot of water on the stove, the system may includes the stove, the pot, and the water and the environment or surroundings may include everything else. Biological organisms are what are called open systems; energy is transferred between them and their surroundings.

### 1st Law of Thermodynamics

The first law of thermodynamics deals with the total amount of energy in the universe. It states that this total amount of energy is constant. In other words, there has always been, and always will be, exactly the same amount of energy in the universe. According to the first law of thermodynamics, energy may be transferred from place to place, but it cannot be created or destroyed. The transfers of energy take place around us all the time. Light bulbs transfer energy from an electrical power station into heat and photons of light. Gas stoves transfer energy stored in the bonds of chemical compounds into heat and light. Heat, by the way, is the amount of energy transferred from one system to another because of a temperature difference. Plants perform one of the most biologically useful energy transfers on earth: they transfer energy in the photons of sunlight into the chemical bonds of organic molecules. In every one of these cases energy is neither made or destroyed and we must try to account for all of the energy when we examine some of these reactions.

### 1st Law and the Energy Story

The first law of thermodynamics is deceptively simple. Students often understand that energy cannot be created or destroyed. Yet, when describing an energy story of a process they often make the mistake of saying things such as "energy is produced from the transfer of electrons from atom A to atom B". While most of us will understand the point the student is trying to make, the wrong words are being used. Energy is not made or produced, it is simply transferred. To be consistent with the first law, when telling an energy story, make sure that you try to explicitly track all of the places that ALL of the energy in the system at the start of a process goes by the end of a process.

### 2nd Law of Thermodynamics

An important concept in physical systems is that of entropy. Entropy is related to the with the ways in which energy can be distributed or dispersed within the particles of a system. The 2nd Law of Thermodynamics states that entropy is always increasing in a system AND its surroundings (everything outside the system). This idea helps explain the directionality of natural phenomena. In general, the notion is that the directionality comes from the tendency for energy in a system to move towards a state of maximal dispersion. The 2nd law, therefore, means that in any transformation we should look for an overall increase in entropy (or dispersion of energy), somewhere. A idea that is associated with increased dispersion of energy in a system or its surroundings is that as dispersion increases the ability of the energy to be directed towards work decreases.

There will be many examples of where the entropy of a system decreases. To be consistent with the second law, however, we must try to find something else (likely a closely connected system in the surroundings) that must compensate for the "local" decrease in entropy with an equal or greater increase in entropy.

The entropy of a system can increase when:(a) it gains energy; (b) a change of state occurs from solid to liquid to gas; (c) mixing of substances occurs; (d) the number of particles increases during a reaction.

Possible discussion

Does the second law say that entropy is conserved?

Possible discussion

Biological systems, on the surface, see to defy the Second Law of Thermodynamics. They don't. Why?

Possible discussion

A fine point. The figure above discusses order and disorder and shows that this is somehow related to a change in entropy (ΔS). It is common to describe entropy as a measure of order as a way to simplify the more concrete description relating entropy to the number of states in which energy can be dispersed in a system. While the idea of measuring order to define entropy has some flaws, it is sometimes a useful, if imperfect, proxy. Consider the figure above. Here order serves as a good proxy for approximating the number of ways to distribute energy in the system. Can you describe why this is the case?

## Free Energy

If we want to describe transformations, it is useful to have a measure of (a) how much energy is in a system and (b) the dispersal of that energy within the system and, of course, (c) how theses change between the start and end of a process. The concept of free energy, often referred to as Gibbs free energy or free enthalpy (abbreviated with the letter G), in some sense, does just that. Gibbs free energy can be defined in several interconvertible ways, but a useful one in the context of biology is the enthalpy (internal energy) of a system minus the entropy of the system scaled by the temperature. The difference in free energy when a process takes place is often reported in terms of the change (Δ) of enthalpy (internal energy) denoted H, minus the temperature scaled change (Δ) in entropy, denoted S. See the equation below.

$ΔG=ΔH−TΔS$

The Gibbs energy is often interpreted as the amount of energy available to do useful work. With a bit of handwaving we can interpret this by invoking the idea presented in the section on entropy that states the dispersion of energy (required by the Second Law) associated with a positive change in entropy somehow renders some of the energy that is transferred less useful to do work. One can say that this is reflected in part in the T∆S term of the Gibbs equation.

To provide a basis for fair comparisons of changes in Gibbs free energy amongst different biological transformations or reactions, the free energy change of a reaction is measured under a set of common standard experimental conditions. The resulting standard free energy change of a chemical reaction is expressed as an amount of energy per mole of the reaction product (either in kilojoules or kilocalories, kJ/mol or kcal/mol; 1 kJ = 0.239 kcal) when measured at a standard pH, temperature, and pressure conditions. Standard pH, temperature, and pressure conditions are generally calculated at pH 7.0, 25 degrees Celsius, and 100 kilopascals (1 atm pressure), respectively. It is important to note that cellular conditions vary considerably from these standard conditions, and so actual ∆G inside a cell will differ considerably from those calculated under standard conditions.

## Endergonic and Exergonic Reactions

Reactions that have a ∆G < 0 means that the products of the reaction have less free energy than the reactants. Since ∆G is the difference between the enthalpy and entropy changes in a reaction a net negative ∆G can arise in different ways. The left panel of the figure below shows a common graphical representation an exergonic reaction. Free energy is plotted on the y-axis and the x-axis in arbitrary units shows model for the progress of a reaction. This type of graph is called a reaction coordinate diagram. In the case of an exergonic reaction, depicted below, the chart indicates two key things: (1) the difference between the free energy of the reactants and products is negative and (2) the progress of the reaction requires some input of free energy (shown as an energy hill). This graph does not tell us how the energy in the system was redistributed, only that the difference between enthalpy and entropy is negative. Reactions that have a negative ∆G are termed exergonic reactions. These reactions are said to occur spontaneously. Understanding which chemical reactions are spontaneous is extremely useful for biologists that are trying to understand whether a reaction is likely to "go" or not.

It is important to note that the term spontaneous - in the context of thermodynamics - does NOT imply anything about how fast the reaction proceeds. The change in free energy only describes the difference between beginning and end states NOT how fast that transition takes. This is somewhat contrary to the everyday use of the term which usually carries the implicit understanding that something happens quickly. As an example, the oxidation/rusting of iron is a spontaneous reaction. However, an iron nail exposed to air does not rust instantly - it may take years.

A chemical reaction with a positive ∆G means that the products of the reaction have a higher free energy than the reactants (see the right panel of Figure 2). These chemical reactions are called endergonic reactions, and they are NOT spontaneous. An endergonic reaction will not take place on its own without the transfer of energy into the reaction or increase of entropy somewhere else.

Exergonic and endergonic reactions result in changes in Gibbs free energy. In exergonic reaction the free energy of the products is lower than that of the reactants; meanwhile in endergonic the free energy of the products is higher than that of the reactants. Attribution: Marc T. Facciotti (own work)

The building of complex molecules, such as sugars, from simpler ones is an anabolic process and is endergonic. On the other hand, the catabolic process, such as the breaking down of sugar into simpler molecules is generally exergonic. Like the example of rust above, while the breakdown of biomolecules is generally spontaneous these reactions don’t necessarily occur instantaneously (quickly). Remember, the terms endergonic and exergonic only refer to the difference in free energy between the products and reactants - they don't tell you about the rate of reaction (how fast it happens). The issue of rate will be discussed in later sections.

An important concept in the study of metabolism and energy is that of chemical equilibrium. Most chemical reactions are reversible. They can proceed in both directions, often transferring energy into their environment in one direction, and transferring energy in from the environment in the other direction. The same is true for the chemical reactions involved in cell metabolism, such as the breaking down and building up of proteins into and from individual amino acids, respectively. Reactants within a closed system will undergo chemical reactions in both directions until a state of equilibrium is reached. This state of equilibrium is one of the lowest possible free energy and a state of maximal entropy. Equilibrium in a chemical reaction, is the state in which both reactants and products are present in concentrations which have no further tendency to change with time. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. NOTE THIS LAST STATEMENT! Equilibrium means that the relative concentrations of reactants and products is not changing in time BUT it does NOT mean that there is no interconversion between substrates and products - it just means that when reactant is converted to product that product is converted to reactant at an equal rate.

Either a rebalancing of substrate or product concentrations (by adding or removing substrate or product) or a positive change in free energy, typically by the transfer of energy from outside the reaction, is required to move a reaction out of a state of equilibrium. In a living cell, most chemical reactions do not reach a state of equilibrium - this would require that they reach their lowest free energy state. Energy is therefore required to keep biological reactions out of their equilibrium state. In this way, living organisms are in a constant energy-requiring, uphill battle against equilibrium and entropy.

At equilibrium, do not think of a static unchanging system. Instead, picture molecules moving, in equal amounts from one area to another. Here, at equilibrium, molecules are still moving from left to right and right to left. The net movement however, is equal. There will still be about 15 molecules in each side of this flask once equilibrium is reached.
Source: https://courses.candelalearning.com/...apter/entropy/