5. Energy

Every organism has to consume energy to survive. In this topic, we will discuss the processes that handle energy in organisms. 

Definition of energy

Before talking about the energy in organisms, I would like to clarify the general meaning of energy. Energy is defined as the capacity to do work. Work is defined as any change in the motion of the matter or any change in the state of the matter. So, energy is the capacity to change the motion or state of matter.

There are two units of energy: the joule and the calorie. They can be converted into each other; 1 calorie is equal to 4.184 joules. Both of them express the same thing, but the joule is the SI unit for all forms of energy, while the calorie is a specific unit historically used for heat energy, particularly in the context of food and nutrition.

Types of energy

We can classify the energy into two types: kinetic energy and potential energy. 

You might imagine that a moving object, like a baseball that a baseball player throws, has a certain amount of energy, and that this energy increases as the ball's speed increases. As you can see from this, motion is a form of energy, and this is called kinetic energy.

On the other hand, potential energy is defined as the energy of a position or state. You can visualize this by imaging a ball at 50 cm high and a ball at 1 meter high. If you release these balls, the ball with a height of one meter will hit the floor stronger than the other ball. From this episode, you can tell that an object in a higher position has more energy. Not only the position of the object but also the state of the object can form potential energy. For example, both cold water and boiling water are water, needless to say. We can easily tell that boiling water has more energy than cold water because it is exerting heat, which is another form of energy. For another example, gasoline binds with oxygen to form carbon dioxide and water during the combustion reaction. It goes without saying that gasoline has more energy than carbon dioxide and water because gasoline literally produces energy for the movement of the car, but carbon dioxide and water cannot form such energy. The difference between gasoline and carbon dioxide + water is their chemical composition. Like this, the chemical composition of the matter can determine its potential energy.

In living organisms, foods like carbohydrates, lipids, proteins, and nucleic acids are digested into simpler substances and eventually converted into carbon dioxide and water, just like in the combustion reaction we saw before. During this chemical reaction, energy is released, and we utilize it to maintain or move our body. In short, we eat food in order to be able to move; in other words, living organisms convert potential energy in the food to kinetic energy in the body.

And we can convert them in the opposite direction, so the cells convert kinetic energy to potential energy. For instance, the cell creates ATP, which has potential energy in the form of a macroergic bond, from the kinetic energy of the proton. We will see the details of this later.

Enthalpy, entropy, and Gibbs free energy

All three terms, enthalpy (H), entropy (S), and Gibbs free energy (G), are fundamental concepts in thermodynamics, the branch of science concerned with energy and its transformations. 

Enthalpy

Enthalpy (H) represents the total internal energy of a system, including its thermal energy (heat) and the potential energy stored within its chemical bonds. In simple terms, it’s a measure of the heat content of a system at constant pressure. Or, we can say, enthalpy is the maximal amount of heat that can be extracted from the system. 

Now, let’s consider the “maximal amount” of extractable energy. Imagine boiling alcohol. If you put this alcohol at room temperature, it will exert some heat in the room. Through this, you could extract heat from the alcohol. After that, you ignite the alcohol. As you imagine, the alcohol will undergo the combustion reaction and release heat. Through this process, you could again extract some energy from the alcohol. After the reaction, the products will remain, which will be carbon dioxide and water. The source of this extracted energy is the chemical bonds between atoms in the alcohol; the chemical bonds in the alcohol were higher than those in carbon dioxide and water. Both carbon dioxide and water are chemically stable, so they won’t undergo further chemical reactions in nature. However, you would be able to extract more energy from carbon dioxide and water if you could break the attraction forces between subatomic particles, like protons and neutrons. (We call such reaction nuclear reaction.)

As you saw above, you can extract heat or energy as much as you break the substance smaller and smaller. And in reality, we cannot define the smallest particles in nature. (Heisenberg Uncertainty Principle) This is the reason why people cannot define the zero point of enthalpy, meaning enthalpy is always a relative value compared with another substance, and you will see only the change in the enthalpy (ΔH) of the system, not the absolute value (H).

Entropy

Entropy (S) is a measure of disorder or randomness within a system. A higher entropy state signifies greater disorder.

In order to visualize this concept, let’s compare water and ice at 0 degrees Celsius. Both of them are composed of water molecules, and their temperatures are the same, so the only difference is the state of matter. Water is a liquid in which the interactions between the water molecules are hydrogen bonds. These hydrogen bonds can be detached and attached randomly, so that liquids have fluidity. On the other hand, ice is a solid in which each hydrogen bond is fixed between two specific water molecules. How do you think which one is more “random”? We may agree that liquid water has more randomness than ice. It can be stated that liquid water possesses a greater degree of entropy compared to ice. 

Let’s compare liquid water and vapor. As I said, liquid water is composed of water molecules connected via temporal hydrogen bonds. In vapour, there is nearly no interaction between water molecules, so water molecules can move freely in the air. We might be able to agree that vapour has more entropy than liquid water because vapour is more random. 

Now, you might already understand the abstract image of "randomness.” However, the concept of randomness in detail is still unclear. Let’s discuss it. The randomness can be defined as the number of possibilities the system has. In the examples above, vapour is composed of water molecules that can freely move across the container. In this case, the locations of the water molecules are variable because they are literally flying. In contrast to that, liquid water is composed of water molecules that can also freely move but are restricted in certain volumes because they are connected to each other. The location of the molecules in this case is less variable than that of vapor because the possible location of the molecules is restricted in the liquid.

Entropy is a very important component of the enthalpy of a system. The higher the entropy, the higher the enthalpy. As in the example above, vapour at 100 °C has a higher entropy than liquid water at 100 °C. Please imagine which of them can exert more heat or energy. We can agree that vapour can exert more heat because it is warmer. Also, you can guess it by thinking that liquid water has to “absorb” heat to evaporate to become vapour. 

Gibb’s free energy

Entropy is a component of the enthalpy of a system. And the other component that occupies the rest of the enthalpy is Gibbs free energy. The maximum amount of work that a system can perform at a constant temperature is known as Gibbs free energy.

How can we determine the maximum amount of work? Well, you may easily imagine this concept by thinking that enthalpy can be divided into entropy and Gibbs free energy. Gibbs free energy is an almighty form of energy which can be converted into any form of energy, like chemical energy, kinetic energy, potential energy, etc. However, when Gibbs free energy is “consumed” to raise the entropy of the system, the converted energy can no longer be converted into any other form of energy. So, we can say Gibbs free energy is a “useful form of energy” in the system.

For example, imagine a container that contains ice and an electric circuit connecting a power cell and a heater. When the circuit is on, the heater transforms the electric energy in the power cell (Gibbs free energy) into heat, which ice then absorbs. As a result, ice melts to become water (entropy increases). Before the reaction, we can use the energy stored in the power cell, but we cannot use the energy stored as the entropy of the system after the reaction. 

To be precise, entropy is not a form of enthalpy; rather, entropy times temperature is a form of enthalpy. The formula below describes the important relationship between enthalpy, entropy, and Gibbs free energy.