Sunday, August 13, 12023 Human Era (HE)
- Continuation Explanation
- The Phantasmagoria of Photosynthesis
Continuation Explanation
This post is a continuation of a series on energy and its relationship to stand up paddleboarding (SUP). The impetus for the post came after a conversation with a client on energy systems. We got into some fairly intricate details, and I realized that a refresher for myself was in order. I found that writing a few posts on anatomy and biomechanics specific to SUP served as a great review of my anatomical knowledge, and I thought it would be a fun exercise to refresh on some energy metabolism physiology.
The first instalment covered some background basics before branching into a rant on the semantics of sex-typing, and it finished by attempting to define what energy is. The second instalment looked at work, enthalpy and free energy, and carbon as the building block of life. This post continues from where we left off in instalment two talking about photosynthesis.
The Phantasmagoria of Photosynthesis
Our energy source as humans is ultimately the Sun. The gigantic nuclear reactor in our backyard supplies us with electromagnetic radiation in the form of packets of light, i.e., photons. As discussed by the BBC‘s Melvin Bragg and his guests on this episode of In Our Time, “The Life of Stars,” there is no Milky Way Galaxy nor us without nuclear. We are the by-product of nuclear waste.
But there was a problem for our present solar energy procurements. We lacked and still lack the capability of consuming sunlight directly as an energy source. Humans are heterotrophs, and we can not produce our own food, biologically speaking. We can only meet our energy requirements by feeding off of organisms capable of converting the Sun’s energy into chemical energy. Fortunately for us, somehow, a long time ago, biology cracked the code of the energy conversion conundrum. From what we can tell, over three billion years ago, photosynthetic organisms evolved. The origins of land plants, which most modern terrestrial animals feed on, would begin around one to half a billion years ago.
A Taste of the Sun
During photosynthesis, an organism uses light energy to assemble sugars from carbon dioxide (CO2) and hydrogen (H) and electron sources such as water (H2O). For a more technical visual description of photosynthesis, check out this video, “The Most Important Process on Earth,” from But Why?.
A Little Help From Our Friends
Autotrophs are organisms capable of creating complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as CO2. Autotrophs do this using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis), thereby creating their own food. Today, through photosynthesis, plants, and other organisms are able to transform light energy into chemical energy. The electromagnetic energy of the Sun is stored in the form of chemical bonds between atoms. At least, that is what I would have said prior to this deep dive into energy. I think a more correct description would be that energy from the Sun is stored in the form of chemical bond systems (i.e., systems bound by chemical bonds). It is the organization/complexity of these systems that becomes our free energy source.
Source: https://opentextbc.ca/biology/chapter/5-1-overview-of-photosynthesis/
A Bit on Bonds (…okay more than a bit)
Perhaps this is a bit of a semantical tirade, but there is some controversy over using the phrase, ‘energy is stored in the bonds between atoms,’ to the point that it has made headlines in popular science writing. While most of us might say that energy is stored in bonds in colloquially ‘science-y’ speak, and that is technically correct, it is missing some of the details. Most specifically, the simplification fails to consider that the formation of bonds releases energy, while the breaking of bonds requires energy.
What is Wrong About Our (Common) Belief About Bonds?
Before researching for this post, I would have said that the energy stored from the Sun in the process of photosynthesis is locked away in the chemical bonds of the compounds created. But, perhaps this isn’t quite correct. At an individual bond level, when a bond is created, energy is released. That released energy is lost to the wider environment as a transfer of energy and thus can not be stored in the molecular bonds. Conversely, when a chemical bond is broken, energy is needed to overcome the molecular attraction. The latter makes more intuitive sense. It takes energy in the macro-world to break something apart, and the same is true in the micro-world of chemistry. While I am sure that I was taught this fundamental principle of chemistry at some point, that chemical bonds release energy when formed and require energy input to be broken, my default explanation drifted to describing a release of energy from the destruction of molecular bonds, e.g., sugar. The simplicity of this summary serves as an explanation for its predominance.
The Kinetic Potential Oxymoron
Here is where things get a bit confusing. We refer to chemical energy as stored potential energy. This is particularly the case when referencing the energy in foodstuffs. Food is described as stored potential energy. Body fat, for example, is often referred to as stored potential energy in the body.
Where things get slightly strange and somewhat confusing is when a more complex chemical is formed. In the formation of glucose during photosynthesis, energy must be added to the system. That is the “sunlight” in the photosynthesis equation above. Sunlight is the electromagnetic radiation energy that powers molecular transformation. But, as we saw above, bond formation releases energy. What’s going on here?
Bringing together CO2 and H2O to make glucose requires energy because CO2 and H2O are in a more stable molecular state, which is a lower energy level. Creating glucose requires energy input to reach a higher, less stable, energy state compared to CO2 and H2O. The bonding of carbon atoms to make the hexose carbon structure for glucose at the individual bond level releases energy. This loss of energy to the surroundings is kinetic at the subatomic level. The covalently bonded electrons enter a hybridized orbital that is shared between atomic nuclei and give up some of their kinetic energy to do so. However, the system of glucose overall is left with greater potential energy from the instability and higher energy level of the newly formed molecule. Given a catalyst, glucose can later be broken down to release this stored energy by disrupting the molecule and forming more stable bonds at lower molecular energy levels, releasing energy from the system to the surroundings, that is the wider system.
Semantics, Systems, and State
To me, the ambiguity about bonds and energy comes down to the energy states and the limits, or boundaries, of systems. The definition of a system is somewhat arbitrary. Draw a line somewhere in the sand, and that is your system. Of course, there are formal definitions of different types of thermodynamic systems, but at the end of the day, it comes down to what your cutoff point is. Ultimately, the system is the Cosmos, but for practical purposes, we like to contain things closer. If the system is an individual molecule of glucose, first energy is added to the system to break down the molecule, but the result is more energy being released. Free energy is available to the surroundings. But a broader view of the system that includes the surroundings would mean that there was no added energy. Rather, the energy within the system just changed forms. A flux between kinetic, thermal, and potential energies. In this case, the metabolism of glucose into CO2 and H2O with the accompanying energy release would sum up to obey the conservation of energy (and still fit the framework of the zero-energy Universe hypothesis). Carbon dioxide and H2O are in a more stable, less energetic state. They’re sitting at the bottom of their energy hierarchy where the electrons are in a happy place.
Musing Metaphors
An analogy that is often used is to parallel the energetic state to gravity since it is a concept we are all familiar with. The analogy also has the added benefit of being a visual, and both are fundamental phenomena of our Universe. If you lift an object to a higher level (i.e., the increased energy state) and the hypothetical perch is on a ledge or slope, it has greater gravitational potential energy (our analogous chemical potential energy). It requires kinetic energy to lift the object to the higher level (this would be the photon’s excitation of electrons in our photosynthesis parallel). The object is less stable at its higher height (perched on the edge) and, given the opportunity, will come down if possible. In a similar fashion, glucose is unstable compared to CO2 and H2O and wants to come down to a more stable state.
Another metaphor that is often used to describe these energy exchanges from potential to kinetic is a roller coaster. Pulling the train car up the track is the energy input that raises the gravitational potential energy of the system, which can be released as kinetic energy when the cart gets over the hump and begins the ride down. Along the way, there are also smaller peaks and troughs where the train accelerates or decelerates. In our chemistry analogy, these peaks and troughs serve as similes of more stable structures between higher and lower energy states, i.e., the intermediate molecules of glycolysis, which we will discuss later in installment five, when we “Enter the Cytosol” of the cell and look at the breakdown of sugar.
Somewhat of a Summary
My take is that generally speaking, it can be regarded as true that energy is stored in the bonds. There is energy in the bond in the sense that there is energy in the field fluctuations between subatomic particles. As far as I can tell (from reading, I don’t truly know), there are fluctuations in the electromagnetic field where the bonds exist, which constitutes energy. So, there is energy in the bonds strictly speaking. Things are bound by the energy they are losing… this is the energy we are using.
Confusing Controversy
Where things get a bit confusing is the basic physiochemistry of bond formation. We know that energy is released during bond formation. Breaking a chemical bond always requires energy input. One interpretation of the release of energy during bond formation is that the bonds can not be directly storing the energy that is later released. Any energy stored in the bond has to be different from the energy released to the surroundings as a result of the bond formation. The question then becomes, is more or less energy put into the reactants side of the equation than is available on the product side. “If it takes more energy to break the original bonds than is released when the new bonds are formed, then the net energy of the reaction is negative.” In this case, the reaction requires an energy input. Such reactions are known as endothermic. Whereas, “if if [sic] takes less energy to break the original bonds than is released when new bonds are formed, then the net energy of the reaction is positive.” Here, energy will be released as the reaction proceeds, but an initial energy input is required. Such reactions are known as exothermic. The resultant gain or loss of the availability of energy is system dependent. That is the net balance between the reactants and the products.
Photosynthesis Energy Accounting
As an example, take the photosynthesis equation from above. This is an endothermic reaction that requires energy input from the Sun to proceed.
The balanced chemical equation below summarizes the photosynthesis chemical reaction:
6 CO2 + 6 H2O + energy → C6H12O6 + 6 O2
The reactants are to the left of the arrow and the products to the right. The arrow represents the moment the reaction happens. At that moment, six carbon dioxide (CO2) molecules and six water (H2O) molecules are converted, using light energy captured by chlorophyll, into a sugar (C6H12O6) molecule, and six oxygen (O2) molecules. Hidden behind the arrow is where the magic happens. That is the light energy capture and molecular conversion. A more nuanced representation might look something like this:
6 CO2 + 6 H2O + a lot energy →
6 C + 12 H + 18 O →
C6H12O6 + 6 O2 + a little energy
The arrow at the end of the first line represents the breaking of bonds. This step requires a large energy input since CO2 and H2O are stable, low-energy state molecules. The second line depicts the atoms broken out of their molecular form and free to react, with the arrow signalling the transition to the products. The third line shows the products that are now in a higher-energy state but less stable. Some of the light energy has been stored in the new system, increasing the molecular complexity (check out the But Why? video above for a great explanation of what is going on subatomically). That instability/complexity is the potential chemical energy that has been stored in the molecular configurations of glucose and O2. You can think of this as them having more kinetic energy at the subatomic level, they are in a higher vibratory state but are keen to give up that energy to get to a lower, more stable state. I included “a little energy” as being released on the product side since I am nearly certain the reaction is not 100% efficient (some heat or enthalpy must escape the system).
Complex Chemistry
Taking all the above into consideration allows for a better explanation of how and where the energy is stored in the products of photosynthesis. The molecular structure of glucose (i.e., sugar), C6H12O6, is more complex and unstable compared to the metabolic reactants of photosynthesis, CO2, and H2O. The energy from sunlight is used to counter the chaos of entropy and create complex capricious chemicals we call “carbohydrates,” that is, carbon plus a hydrate complex. The so-called ‘bond energy’ of chemistry is potential energy that is stored in the complexity of the molecule’s design. There is subatomic kinetic energy locked away in the molecular chemical potential energy.
What’s Missing?
The above is a slight oversimplification and leaves out how energy is absorbed and transferred, the light-dependent versus the light-independent reactions (a.k.a., the Calvin Cycle). Here is as good a time as any to introduce adenosine triphosphate or ATP. Often referred to as the universal cellular energy currency, ATP functions as an energy store and transfer within the cell (more on ATP in instalment four). During photosynthesis, ATP alongside nicotinamide adenine dinucleotide phosphate [NADPH, a reduced (i.e., electron-bearing) electron carrier] formed during the light-dependent reactions allows for the transfer of energy and electrons to the Calvin Cycle, where CO2 is fixed to ultimately form glucose.
Image Source: Khan Academy. “Intro to photosynthesis.”
The next instalment will focus on A-T-P spe-ci-fically…
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