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Nature’s Energy Mining Relies on Molecular Design

Emily Reeves
Photo: Chloroplasts, by Juan Carlos Fonseca Mata, CC BY-SA 4.0 , via Wikimedia Commons.

Yesterday I began to explain why, if nature is designed, it may exhibit optimal green energy solutions. (See here.) Today we will look at how energy mining happens in the natural world. Let’s take a step back, though, and first ask: Where does energy come from?

In Einstein’s famous equation, energy is related to mass and light.

E = mc2

Therefore both matter (coal, wood, sugar) and light (sunshine, heat) are energy sources. Because energy transformation is so important  these different sources are sorted and categorized by whether or not energy has been converted or transformed.

Primary and Secondary Energy Sources

Primary energy sources are considered to be energy forms that haven’t been transformed by human engineering or living organisms. Such sources include fossil energy, nuclear energy, and renewable energy (e.g., solar, wind, and water). Secondary energy sources are forms of energy that have been converted from a primary source (electricity, sugar). 

Energy Transformation

Plants mine energy from a primary source (sun) and transform that energy into a secondary source (sugar) which other living organisms use for growth and reproduction. Similarly, solar power (photovoltaics) transform a primary source (sun) into a secondary source (electricity) which flows through power lines and allows for modern infrastructure.

Because the plant’s mechanism of energy transformation (photosynthesis) does not negatively affect sustainability, it is classified as green energy transformation. Photovoltaics are also said to produce green energy transformation because they do not efflux CO2 into the atmosphere. However, their production is not necessarily without environmental impact and the current majority do not recapture carbon from the atmosphere.

The burning of fossil fuels to create electricity is an example of energy transformation from a primary source to a secondary source that increases carbon emissions into the atmosphere without a compensatory recapture mechanism.

While we probably do not currently appreciate all the mechanisms of ecosystem balance, it is still advisable to note that nature’s design does recapture CO2 from the atmosphere and turn it into chemical bonds which power the energy demands of living organisms and keep a balance of carbon in the atmosphere.

Ecosystem Balance

Plants do not generate electricity, but they do still extract energy from sunlight. Plants mine light energy to produce ATP (chemical energy), oxygen (important for ecosystem balance), and sugars (stored chemical energy). In addition, plants remove CO2 from the atmosphere, moving the carbon back into delicious fruits and veggies for us to consume.

Everything about photosynthesis speaks to ecosystem design as plants satisfy the needs of living organisms by providing oxygen and transformed energy (carbohydrates) while recycling a waste product of living organisms (namely CO2) that needs to be balanced in an ecosystem. The mechanism plants employ to generate these products is carefully engineered, down to the design of molecules.

Molecular Machines Drive Photosynthesis

Plants are able to make these useful products for ecosystem balance because of individual units called chloroplasts. Chloroplasts are a lot like solar cells. They contain thousands of antennas (molecules specially designed for absorbing light) that can receive photons. Once a photon is captured by an antenna, it causes electron excitation and the transfer of a high-energy electron from a chlorophyll molecule to a specialized molecular machine that carries electrons. This molecular delivery system drives the excited electron to another molecular machine which uses it to pump protons across a membrane. The process appears to be driven by attraction properties intrinsic to the design of these macromolecular machines.

I want to emphasize here that specialized molecular machinery is responsible for both electron capture and movement. Properties of physics (diffusion, attraction, and repulsion) are exploited as well, but unlike with solar cells, exploitation of physical properties is not all that’s going on. There are specific molecules and dynamic macromolecular complexes designed to facilitate electron capture and transfer.

Biological organisms engage an army of enzymes and reductive pathways to produce long-chain hydrocarbons from naturally available constituents including CO2, H2O and N2. Enzymes and proteins within the metabolic pathways of cells benefit from an ingrained building code in genetic information and are repaired and replicated as necessary.

Cestellos-Blanco et al. 2020

When protons are pumped across the membrane of the chloroplast, this creates voltage which is harnessed by a molecular machine to power the production of ATP using rotational energy. To replenish its lost electron, the chlorophyll molecule, in cooperation with another molecular machine, steals back an electron by splitting H2O. It is this reaction that ultimately results in plants breathing out oxygen.

Photovoltaics Use Physics, Not Molecular Machines

Man-made solar panels lack these specialized molecular machines, instead relying solely on the exploitation of physics and properties of matter. Solar panels are composed primarily of silicon (sand) instead of a molecular factory. The silicon is highly purified, doped with specific chemicals, and then thinly sliced to create silicon wafers. The wafers have a positive layer caused by boron doping and a negative layer caused by phosphorus doping. This causes polarity in the silicon wafer. When a photon of sunlight strikes the wafer, it may displace an electron (negative charge) which will then flow to the positive layer (exploiting a physical property). So movement of electrons occurs due to properties of the materials, not because of dynamic molecular machines. The result is still charge separation or voltage. However, because the molecular actors are missing, there isn’t the capability to do other cool things simultaneously like remove carbon from the atmosphere.

Speaking of cool things, tomorrow I’ll talk about the incredible molecular machine that has received much unfair criticism but is responsible for all current carbon balance. Stay tuned.