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Sustainability Matters 7: Why are Plants Called Nature’s Solar Collectors?


Why are Plants Called Nature’s Solar Collectors?


It is believed that the first plants were a kind of algae-like mat, which took advantage of available carbon dioxide and water to store energy in the form of glucose.

The reaction, called Photosynthesis, is as follows:

6CO2 + 12 H20 673 kilocalories C6H1206 + 6O2 + 6H2O

Plants convert 6 molecules of carbon dioxide (CO2) and 12 molecules of water (H2O) which, fueled by 673 kilocalories of energy in the form of sunlight, produce 1 molecule of glucose (C6H1206), 6 molecules of oxygen, and 6 molecules of water. This simple reaction converts sunlight (kinetic energy) to glucose (potential energy). Glucose, a form of sugar, is a molecule which effectively stores energy in a form which may be broken down and used by living organisms.

How has Photosynthesis affected the earth?

It is a great evolutionary advantage to be able to store energy for times of little or no sunlight, such as nights or winters. It turns out this evolutionary adaptation was so successful that plants colonized every surface of the earth and adapted to virtually every set of conditions.

There was little or no oxygen in the atmosphere until plants came to dominate the earth.

However, their waste, oxygen (the byproduct of photosynthesis), changed the chemical balance of the atmosphere as it accumulated. Today, oxygen accounts for approximately 21% of the atmosphere. It was not until a significant amount of oxygen had built up in the atmosphere that animal life could evolve and prosper.

Oxygen in the atmosphere had another effect.

Lightning in the atmosphere provided the charge needed to turn Oxygen (O2) into Ozone (O3). It turns out that ozone In the upper atmosphere blocks harmful ultraviolet (UV) radiation from the sun. Ozone slowed down the rate of genetic mutation of living things which was a factor in stabilizing the environment on Earth.

Plants were so successful at converting the sun’s energy to stored energy in the form of glucose that they stored a surplus compared to what they actually used. Great forests grew, lived and died, over time and were buried deep below the surface by ongoing tectonic forces before they could decompose.

“…oil and natural gas were created from organisms that lived in the water and were buried under ocean or river sediments. Long after the great prehistoric seas and rivers vanished, heat, pressure and bacteria combined to compress and “cook” the organic material under layers of silt. In most areas, a thick liquid called oil formed first, but in deeper, hot regions underground, the cooking process continued until natural gas was formed. Over time, some of this oil and natural gas began working its way upward through the earth’s crust until they ran into rock formations called ‘caprocks’ that are dense enough to prevent them from seeping to the surface. It is from under these caprocks that most oil and natural gas is produced today.”

The energy stored by photosynthesis remained in the unbroken bonds of that matter until humans discovered millions of years later that “fossil fuels”, those remains which form coal, oil and gas, were a valuable source of high-quality energy.

A more stable environment and an abundance of stored energy was just what animals needed to make the transition from water to land. I always envisioned it rather like my favorite Far Side cartoon by Gary Larson.

Millions of years of photosynthesis has continued to add oxygen to the earth’s atmosphere.

It was only after plants had contributed a significant amount of oxygen to the atmosphere that animals evolved, fueled by a process called respiration which converts glucose, the energy stored by photosynthesis in plants, into useable energy.

C6H1206 + 6O2 + 6H2O673 kilocalories released 6CO2 + 12H20

Notice that the respiration reaction is the exact opposite of the photosynthesis reaction described above. Taken together, the two reactions form a cycle converting carbon dioxide to oxygen (as plants do) and then converting oxygen back to carbon dioxide (as animals do).

Figure 2-5 A Simplified Plant-Animal Energy Cycle

As Figure 2-6 shows, the photosynthesis-respiration cycle is complex. When vegetation or dead organic material gets buried, the carbon contained in those sediments is stored in the ground long term as part of the Carbon Cycle.

geologic time scaled portion of the carbon cycle.


Notice that the photosynthesis-respiration cycle is actually a part of the larger Carbon Cycle.

On a short time-scale, carbon (in the form of carbon dioxide) is cycled from the atmosphere into organic material via photosynthesis. It is returned to the atmosphere via respiration from plants and decaying material in the soil. The terrestrial portion of the cycle shows a net gain of 1.4 billion metric tons of carbon per year. This is represented by the white box over the trees.

Approximately half of the carbon is removed from the atmosphere into the soil and oceans.

Some of that carbon will become part of the sediments and remain in the long-term cycle. However, most of the carbon removed from the long-term Carbon Cycle by humans becomes part of the short term, biologically dominated cycle. This means that instead of being sequestered for millions of years, the additional carbon is now part of a cycle with times being measured in months and years as opposed to millennia. This continued addition of carbon into the atmosphere increases the overall carbon concentration.

There is also a net gain of 1.7 billion metric tons of carbon per year to the ocean. So, through natural processes, the ocean and earth gain about 3.1 billion metric tons of carbon per year.

Review of the Physical Laws of Matter and Energy

A course in ecological theory would include more details on these and other biogeochemical cycles such as Nitrogen, Phosphorous, Potassium that affect ecosystems.  However, our examination of the rock, water, carbon and oxygen cycles provides us with the crucial information we need for this discussion.

What we have learned about the laws of matter and energy can be summarized by the phrase: Energy Flows and Matter Cycles. The sun is the source energy which drives biogeochemical cycles with additional gravitational and heat energy from the earth.

It is important to understand these fundamental scientific laws because the earth’s environment and all life evolve within the constraints of these laws. This goes for human activities too!

In the next week’s blog, let’s look more closely at how living systems use energy and matter.

Giraffe near Kruger NP

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