Photosynthesis and the Flow of Solar Energy to Living Organisms

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As was mentioned at the beginning of this section, life on Earth is primarily fueled by solar radiation energy. Plants use the sun’s light energy (photons) in a process called photosynthesis in order to produce their own “food”. The term “photosynthesis” comes from the Latin words photo, which means “light,” and synthesis, which means “putting together”.

The first photosynthetic organisms on Earth came into existence roughly 3.4 billion years ago. This is quite amazing considering the fact that the complex metabolic system used in photosynthesis evolved less than 1 billion years after the Earth was formed, approximately 4.5 billion years ago. This metabolic system is a complex series of biochemical reactions in the plant cell that allow the plant to use solar radiation as an energy source to convert carbon dioxide into sugar. Therefore, the input energy to photosynthesis is solar radiation, and the output energies are heat (which is lost to the environment), and sugar which is a chemical form of energy that plant tissues utilize to grow and reproduce.

Unlike the photosynthetic mechanisms utilized by today’s plants (which you will explore later in this section), these early organisms did not absorb the sun’s visible light. Instead these organisms absorbed a different portion of the solar spectrum (see Figure 11), called infrared radiation. They also did not produce oxygen as a by-product like plants do today, but rather produced sulfur containing compounds as by-products.

Figure 2: Cyanobacteria, often called blue-green algae, are ancient, single-celled prokaryotic organisms that are the photosynthetic ancestors of modern day plants. They are the first photosynthetic organisms to produce oxygen, which vastly changed the earth’s atmospheric composition, allowing for the highly efficient aerobic metabolism to evolve.1

It would not be for another billion years (~ 2.7 billion years ago) that single-celled cyanobacteria (Figure 2) would become the first photosynthetic organisms to absorb light from the visible portion of the spectrum and produce oxygen. Over the course of the last 2.7 billion years, thousands of different forms of tiny single-celled plant-like organisms called algae have evolved, transitioning the photosynthetic apparatus through the process of evolution to larger, more complex modern day land and aquatic vascular plants.

Looking Ahead

The process of photosynthesis is described in detail in Chapter 4.

As mentioned above, in photosynthetic processes today, photons (the sun’s light energy) are captured by a plant and metabolically converted into a chemical source of energy, such as glucose or simple sugars. Once converted, this energy fuels the plants’ metabolism, growth, and reproduction.

Closer Look

Watch this video for a summary of the photosynthesis reactions.

Plants can also metabolically transform the simple sugar produced by photosynthesis into more complex, higher energy molecules such as starches and lipids. The chemical energy (sugars, starches, lipids, etc.) stored in the tissues of plants (leaves, stem, seeds, etc.) may then be consumed by animals, including humans, who eat plants as food. After ingesting plants and the chemical energy present in them, an animal’s metabolic system then transforms that chemical energy into fuel for its own growth, reproduction, and physical activities.

We can observe the First Law of Thermodynamics at work in the process of photosynthesis. Solar energy fuels photosynthetic plants and algae, which in turn act as a fuel base that supports entire food webs of life on Earth. As mentioned earlier, this fuel base originates when energy from the sun is converted into sugars, starches, and lipids by plants that will then be consumed by animals and humans as food. The Second Law of Thermodynamics is also at work here. As energy is being transferred through the food chain, it is being transformed from one chemical form to another. In this process, much of the food energy is lost as metabolic heat.

Figure 3: Cross section of a deciduous tree leaf with palisade mesophyll cells designed to focus sunlight into the spongy mesophyll, where both types of mesophyll contain chloroplasts and conduct photosynthesis.1

Modern plants typically undergo photosynthesis within specialized organs called leaves, which are green and usually thin, providing optimal exposure to light. Figure 3 displays the cross-section of a typical deciduous tree leaf broken down into the layers of specialized cells, most of which are made green by the photosynthetic pigment called chlorophyll.

Figure 4: These plant cells are shaped like 6-sided hexagons, each containing multiple green spherical organelles called chloroplasts. The process of photosynthesis takes place within the chloroplasts.1

Chlorophyll is the most common photosynthetic pigment. It primarily absorbs red and blue solar radiation from the visible spectrum of light, and reflects back green light, which is what gives plants their green color. In eukaryotic algae and plants (see Figure 4), the chlorophyll is housed within specialized compartments, or organelles, called chloroplasts.

The process of photosynthesis involves a complex biochemical reaction that is split into two parts, the light-dependent reactions and the light-independent reactions, the latter is also known as the Calvin Cycle (Figure 5). Both are summed up with the overall equation of photosynthesis:

6CO2 + 12H2O + Solar Energy ⇒ C6H12O6 + 6O2 + 6H2O + Heat

Closer Look

Read more about the Calvin Cycle.

Six carbon dioxide molecules from the atmosphere plus twelve liquid water molecules from the soil plus light energy from the sun are metabolized through the process of photosynthesis in the plant’s chloroplasts to yield one molecule of glucose, six molecules of oxygen gas, and six water molecules. Heat is lost as a byproduct of the photosynthetic reaction.

In the light-dependent reaction of photosynthesis, solar radiation in the visible portion of the spectrum travels from the sun to the chloroplasts. A flow of electrons is initiated once energy is absorbed by the chlorophyll. Electrons are removed from water molecules and passed to an electron transport system, where they facilitate the synthesis of a molecule of ATP (adenosine triphosphate; cellular energy) and NADPH (an electron source). The ATP and NADPH are reactants needed to begin the Calvin Cycle.

Figure 5: An overview of photosynthesis with the light-dependent reactions and the Calvin Cycle both occurring within the chloroplast of the plant cell. Note how energy in the form of ATP from the light-dependent reactions is used to power the Calvin Cycle, and reducing agents (NADP and ADP) from the Calvin Cycle are used to aid the transformation of solar energy into ATP within the chloroplast’s stroma.1

The purpose of the Calvin Cycle is to produce the familiar 6-carbon sugar called glucose (C6H12O6) which it does by turning the biochemical cycle 6 times, each time bringing in one more carbon in the form of CO2 to build the 6-carbon glucose molecule.

Questions to Consider

Since ancient times, human beings have tried to invent a machine that continues indefinitely without any exterior source of energy, a so-called 'perpetual motion machine'. Can you explain why such an invention is impossible according to the laws of thermodynamics?

If you would like some assistance, visit this website.


The laws of thermodynamics are essential for understanding energy. People often have more difficulty understanding the second law than the first and third. To test your understanding of the second law, try explaining it with these phenomena:

  • hot pans cooling
  • water flowing down a waterfall
  • air blowing out into the atmosphere when a bicycle tire is punctured