Note: In this Investigation, we wish to clearly establish the unbroken flow of energy from the Sun to autotrophs to heterotrophs. In doing so, we will refresh students’ memories from the CELL Space, of the importance and details of the nuclear fusion reaction that takes place deep within the interior of the Sun. The product of the nuclear fusion reaction, photons, radiate from the Sun’s surface. Photons are high energy particles of light.

On Earth, many of the Sun’s photons are captured by the photosynthesis machinery of plants and algae. Some bacteria are also capable of photosynthesis as well. However, in our discussions and illustrations here, we will focus on “plants” for simplicity’s sake.

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Note: In this slide, and the next slide, we present two illustrations first seen in Investigation 1, in order to clearly put new information into the context of what students have just learned about trophic levels.

• The point of emphasis of this slide is the concept that a chain of feeding exists within all ecosystems. The “Producers” are the plants in the ecosystem that are capable of photosynthesis. Plants are autotrophs; they can produce their own food through photosynthesis. The consumers, on the other hand, are all heterotrophs; they cannot photosynthesize and therefore cannot produce their own food. They must feed on plants or other animals to survive.

• The different trophic levels of the food chain simply demonstrate that there is a feeding relationship between consumers in an ecosystem. As one moves up the pyramid or chain, the animals at a given trophic level are capable of feeding on the animals below them. Thus, the mice at the secondary consumer level in this illustration can feed on the beetles but not the other way around. However, keep in mind that sometimes a whole tropic level can be skipped in an ecosystem. That is, a snake can not only eat mice at the trophic level immediately beneath them but may also consume beetles two trophic levels below. Birds of prey can consume not only snakes in this ecosystem but can also consume mice or even beetles in some instances.

• In any case, there is an overall feeding chain that comes from the top down. Thus, in this case, one might observe that among the animals in this ecosystem that the primary consumers, the beetles, are “at the bottom of the food chain”. Again, the detritivores are included because they feed on the dead bodies of every trophic level, including the plant producers.

Note: The lettering in red to the left of the pyramid indicates that both the number of individual organisms and their total biomass decreases as one moves up the pyramid. This should be pointed out to students here, but we will discuss biomass in further detail in Investigation 3.

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Note: This is another slide that students saw in the last Investigation as well.

• In this slide we see the food chain pyramid again, this time turned on its side to include the Sun. Recall that the important concept here is that all energy that flows through an ecosystem comes from the Sun and then flows up from the producers through the trophic levels.

• Photons are represented as leaving the Sun and entering the ecosystem through the plant producers that capture some of this energy through photosynthesis.

• In Investigation 2 lab, we will discover that energy flow between the trophic levels is not efficient and that some energy is lost as it makes its way to the next higher trophic level. Consequently, we find less and less total energy at higher and higher trophic levels, just as we find fewer organisms and lower total biomass at those levels.

Note: Now that this and the previous slide from Investigation 1 have been reviewed, we will focus on how energy from the Sun enters and flows through the trophic levels of an ecosystem.

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• This slide points out that, while the surface of the Sun is a blazing 5,600^oC, the interior of the star is some two and a half thousand times that hot, at about 15,000,000^oC!

• Also important is the fact that, due to the Sun’s extreme size and therefore the tremendous force of gravity on such a massive object, causes incredibly high pressure and density at the Sun’s interior. The Sun’s internal temperature and pressure are of paramount importance, as only under these extreme conditions can the nuclear fusion reaction take place. Notice that the Sun is composed almost entirely of hydrogen and helium atoms. This composition is also relevant to the nuclear fusion reaction that we will discuss on the following slide.

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• The nuclear fusion reaction is rather simple to understand. Remember the hydrogen atom (H, pictured at the top right) has a single proton in its nucleus.

• At the very high temperatures and pressure that exist at the Sun’s core, 4 hydrogen atoms are forced together in a manner that results in the formation of a new element, helium (He).

• Notice, based on atomic mass (lower left), that 4 atoms of hydrogen should add up to an atomic mass of 4.0316 (4 X 1.0079). However, the atomic mass of the one helium formed by nuclear fusion is only 4.0026. The difference reflects that some of the atomic mass has been converted into the energy of a photon. It is this energy that radiates from the Sun and will be converted back into chemical energy by photosynthesis in plants on Earth.

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• This slide reviews the photosynthesis reaction. As shown, photosynthesis takes place exclusively in one organelle of plant cells (or algal cells), the chloroplast. It is in these structures that the green pigment chlorophyll is concentrated and where the energy produced by nuclear fusion in the Sun is converted into chemical energy for storage and use by plants.

• Notice that a photon of energy from the Sun must be added to the photosynthesis reaction to convert the gas carbon dioxide and water into the sugar glucose and the gas oxygen. This is why the reaction is called photo-synthesis, because a photon was used to “synthesize” a molecule of the sugar, glucose. The plant can use the energy stored in the chemical bonds of glucose to supply energy for its own metabolism and growth.

• In the next slide, we will see how this stored energy from the Sun can be transferred to heterotroph consumers who feed on plants and other animals.

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• Cellular respiration is the name given to the primary complex reaction(s) that takes place in heterotrophs. In the respiration reaction, nutrients, including glucose and other energy-containing molecules made from it, are converted into energy in the form of ATP (adenosine triphosphate) and carbon dioxide. In order for the reaction to occur, a molecule of oxygen gas (O₂) must also be added as a reagent. This is why animals require oxygen to live.

• Thus, a gas given off by autotrophs (O₂) is used by heterotrophs in cellular respiration, and a gas given off by heterotrophs (CO₂) is used by autotrophs to perform photosynthesis!

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• This slide shows both photosynthesis and cellular respiration together, stressing the interrelationship between the autotrophic and heterotrophic systems.

• As hard as it may be to believe, scientists are still not sure which kind of cells evolved first, heterotrophs or autotrophs. Or perhaps they both evolved from a yet earlier form of cells.

• As they exist today, autotrophs not only provide the nutrients and energy-rich molecules for heterotrophs to metabolize, they also supply the heterotrophs the oxygen they need to do so. Heterotrophs, on the other hand, give off CO₂ gas during respiration, which, in turn, can be used by autotrophs to perform photosynthesis. This arrangement has existed on Earth for billions of years.  

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• This final slide is a very good summary of our discussion of ecosystems thus far. In the upper left is the nuclear fusion reaction the takes place in the interior of the Sun. Nuclear fusion provides the light energy of photons that constantly leave the Sun’s surface and rain down on Earth.

Note: While it only takes a photon traveling at the speed of light about 8 minutes to reach Earth once it leaves the Sun, it may take well over 4,000 years for that same photon, formed at the Sun’s interior, to actually reach the surface of the Sun! 

• The next step in this review slide shows a plant cell and the photosynthesis reaction that it is responsible for. Finally, at the lower right, we see an animal cell and the cellular respiration reaction that it carries out.

Note: The air we inhale on Earth is mainly nitrogen (about 78%). The oxygen we inhale is present at about 21% of the air and CO₂ is only present at about 0.03%. On the other hand, as the result of cellular respiration, we exhale about 15% O₂ (down from 21% in the air around us) and 3.6% CO₂ (up from 0.03% in the air around us). When we breathe out oxygen, we complete a series of reactions that began many thousands of years ago at the very center of the Sun.

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