In this Investigation, we wish to introduce students to the concept of photosynthesis. We wish to first emphasize the abundance and importance of photosynthesis on Earth.

We will then turn to the chemical formula for photosynthesis, undoubtedly one of the most important chemical reactions on our planet. We will review where photons come from and where they obtain the energy that is captured by the photosynthetic reaction through the pigment chlorophyll. We will also remind students that chlorophyll is located in the chloroplasts of plant and algal cells.

Finally, we will introduce the procedure of paper chromatography, which students will use in Investigation 1 lab to separate pigments extracted from spinach leaves.  

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• This is essentially an introductory slide for photosynthesis. It shows a tropical jungle. Notice the lush vegetation and all of the green color. Essentially all of that green comes from the presence of chlorophyll in plant cells. All of the chlorophyll, in turn, is involved in photosynthesis.

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• This interesting slide is from NASA’s Visible Earth Project. It shows the abundance of chlorophyll and plant/algal concentrations on planet Earth.

• Photosynthesis on land: Use the key on the lower right to consider the plant concentration and photosynthesis activity on the various continents. Notice the intense green at the equator, Indonesia, much of South America, equatorial Africa, and Asia. These represent the bulk of the world’s jungles. Also, notice the large gold bands of desert to the immediate north and south of the equatorial jungles. This is perhaps best seen on the African continent. Further to the north and south, the temperate forests of the world are found.

• Photosynthesis in the oceans: Notices that the greatest concentration of chlorophyll and therefore photosynthesis occurs at latitudes roughly equivalent to the highest photosynthetic activity on the land. Thus, higher areas of chlorophyll are found (light blues and greens, using the key at the lower left) near the equator and in areas adjacent to the temperate forests. The large blue areas of the oceans have low chlorophyll concentrations because, while the ocean may be well over a kilometer deep in such areas, the sunlight required for photosynthesis only penetrates down to a range of tens of meters. All photosynthesis occurs near the surface of the oceans. Finally, notice that the highest photosynthetic activity occurs near shorelines, particularly at the mouths of rivers. This is largely due to the runoff of nutrients from land surfaces and relatively shallow waters.

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Note: Students will see slides very similar to this one in their next CELL, Ecosystems.

• This slide dramatizes the direct interaction of photons in sunlight with plants. This energy is the basis for all life on Earth. Through photosynthesis, plants will convert this light energy into chemical energy for growth. As students will learn in Ecosystems, this chemical energy is passed on up the “food chain” and sustains essentially every living organism on the planet.

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• This slide presents the photosynthesis reaction. In words, it is a quite simple reaction: carbon dioxide and water are transformed by light (photons) and chlorophyll into glucose and oxygen. Glucose is a sugar where the light energy from photons is stored as chemical energy. The O₂ is used by animals for respiration.

• Notice that the balanced equation includes 6 molecules both CO₂ and H₂O on the reactant side and one molecule of glucose and 6 molecules of oxygen gas (O₂) on the product side. The number and approximate molecular size of the individual components of the reaction are shown as ball and stick models.

• The squiggly arrow and the Greek letter gamma symbolize light energy. Importantly, notice that chlorophyll is included as a catalyst in the reaction. As students already know but will see again in this CELL, the pigment chlorophyll is concentrated in plant organelles, chloroplasts.

• At the bottom of the slide is the question: Where does the energy of a photon come from? We will devote the next several slides to reviewing photon production by the Sun to answer this question.

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• Students have seen this slide in the past, in the Space CELL. It is included to point out the extreme conditions of temperature and pressure at the Sun’s center as well as to show the almost limitless amount of hydrogen that is present on our nearest star.

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Note: Students will have seen a similar slide previously.

• Nuclear fusion: At the very center of the Sun, where temperature and pressure are intense, nuclear fusion reactions take place.

• Under such conditions, the nuclei of four atoms of hydrogen (H) fuse to form a single helium (He) atom. In the process, a single photon is formed. On the following slide, we will see how the nuclear fusion reaction produces energy.

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• Here again, we have the nuclear fusion reaction where four hydrogen atoms fuse to form a single helium atom.

• A Periodic Table is included with the elements H and He enlarged. Notice that according to the Periodic Table, each hydrogen atom has a molecular mass of 1.0079. The mass of the four hydrogen atoms entering into a fusion reaction is therefore 4.0316 (4 X 1.0079).

• After nuclear fusion has occurred and a helium atom is formed, we see that its mass is only 4.0026. That is, it has a mass less than the four hydrogen atoms that combined through fusion to form it!

• The Law of Conservation of Matter tells us that matter can neither be created nor destroyed. Nonetheless, matter has decreased in the nuclear fusion reaction. What happened?

• It turns out that there is also a relationship between mass and energy that became clear through the work of Albert Einstein and others about one hundred years ago. During the nuclear fusion reaction, a small amount of matter was transformed into a particle of light energy, called a photon. It is the photons produced by nuclear fusion in the Sun that bathe the Earth with energy for photosynthesis.

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• In this slide, we return to the photosynthesis reaction. It may take thousands of years, perhaps as many a 10,000 years, for a photon formed at the Sun’s dense and superheated center to reach its surface and be released from the star. As a reference, it was also about 10,000 years ago when human beings on Earth first began to domesticate and grow plants, paving the way for civilization.
• However, once released, it only takes a photon about 8 minutes to reach the Earth’s surface. By comparison, a modern commercial jet, flying at 885 kph (kilometers per hour), directly at the Sun, would take nearly 20 years to cover the same distance a photon travels in only eight minutes!
• As shown in the chemical formula in this slide, photons provide the energy for the photosynthesis reaction to occur.
• Circled in this slide, is the pigment chlorophyll. This somewhat complex molecule is shown on the next slide.

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• The chlorophyll molecule contains atoms of hydrogen, carbon, oxygen, nitrogen, and magnesium. Its molecular mass is 893.49. That is, if you added up the mass of all of the different atoms in the chlorophyll molecule, it would total 893.49. By comparison, a single hydrogen atom has a molecular mass of 1.0079 and an atom of carbon and oxygen have molecular masses of 12.011 and 15.99, respectively.

• Once again, the chloroplast is included on this slide to remind students that chlorophyll is located in these intracellular organelles. In fact, the structure of the chloroplast, which contains hundreds of different proteins and other molecules, as well as a complex membrane system, begins to convert the energy of glucose molecules, made by photosynthesis, into many other compounds that are used by plants for energy storage and growth.

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Note: In this slide, we introduce a very useful procedure – paper chromatography. Students will use this procedure in Investigation 1 lab.

• As indicated, a sample is applied to a strip of filter paper. It is allowed to dry and then the tip is dipped into a solvent. The solvent will be absorbed by the filter paper and move up the strip like a wick in a lamp. This is called “capillary action”.

• When the rising solvent reaches the spot of the sample, the molecules in the sample are solubilized by the solvent. Solvents are chosen so that components of the sample are soluble in it, otherwise, the sample will not move with the solvent as it moves up the strip.

• How far and fast molecules in the sample move up the strip is dependent on both how soluble they are in the solvent and how strongly they interact with the filter paper itself. Since these factors may differ from one type of sample molecule to another, paper chromatography allows us to separate and study them. That is one of the reasons that this procedure is so useful to scientists.

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• This final slide shows a simulation of what one might expect in the experiment in Investigation 1 lab. It shows the movement of the sample as the solvent makes its way up the filter paper strip.

• If a given sample contains multiple different molecules, they may be separated by this procedure. This is because the physical properties of two different molecules can affect both their relative solubility in the solvent and their relative attraction to the filter paper.

• By using various combinations of solvents and solid absorptive surfaces (the filter paper in this case), scientists use chromatography to separated and study individual molecules that were once components of a molecular mixture in the sample.

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