Photosynthesis: An Overview Read the lesson title aloud to students.

Learning Objectives Explain the role of pigments in the process of photosynthesis. Describe the role of electron carrier molecules. Identify the reactants and products of photosynthesis. Click to show each learning objective. To prepare students for this lesson, pass a light through a prism onto a surface where the colors can be seen. Ask: What do you observe about the light as it comes out of the prism? Answer: The light separates into the colors of the rainbow. Ask: What does that tell us about white light? Answer: That it is made up of different colors of light. Explain that the separation occurs because each different wavelength of light refracts, or bends, a different amount.

Chlorophyll and Chloroplasts Light energy from the sun must be captured for photosynthesis to occur. Sunlight is “white” light—actually a mixture of different wavelengths. Photosynthetic organisms capture energy from sunlight with pigments—principally with chlorophyll. Tell students: The lives of nearly every living thing on the surface of Earth are made possible by the sun and the process of photosynthesis. Display the prism again and pass the light through it. Ask: What colors do you see? Answer: Violet, blue, green, yellow, orange, and red. Tell students: The wavelengths we can perceive with our eyes are known as the visible spectrum. Explain that plants gather the sun’s energy with light-absorbing molecules called pigments. Tell students: The plants’ principal pigment is chlorophyll, and the two types of chlorophyll found in plants are chlorophyll a and chlorophyll b. Make the connection for students that chlorophyll in the leaves of plants does not absorb green light well—instead it reflects it—which is why plants look green. Tell students: Plants also contain red and orange pigments such as carotene that absorb light in the blue region of the spectrum. Most of the time, the intense green color of chlorophyll overwhelms the accessory pigments, so we don’t notice them. As temperatures drop late in the year, chlorophyll molecules break down first, leaving the reds and oranges of the accessory pigments for all to see. The beautiful colors of fall in some parts of the country are the result of this process.

Chloroplasts Photosynthesis takes place inside organelles called chloroplasts. Plant Cell Chloroplast Remind students that in plants and other photosynthetic eukaryotes, photosynthesis takes place inside organelles called chloroplasts. Click to highlight the chloroplast. Ask: What’s so special about chlorophyll that makes it important for photosynthesis? Answer: Because light is a form of energy, any compound that absorbs light absorbs energy. Chlorophyll absorbs visible light especially well. Explain to students that, in addition, when chlorophyll absorbs light, a large fraction of that light energy is transferred directly to electrons in the chlorophyll molecule itself. By raising the energy levels of these electrons, light energy can produce a steady supply of high-energy electrons, which is what makes photosynthesis work.

Chloroplast Structure In plants, photosynthesis takes place inside chloroplasts. Stroma Remind students that in plants and other photosynthetic eukaryotes, photosynthesis takes place inside organelles called chloroplasts. Point out that chloroplasts contain an abundance of saclike photosynthetic membranes called thylakoids. Request a volunteer to point out the thylakoid and write the name on the appropriate line. Click to reveal the correct answer location. Thylakoids are interconnected and arranged in stacks known as grana. Ask for a volunteer to point out a granum and write the label in the appropriate location. Pigments such as chlorophyll are located in the thylakoid membranes. Click to highlight the thylakoid membrane. The fluid portion of the chloroplast, outside of the thylakoids, is known as the stroma. Ask for a volunteer to point out the stroma and write the label in the appropriate location. Thylakoid Granum Thylakoid membrane

Electron Carriers The high-energy electrons produced by chlorophyll are highly reactive and require a special “carrier.” Encourage students to think of the high-energy electron as being similar to a hot coal. Like a pan being used to carry hot coals, NADP+ carries pairs of electrons and an H+ ion from place to place. Plant cells treat high-energy electrons in much the same way. Instead of a pan, they use electron carriers to transport high-energy electrons from chlorophyll to other molecules.

Electron Carrier An electron carrier is a compound that can accept a pair of high-energy electrons and transfer them, along with most of their energy, to another molecule. NADPH can carry the high-energy electrons that were produced by light absorption in chlorophyll to chemical reactions elsewhere in the cell. NADP+ is a carrier molecule that transports pairs of electrons (and an H+ ion) in photosynthetic organisms, similar to how an oven mitt is used to transport a hot object such as a baked potato. Ask: In the conversion of NADP+ to NADPH, what happens to the energy absorbed by chlorophyll from sunlight? Answer: The conversion of NADP+ to NADPH traps the energy of sunlight in chemical form. Explain to students that NADP+ accepts and holds two high-energy electrons, along with a hydrogen ion (H+). This converts the NADP+ into NADPH. The conversion of NADP+ into NADPH is one way in which some of the energy of sunlight can be trapped in chemical form.

An Overview of Photosynthesis Photosynthesis uses the energy of sunlight to convert water and carbon dioxide (low-energy reactants) into high-energy sugars and oxygen (products). Carbon dioxide + Water → Sugars + Oxygen light 6CO2 6H2O C6H12O6 6O2 light → + + Read aloud the sentence on the slide that summarizes photosynthesis. Tell students: Plants then use the sugars to produce complex carbohydrates, such as starches, and to provide energy for the synthesis of other compounds, including proteins and lipids. Explain that photosynthesis usually produces 6-carbon sugars (C6H12O6). Ask for a volunteer to write this chemical formula on the appropriate line of the equation. Then click to confirm this information. Ask for a volunteer to write in the chemical formulas for carbon dioxide, water, and oxygen. The student should write CO2, H2O, and O2. Then point out that in order for a chemical equation to be balanced, there needs to be equal amounts of each atom on either side of the equation. The total number of carbon, oxygen, and water atoms that exist on each side of the equation must remain the same. Click to reveal the remainder of the balanced equation.

Photosynthesis and Light Photosynthesis involves two sets of reactions: Light-dependent reactions Light-independent reactions Explain to students that, although the equation for photosynthesis looks simple, there are many steps to get from the reactants to the final products. Photosynthesis actually involves two sets of reactions: Light-Dependent Reactions (Click to highlight that portion of the illustration.) and Light-Independent Reactions (Click to highlight that portion of the illustration.) Ask: What happens to the ATP and NADPH produced in the light-dependent reactions? Answer: Both ATP and NADPH from the light-dependent reactions are used to produce high-energy sugars in the light-independent reactions.

Light-Dependent Reactions Light-dependent reactions require the direct involvement of light and light-absorbing pigments. Water Light-dependent reactions use energy from sunlight to produce energy-rich compounds such as ATP. These reactions take place within the thylakoids—specifically, in the thylakoid membranes—of the chloroplast. Water is required in these reactions as a source of electrons and hydrogen ions. Click to illustrate. Oxygen is released as a by-product. Ask: Why is the first set of reactions called the light-dependent reactions? Answer: Because these reactions require the direct involvement of light. Oxygen

Light-Independent Reactions Light-independent reactions use ATP and NADPH molecules produced in the light-dependent reactions to produce high-energy sugars from carbon dioxide Carbon Dioxide Remind students that plants absorb carbon dioxide from the atmosphere. Click to illustrate. Tell students: Plants complete the process of photosynthesis by producing carbon-containing sugars and other carbohydrates. Explain that no light is required to power the light-independent reactions. The light-independent reactions take place outside the thylakoids in the stroma. Sugars and Other carbohydrates

Interdependence of Reactions Light-dependent and light-independent reactions have an interdependent relationship. Point out to students that the two sets of reactions work together to capture the energy of sunlight and transform it into energy-rich compounds such as carbohydrates. Ask: What compounds are “recycled” between these reactions? Answer: NADPH/NADP+ and ATP/ADP are “recycled” between light-dependent and light-independent reactions. Click to highlight. Ask: What happens if an inhibitor of the light-independent reaction is added to chloroplasts? Answer: The light-dependent reaction is also inhibited because electron acceptors and ADP are not recycled.