1. The Process of Photosynthesis
Read the lesson title aloud to students.

2. Learning Objectives
Explain what happens during the light-dependent reactions.
Explain what happens during the light-independent reactions.
Identify the factors that affect photosynthesis.
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Refresh students’ memories of what they learned in the last lesson.
Ask: Which stage of photosynthesis requires light?
Answer: Light-dependent reactions require light.
Let students know that, in this lesson, they will learn more about what specifically happens during the light-dependent and the light-independent reactions. By the end of this lesson, they will also be able to identify the factors that affect photosynthesis.

3. Two Sets of Reactions
Photosynthesis involves two primary sets of reactions: light-dependent reactions and light-independent reactions. 
Remind students that photosynthesis involves both light-dependent and light-independent reactions.
Ask: What do the light-dependent reactions produce?
Answer: Oxygen.
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Ask: What is converted in the process?
Answer: ADP and NADP+ are converted into the energy carriers ATP and NADPH.

4. Light-Dependent Reactions
Light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Ask: Do you recall from our last lesson where the light-dependent reactions occur?
Answer: The light-dependent reactions occur in the thylakoids of chloroplasts.
Request a volunteer to point out a thylakoid.
Click to point out a thylakoid.
Explain that thylakoids are saclike membranes containing most of the machinery needed to carry out light-dependent reactions.
Thylakoid

5. Thylakoid
Thylakoids contain clusters of chlorophyll and proteins known as photosystems.
Explain to students that photosystems, which are surrounded by accessory pigments, are essential to light-dependent reactions.
Light absorption by photosystems is just the beginning of photosynthesis.
Ask: Why are most plants green?
Answer: The color green is caused by the reflection of green light by the pigment chlorophyll. Pigments capture light energy during the light-dependent reactions of photosynthesis.

6. Photosystem II Absorbs light energy and produces high-energy electrons
Splits water molecules, releasing H+ ions and oxygen
Tell students: Light energy is absorbed by electrons in the pigments found within photosystem II, increasing the electrons’ energy level. These high-energy electrons (e–) are passed to the electron transport chain.
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Explain that, as light continues to shine, more and more high-energy electrons are passed to the electron transport chain. The thylakoid membrane contains a system that provides new electrons to chlorophyll to replace the ones it has lost.
Ask: Where do you think these new electrons come from?
Answer: From water. Enzymes on the inner surface of the thylakoid break up each water molecule into two electrons, two H+ ions, and one oxygen atom.
Explain that, as plants remove electrons from water, oxygen is left behind and is released into the air.
Point out that this reaction is the source of nearly all of the oxygen in Earth’s atmosphere.

7. Electron Transport Chain
A series of electron carrier proteins shuttle high-energy electrons during ATP-generating reactions.
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Ask: What happens to the electrons as they move down the electron transport chain?
Answer: Energy from the electrons is used by the proteins in the chain to pump H+ ions from the stroma into the thylakoid space.
Request a volunteer to point out where this is illustrated.
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Tell students: At the end of the electron transport chain, the electrons themselves pass to a second photosystem called photosystem I.

8. Photosystem I Electrons are reenergized
Second electron transport chain transfers electrons to NADP+, producing NADPH
Point out to students that because some energy has been used to pump H+ ions across the thylakoid membrane, electrons do not contain as much energy as they used to when they reach photosystem I.
Direct students to the photosystem I portion of the illustration.
Tell students: The pigments in photosystem I use energy from light to reenergize the electrons.
At the end of a short second electron transport chain, NADP+ molecules in the stroma pick up the high-energy electrons, along with H+ ions, at the outer surface of the thylakoid membrane, to become NADPH.
Ask: How many molecules of NADPH are produced per water molecule used in photosynthetic electron transport?
Answer: Two NADPH molecules are produced for every two water molecules split, so one per water molecule.
Click to highlight the portion of the illustration that shows the production of NADPH.
Ask: Why do you think this NADPH is important?
Answer: This NADPH is very important in the light-independent reactions of photosynthesis.

9. Hydrogen Ion Movement/ATP Formation
The difference in both charge and H+ ion concentration across the membrane provides the energy to make ATP.
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Explain that, as the thylakoid space fills up with positively charged H+ ions, the inside of the thylakoid membrane becomes positively charged relative to the outside of the membrane.
H+ ions pass back across the thylakoid membrane through ATP synthase down a concentration gradient.
Request a volunteer to point out where the passing of the hydrogen ion through the membrane is illustrated.
Then click to highlight.
As the ions pass through, the ATP synthase molecule rotates and the energy produced is used to convert ADP to ATP.
Click twice to highlight the synthase rotation and ATP generation as you explain.

10. Light-Dependent Reactions Summary
The light-dependent reactions produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH.
Ask: What good are the compounds produced in light-dependent reactions?
Answer: They have an important role to play in the cell: They provide the energy needed to build high-energy sugars from low-energy carbon dioxide.

11. Light-Independent Reactions
ATP and NADPH are used to synthesize high-energy sugars.
Tell students: The light-independent reactions are commonly referred to as the Calvin cycle.
Ask: What products of the light-dependent reactions are used in the light-independent reactions?
Answer: The reactions use the electron carriers ATP and NADPH from the light-dependent reactions to produce high-energy sugars such as glucose.
Ask for a volunteer to point out the locations where ATP and NADPH are used.
Then click to highlight.
Ask: How many molecules of ATP are needed for each “turn” of the Calvin cycle?
Answer: Eighteen ATP molecules are needed for each turn of the Calvin cycle.
Ask: Where do the light-independent reactions occur?
Answer: In the stroma of the chloroplast.
Ask: What is the main product of the Calvin cycle?
Answer: Sugars and other compounds.
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12. Carbon Dioxide Enters the Cycle
Six carbon dioxide molecules from atmosphere combine with six 5-carbon molecules
Produces 12 3-carbon compounds.
Explain to students that carbon dioxide molecules enter the Calvin cycle from the atmosphere.
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An enzyme in the stroma of the chloroplast combines these carbon dioxide molecules with the 5-carbon compounds that are already present in the organelle.
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Ask: How many 3-carbon compounds are produced for every 6-carbon dioxide molecules that enter the cycle?
Answer: A total of 12 3-carbon compounds are produced.
If students are having difficulty following this event, help them understand by working through the math with them on the board.
Ask: How many carbon atoms does a molecule of CO2 contain?
Answer: One.
Ask: When a carbon dioxide molecule combines with a 5-carbon compound, how many 3-carbon
compounds are produced?
Answer: Two, because = 6; 6 ÷ 3 = 2.
Ask: If for every carbon dioxide molecule that enters the cycle, two 3-carbon compounds are produced,
then how many 3-carbon compounds are produced when 6-carbon dioxide molecules enter the cycle?
Answer: Twelve, because 2 × 6 = 12.

13. Sugar Production
Energy from ATP and high-energy electrons from NADPH are used to convert the 3-carbon molecules to higher- energy forms.
Point out to students that the two 3-carbon molecules that were removed become the building blocks that the plant cell uses to synthesize sugars, lipids, amino acids, and other compounds.
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Explain that the remaining 10 3-carbon molecules are converted back into six 5-carbon molecules.
Ask: What becomes of the six 5-carbon molecules?
Answer: These molecules combine with six new carbon dioxide molecules to begin the next cycle.